Intro to CDMA Paper

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RF EngineeringContinuing Education & Training

Introduction to CDMA

Prepared by:

SAFCO Technologies, Inc.

600 Atlantis Rd.

Melbourne, FL 32904 USA

Phone: (407) 952-8300

Fax: (407) 725-5062

www.safco.com

Revision 3

Copyright 1997 by SAFCO Technologies, Inc.

All rights reserved. No part of this book shall be

reproduced, stored in a retrieval system, or transmitted

by any means, electronic, mechanical, photocopying,

recording, or otherwise, without written permission

from SAFCO Technologies, Inc.



Approximate Unit Length: 8 hr.

The purpose of this unit is to expose personnel unfamiliar with CDMA Technology to the basic

properties of CDMA. This unit assumes that the attendees have a basic understanding of wireless

digital and analog communications systems. The target audience for this unit includes Associate

level and above RF engineers as well as engineering managers.

Upon successful completion of this unit, the student should be able to describe:

• The definition of CDMA and its theoretical advantages

• The direct sequence modulation technique

• The concept of physical and logical channels

• The concept of call quality, how it is measured, and how it affects system capacity

• The CDMA advantage as provided by the utilization of the RAKE receiver

• The factors affecting the capacity of CDMA systems

• The various handoffs associated with CDMA

• The basic reverse link and forward link processes of a CDMA system• Some basic concerns associated with engineering a CDMA system



RF Engineering Continuing Education & Training


Table of Contents

1 DEFINITION OF CDMA .................................................................................................................................... 8

1.1 CDMA BASICS ....................................................................................................................................................8

1.2 CDMA POWER SPECTRAL DENSITY & NOISE .....................................................................................................8

1.3 ADVANTAGES OF CDMA...................................................................................................................................10

1.3.1 Frequency Reuse .....................................................................................................................................10

1.3.2 Coherent Signal Combination .................................................................................................................10

1.3.3 User Privacy............................................................................................................................................11

1.4 COVERAGE AND CAPACITY LIMITATIONS ..........................................................................................................11

1.5 COMPARISON OF MULTIPLE ACCESS TECHNIQUES.............................................................................................11

1.5.1 FDMA......................................................................................................................................................11

1.5.2 TDMA ......................................................................................................................................................11

1.5.3 Multiple access: division by code............................................................................................................12

2 CDMA SPREAD SPECTRUM TERMINOLOGY ......................................................................................... 13

2.1 IS-95 AND IS-95-A CDMA:..............................................................................................................................13

2.2 FORWARD AND R EVERSE LINKS.........................................................................................................................13

2.3 CORRELATION AND ORTHOGONALITY ...............................................................................................................13

2.4 PN SEQUENCE ...................................................................................................................................................14

2.5 CHIPS AND CHIP R ATE .......................................................................................................................................15

2.6 BIT R ATE ...........................................................................................................................................................15

2.7 TRAFFIC FRAME .................................................................................................................................................15

2.8 PROCESSING GAIN .............................................................................................................................................15

2.9 EB/NT, BER, AND OTHER FIGURES OF MERIT .....................................................................................................16

2.10 SUMMARY OF CODES....................................................................................................................................16

2.10.1 PN Long Code .........................................................................................................................................16

2.10.2 PN Short Codes .......................................................................................................................................17

2.10.3 Walsh Codes ............................................................................................................................................17

2.11 CDMA CALL QUALITY (EB/NT) ....................................................................................................................18

2.12 COHERENT VS. NON-COHERENT DETECTION ................................................................................................19

3 CDMA PHYSICAL AND LOGICAL CHANNELS........................................................................................ 20

3.1 PHYSICAL CHANNEL ..........................................................................................................................................20

3.2 LOGICAL CHANNEL............................................................................................................................................20

3.2.1 Forward Link (Downlink)........................................................................................................................203.2.1.1 Pilot......................................................................................................................................................................21

3.2.1.2 Sync Channel .......................................................................................................................................................21

3.2.1.3 Paging Channel ....................................................................................................................................................21

3.2.1.4 Traffic Channel ....................................................................................................................................................21

3.2.1.5 Power Control Sub-Channel.................................................................................................................................22

3.2.2 Reverse Link (Uplink)..............................................................................................................................223.2.2.1 Access Channel ....................................................................................................................................................23

3.2.2.2 Traffic Channel ....................................................................................................................................................23

4 CDMA MODULATION & DEMODULATION ............................................................................................. 24

4.1 TYPES OF SPREAD SPECTRUM MODULATION .....................................................................................................24

4.1.1 Frequency Hopping.................................................................................................................................24

4.1.2 Direct Sequence.......................................................................................................................................24

4.2 SPREAD SPECTRUM (CDMA) MODULATION EXAMPLE: E NCODING AND DECODING OF I NFORMATION ...........25

4.2.1 Spread Spectrum Transmit Process.........................................................................................................25

4.2.2 Spread Spectrum Receive Process...........................................................................................................26

4.2.3 Multiple Signal Case ...............................................................................................................................27

Copyright © 1997 by SAFCO Technologies, Inc. 3 Version 3.0





5 THE CDMA ADVANTAGE - THE RAKE RECEIVER AND THE MULTIPATH ENVIRONMENT ... 29

5.1 A BRIEF REVIEW OF MULTIPATH AND ITS EFFECT ON A NALOG AND DIGITAL TRANSMISSIONS..........................29

5.2 THE RAKE R ECEIVER .......................................................................................................................................31

5.3 COMPARISON OF THE EFFECTS OF MULTIPATH ON FDMA, TDMA, AND CDMA..............................................35

5.3.1 FDMA......................................................................................................................................................35

5.3.2 TDMA ......................................................................................................................................................35

5.3.3 CDMA......................................................................................................................................................37 5.3.4 Summary of Multipath Effects .................................................................................................................37

5.4 RAKE R ECEIVER EXAMPLE: IMPROVEMENT IN CALL QUALITY (EB/NT) ..........................................................37

6 DYNAMIC POWER CONTROL ..................................................................................................................... 38

6.1 THE “NEAR -FAR ” PROBLEM..............................................................................................................................39

6.2 R EVERSE LINK ...................................................................................................................................................39

6.2.1 Open-Loop...............................................................................................................................................39

6.2.2 Closed-Loop ............................................................................................................................................39

6.3 FORWARD LINK .................................................................................................................................................40

7 CDMA IMPLEMENTATION AND DIGITAL RADIO LINK PROCESSES............................................. 41

7.1 FORWARD LINK .................................................................................................................................................41

7.1.1 Variable Rate Speech Coding..................................................................................................................427.1.2 Channel Coding.......................................................................................................................................43

7.1.3 Bit Interleaving........................................................................................................................................44

7.1.4 Encryption: Long Code Scrambling.......................................................................................................447.1.4.1 Paging Channel Encryption..................................................................................................................................45

7.1.4.2 Access Channel Encryption .................................................................................................................................46

7.1.4.3 Traffic Channel Encryption..................................................................................................................................46

7.1.5 Walsh Function Modulation ....................................................................................................................46 7.1.5.1 Power Control Signaling Subchannel Modulation ...............................................................................................46

7.1.5.2 Forward Link Base Station Transmit Power Control ...........................................................................................47

7.1.6 Quadrature Spreading & Carrier Modulation........................................................................................48

7.2 R EVERSE LINK ...................................................................................................................................................49

7.2.1 Variable Low Bit Rate Speech Coding ....................................................................................................50

7.2.2 Channel Coding.......................................................................................................................................517.2.3 Bit Interleaving........................................................................................................................................52

7.2.4 64-ary Orthogonal Walsh Symbol Modulation .......................................................................................52

7.2.5 Encryption: Long Code Spreading.........................................................................................................53

7.2.6 Quadrature Spreading & Carrier Modulation........................................................................................54

7.3 SYSTEM BLOCK DIAGRAM .................................................................................................................................55

8 CDMA CAPACITY............................................................................................................................................ 56

8.1 THE GENERAL CASE ..........................................................................................................................................56

8.2 ADJUSTMENTS TO THE GENERAL CASE ..............................................................................................................57

8.2.1 Sectorization Gain ...................................................................................................................................57

8.2.2 Voice Activity Factor ...............................................................................................................................58

8.2.3 Frequency Reuse Efficiency (I ADJ. ) ..........................................................................................................58

8.3 DEFINITION OF

POLE

POINT

...............................................................................................................................588.4 THE POLE POINT EQUATION...............................................................................................................................59

9 CDMA HANDOFF............................................................................................................................................. 60

9.1 HANDOFF TERMINOLOGY ..................................................................................................................................60

9.1.1 Introduction to T ADD , T DROP & T COMP .......................................................................................................60

9.1.2 Handoff Candidate Classification ...........................................................................................................61

9.2 TYPES OF HANDOFFS .........................................................................................................................................61

9.2.1 Soft Handoff.............................................................................................................................................629.2.1.1 Forward Link........................................................................................................................................................62






9.2.1.2 Reverse Link ........................................................................................................................................................62

9.2.1.3 Joint Power Control..............................................................................................................................................62

9.2.2 Soft - Soft Handoff ...................................................................................................................................63

9.2.3 Softer Handoff .........................................................................................................................................63

9.2.4 Soft - Softer Handoff................................................................................................................................63

9.2.5 Hard Handoff ..........................................................................................................................................63

9.2.6 CDMA to Analog Handoff .......................................................................................................................63

9.3 HANDOFF CRITERIA ...........................................................................................................................................63

9.4 HANDOFF PROCESS ............................................................................................................................................64

9.4.1 Example 1 ................................................................................................................................................64

9.4.2 Example 2 ................................................................................................................................................64

9.4.3 Example 3 ................................................................................................................................................66

10 CDMA CALL EXAMPLE................................................................................................................................. 67

10.1 I NITIAL SYSTEM ACCESS...............................................................................................................................67

10.2 CALL I NITIATION AND SETUP ........................................................................................................................67

10.3 SOFT HANDOFF .............................................................................................................................................67

10.4 CALL TERMINATION......................................................................................................................................68

11 BASIC SYSTEM ENGINEERING ISSUES .................................................................................................... 68

11.1 PROPAGATION MODELING OF THE WIDEBAND CDMA RF SIGNAL ...............................................................69

11.2 LINK BUDGET................................................................................................................................................70

11.3 NOMINAL CELL CONFIGURATIONS& NOMINAL CELL R ADII CALCULATIONS...............................................71

11.4 NOMINAL SYSTEM PARAMETERS ..................................................................................................................74

11.5 COVERAGE & CAPACITY R ELATIONSHIP.......................................................................................................74

11.5.1 Sensitivity Analysis: Effects of Loading on the System..........................................................................74

11.5.2 Sensitivity Analysis Example ...................................................................................................................75

11.6 PN OFFSET PLANNING ..................................................................................................................................75

11.7 PN I NTERFERENCE........................................................................................................................................77

11.8 NOMINAL ASSIGNMENT OF PN (RAKE) SEARCH WINDOW..........................................................................77






List of FiguresFIGURE 1-1: COMPARISON OF I NFORMATION AND TRANSMISSION BANDWIDTH ...............................................................9

FIGURE 1-2: NOISE IN NARROW BAND AND SPREAD SPECTRUM COMMUNICATION SYSTEMS...........................................9

FIGURE 1-1: COMPARISON OF MULTIPLE ACCESS TECHNIQUES......................................................................................12

FIGURE 2-1: AUTOCORRELATION OF PSEUDO- NOISE BIT SEQUENCE ................................................................................14

FIGURE 2-2 FOUR -STAGE SHIFT REGISTER : GENERATION OF PN SEQUENCE ...................................................................15

FIGURE 2-1: SUMMARY OF SEQUENCES USED IN CDMA SPREAD SPECTRUM .................................................................18

FIGURE 2-1: EXAMPLE OF FER TO EB/NT RELATION: DIFFERENT FOR FORWARD AND R EVERSE LINK ................19

FIGURE 3-1: FORWARD LINK CHANNEL ASSIGNMENTS...................................................................................................20

FIGURE 3-2: R EVERSE LINK CHANNEL ASSIGNMENTS ....................................................................................................22

FIGURE 4-1: SPREAD SPECTURM TRANSMIT PROCESS.....................................................................................................25

FIGURE 4-2: SPREAD SPECTRUM R ECEIVE PROCESS .......................................................................................................26

FIGURE 5-1: DESTRUCTIVE I NTERFERENCE DUE TO MULTIPATH.....................................................................................30

FIGURE 5-1: SINGLE TRANSMITTER WITH MULTIPATH....................................................................................................31

FIGURE 5-2: TYPICAL SINGLE TRANSMITTER BAND-LIMITED CHANNEL IMPULSE R ESPONSE WITH FIVE DISCRETE

MULTIPATH COMPONENTS .....................................................................................................................................32

FIGURE 5-3: COHERENT COMBINATION OF THREE STRONGEST MULTIPATH COMPONENTS FROM A SINGLE TRANSMITTER

...............................................................................................................................................................................33

FIGURE 5-4: MULTIPLE TRANSMITTERS WITH MULTIPATH .............................................................................................34

FIGURE 5-5: TYPICAL MULTIPLE TRANSMITTER BAND-LIMITED CHANNEL IMPULSE R ESPONSE WITH DISCRETE

MULTIPATH COMPONENTS .....................................................................................................................................34

FIGURE 5-6: COHERENT COMBINATION OF THREE STRONGEST COMPONENTS OF A TYPICAL MULTIPLE TRANSMITTER

BAND-LIMITED CHANNEL IMPULSE R ESPONSE WITH DISCRETE MULTIPATH COMPONENTS...................................35

FIGURE 5-1: TIME DISPERSION........................................................................................................................................36

FIGURE 7-1: CDMA DIGITAL R ADIO FORWARD LINK PROCESS .....................................................................................42

FIGURE 7-2: FORWARD LINK SPEECH PROCESSING AT THE NETWORK SIDE.....................................................................43

FIGURE 7-3: CHANNEL CODING PROCESS .......................................................................................................................44

FIGURE 7-4: BIT I NTERLEAVING......................................................................................................................................44

FIGURE 7-5: FORWARD LINK SCRAMBLING FOR TRAFFIC AND PAGING CHANNELS ........................................................45

FIGURE 7-6: POWER CONTROL SIGNALING SUBCHANNEL...............................................................................................47

FIGURE 7-7: FORWARD LINK BASE STATION TRANSMIT POWER CONTROL ....................................................................48

FIGURE 7-8: FORWARD LINK QUADRATURE SPREADING AND CARRIER MODULATION...................................................49

FIGURE 7-1: CDMA R EVERSE LINK R ADIO PROCESS .....................................................................................................50FIGURE 7-2: SPEECH PROCESSING AT MOBILE SIDE ........................................................................................................51

FIGURE 7-3: R EVERSE LINK CHANNEL CODING PROCESS ...............................................................................................52

FIGURE 7-4: R EVERSE LINK BIT I NTERLEAVING .............................................................................................................52

FIGURE 7-5: R EVERSE LINK TRAFFIC CHANNEL SPREADING, POWER CONTROL GROUP GATING, AND E NCRYPTION.....54

FIGURE 7-6: R EVERSE LINK QUADRATURE SPREADING AND CARRIER MODULATION ....................................................55

FIGURE 7-1: CDMA FORWARD LINK (BASE TO MOBILE) PHYSICAL LAYER ..................................................................55

FIGURE 7-2: CDMA R EVERSE LINK (MOBILE TO BASE) PHYSICAL LAYER ....................................................................56

FIGURE 9-1: MOBILE U NIT TRANSITIONS INTO A REGION DEFINED BY TWO PILOT CHANNELS GREATER THAN T_ADD

(SOFT HAND-OFF)...................................................................................................................................................64

FIGURE 9-2: MOBILE U NIT TRANSITIONS INTO A REGION DEFINED BY FOUR OR MORE PILOT CHANNELS GREATER THAN

T_ADD..................................................................................................................................................................65

FIGURE 9-3: MOBILE U NIT TRANSITIONS THROUGH A REGION DEFINED BY TWO PREVAILING PILOTS GREATER THAN

T_ADD. .................................................................................................................................................................66FIGURE 11-1: TYPICAL CDMA SYSTEM PARAMETERS...................................................................................................74

FIGURE 11-1: COMPARISON OF COVERAGE DUE TO CHANGE IN TRAFFIC (5% TO 80% OF THEORETICAL CAPACITY) ......75






LIST OF TABLESTABLE 4-1: SUMMARY OF FREQUENCY HOPPING QUALITIES ..........................................................................................24

TABLE 4-2: SUMMARY OF DIRECT SEQUENCE SPREAD SPECTRUM QUALITIES ...............................................................25

TABLE 5-1: CALL QUALITY DB TO LINEAR CONVERSION TABLE....................................................................................38

TABLE 6-1: FORWARD LINK TCE ATTENUATION LEVEL VS. VOICE CODING R ATE........................................................40

TABLE 6-2: BASE STATION NOMINAL CHANNEL POWER ALLOCATIONS.........................................................................40TABLE 7-1: BASE STATION TRANSMIT POWER VS. DATA R ATE ......................................................................................48

TABLE 7-2: I AND Q BITS AND CORRESPONDING PHASE MODULATION STATE ...............................................................49

TABLE 7-1: I AND Q BITS AND CORRESPONDING PHASE MODULATION STATE ...............................................................54

TABLE 9-1: PILOT SEARCH PARAMETERS........................................................................................................................61

TABLE 11-1: R ECEIVER SENSITIVITY FOR DIFFERENT CDMA CHANNEL TYPES..............................................................70

TABLE 11-2: SIMPLIFIED EXAMPLE OF IS-95 CDMA LINK BUDGET FOR I N-VEHICLE COVERAGE .................................71

TABLE 11-1: SUMMARY OF PARAMETERS USED TO CALCULATE NOMINAL CELL RADIUS, AND CALCULATED CELL RADIUS

FOR EACH AREA TYPE AND ANTENNA CONFIGURATION OF A TYPICAL SYSTEM AT 50% LOADING. ..........................73

TABLE 11-1: TYPICAL DELAY SPREAD VALUES FOR DIFFERENT E NVIRONMENT TYPES.................................................76

LIST OF EQUATIONSEQUATION 2-1: DEFINITION OF CORRELATION................................................................................................................14

EQUATION 2-1: PROCESS GAIN......................................................................................................................................16

EQUATION 2-1: FRAME ERROR R ATE ..............................................................................................................................18

EQUATION 5-1: ∆ PATH LENGTH.....................................................................................................................................32

EQUATION 5-1: CALL QUALITY DB TO LINEAR CONVERSION.........................................................................................38

EQUATION 8-1: CAPACITY EQUATION (GENERAL FORM) ............................................................................................57

EQUATION 8-1: POLE POINT EQUATION ..........................................................................................................................59

EQUATION 11-1: CALCULATION OF NOMINAL CELL R ADII ..............................................................................................73






1 Definition of CDMA

Cellular and Personal Communications Services (PCS) face an ever-increasing number of users

sharing a limited amount of spectrum. In order to accommodate this increasing demand for communication services, providers must increase system capacity without degrading the quality of

service to an unacceptable level. One approach for meeting increased subscriber demand is the use

of Code Division Multiple Access (CDMA). CDMA is a digital spread spectrum technology that

has been used for military and satellite communications for several decades. CDMA, as it applies

to the land mobile telephone environment, is new and is most easily defined or explained by

comparison with more familiar technologies and simple example. Section 1 addresses some basic

characteristics and parameters associated with and unique to CDMA.

1.1 CDMA Basics

CDMA is a Multiple Access Direct Sequence Spread Spectrum Modulation Technique. Thistype of modulation takes narrow band (10 kHz) user information (voice or data) and transmits it

over a very wide RF bandwidth (1.23 MHz). Many users occupy the same RF transmission band.

This is very different from standard AMPS cellular in which each user is assigned a unique narrow

band (10 or 30 kHz) channel. CDMA uses correlative codes to distinguish each individual user in

the system. Each CDMA channel or Traffic Channel Element (TCE) is defined by a unique

correlative code and an associated center frequency. When the signal is received, a correlator

recovers the desired signal and rejects the other signals and interference. This is possible because

all interference sources (including other CDMA users) are uncorrelated with the desired signal.

1.2 CDMA Power Spectral Density & Noise

In a narrow band communication system, the energy used to transmit information is confined to a

relatively small bandwidth – on the order of the information bandwidth. The underlying concept of

spread spectrum communication system is the spreading of the transmitted energy over a wide

bandwidth. The “effective” transmission bandwidth of a direct sequence spread spectrum system is

related to the rate of the final spreading sequence and the type of modulation used. Relatively wide

(i.e. large time duration) pulses in the time domain result in energy being transmitted over a narrow

frequency range. Much shorter pulses (used in CDMA PN spreading sequences) result in energy

being transmitted over a wide range of frequencies. This is illustrated in Figure 1-1.






Time Domain

τ

τ

t

t

Frequency Domain

nτ

nτ

f

f

Figure 1-1: Comparison of Information and Transmission Bandwidth

The thermal noise encountered in a narrow band communication system is typically considered to

be constant (for a given temperature) over frequency. This level of background noise power

contained in a given bandwidth is called the noise floor. In the case of narrow bandcommunications, concentrating the transmitting energy in a narrow frequency band provides a

received RF signal that is above the noise floor. Having the signal sufficiently above the noise floor

is critical to being able to detect and receive (demodulate) the narrow band signal. This is measured

as ratio of the desired signal energy per bit (E b) to total system noise (Nt). For spread spectrum

systems, the transmitted energy is spread over such a wide bandwidth that the received signal

density may be below the noise floor – yet it is still recoverable knowing the correct spreading

sequence (code). This is illustrated in Figure 1-2.

Narrow Band & Wide Band Signal/Noise

-140

-120

-100

-80

-60

-40

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Distance

Pwr

(dBm)

RSL

Narrow BandNoise Floor

(1.23 MHz)

(30 kHz)

Wide BandNoise Floor

Figure 1-2: Noise in Narrow Band and Spread Spectrum Communication Systems






Unique Features

The following is a list of features that differentiate CDMA from analog cellular telephone (AMPS).

These features will be explained in later sections.

• Spread Spectrum Modulation – Narrow band information is transmitted

over a wide band RF channel.

• N=1 Frequency Reuse – Multiple users (in adjacent cells) operate on the

same frequency.

• Code Division Access – Each user and base station is associated with a

unique code rather than a frequency or time slot.

• Coherent Multiple Transmission (CMT) – Multiple base stations

simultaneously transmit to a given mobile user.

• Coherent Multiple Reception (CMR) – mobile units coherently combine

multipath components and signals from multiple base stations.

• Dynamic Power Control – Forward and reverse link transmit power is

controlled to the minimum required to achieve the link.• Variable Rate Speech Encoding – Voice is encoded at a slower rate when

the user is not speaking in order to minimize transmitted power and system

interference.

1.3 Advantages of CDMA

The use of CDMA technology offers several advantages including:

• Increased capacity due to adjacent cell frequency reuse (N=1),

• Coherent combination of signals, and

• User privacy.

These features are described in the following sections.

1.3.1 Frequency Reuse

Capacity gain is achieved with CDMA’s inherent N=1 frequency reuse pattern. This is distinctly

different from the typical AMPS N=7 frequency reuse pattern in which only one-seventh of the

available frequencies are used in a given cell. N=1 indicates that the same (wide band) frequencies

are used in each cell. When sectored cells are used, the same frequencies can be used in each

sector. Adjacent cell frequency reuse is possible because each signal in the system is associated

with a unique code – not a frequency.

1.3.2 Coherent Signal Combination

CDMA has the ability to coherently combine signals from multiple sources. This multiple

correlation system employs a RAKE receiver . The RAKE receiver combines signals arriving at a

given location, with different time delays, thus mitigating fading due to multipath. In addition, this

feature allows the mobile receiver to use signals from multiple base station transmitters, thus

improving cell-boundary performance and minimizing dropped calls.






1.3.3 User Privacy

CDMA’s spread spectrum modulation technique distributes the user information over an RF

bandwidth that is much larger than the information bandwidth. The resulting power spectral density

(PSD) of the transmitted wide band signal resembles thermal noise making the signal very difficult

to detect. In addition, a unique address code is required to recover user information.

1.4 Coverage and Capacity Limitations

The capacity of a CDMA cell site is effectively limited by the amount of interference in the

environment. Interference is generated by several sources including:

• Users of the given cell sight interfering with each other,

• Users of adjacent cell sites interfering with users of the given cell site,

• Adjacent base stations interfering with users of the given cell site, as well as

• Thermal and spurious noise.

It will be shown that system interference is a function of the number of users and their transmit power. Dynamic power control is used to minimize forward and reverse link transmit power to

mitigate interference. The dynamic nature of interference due to system load must be carefully

considered during system design.

1.5 Comparison of Multiple Access Techniques

In addressing CDMA, it is useful to understand other commonly used multiple access techniques

such as FDMA and TDMA. CDMA can be considered a combination of these techniques as it

possesses elements of frequency and time diversity.

1.5.1 FDMA

Frequency Division Multiple Access (FDMA) is used in conventional analog cellular systems (e.g.

AMPS, NMT). The FDMA process assigns discrete frequencies (i.e. channels) to individual users.

It is considered multiple access in that a number of users can simultaneously use the system

providing there is sufficient spectrum to accommodate each user. Accordingly, the capacity of this

system is limited by the amount of available spectrum.

1.5.2 TDMA

Time Division Multiple Access (TDMA) is employed in digital communication systems. TDMA is

used in cellular systems such as Digital-AMPS and GSM. It is considered multiple access in that anumber of encoded messages can be transmitted over time on a common carrier frequency. TDMA

assigns discrete time slots on a common carrier frequency to each user. During the time slot

designated for a specific user, digital information is burst out using the entire allocated RF channel.

Information is recovered by the receiver which decodes information only in its designated time slot.

As the number of users increases, the transmission bit rate and associated bandwidth increases.

Hence, TDMA is also limited by the amount of available spectrum.






Note that TDMA may be coupled with FDMA to further increase system capacity. Each channel in

an FDMA system may be time-division multiplexed between several users.

1.5.3 Multiple access: division by code

In the CDMA scheme, the digital information from each user is allowed to access the systemsimultaneously (as each user requests) using the same frequency spectrum. Frequency division is

still used, but a large bandwidth is used for each carrier. A user “channel” in CDMA is defined by

a specific code and an associated carrier frequency. The user code is correlated against the receive

signal to recover only the information specific to that user. The capacity of a CDMA system is

governed by the amount of interference in the environment that the receiver can tolerate before it is

unable to recover the desired user information.

U s e r 3

U s e r 1

F r e q u e n c y

U s e r

4 U s e r 3

F r e q u e n c y

A l l o c a t e d B a n d w i d t h

FDMA

C o

d e

U s e r 2 U s

e r 1

U s e r 1

U s e r 2

TDMA

T i m e

C o

d e

U s e r 1

CDMA

U s e r 2

. . .

T i m e

C o

d e

U s e r 3

U s e r 4 U s e r 2

.

.

.

F r e q u e n c y

T i m e

Figure 1-1: Comparison of Multiple Access Techniques






2 CDMA Spread Spectrum Terminology

There are several key words and tricky phrases that are used in discussing CDMA processing and

Spread Spectrum modulation. The following sections define some of the common terms that will

be used in the following sections.

2.1 IS-95 and IS-95-A CDMA:

CDMA as described in this document is based on an document known as IS (Interim Standard) -95.

IS-95 is the “Mobile Station - Base Station Compatibility Standard for Dual-Mode Wideband

Spread Spectrum Cellular.” IS-95 is also known as the CDMA “Air Interface” specification1.

CDMA air interface for PCS applications is described in Interim Standard 95-A (IS-95-A). The

basic CDMA process is the same in both standards. Note however that IS-95-A specifies a

maximum data rate of 14.4 kbps where as IS-95 specifies a maximum rate of 9.6 kbps.

Some standards that encompasses are:

IS-96-A Voice Encoder Spec

IS-97 Base Station Performance Spec

IS-98 Mobile Station Performance Spec

J - STD 8 Defines RF requirements at 1900 MHz

J - STD 18 Recommends minimum performance for 1900 MHz personal stations

2.2 Forward and Reverse Links

The definitions of the forward and reverse links are the same in CDMA as in other cellular systems.

The Forward link (also known as the Downlink) refers to transmissions from the base station(cell/sector) to the mobile user. The Reverse link (also known as the Uplink) refers to

transmission from the mobile user to the serving base station (cell/sector).

2.3 Correlation and Orthogonality

In discussing spread spectrum CDMA modulation, we often refer to the “correlation” properties of

different signals or sequences. In conceptual terms, two binary sequences that are being received

are correlated if their patterns of 1s and 0s are “alike” as they are received over time. If their

received bit patterns are different or “random” with respect to each other, the sequences or signals

are said to be uncorrelated. Correlation can be thought of as the “degree of similarity” of signals as

they are received over time.

1 Term cdmaOne has been adopted by CDG as a designator for CDMA technology based on IS-95 and accompanying

standards.






The correlation of two sequences can be determined by multiplying the received signals and

summing them over time. Correlation of two bit sequences is defined by

∑=

+⋅⋅= L

k

AB nk Bk A L

n R1

)()(1

)(

Equation 2-1: Definition of Correlation

Where:

is a relative shift (offset) of the two sequencesn

and are bit sequences of the length L )(k A )(k B

For some offset n, two bit sequences are totally correlated if is 1. If the correlation

is zero, sequences are orthogonal .

)(n R AB

)(n R AB

2.4 PN Sequence

The Pseudo-Noise (PN) Sequence (periodic and noise like) is fundamental to all direct sequence

spread spectrum systems. The PN sequence is a finite length binary sequence (code) that exhibits

properties similar to those of an infinite length random sequence. A good PN sequence is such that

the number of 1's versus the number of 0's (or -1's) are equal. The correlation of a PN sequence

with itself results in only 1 peak. It is illustrated in Figure 2-1, for any offset other than zero PN

sequence is totally uncorrelated with itself. This property is the foundation for finding the desired

code among all other PN codes.

time [Tc]

RAA(n)

0

1 2 3 L

1

-1/L

4-1-2-3-4-L

Figure 2-1: Autocorrelation of pseudo-noise bit sequence






An example of PN sequence generator (four-stage shift register):

1 2 3 40001

10001100

1110

1111

0111

1011

0101

1010

1101

0110

0011

1001

0100

0010

MAXIMUM LENTGH OF

SEQUENCE: 24-1=15

Figure 2-2 Four-stage shift register: generation of PN sequence

2.5 Chips and Chip Rate

A Chip is a “bit” (1 or 0) of a PN sequence. The Chip Rate is the rate at which the PN sequence isgenerated. For CDMA per IS-95, the chip rate is 1.2288 * 106 cps (chips per second).

2.6 Bit RateThe Bit Rate (R b) is the rate of the digitized baseband user information (i.e. user voice). In CDMA,voice is digitized at different rates depending on the speech activity level. The system parameters presented in this discussion are based on a maximum bite rate of 9.6 kbps per IS-95 and 14.4 for PCS CDMA systems (per IS-95-A).

2.7 Traffic Frame

A traffic frame is a 20 ms burst of data (i.e. user voice, error correction coding and controlinformation) from either the base station or the mobile unit.

2.8 Processing Gain

Processing (or Process) Gain is a term common to all direct sequence spread spectrum system.Process gain is defined as the ratio of the Chip Rate (R c) to the information bit rate (R b). This provides a measure of the amount of “spreading” in the system.






Process Gain =R

Rc

b

Equation 2-1: Process Gain

For CDMA as defined in IS-95:

R c = 1.2288 Mcps,

R b = 9.6 kbps (max), resulting in

Process Gain = 128 or 21.07 dB.

2.9 Eb /Nt, BER, and other Figures of Merit

There are several figures of merit that are bantered about when discussing CDMA as well as digital

communication systems in general.

• E b/No = Ratio of Transmitted energy per bit (E b) to Thermal Noise (No) usually

expressed in dB.

• E b/Nt = Ratio of Transmitted energy per bit (E b) to Total Noise (Nt) including

thermal, spurious, and interference from other CDMA users usually expressed in dB.

• Ec/Nt = Ratio of Transmitted energy per chip (Ec) to Total Noise (Nt) usually

expressed in dB.

• Ec/Io = Ratio of Transmitted energy per chip (Ec) to Total Noise including self-

interference (Io) usually expressed in dB.

• BER (Bit Error Rate) = Probability that a transmitted bit will be received incorrectly

(i.e. 1 received as a 0 or a 0 received as a 1)

• FER (Frame Error Rate) = Probability that a transmitted frame will be received

incorrectly.

2.10 Summary of Codes

In discussing CDMA modulation, several different PN sequences or “codes” are bantered about

incessantly. In attempting to make sense out of CDMA modulation, it is helpful to know the

relative length (time period) of these codes as well as what they are used for.

2.10.1 PN Long Code

The Long Code is a PN sequence that is 242

- 1 bits (chips) long. It is generated at a rate of 1.2288

Mbps (or Mcps) giving it a period (time before the sequence repeats) of approximately 41.4 days.

The long code is used to encrypt user information. Both the base station and the mobile unit have

knowledge of this sequence at any given instant in time based on a specified private “long code

mask” that is exchanged.

The generation of a Long Code is governed by Long Code Mask . A long code mask is a 42 bit

code which define the initial values used by the long code generator. Knowledge of this long code

mask allows the base station or mobile user to generate the same PN Long Code. Generating the






same long code (synchronized in time) at both end of the link allows information to be encrypted

and decrypted.

A unique and private, long code mask (thus, PN long code) is assigned to each CDMA user. This

code is referred to as a “user mask”. The user mask is exchanged between the mobile and the

serving cell(s)/sector(s), which allows user traffic data to be encrypted on both the forward andreverse links.

A different long code mask is used to generate the long code for encryption and decryption of

Access and Paging information – more on this later.

2.10.2 PN Short Codes

The Short Code is a PN sequence that is 215

bits (chips) in length. This code is generated at 1.2288

Mbps (or Mcps) giving a period of 26.67 ms. This code is used for final spreading of the signal and

is transmitted as a reference known as the “Pilot Sequence” by the base station. All base stations

use the same short code. Base stations are differentiated from one another by transmitting the PN

short code at different “offsets” in absolute. This time offset is known as a “PN Offset”. All base

stations and mobiles have knowledge of this code, however, mobile units do not have initial

knowledge of absolute time. Mobile units initially search (in time) until they synchronize with a

pilot code transmitted by a base station. The base station then conveys timing information to the

mobile – more on this stuff later.

2.10.3 Walsh Codes

CDMA defines a group of 64 orthogonal sequences, each 64 bits long, known as Walsh Codes.

These sequences are also referred to as Wash Functions. These codes are generated at 1.2288 Mbps

(Mcps) giving them a period of approximately 52 µs. These are used to identify users on theforward link. For this reason they are loosely referred to as CDMA channels. All base stations and

mobile users have knowledge of all Walsh codes.






Walsh Codes

64 bits

64 bit Walsh Codesused to identify userson downlink

242 - 1 bits

42 bit user maskidentifies user on uplink

64 chip offsetsused to identifybase station/sector to the mobile

PN Long

Codes

215 bits

PN short codes: PN-i(t) = PN-0 (t - i x 64Tc)

Figure 2-1: Summary of Sequences used in CDMA Spread Spectrum

2.11 CDMA Call Quality (Eb /Nt)

With CDMA the raw channel bits have no inherent information and are not available outside of the

spread spectrum receiver. For this reason the fundamental performance measure is the frame error

rate (FER) rather than the bit error rate. Note that a frame includes signaling information and error detection bits as well as user voice/data. This metric includes the error detection/correction coding

inherent in the system. Frame error rate is defined as:

X rateat d transmitte framesof number

X rateat correctlyreceived framesof number FER x ⋅⋅⋅⋅⋅⋅

⋅⋅⋅⋅⋅⋅⋅−=1

Equation 2-1: Frame Error Rate

The “rate X ” term refers to the specific rate at which voice information is being encoded by the

variable rate vocoder.

System performance is typically characterized by plotting Frame Error Rate vs. Received signalE b/Nt. These plots are known as “waterfall curves” due to their shape. These are similar to Bit

Error Rate (BER) curves for other digital communication systems. An example plot of this type is

shown in Figure 2-1 for different modulation types. Specific CDMA performance curves are not

shown as they are specific to vendor hardware. CDMA systems require a Frame Error Rate of less

than 1% for acceptable call quality. This roughly corresponds to a Bit Error Rate (BER) of 10-3

.






10 -2

10 -1

10 -3

10 -4

10 -5

10 -6

10 -0

5 7 9 1131-1-3

P r o b a b i l i t y o f F r a m e

E r r o r

Avera e Des read Eb/Nt

Reverse Link FER

Performance

Forward Link FER Performance

“Good Call ualit ”

Figure 2-1: Example of FER to Eb/Nt relation: different for Forward and Reverse Link

2.12 Coherent vs. Non-Coherent Detection

The typical values Eb/Nt required to maintain a 1% FER have a more-or-less Log Normal

distribution with a standard deviation of 2.5. A 1% FER corresponds to a mean E b/Nt of 5 dB for

the forward link and 7 dB on the reverse link. The difference in the required signal strength is due

to the use of coherent reception on the forward link and non-coherent reception on the reverse link.

Coherent reception implies knowledge of the received signals phase (or timing). In the case of the

forward link, this is provided by the Pilot sequence which is transmitted by each cell/sector.

Non-coherent reception implies detection of only the magnitude of received signals. The phase of

the incoming signals is not known. As there is no pilot sequence transmitted on the reverse link,

this type of receiver must be used. CDMA systems are therefore considered to be reverse link

limited with regards to call quality.






3 CDMA Physical and Logical Channels

3.1 Physical Channel

Physical channels are described in terms of a wideband RF channel and code sequence. As definedin IS-95, each RF channel is 1.2288 MHz wide. For each RF channel, there are 64 Walsh

sequences (W0 through W63) available for use on the forward link. These Walsh sequences are

commonly referred to as CDMA channels (though this is not correct for the uplink).

3.2 Logical Channel

Divisions on the physical channel that carry specific types of information are known as logical

channels. Logical channels in CDMA are divided into two categories: Traffic Channels and

Control Channels. For the forward link there are three types of Control/Signaling channels and one

Traffic Channel (per user). For the Reverse Link there is one type Signaling Channel and one

Traffic Channel per user.

It is important to note that signals on the forward link are identified by Walsh codes, however, signals

on the reverse link are identified by Long Codes.

3.2.1 Forward Link (Downlink)

The logical channels for the Forward Link must provide identification of the Base station, timing

and synchronizing of the transmissions between the base station and mobile station, “hailing” of

mobile units in the area, and the voice/data transmission from the base station to the mobile unit.

The forward link is comprised of:

• The Pilot Channel,• Up to one Sync Channel,

• Up to seven Paging Channels, and

• Up to 55 Traffic Channels.

Forward CDMA Channel(1.23 Mhz radio channel

transmitted b base station

Pilot

Chan

Paging

Ch 1

Sync

Chan

Paging

Ch 7...up toW0 W32 W1 W7

Traf

Ch 1

Traf

Ch n

Traf

Ch 24

W8

... ...up to

W31

Traf

Ch 25

W33

Traf

Ch 55

W63

...up to

Traffic

Data

Overhead

Control Bits

Figure 3-1: Forward Link Channel Assignments






3.2.1.1 Pilot

The Pilot Channel allows a mobile station to acquire the timing of the Forward Traffic Channel -

user information. It provides a phase reference for coherent demodulation and provides a means for

signal strength comparisons between base stations, which is used to determine when to handoff. It

consists of the unmodulated final spreading sequences (PN short codes). The Pilot signal is

transmitted continuously on Walsh 0 by each CDMA base station at the transmitter (cell/sector)

level.

3.2.1.2 Sync Channel

The Synchronization Channel is an encoded, interleaved and modulated spread spectrum signal

that is used with the Pilot Channel to acquire initial system time and synchronization. The sync

channel is always transmitted on Walsh 32.

3.2.1.3 Paging Channel

The Paging Channel is used for transmission of control information to the mobile. When a mobile

is to receive a call it will receive a “page” from the base station. Up to seven (7) channels may be

configured for paging depending on the expected demand.

Page channel messaging to each user takes place in an 80 ms “slot”. The 80 ms slots are grouped

into cycles of 2048 slots (cycle duration 163.84 s) referred to as maximum slot cycles. The base

station can limit the maximum slot cycle used by the mobile. The mobile randomly picks a “slot

cycle index” and informs the base station of its choice when it registers. The mobile now only

monitors the Page channel during its assigned 80 ms slot defined by:

Slot Cycle = 1.28 x 2SLOT_CYCLE_INDEX

(in seconds)

where: SLOT_CYCLE_INDEX is {0 … 7}

That is to say… for a slot cycle index of 5, the mobile “powers up” and monitors the Page channel

for 80 ms once every 1.28 x 25

= 40.96 seconds. This process of periodic monitoring allows

considerable power savings by the mobile unit.

3.2.1.4 Traffic Channel

The Traffic Channel or Traffic Channel Element (TCE) carries all the phone calls (voice or data

signal) from a given base station to all the mobile units active in the coverage area. Each user has a

dedicated TCE, and corresponding Walsh code, on the down link. The forward traffic channelmessage consists of user voice (or data), power control data, and error correction bits. The message

is transmitted as a series of traffic frames. The traffic channel may also carry signaling information

with or in place of user voice (or data). A Walsh code is assigned by the base station for each

Traffic Channel in use.






3.2.1.5 Power Control Sub-Channel

A Power Control Sub-Channel is continuously transmitted on the forward traffic channel as part of

the traffic frame. Information on this channel commands the mobile unit to adjust its transmitted

power + 1 dB every 1/16 of a speech frame (800 times per second).

3.2.2 Reverse Link (Uplink)

The logical channel requirements of the reverse link must provide for the identification and access

request by the mobile unit to the base stations in the area and the voice/data transmission from the

mobile unit to the base station. The reverse link is composed of:

• Access Channels and

• Traffic Channels.

These channels share the same CDMA center frequency on the reverse link (a different frequency is

used for forward link transmissions). The total number of channels is determined by base station

activity. The example in Figure 3-2 shows 55 Traffic Channels available for all reverse links at a

given base station in accordance with the previous forward link channelization discussion. In

actuality, an individual subscriber unit is limited to one access channel and one traffic channel. The

reverse link capability of a given base station is limited by the number of traffic channels assigned

(up to 55) and up to seven (7) access channels (correlating to a maximum of 7 paging channels).

Note that a mobile does not “tie up” an access channel, it only borrows it for a short amount of

time.

Reverse CDMA Channels

(1.23 Mhz radio channel

received by base station)

Access

Ch 1

Access

Ch n...up to

Traf

Ch 1

Traf

Ch 55.................

User Dataand/or

Control

Addressed by Long Codes

Figure 3-2: Reverse Link Channel Assignments






3.2.2.1 Access Channel

The Access Channel is used for the transmission of control information to the base station. When a

mobile is to place a call it uses the “access” channel to inform the base station. This channel is also

used when responding to a “page”. Each Access Channel is identified by a distinct “Access

Channel Long PN Code ”. An Access Channel is selected randomly by the mobile unit from the

total number of access channels available from the serving cell/sector.

3.2.2.2 Traffic Channel

The Traffic Channel for the reverse link is identical to the forward link Traffic Channel Element in

function and structure. Each traffic channel is identified by a “User Long PN Code” which is

unique to each CDMA user.






4 CDMA Modulation & Demodulation

4.1 Types of Spread Spectrum Modulation

CDMA is a spread spectrum modulation scheme. This implies that the transmission bandwidth ismuch larger than the information bandwidth. The types of spread spectrum modulation commonly

used in communication systems are classified as:

• Frequency Hopping

• Direct Sequence

GSM and PCS-1900 are TDMA systems with the ability to frequency hop. CDMA is a direct

sequence technique. These modulation schemes are described further below.

4.1.1 Frequency Hopping

The carrier frequency is varied and the bandwidth of the transmitted signal is comparable to the bandwidth of the information signal. Information is modulated on top of a rapidly changing carrier

frequency. Some advantages and disadvantages of frequency hopping systems are listed in Table

4-1.

Table 4-1: Summary of Frequency Hopping Qualities

Advantages Disadvantages

• Carrier can be hopped over large

portions of the spectrum

• Complex Frequency Synthesizer

• Can be programmed to avoid portions

of the spectrum

• Not useful for location and velocity

measurements

• Shorter Acquisition Time than direct

sequence

• Error correction required

• Less affected by near-far problem than

direct sequence

4.1.2 Direct Sequence

In direct sequence modulation the carrier frequency is fixed and the bandwidth of the transmitted

signal is larger and independent of the bandwidth of the information signal. Some properties of

direct sequence spread spectrum systems are listed in Table 4-2.






Table 4-2: Summary of Direct Sequence Spread Spectrum Qualities

Advantages Disadvantages

• Better noise & anti-jam performance

than frequency hopping for a fixedtransmission bandwidth.

• Requires wide band channel with little

phase distortion

• More difficult to detect than

Frequency hopping or narrow band

transmissions.

• Longer acquisition time than

frequency hopping systems

• Best discrimination against multipath

due to inherent frequency diversity

• Fast code generator needed

4.2 Spread Spectrum (CDMA) Modulation Example: Encoding andDecoding of Information

This section provides a simple example of CDMA spread spectrum modulation. The example

illustrates how information bits are encoded by a PN sequence an the recovered in presence of

another spread spectrum signal.

4.2.1 Spread Spectrum Transmit Process

Transmitting a spread spectrum signal involves

• Modulating the information signal with the spreading PN sequence,

• Modulating the resulting signal with the desired carrier wave,• Band Pass Filtering the output, and

• Transmitting the resulting RF signal.

This is illustrated below in Figure 4-1

BPF

CosωctC

1(t)

S(t)

S(t)C1(t)Cos(ω

ct)

InformationSignal

SpreadingCarrier

ModulationBand Pass

Filter

RF Signal

Figure 4-1: Spread Specturm Transmit Process






Where:

S(t) = Desired information signal as a function of time (digital signal).

C1(t) = CDMA PN code as a function of time (comprised of a known binary

pattern).

Cos(ωct) = Desired RF carrier frequency.

S(t)*C1(t)*Cos(ωct) = Transmitted RF signal.

4.2.2 Spread Spectrum Receive Process

Receiving a spread spectrum signal involves

• Demodulating the signal with the RF carrier,

• Low Pass Filtering the resulting wide band signal,

• Demodulating with the signal with the known spreading sequence, and

• Integrating the de-spread signal over a bit time to recover the information signal

This process is illustrated below in Figure 4-2.

LPF

Cos(ωct) C1(t)

S(t)

S(t)C1(t)Cos(ω

ct)

InformationSignal

De-Spreading

Carrier De-Modulation

Low PassFilter

ReceivedRF Signal

t

t +

∫ τ

Integrate over Bit Time &

Dump

Correlationwith the PNsequence

Figure 4-2: Spread Spectrum Receive Process

[S(t)*C1(t)*Cos(ωct)]*Cos(ωct) = [S(t)*C1(t)*Cos(ωct)]*Cos(ωct)

= 1/2*[S(t)*C1(t)] + 1/2[S(t)*C1(t)*Cos(2ωct)] → LPF

= 1/2*[S(t)*C1(t)]

1/2*[S(t)*C1(t)] *C1(t) = 1/2*[S(t)] → after integration over the information period

Where:

[S(t)*C1(t)*Cos(ωct)] = Received RF signal

LPF = Low Pass Filter with bandwidth equal to the spread bandwidth (W)S(t) = Signal as a function of time (Digital)

C1(t) = PN code as a function of time (comprised of pseudo random binary

sequence)

Cos(ωct) = Desired RF carrier frequency.

C1(t)*C1(t) = 1 when the codes are aligned in time because of correlation properties of

the PN codes.






4.2.3 Multiple Signal Case

What if the Code 1 signal was also received by a Code 2 receiver ?

C1(t)*C2(t) = C3(t) because of correlation properties of the PN codes. Knowledge of (and timesynchronization to) the PN code associated with a specific information signal allows us to recover

that signal from among other spread spectrum transmissions.

A simple example illustrates how the CDMA signal is transmitted and then recovered in the

presence of another CDMA signal. In the example shown, two (2) information bits are encoded

onto a repeating 7 chip CDMA like code sequence. Note that the effects of noise and interference

are not considered.

Question: What is the processing gain of the spread spectrum signal in this example?

Hint: R c/R b

Information bits from two different transmitters

S1 S2

Encoding PN Sequences from those two transmitters

C1 C2

Encoded Information

S1*C1 S2*C2






Receipt of Multiple Encoded Signals

S1*C1 + S2*C2 S1*C1 + S2*C2

Receiver Decoding Sequences (Same as TX Sequences)

C1 C2

Decoding (Correlation) of Received Signals

(S1*C1 + S2*C2)*C1 (S1*C1 + S2*C2)*C2






Integration of the Correlated Received Signals

Integrator Output Integrator Output

Output of Translated Information Bits (at T + 1 Bit)

Comparitor Output Comparitor Output

5 The CDMA Advantage - The RAKE Receiver and theMultipath Environment

The land based wireless telephone environment is a multipath environment. Multipath is generally

a destructive force in TDMA and FDMA systems and has been factored as a loss in the engineeringof those networks. In CDMA systems, as proposed by the interim standards and proponents of the

technology, multipath is converted to a positive force through the application of the RAKE receiver.

In order to clearly illustrate the benefits associated with the RAKE receiver’s unique ability to

demodulate signals in a multipath environments, it is prudent to briefly review the additive

properties of waves and the multipath phenomena.

5.1 A Brief review of Multipath and its effect on Analog and DigitalTransmissions.

Multipath, as it is referred to in RF engineering, is the result of reflections and scattering of radio

waves off of buildings, water towers, mountains, etc. Multipath will exist anywhere the incidentwave and one or more reflected and/or defracted waves can reach the receiver as shown in Figure

5-1

Multipath, in effect, creates “multiple versions” of the transmitted signal which arrive at the

receiver at different times. These “multiple versions” of the transmitted signal are known as

multipath components. The arrival of multipath components results in destructive interference due

to the superposition of the various waves. The received signal for a given frequency will be the






sum of all the multipath components. When the components arrive perfectly in phase, the overall

Received Signal Level (RSL) will be stronger than any of the individual components. When they

arrive out of phase, as a result of the reflective/defractive process, the overall RSL is less than the

strongest individual component.

Lets consider a single transmitted wave that is scattered such that the receiver detects thetransmitted wave and three multipath components of differing magnitudes and relative phase angles

from the incident wave. Mathematically these waves are given as:

f(t )Incident (Direct) Wave = 2.0 sin (ωt )

f(t )Multipath 1 = 1.5 sin (ωt + 90o)



The figure below provides a graphic representation of the incident waveform, multipath waveforms

and the resultant waveform. Notice that magnitude of the resultant waveform is less than the

incident waveform as a result of the superpositioning of the multipaths on the incident wave.

Destructive Interference Due to Multipath

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0 2 4 6 8 1 0

1 2

1 4

1 6

1 8

2 0

2 2

2 4

Time

R e l a t

i v e A m p l i t u d e Incident Wave

Multipath 1

Multipath 2

Multipath 3

Resultant Wave

Figure 5-1: Destructive Interference due to Multipath

Destructive (and constructive) interference due to the arrival of equal amplitude and random phase

multipath components is referred to as Rayleigh Fading. The significance or degree that RayleighFading affects system operation is determined by the surrounding environment. If we assume four

(4) different land classifications based on the concentration and size of structures in a given area

and designate them in decreasing concentration as Dense Urban, Urban, Suburban, Rural. In

general we would expect to see the greatest effects of Rayleigh fading in the Dense Urban

environment and the least in a Rural Environment. This is due to the greater concentration of

scattering structures in a Dense Urban Environment than in rural areas






5.2 The RAKE Receiver

The RAKE receiver is the optimum demodulator structure for multipath propagation paths in a land

mobile telephone environment. It was first implemented in static form in the late 1950’s.

Essentially this device has the capability of “looking” at a given window in time, picking out

multipath components of a given signal and lining them up so that they are in phase again. This process is referred to as coherent addition and results in a greater probability of making or

maintaining the forward link in areas where it would otherwise be prohibited. The RAKE receiver

is also applied to reverse link, however, because of a lack of a coherent reference (pilot signal) the

reverse link uses a non-coherent RAKE demodulator.

To explain the conceptual processes of the RAKE receiver, consider the forward link scenario in

Figure 5-1below in which a mobile unit (in the car), is being served by the nearby base stations

designated BSA.

Figure 5-1: Single Transmitter with Multipath θ

θ1= θ2

θ1

Direct wave

For a single pulse transmitted from BSA, the mobile receives many copies of the pulse, delayed in

time, with amplitude which depend on the interaction with buildings, terrain, and the antenna. The

plot of the received signal vs. Time for a pulse is called an impulse response. In theory, a pulse of

zero time duration requires an infinite bandwidth. In practice this is not possible, therefor, the

transmitted pulse has a finite time duration resulting in a finite bandwidth. For this reason the plot

of receive signal vs. time for a pulse is referred to as a “Band Limited Impulse Response”.

A typical band-limited channel impulse response for the above scenario would be composed of

multipath components from BSA arriving at MU1 at different points in time as shown below in

Figure 5-2. The spikes indicate discernible multipath signals. The surrounding envelope is caused by smaller multipath components, scattering, and background noise.






A

A|h(t )

| 2A

1

3

A

A4

5

t

Figure 5-2: Typical Single Transmitter Band-Limited Channel Impulse Response with Five

Discrete Multipath Components

The time delay between the received components is related to the different distances traveled by the

various components as they propagate from BSA to MU1. The difference in path length between

two signals can be found by multiplying the time difference between the received signals by the

speed of light.

∆ Path Length Between A1 and A2 = (T2 - T1) 3 x 108

m/s

Equation 5-1: ∆ Path Length

In the time domain, these multipath components differ in amplitude and time shift. In the frequency

domain, these differences correspond to differences in amplitude and phase. In IS-95 CDMA, the

function of the RAKE receiver is to align up to a maximum of three multipath components in time

by selectively adjusting the phase of the multipath components so that they are all equal. When

correctly adjusted and put in a summing device the result is the coherent addition of the multipath

signals as shown in Figure 5-3. This figure shows the magnitude of the received and combined

signals, however the phase information of the signals is also maintained.






Received Signals

A1

|h(t)|

2A

A3

A5

A4

A4

t

Magnitude of CoherentlyCombined Multipath

A2

A1

Figure 5-3: Coherent Combination of Three Strongest Multipath Components from a Single

Transmitter

It is important to note that the only means of adjusting these components is by having a reference

that is also transmitted by BSA along with the traffic information. All IS-95 CDMA base stations

within a given system continuously transmit a pseudorandom (PN) binary (short) code for the

purpose of synchronization and timing (Pilot Channel). Synchronization to the pilot signal allows

the RAKE receiver to operate in an efficient manner.

Each base station starts the PN short code at a unique time which is offset from the system

reference (which is maintained by GPS time). The PN offset makes it appear to a mobile that each

base station is transmitting a unique code because of the correlation properties of the PN sequence.

Note that the PN Code has properties such that when the received PN short code and the PN short

code generated by the mobile unit are aligned in time, a correlation peak occurs. When they are not

aligned, the correlation between the codes is noise.

The RAKE receiver provides for the coherent combination of multipath components from a single

base station and multiple cells/sectors jointly in a CDMA Handoff scenario (see Section 9). In IS-

95 CDMA, the RAKE receiver is limited to resolving and combining a maximum of three multipathcomponents from either a single transmitter, multiple transmitters, or a combination of both. The

limit of resolution in time of the received signals may be as small as ½ of a chip. The maximum

number of signals considered is defined in the system specification and results from the fact that

there is very little added benefit from using more than three components. Typically the RAKE

receiver processes the three strongest three signal components, however, the precise determination

of which signals will be process depends on the handoff type, desired traffic flow, and relevant

thresholds seat at each serving cell/sector.






Consider the forward link scenario given below in which a mobile unit, MU1, is being served by

three base stations designated BSA, BSB, BSC. The lines from the base stations indicate multipath

that could exist for the geometry indicated.

BSC

BSB

BSA

MU1

Figure 5-4: Multiple Transmitters with Multipath

A typical band-limited channel impulse response for the above scenario could be composed of

multipath components from serving base stations BSA, BSB, BSC and arriving at MU1 at different

points in time as shown below.

time

|h(t)|

A2

A3

A1B1

B2

C1

C2

C3

C4

Figure 5-5: Typical Multiple Transmitter Band-Limited Channel Impulse Response with

Discrete Multipath Components

Given that the RAKE receiver MU1 has knowledge that BSA, BSB, and BSC are all serving base

stations (See Section 9 for details on joint handoffs), the receiver performs the following functions:






• Identifies the components which are the strongest (maximum of three),

• Performs time alignment of the select components, and

• Sums the components.

When correctly time aligned and put into a summing device, the result is the coherent combination

of the multipath signals as shown in Figure 5-6.

C4 A3

B2

C2

A2

A3

A1B1

B2

C1

C2

C3

time

Relative

Power

Received Multipaths

Magnitude of Coherently Combined

Multipaths

Figure 5-6: Coherent Combination of Three Strongest Components of a Typical Multiple

Transmitter Band-Limited Channel Impulse Response with Discrete Multipath Components

5.3 Comparison of the effects of Multipath on FDMA, TDMA, andCDMA.

5.3.1 FDMA

The quality of service provided by a Frequency-Division Multiple-Access System is a function of

the received signal level and proper frequency planning. Assuming no frequency reuse or the

assignment of adjacent channels within the system, the problem becomes one dimensional as a

function of signal strength. In FDMA, the carrier wave is subjected to the multipath fading

(Rayleigh fading) as discussed above. The human ear is an excellent discriminator of echoes, noise,

fading. Multipath may greatly impact voice quality.

5.3.2 TDMAMultipath in a digital system adversely effects the performance in two ways that must be

compensated for in the design and implementation of the hardware. First, multipath fading of the

carrier wave results in reduced signal strength. The reduction in signal strength results in increased

bit error rate as E b/Nt falls below what is required for acceptable call quality.

The second effect of multipath, the time delay in arrival over which multipath components arrive

(delay spread), can be large enough to create Inter Symbol Interference (ISI). This effect is known










×

= dBt

b

N

E

e LinearValu1.0

10

Equation 5-1: Call Quality dB to Linear Conversion

The linearized values for each of the multipath components are 3.16, 2.00, 1.58 respectively.Assuming perfect phase alignment and zero processing losses, the combined value for all of the

components is 6.74 which corresponds to a calculated E b/Nt of 8.29 dB which provides the desired

level of call quality.

Additional examples can be made up and solved using Equation 5-1 or Table 5-1 for the

linearization of E b/Nt.

Table 5-1: Call Quality dB to Linear Conversion Table

Eb/Nt

(dB)

Linearized

Value

Eb/Nt

(dB)

Linearized

Value

Eb/Nt

(dB)

Linearized

Value

Eb/Nt

(dB)

Linearized

Value

0.1 1.02 2.1 1.62 4.1 2.57 6.1 4.070.2 1.05 2.2 1.66 4.2 2.63 6.2 4.17

0.3 1.07 2.3 1.70 4.3 2.69 6.3 4.27

0.4 1.10 2.4 1.74 4.4 2.75 6.4 4.37

0.5 1.12 2.5 1.78 4.5 2.82 6.5 4.47

0.6 1.15 2.6 1.82 4.6 2.88 6.6 4.57

0.7 1.17 2.7 1.86 4.7 2.95 6.7 4.68

0.8 1.20 2.8 1.91 4.8 3.02 6.8 4.79

0.9 1.23 2.9 1.95 4.9 3.09 6.9 4.90

1.0 1.26 3.0 2.00 5.0 3.16 7.0 5.01

1.1 1.29 3.1 2.04 5.1 3.24 7.1 5.13

1.2 1.32 3.2 2.09 5.2 3.31 7.2 5.25

1.3 1.35 3.3 2.14 5.3 3.39 7.3 5.37

1.4 1.38 3.4 2.19 5.4 3.47 7.4 5.50

1.5 1.41 3.5 2.24 5.5 3.55 7.5 5.62

1.6 1.45 3.6 2.29 5.6 3.63 7.6 5.751.7 1.48 3.7 2.34 5.7 3.72 7.7 5.89

1.8 1.51 3.8 2.40 5.8 3.80 7.8 6.03

1.9 1.55 3.9 2.45 5.9 3.89 7.9 6.17

2.0 1.58 4.0 2.51 6.0 3.98 8.0 6.31

6 Dynamic Power Control

One of the fundamental requirements for successful IS-95 CDMA operation is the implementation

of Dynamic Power Control (DPC) on the forward and reverse links. Using DPC the power of all

mobile units is controlled so their transmitted signals arrive at the base station at an equal andminimum received power level. In addition, the traffic channel power on the forward link is varied

as a function of voice coding rate. In this way, the interference generated from one mobile unit to

another is kept to a minimum resulting in increased system capacity.






6.1 The “Near-Far” Problem

The “near-far” problem in spread-spectrum systems relates to the problem of very strong signals at

a receiver swamping out the effects of weaker signals located on the edge of the coverage area in a

CDMA system resulting in dropped and blocked calls. The direct-sequence spread spectrum

(CDMA) technology is the most susceptible to “near far” due to the ‘N = 1’ frequency reusescheme. A frequency-hopping system is much less susceptible to the near-far problem because it is

an avoidance system. Interference will result only when there is simultaneous occupancy of a given

frequency slot. FDMA and TDMA are virtually immune to “near-far” because of frequency

isolation for FDMA and much lower baud rates for TDMA.

Conceptually, the near-far problem is overcome in CDMA systems by making the base station

receive all signals of equal strength. For a static system, the reverse link transmit powers would be

selectively optimization so that an individual base station receives equal power from all subscribers.

Overcoming “near-far” in the mobile environment requires that the reverse link transmit power for

all subscribers be continuously adjusted. The rate and degree of adjustment should be commiserate

with the maximum anticipated rate and magnitude of change in required power to maintain a

constant RSL at the base station. This is accomplished through the implementation of dynamic

power control.

6.2 Reverse Link

Two forms of power control are used for the reverse link:

• Open-loop, and

• Closed-loop.

6.2.1 Open-Loop

Open loop power control involves only the mobile unit. Open-loop control sets the sum of the

transmit (Access Channel) and receive (Pilot Channel) powers (in dBm) to a constant, nominally

-73 dBm. A reduction in received signal power from the base station results in increased transmit

power from the mobile unit. For example, if the received pilot power from the base station is -85

dBm, the open-loop transmit power setting would be (-73) - (-85 dBm) = 12 dBm. This process is

used for reverse link transmissions made on the access channel prior to setting up a user call. Note

that access attempts are made at successively higher power levels until a response is received from

the base station or a maximum threshold is reached. Once a user call is initiated, closed-loop power

control takes effect.

6.2.2 Closed-Loop

Close-loop power control is used to allow the power from the mobile unit to deviate from the

nominal as set by open-loop control. The base station monitors the power received from each

mobile station and commands each mobile unit to raise or lower its power by a fixed step

(nominally 1 dB) to keep the received signal at the minimum acceptable level. Acceptable signal is

defined by < 1% FER. This process is repeated 800 times per second, or every 1.25 ms. This is

accomplished by dividing each 20 ms traffic frame into 16 power control groups. Each power






control group is preceded by a power control bit. Mobile units support a dynamic range of about

80 dB and can be controlled to transmit as little as -60 dBm.

6.3 Forward Link

Forward traffic channel (TCE) power is attenuated (for each TCE) based on voice coding rate thatis being used. As the data rate is lowered, the output signal is attenuated. This provides a constant

E b for the output signal.

Table 6-1 lists the attenuation levels for the available Vocoder rates.

Table 6-1: Forward Link TCE Attenuation Level vs. Voice Coding Rate

Vocoder

Rate

Data Rate (R b) kbps

(per IS-95)

Attenuation Level

(dB)

1 9.6 0

½ 4.8 3

¼ 2.4 6

1/8 1.2 9

In addition, the available base station transmit power is divided among the pilot, sync, paging, and

traffic channels in use. Table 6-2 lists the nominal power allocations. These allocations are not

dynamic with time but may be adjusted on a per transmitter basis as necessary by the operator.

Table 6-2: Base Station Nominal Channel Power Allocations

Logical Channel Relative Power Allocation Nominal

Allocation

Pilot 0.2 of total power (linear) 20 %Sync + Paging +

Traffic

Remainder (0.8) of total power (linear) 80 %

Sync 3 dB less than one Traffic Channel;

always 1/8 rate

3 %

Paging 3 dB greater than one Traffic Channel;

full rate only

2 %

Traffic Equal power in each Traffic Channel:

full rate only (or specified maximum

per TCE)

75 %






7 CDMA Implementation and Digital Radio Link Processes

The following sections provide a general explanation of how a CDMA radio link is implemented.

A detailed hardware description is not discussed here and is provided in Unit 2 of The IS-95 CDMA

Digital Cellular Communication System. Note that the following discussion assumes a maximum bit rate of 9.6 kbps as specified in IS-95 for 850 MHz systems. PCS CDMA systems using a 14.4

kbps maximum data rate, as specified in IS-95-A, follow the same implementation procedure as

discussed below.

The forward and reverse links are broken into functional blocks and a qualitative description of

each block is provided. The digital processing for the forward link and reverse link are not

identical. Pilot signals on the forward link allow more robust detection techniques to be

implemented (e.g. coherent demodulation). A pilot signal is not transmitted on the reverse link,

requiring the use of non-coherent detection at the base station. This necessitates 2 to 3 dB higher

E b/Nt at the base station receiver than at the mobile unit.

7.1 Forward Link

The forward link or downlink describes the communication from the base station to the mobile user.

A block diagram of the transmit path of the base station and the receive path of the mobile unit is

shown in Figure 7-1. Note that the demodulation process includes a RAKE receiver to combine

multipath signals. The operation of the RAKE receiver is omitted for clarity in the following

sections, however, the RAKE receiver is discussed Section 5.2.






VariableLow Bit Rate

SpeechCoding

ChannelCoding

BitInterleaving

Variable LowBit

Rate SpeechDecoding

ChannelDecoding

BitDeinterleaving

Encryption:Long CodeScrambling

WalshFunction

Modulation

QuadratureSpreading andMultiplexing

QuadratureCarrier

Modulation

Decryption:Long Code

Descrambling

WalshFunction

Demodulation

QuadratureDespreading

QuadratureCarrier

Demodulation

Transmit Path in Base Station Receive Path in Mobile

DownlinkSpeech/Channel

Processing

RFChannel

Figure 7-1: CDMA Digital Radio Forward Link Process

The following sections describe the forward link processing with respect to the transmit side.

Reception of the signal at the mobile unit employs coherent detection using the base station pilot

signal and is essentially the reverse of the described transmit process.

7.1.1 Variable Rate Speech Coding

When voice is transmitted over the commercial telephone system (land line) it is assumed to be

band limited to the frequency range of 200 to 3300 Hz. The voice signal is initially sampled 8000times per second, logarithmically (µ-law) quantized to 8 bits, and transmitted at 64 kbps. In

CDMA, as well as conventional Digital AMPS cellular, speech is sampled at 8 kHz and uniformly

quantized to 13 bits. This data is divided into 20 ms frames and transmitted at 104 kbps. The first

step in the Speech Coding process is to transcode and rate adapt (modify the quantization and

data rate) to the cellular standard 104 kbps bit stream. Note that transcoding is not required by

mobile unit for the reverse link voice transmissions. The transcoded data is then fed to the Code-

Excited Linear Predictive (CELP) coder. This is illustrated in Figure 7-2.






8-bit µ-law to13-bit uniform

transcoder

CELPSpeechEncoder

64 kbps

Digital Speech

Input From

Land Line

uniform

104 kbpsVariable Rate

1 to 1/8

Figure 7-2: Forward Link Speech Processing at the Network Side

The CELP speech encoder produces a variable output data rate based on speech activity. The

encoder generates one frame (a.k.a. packet, a.k.a. block) every 20 ms. The coded data frame is at

one of the following data rates:

• Rate 1: 171 bits / packet (8.55 kbps)

• Rate 21 : 80 bits / packet (4.0 kbps)



The advantage of using lower bit rates when there is little or no speech activity is that it allows the

transmit power to be decreased while maintaining a constant E b/Nt. A reduction in transmit power

decreases the level of interference imposed on other users of the system.

7.1.2 Channel Coding

Channel coding involves converting the 20 ms speech frames into traffic blocks and applying ½ rate

convolutional coding. Generating traffic blocks involves incorporating overhead data transmissions

(signaling information) with the voice data. The Mixed Mode (MM) bit indicates the insertion

(MM=1) or non-insertion (MM=0) of signaling data into the traffic frame as required by the system.

A 20 ms frame of voice at coding Rate 1 may be replaced entirely with signaling information. Thisis known as blank-and-burst. Alternatively, signaling data transmitted at Rate 1 may share a

frame with lower rate voice data. This process is known as dim-and-burst. Traffic Frames are

then generated by adding Cyclical Redundancy Check (CRC) code bits which provide error

detection capability.

The resulting traffic frame is fed to a convolutional coder of rate ½ with a constraint length of 9.

This coder uses an 8 bit shift register and outputs 2 bits for every input bit. Convolutional coding

provides channel bit error detection and correction capability. For data rates below 9.6 kbps (Rate 1

+ overhead), output bits are repeated to bring the number of bits in a 20 ms block to 384 for a

constant output rate of 19.2 kbps. Remember that the user data (voice information) is still input to

the system at a variable rate – the change to 19.2 kbps represents a change in sampling rate. Thischannel coding process is illustrated below in Figure 7-3.






ΣTrafficBlock

Generator

ConvolutionalCoder

FromVariable

RateSpeechCoder

Signaling

Tail bits

MM bit

To Interleaver

Speechblocks

Traffic blocks Traffic frame

CRC

MM = 0 No SignalingMM = 1 Signaling present

at VariableRate

19.2 kbps

Figure 7-3: Channel Coding Process

7.1.3 Bit Interleaving

The effect of interleaving is to spread a burst of bit errors that occur in the transmission channel

over several data blocks as well within a data block. The coded transmissions used in CDMA areless susceptible to random errors than to burst errors. . Interleaving consists of writing bit stream

into the buffer matrix using one pattern and reading the bit stream from the matrix using different

pattern. Forward link uses 24 by 16 matrix. This is illustrated below in Figure 7-4. The process is

reversed to de-interleave the data.

Block Interleaver

Write block into matrixaccording to pattern

Read block from matrixaccording to pattern384 bits 384 interleaved bits

From Channel Coder

20 ms block

19.2 kbps 19.2 kbps

20 ms block

To Encryption (Scrambling)

Coded Blocks In Interleaved Blocks Out

Figure 7-4: Bit Interleaving

7.1.4 Encryption: Long Code Scrambling

The data transmitted on several CDMA channels is encrypted (scrambled). Encryption provides

• Privacy of user traffic information

• Identification of signals on the reverse link (reverse link traffic and access channels)

The encrypted channels are:

• Paging channel (forward link)

• Access channel (reverse link)

• User Traffic channels (reverse link and forward link)






Data on these channels is encrypted by modulating it with a PN sequence with a length of 242

- 1

chips at a chip rate of 1.2288 Mbps. This PN sequence is referred to as a Long Code. All long

codes are generated using a 42 bit Long Code Mask . The long code mask is used in conjunction

with a 42 bit state vector of a PN sequence generator to generate a long code. In the case of the

forward link, the long code is converted to 19.2 kbps by keeping and holding the first chip of every

64 long code chips. This is used to encrypt the interleaved bits using a modulo 2 addition. This process is illustrated for the forward link in Figure 7-5. At the receiver, the encrypted signal is

operated on by the inverse process, using the same long code mask to generate the equivalent long

code and thus, reproduce the original forward link / reverse link coded data streams.

Block Interleaver

Page Channel or

Forward Traffic

Channel Coded Bits

19.2 kbpsscrambled bit (to

Walsh Function

modulation)

19.2 kbps

19.2 kbps

Long CodeGenerator

Take 1st of 64bits and hold

for 64 bits

Page Channel

or User Long

Code Mask

19.2 kbps

1/64 long

code

1.2288 Mbps

Modulo 2

Add

Figure 7-5: Forward Link Scrambling for Traffic and Paging Channels

Knowledge of a specific long code mask allows the user (and base station) to encrypt or decrypt

the information associated with that mask. Masks for the Paging, Access, and traffic channels are

based on the knowledge of different information.

7.1.4.1 Paging Channel Encryption

The information transmitted by the base station to the mobile user on the paging channel is

encrypted using a Long Code. To generate the Long Code that will decrypt this data, the mobile

user must formulate the 42 bit mask that corresponds to the paging channel that it is listening to. To

formulate this mask, the mobile must have knowledge of :

• Pilot channel PN offset index (PILOT_PN), and

• the Page channel number that it is listening to (PCN)

The Pilot channel PN offset is transmitted to the mobile unit on the Sync channel. The mobile

initially defaults to Page Channel 1 (PCN=1) until it is reassigned.






7.1.4.2 Access Channel Encryption

The information transmitted by the mobile user to the base station on the access channel is

encrypted using a Long Code. To generate the Long Code that will encrypt this data, the mobile

user must formulate the 42-bit mask that corresponds to the selected access channel it will transmit

on. To formulate this mask, the mobile must have knowledge of :

• Pilot channel PN offset index (PILOT_PN),

• Number of the page channel that it is listening to (PCN),

• Selected Access Channel Number (ACN), and

• the Base Station Identification Number (BASE_ID).

The Pilot channel PN offset and the Base station ID number are transmitted to the mobile unit on

the Sync channel. The mobile initially defaults to Page Channel 1 (PCN=1) until it is reassigned by

a page channel message. The access channel number is randomly selected based on the maximum

number of access channels associate with the paging channel that the mobile is listening to. The

maximum number of access channels is provided by a page channel message.

7.1.4.3 Traffic Channel Encryption

The information transmitted on the forward and reverse traffic is encrypted using a Long Code. To

generate the Long Code that will encrypt and decrypt this data, the mobile and the base station must

have knowledge of the 42-bit mask that corresponds to that specific user. This mask is exchanged

and authenticated through Page and Access channel messages. The details of this process are not

public information.

7.1.5 Walsh Function Modulation

The “narrow band” data transmitted on the forward link is spread over a wide bandwidth by

modulating it with a Walsh function at a fixed rate of 1.2288 Mbps. There are 64 orthogonal Walsh

functions (loosely referred to as channels). Standard assignments are:

• Pilot channel: Walsh 0

• Sync Channel: Walsh 32

• Page Channel: Walsh i, i = 1 to 7,

• Traffic Channel: Walsh i, i = 8 up to 63 , ≠ 32

The specific Walsh function on to which the data is modulated defines the forward link

channelization.

7.1.5.1 Power Control Signaling Subchannel Modulation

To facilitate closed loop power control, the base station commands the mobile to increase or

decrease its transmit power to maintain the Received Power Level (RSL) at the base station at a

constant and minimum acceptable level . This information is encoded on each traffic channel just

prior to Walsh code modulation as illustrated in Figure 7-6. Every interleaved and encrypted 20 ms

frame is divided into 16 power control groups. Each power control group is preceded by a power

control bit. A “1” power control bit requests the mobile to decrease its transmit power by 1 dB.






Conversely, a “0” power control bit requests the mobile to increase its power by 1 dB. This

signaling format allows the mobile unit output power to be changed 800 times per second.

Replace 2

consecutive input

bits by one power

control bit every

1.25 ms

Forward TrafficScrambled Interleaved

Output Bits

19.2 kbps

1/64 long

code

19.2 kbps

Walsh Function

Wi

1.2288 Mbps

Power Control

Bits

800 bps

To Quadrature

Spreading &

Carrier

Modulation

Figure 7-6: Power Control Signaling Subchannel

7.1.5.2 Forward Link Base Station Transmit Power Control

As mentioned previously, voice data is coded at varying rates based on the level of speech activity.The base station seeks to transmit signals at a constant Energy per Bit (E b). Since the bit rate isvarying, data coded at a high rate (Rate 1) must be transmitted at a higher power than data coded ata lower rate (e.g. Rate 1/8) in order to maintain a constant E b.

Forward link transmit power control accomplished using a Variable Attenuator which isimplemented immediately following Walsh function modulation as shown in Figure 7-8. Thetransmit power attenuation level vs. Voice encoding data rate is given in Table 7-1. Reducingtransmit power in this manner reduces the interference introduced into the system.






Bit

Puncturer

Variable

Attenuator

1/64 Long Code

Foward Traffic

Channel

Scrambler Output

Stream of ith user

Power Control

Bits 800 bps

19.2 kbps 1.2288 Mbps

Rate

to Quadrature

Spreading

WalshFunction

Wi

19.2 kbps

Figure 7-7: Forward Link Base Station Transmit Power Control

Table 7-1: Base Station Transmit Power vs. Data Rate

Voice Coding RateData Rate (R b) kbps

(per IS-95) Base Station Transit Power

Attenuation Level (dB)

19.6

0

214.8

3

41

2.4

6

811.2

9

7.1.6 Quadrature Spreading & Carrier Modulation

After the appropriate Walsh function modulation (spreading) is performed, the pilot, paging, andtraffic channels are summed together. The composite signal is then spread in quadrature bydividing the signal in quadrature and phase modulating the I and Q channels with a “short code” of

length 215 chips at a chip rate of 1.2288 Mbps. Note that this sequence repeats every 26.66 ms. TheBinary “0” and “1” for the I and Q channels are mapped according to phase states as specified inTable 7-2. This process result in a QPSK modulated signal with a bandwidth of 1.2288 MHz.






Table 7-2: I and Q bits and Corresponding Phase Modulation State

I Q Phase

0 0 π/41 0 3π/41 1 −3π/40 1 −π/4

These spreading sequences are referred to as Pilot PN Sequences and can be noted in the following

way:

PN-I-i(t) = PN-I-0 (t - i x 64 T )c

PN-Q-i(t) = PN-Q-0 (t - i x 64 Tc)

Where: i = 0, 1, 2, … 511

t = Time

Tc = Chip Period = 1/1.2288 MHz = 814 ns

This means that including the zero offset sequence, PN-I-0(t) and PN-Q-0(t), there are 512

possible time offset indices, i, to identify cells. There are referred to as “PN Offsets”. Each PN

Offset is 64 chips long. The assignment of PN offsets to specific base stations is known as PN

Offset Planning. This is discussed further in Section 11. The offset I and Q channels are

quadrature modulated with the RF carrier (cos (ωc t) and sin (ωc t)), summed, and transmitted as

illustrated in Figure 7-8

Σ

Σ

Σ

LPF

LPF

from Walsh function i

modulation

(after scrambling)

1.2288

MbpsPN-Q-i(t)

Q

I

PN-I-i(t)

cosωc

sinωc

1.2288 MbpsRF

Figure 7-8: Forward Link Quadrature Spreading and Carrier Modulation

7.2 Reverse Link

The reverse link or “uplink” describes the communication from the mobile unit to the base

station(s). A block diagram of the transmit/receive is shown in Figure 7-1. Note that the

demodulation process includes a RAKE receiver to combine multipath signals. The operation of

the RAKE receiver is omitted for clarity in the following sections, however, the RAKE receiver is

discussed in Section 5 of this document.








uniform

104 kbpsLPF A/D

CELPSpeechEncoder

Mouthpiece

Figure 7-2: Speech Processing at Mobile Side

The CELP speech encoder produces a variable output data rate based on speech activity. The coder

generates one frame, or packet, every 20 ms. The available output rates are:

• Rate 1: 171 bits / packet (8.55 kbps)



• Rate 81 : 16 bits / packet (0.8 kbps)As with the forward link, the advantage of using lower bit rates when there is little or no speech

activity is that it limits the amount of extraneous information transmitted. Decreasing the bit rate

allows the transmit power to be reduced while maintaining a constant E b/Nt resulting in less

interference imposed on other users of the system.

7.2.2 Channel Coding

The channel coding process for the reverse link is identical to that on the forward link with the

exception of the convolution coding rate. Channel coding involves converting the 20 ms speech

frames into traffic blocks and applying 1/3 rate Convolutional coding. Generating traffic blocks

involves incorporating overhead data transmissions (signaling information) with the voice data.The Mixed Mode (MM) bit indicates the insertion (MM=1) or non-insertion (MM=0) of signaling

data into the traffic frame as required by the system. A 20 ms frame of voice at coding Rate 1 may

be replaced entirely with signaling information. This is known as blank-and-burst. Alternatively,

signaling data transmitted at Rate 1 may share a frame with lower rate voice data. This is known as

dim-and-burst. Traffic Frames are then generated by adding Cyclical Redundancy Check (CRC)

code bits that provide error detection capability.

The resulting traffic frame is fed to a convolutional coder of Rate 1/3 with a constraint length of 9.

This coder uses an 8 bit shift register and outputs 3 bits for every input bit. Convolutional coding

provides channel bit error detection and correction capability. For data rates below 9.6 kbps (Rate 1

+ overhead), output bits are repeated to bring the number of bits in a 20 ms block to 576 for aconstant output rate of 28.8 kbps. Remember that the user data (voice information) is still input to

the system at a variable rate – the change to 19.2 kbps represents a change in sampling rate. This

channel coding process is illustrated below in Figure 7-3.






ΣTraffic

BlockGenerator

Convolutional

Coder

From

Variable

RateSpeech

Coder

Signaling

Tail bits

MM bit

To Interleaver

Speech

blocks

Traffic blocks Traffic frame

CRC

MM = 0 No Signaling

MM = 1 Signaling present

Variable

Rate

28.8 kbps

Figure 7-3: Reverse Link Channel Coding Process

7.2.3 Bit Interleaving

The bit interleaving process on the reverse link is very similar to that used on the forward link. The

effect of interleaving is to spread a burst of bit errors that occur in the transmission channel over

several data blocks as well within a data block. The coded transmissions used in CDMA are less

susceptible to random errors than burst errors. Interleaving consists of writing bit stream into the

buffer matrix using one pattern and reading the bit stream from the matrix using different pattern.

Reverse link uses 32 by 18 matrix. This is illustrated below in Figure 7-4. The process is reversed

to de-interleave the data.

To 64ary Symbol Modulation

576 bits

Block Interleaver

--Write block into matrixaccording to pattern

--Read block from matrixaccording to pattern

From Channel Coder

28.8 kbps

Coded Blocks In

28.8 kbps

20 ms block20 ms block

576 interleaved bits

Interleaved Blocks Out

Figure 7-4: Reverse Link Bit Interleaving

7.2.4 64-ary Orthogonal Walsh Symbol Modulation

The process of 64-ary orthogonal Walsh symbol modulation is not really that scary. To improve

error performance, and aid in non-coherent detection, groups of 6 bits coming from the interleaver

are mapped to one of 64 orthogonal Walsh Codes. The index to the specific Walsh Code is

determined by the decimal equivalent of the binary number consisting of the 6 incoming bits. This 6






bit, binary number has decimal equivalent ranging from 0 to 63. The selected Walsh Code becomes

the “modulation symbol” representing 6 binary bits. Note that on the reverse link Walsh functions

Do Not designate channels.

In summary, the input 20 ms frame of data consists of 576 bits. This frame gets converted

(“modulated”) to 96 Walsh functions. Each group of 6 Walsh functions is called a “power controlgroup”.

Power Control Group Gating

As it turns out, several of the power control groups are repeated bits when the traffic frame rate is

less than Rate 1 (9.6 kbps). The power control groups with repeated bits are removed by gating off

their transmissions with a data burst randomizer. The long code is used by the data burst

randomizer to determine which power control groups are to be gated off. The gating of repeated

bits decreases the self interference to all mobiles transmitting on the same CDMA RF carrier

frequency. The resulting output of the data burst randomizer is still at 307.2 kbps and is then

encrypted. This process is illustrated in Figure 7-5.

7.2.5 Encryption: Long Code Spreading

The data transmitted on several CDMA channels is encrypted (scrambled). Encryption provides

• Privacy of user traffic information

• Identification of signals on the reverse link (reverse link traffic and access channels)

The encrypted channels are:

• Paging channel (forward link)• Access channel (reverse link)

• User Traffic channels (reverse link and forward link)

Data on these channels is encrypted by modulating it with PN sequences with a length of 242

- 1

chips at a chip rate of 1.2288 Mbps. This PN sequence is referred to as a Long Code. All long

codes are generated using a 42 bit Long Code Mask . The long code mask is used in conjunction

with a 42 bit state vector of a PN sequence generator to generate a long code.

On the reverse link, The 64-ary modulated symbol at 307.2 kbps is modulated with the long code at

1.2288 Mbps. The output stream is encrypted (as well as spread) data at 1.2288 Mbps with 4 chips

for each 64-ary data bit within the symbol. This process is illustrated for the reverse link Traffic

Channel in Figure 7-5. At the receiver, the reverse link data is identified by the long code usedto encrypt it -- not a Walsh Function. The received signal is operated on by the inverse process,

using the same long code mask to generate the equivalent long code and thus, reproduce the original

forward link / reverse link coded data streams.






BlockInterleaver

6 BitsTo WalshFunction

Data BurstRandomizer

Long CodeGenerator

Traffic Frame Rate Value

User Long Code Mask

Repeated bits gated off (below 9.6 kbps)

Interference to other mobiles reduced when off

1.2288 Mbps

307.2 kbps

4 PN chips

per Walsh bit

307.2

kpbs28.8

kps

on on on on

off off

gated

power

control

groups

Power Control

Group Gating

1.2288 Mbps

(to Quadrature

Spreading)

Figure 7-5: Reverse Link Traffic Channel Spreading, Power Control Group Gating, and

Encryption

Note that the Access Channel and the Traffic Channel are modulated with different long codes

generated with different Long Code Masks. Knowledge of a specific long code mask allows the

user (and base station) to encrypt or decrypt the information associated with that mask. Masks for

the Paging, Access, and Traffic channels are based on the knowledge of different information.

These masks are discussed in Section 7.1.4.

7.2.6 Quadrature Spreading & Carrier Modulation

In the reverse link transmitter, following the direct sequence spreading by the long code, the

forward link zero offset PN codes, PN-I-0(t) and PN-Q-0(t) are used, where PN-Q-0(t) is delayed by

one-half chip time. This delay (406.9 ns) results in an offset quadrature spreading eliminating the

180 deg phase transitions from the reverse link, allowing the use of a more nonlinear amplifier,

without incurring serious intermodulation problems. Nonlinear amplifiers are cheaper and simpler

to build. The reverse link modulation process is illustrated in Figure 7-6. The Binary “0” and “1”

for the I and Q channels are mapped according to phase states as specified in Table 7-1. This

process results in an Offset QPSK modulated signal. The offset I and Q channels are modulated

with the RF carrier (cos(ωc t) and sin(ωc t)), summed, and transmitted.

Table 7-1: I and Q bits and Corresponding Phase Modulation State

I Q Phase

0 0 π/41 0 3π/41 1 −3π/40 1 −π/4








VOCODEDSPEECHDATA

LONG

CODE

I SHORT

CODE

FIR

FIR

Q SHORT

CODE

Q

I9.6

kbps28.8

kbps

28.8

kbps

307.2

kbps

INTERLEAVER

CONVOLUTIONAL

ENCODER

Rate 1/3

64-ary

Modulator

1.2288

Mbps 1.2288 Mbps

1.2288 Mbps

20msec

blocks1 to 64 Walsh

Codes

1.2288

Mbps

1/2

1/2 Chip

Delay

Figure 7-2: CDMA Reverse Link (Mobile to Base) Physical Layer

8 CDMA Capacity

CDMA technology offers a significant capacity advantage over other multiple access systems. The

capacity of FDMA and TDMA systems is limited by the finite amount of spectrum allocated to

cellular and PCS services with the corresponding frequency reuse requirements. CDMA is

different in that many users operate on a single wideband RF carrier. This carrier frequency may be

reused by the adjacent cell (N=1 reuse). CDMA capacity is only interference limited, therefore anyreduction in interference converts directly and linearly into an increase in capacity. Interference is

introduced from several sources including:

• Co-cell mobile users,

• Adjacent cell mobile users,

• Adjacent cell base stations, as well as

• Thermal and spurious noise.

CDMA employs several techniques to reduce these interference sources including:

• Suppressing or squelching transmissions during quiet periods of each speaker.

• Using sectored base station antennas.

• Dynamic power control to keep transmit levels to the minimum required to close the

link.

8.1 The General Case

For the simplest case of the single CDMA cell site, the approximate capacity in terms of number of

users can be written as:






N W R

E N S b

= + −10

η

Equation 8-1: Capacity Equation (General Form)

Where:

W = Spread Spectrum bandwidth (Hz)

R = Information bit rate (Hz)

E b = Energy per bit (J)

No = System (thermal) noise energy (J)

N = Number of users

S = Received power of user signals at the base station (Watts)

(not including serving signal)

η = Received background noise level at the base station (Watts)

W/R is known as the processing gain and the value of E b/No is the value required for adequate

performance of the receiver. For the case of digital voice, this implies a Frame Error Rate of 1% or

better which corresponds to a BER of 10-3

or less.

We can see that the number of users (i.e. TECs that may be assigned) is proportional to the system

processing gain and inversely proportional to the required E b/No (or E b/Nt as the case may be). In

addition, capacity is reduced by the inverse of the per user signal-to-noise ratio in the total system

spread bandwidth.

8.2 Adjustments to the General Case

Short of reducing the required E b/Nt through improved coding or modulation techniques, we can

only increase capacity by reducing interference. We must consider the interference generated by

other users with in the given cell and as well as interference from adjacent cell sites. Adjustments

are made to the general capacity equation to reflect these factors.

8.2.1 Sectorization Gain

A common technique for reducing interference is sectorization at the base station. Sectorization

refers to using directional antennas at the cell site for both receiving and transmitting. For a three

sectored cell site, the number of interferers seen by any antenna is, theoretically, a third of the

number seen by an omni directional antenna. An adjustment term called Sectorization Gain (Gs) is

incorporated into the capacity equation to reflect the resulting increase in system capacity. The

sectorization gain for an omni site is 1 and approximately 2.55 for a three-sectored site.

Sectorization gain is slightly less than three due to some overlap in coverage (antenna patterns)

between sectors.






8.2.2 Voice Activity Factor

Studies have shown that either speaker is active only 35% to 40% of the time when making a phone

call. The percentage of time that a user is actually speaking is called the voice activity factor

(VAF). Statistically determined VAF values range from approximately 0.25 to 0.6 with 0.4 to 0.5

being the most common. CDMA uses a variable rate voice encoder that monitors the voice activity

level and suppresses transmission when no speech is taking place. This process reduces the level of

interference introduced into the system. The VAF is incorporated into the capacity equation to

reflect this advantage.

8.2.3 Frequency Reuse Efficiency (IADJ.)

The Frequency Reuse Efficiency, (IADJ) accounts for the interference caused by other mobile units

as well as base stations in the surrounding cells. Dynamic power control is used on the forward and

reverse links to minimize adjacent (as well as co- ) cell interference. IADJ is statistically dependent

on the loading of adjacent cells as well as the location of users within those cells.

This factor is stated as a fraction of the noise experienced nominally by the cell under

consideration. A typical value for IADJ is given as 0.66. This value implies that a cell located in the

center of a seven cell cluster is subject to a noise floor that is 160% of that which would be

observed if a cell is operating in total isolation.

8.3 Definition of Pole Point

The pole point is frequently referred to in CDMA capacity analysis. It is best described

conceptually by visualizing the operation of a single CDMA Forward channel transmitter. When

the transmitter is idle, only Pilot, Paging, and Sync are transmitted. As a single conversation (TCE)

becomes active, some power is added to the total transmitted signal to service that conversation. It

may be a minority of the total transmitter power, but the CDMA processing gain, (roughly 19.3 dB

for a 14.4 kbs information rate) will make the received de-spread E b/Nt sufficiently large that the

data can be demodulated. As path loss (distance) increases, the subscriber will eventually lose the

signal, as No (thermal noise) begins to become the dominant part of Nt, and it overwhelms E b.

As the number of users (TCEs) increases on the single CDMA RF carrier, the call we wish to

decode becomes a smaller and smaller fraction of the total transmitter power, and the total

transmitter power will increase to provide an adequate signal for each active call (TCE). At large

distances, Nt is significantly above that which was observed with only one active call, and the cell's

maximum range is gradually reduced. The available fixed processing gain of the system is less

effective in eliminating the transmitter-produced portion of Nt because of the correlated effect of multiple users.

At some point, the increasing number of active calls (TCEs) becomes large enough that No no

longer matters. The noise resulting from Sync, Paging, Pilot, and the other active calls overwhelm

the processing gain, and the desired call can no longer be decoded, at any range, regardless of how

high the transmit power is raised. In other words, the cell jams itself with its own co-channel (i.e.






co-frequency) transmissions. This number of users at which this condition occurs is known as the

Pole Point.

8.4 The Pole Point Equation

The pole point equation estimates the maximum number of traffic channels that may be assigned toa single CDMA base station (or sector) on a single carrier frequency. This equation includes the

effects of sectorization, voice activity, and adjacent cell interference.

Equation 8-1: Pole Point Equation

#ofTCEs at Pole Point = ++

11

( )

( )( )( )(

W R

I VAF E N Gb

ADJ b t s)

where:

W = The Spread Bandwidth in Chips/sec = 1.2288 x 106 for IS-95 derivatives,

R b = The Information Bit Rate = 14.4 x 10

3

bps (IS-95-A), (9.6 kbps for IS-95)IADJ = The additional interference contributed by adjacent cells = 0.6,

VAF = Voice Activity Factor = 0.5,

E b/Nt = Minimum E b/Nt required (after despreading) to provide specified voice quality,

Gs = Sectorization Gain

= 1 for omni cells

= 1.18 (that is 3/2.55) for 3-sector cells

To explain these variables further,

• The spread bandwidth (W) is the actual number of chips transmitted on the RF channel after the

data signal is spread by a direct sequence technique, as it is in IS-95 and PCS derivatives.

• The information bit rate (R b) is the channel information bit rate, including both the voice

channel and system overhead bits to support a single voice channel

• The additional interference contributed by adjacent cells (IADJ) is an adjustment factor that has

been stated by equipment vendors to be the nominal amount of extra system-generated noise

contributed by adjacent cells. This is stated as a fraction of the noise generated by the cell

under consideration. In other words, 0.6 means that Nt (neglecting the No component) is 160%

of that which would be observed if a cell is operating in total isolation. This factor is a function

of cell loading, propagation characteristics, and voice activity factor.

• The voice activity factor (VAF) is the fraction of the time that a person is actually speaking (and

transmitting full-rate data) during an average conversation. If a person spends 50% of the time

talking, VAF=0.5.






• Minimum E b/Nt is the E b/Nt required to maintain a 1% frame error rate, that which has been

specified as the minimum acceptable to maintain call quality. This is normally expressed as a

linear energy ratio, not in dB.

• Sectorization gain (Gs) is somewhat similar to the additional interference contributed by

adjacent cells except that it is a factor to describe the noise introduced by adjacent sectors within the same cell. In other words, it is intended to adjust for the fact that sectorizing a cell

does not quite increase the available number of TCEs available at a 3 sector cell by a factor of 3.

For the assumptions stated previously, the # of TCEs at Pole Point = 19.09, or 19 when truncated to

the next lower integer

It is important to note that pole point is expressed per sector, not per cell.

It is also possible to show that the accuracy of the closed loop power control plays a part in the pole

point, as it affects the ratio of the desired signal's power to the total noise. The pole point equation

shown above assumes that perfect power control is maintained. At this time, the specifications isfor ±2.5dB which results in a 20% reduction in the available maximum number of TCEs

2.

9 CDMA Handoff

A CDMA cellular network handles mobile unit call processing transitions more subtly than the

other technologies used for mobile communications networks. CDMA Handoffs require that the

mobile unit maintain an ongoing list of possible base station sites that it may use for Handoffs as it

travels through the system. CDMA offers the unique feature of allowing mobile users to process

signals from multiple (up to 3) base stations simultaneously. The terminology and various types of Handoffs associated with CDMA are described below.

9.1 Handoff Terminology

Handoffs are initiated and terminated as a result of the pilot signal strength as measured by the

mobile unit in terms of Ec/Nt (Energy per chip to Total Noise). The parameters and classifications

associated with CDMA Handoffs are provided below.

9.1.1 Introduction to TADD, TDROP & TCOMP

TADD is the value of the Pilot signal strength, Ec/Nt, in dB received by the mobile unit at which themobile will recognize the cell/sector as a possible contributor to the call processing activities.

Values provided by vendors are typically on the order of -13 dB.

2Reference Robert Padovani, “Reverse Link Performance of IS-95 Based Cellular Systems,” IEEE Personal

Communications, Third Quarter 1994






TDROP is the value of the Pilot signal strength, Ec/Nt, in dB received by the mobile unit at which the

mobile will drop the cell/sector as a possible contributor to the call processing activities. Values

provided by vendors are typically on the order of -17 dB. Note that the received pilot strength must

fall below TDROP for some specified length of time before the cell/sector is dropped in order to keep

from “toggling” the cell on and off. This length of time (T_TDROP) is an addressable parameter

with values ranging from 0.1 to 319 seconds.

Note that both TADD and TDROP are assigned on a per transmitter (i.e. per cell or sector) basis. These

terms need not be the same for every cell in the system.

T _COMP is the Active Set versus Candidate Set comparison threshold. Mobile Stations transmit a

Pilot Strength Measurement Message when the strength of a pilot in the Candidate Set exceeds that

of a pilot in the Active Set by this margin. The base station shall set this field to the threshold

Candidate Set pilot to Active Set pilot ratio, in units of 0.5 dB.

9.1.2 Handoff Candidate Classification

The mobile station continuously searches for Pilots to detect the presence of other CDMA signals

that have the same carrier frequency and measures the strength (received Ec/Nt) of the pilots. When

the mobile station detects a Pilot of sufficient strength that is not associated with the serving

cell/sector, it sends a message to the serving base station. The cellular network decides which

neighbor base stations can be involved in a Handoff. In doing this, the all of the base stations are

classified into one of the categories described in the following table.

Table 9-1: Pilot Search Parameters

Classification Description

Active Set The pilots associated with the Forward Traffic Channels assigned to the mobile

station.Candidate Set The pilots that are not currently in the active set but have been received by the

mobile station with sufficient strength to indicate that the associated Forward

Traffic Channels could be successfully demodulated.

Neighbor Set The pilots that are not currently in the Active Set or the Candidate Set and are

likely candidates for Handoff.

Remaining Set The set of all possible pilots in the current on the current CDMA frequency

assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the

Active Set.

9.2 Types of HandoffsThe RAKE receiver allows the mobile unit to coherently combine bit energy from up to three

different sources to complete the forward link. Conversely base station processing software allows

for completion of the reverse link by evaluating and selecting the best data received by up to three

base stations. The differences in the types of Handoffs stems from the number of contributing

entities and the relative locations of the contributing base stations (i.e. adjacent cells or adjacent

sectors).






9.2.1 Soft Handoff

The condition where two cells are in simultaneous communication with the mobile is called

Soft Handoff. Soft Handoff will continue until the pilot signal from one of the contributing cells

drops below a predefined threshold (TDROP). At that time the call will be transferred to the

remaining cell.

The mobile station typically initiates soft Handoffs. The mobile station continuously searches for

pilots to detect the presence of other CDMA signals that have the same carrier frequency and

measures the strength of the pilots. When the mobile station detects a pilot of sufficient strength

that is not associated with the serving cell, it sends a message to the serving base station. The

cellular network decides which neighbor base stations can be involved in a Handoff and selects an

idle Walsh function associated with the selected site, effectively selecting a traffic channel. The

selected site is given the mobile’s long code mask. The serving base station is directed to send the

mobile a message to initiate Soft Handoff. The simultaneous communication with the two base

stations is handled differently on the forward link and reverse link.

9.2.1.1 Forward Link

When the Soft Handoff is initiated, the two base stations begin transmitting data to the mobile. The

mobile receives information from the two forward links and uses the RAKE receiver to coherently

combine the signals using the pilot sequence transmitted by each cell/sector as its reference. This

combination of multiple forward link signals improves overall link performance.

9.2.1.2 Reverse Link

In the case of the reverse link, both base stations are receiving the transmitted speech frames from

the mobile. However, these signals are not coherently combined at the MTSO. Instead, the highest

quality traffic frame received from among the two base stations is selected on a frame-by-frame

basis. Improved reverse link performance results since it is more probable that a traffic frame of

acceptable quality will be receive by one of the two base stations than by a single base station. The

Soft Handoff reverse link process is modeled in terms of a “joint probability”.

9.2.1.3 Joint Power Control

During a Soft Handoff, closed-loop power control of the mobile user is handled “jointly”

between the serving base stations. The base stations send identical traffic frames with the

exception of the power control bits. If all of the serving base stations request the mobile increase

its power, the mobile will increase its output power by 1 dB (nominal). However if any one of the

serving stations request a decrease in power, the mobile will drop its output by 1 dB. As with the

normal closed-loop power control process, adjustments are made for each power control group

(1/16 of a 20 ms frame or 800 time per second).






9.2.2 Soft - Soft Handoff

Soft-Soft Handoffs are identical in function and process to that of the soft Handoff described in

Section 9.2.1, however, Soft-Soft Handoffs entail the simultaneous serving of a mobile unit by

three cell sites. Three is the maximum number of serving signals due to mobile (RAKE) receiver

specification.

9.2.3 Softer Handoff

Softer Handoffs are identical in function and process to that of the soft Handoff described in

Section 9.2.1, however Softer Handoffs entail the simultaneous serving of a mobile unit by two

sectors of the same cell.

9.2.4 Soft - Softer Handoff

Soft - Softer Handoffs are identical in function and process to that of the Soft Handoff described in

Section 9.2.1, however, Soft-Softer Handoffs are the simultaneous serving of a mobile station

by the original sector, an adjacent sector, and an adjacent or neighboring cell.

9.2.5 Hard Handoff

The mobile unit will initially seek to perform a soft Handoff. If the cellular network cannot perform

a soft Handoff, a hard Handoff is necessary. A hard Handoff occurs when a CDMA call is

transferred from one base station to another base station transmitting on a different carrier

frequency. Hard Handoff is analogous to the Handoff procedure that takes place in standard

AMPS Cellular. When the serving base station directs the mobile unit to perform a hard handoff, it

provides the mobile with the new CDMA frequency assignment, new Walsh Code assignment, and

new Active Set of base stations. The mobile then disables its transmitter, switches its receiver to

the new CDMA frequency. It then acquires the Pilot signals from the base stations in the newly

specified active set. Once the mobile station has received a predetermined number of correct traffic

frames from the new base station, it enables its transmitter on the new CDMA frequency and

continues the conversation.

9.2.6 CDMA to Analog Handoff

If there are no CDMA channels to Handoff to, then the call would be handed off to an available

analog channel at the serving base station and switched to the analog mode of processing. From

that point on, the call will be handled as any other analog call at that base station. The CDMA to

Analog Hand off is applicable to 850 MHz cellular CDMA, however, it is not currently defined for

PCS applications.

9.3 Handoff Criteria

It is important to note that current system implementations base handoff decisions solely on Pilot

signal strength. Call quality is not considered a handoff parameter. Systems put mobile stations

into soft handoff with as many base stations as possible. That is, the system will seek to put mobile






users in soft handoff with the with all the base stations (max. of three) with pilots signals exceeding

TADD. Indeed; resources (TCEs) are allocated on the ability to allocate them rather than the need to

allocate them. Go figure. In short:

• Handoffs are based solely on Pilot Strength – not call quality

• If a mobile station can be into a soft handoff – it will

9.4 Handoff Process

Three examples are provided to illustrate IS-95 CDMA Soft Handoff Processing.

9.4.1 Example 1

Figure 9-1 walks through the processes associated with two Pilot Channels greater than T _ADD. It is

easily extended to the case of three Pilot Channels greater than T _ADD.

Mobile Station Base Station(User conversation using A)

• Pilot B strength exceeds T_ADD

(User conversation using A)

• Sends Pilot Strength Measurement

Message⇒ Reverse Traffic

Channel⇒ • A receives Pilot Strength

Measurement Message

• B begins transmitting traffic on the

Forward Traffic Channel and

acquires the Reverse Traffic

Channel

• Receives Handoff Direction

Message⇐ Forward Traffic

Channel⇐ • A and B send Handoff Direction

Message to use A and B

• Acquires B; begins using Active

Set (A,B)• Sends Handoff Completion


Channel⇒ • A and B receive Handoff

Completion Message

Figure 9-1: Mobile Unit transitions into a region defined by two Pilot Channels greater than

T _ADD (Soft Hand-off)

9.4.2 Example 2

IS-95 permits up to three Pilots to be assigned to the Active Set. There will be situations in which a

fourth Pilot Channel is greater than T _ADD. IS-95 deals with this situation by favoring the prevailing

Pilot Channels greater than T _ADD through the use of T _COMP. T _COMP compares the value of the

incoming pilot to the weakest Pilot in the Active Set and will demote promote the incoming Pilot if

its Ec/Io value exceeds the active pilot by some specified margin and demotes the weaker Pilot to

the Candidate Set. A simplified example is given in Figure 9-2 where base station C is the weakest

Active Pilot.

Mobile Station Base Station






(User conversation using A, B, C)

• Pilot D strength exceeds T _ADD

(User conversation using A, B, C)



Channel⇒ • A, B, C receives Pilot Strength

Measurement Message.

• T _COMP is applied to C & D.

• D begins transmitting traffic on

the Forward Traffic Channel and


Channel



Channel⇐ • C and D send Handoff Direction


• Acquires D; begins using Active

Set (A,B,D)

• C is relegated to Candidate Set

• Sends Handoff Completion


Channel⇒ • C and D receive Handoff

Completion Message

• Handoff drop timer of pilot Cexpires



Channel⇒ • A, B, C and D receive Pilot

Strength Measurement Message



Channel⇐ • C and D send Handoff Direction

Message to use D only

• Stops diversity combining with C;

begins using Active Set (A,B, D)


Message⇒ Forward Traffic

Channel⇒ • A, B, C and D receive Handoff

Completion Message

• C stops transmitting on the

Forward Traffic Channel andreceiving on the Reverse Traffic

Channel

(User conversation using A, B, D) (User conversation using A, B, D)

Figure 9-2: Mobile Unit transitions into a region defined by four or more Pilot Channels

greater than T _ADD






9.4.3 Example 3

Figure 9-3 illustrates the basic processes associated with a transition involving two Pilots. This is

easily extended to handle three Pilots

Mobile Station Base Station(User conversation using A)

• Pilot B strength exceeds T_ADD

(User conversation using A)



Channel⇒ • A receives Pilot Strength

Measurement Message

• B begins transmitting traffic on the



Channel


Message

⇐ Forward Traffic

Channel⇐ • A and B send Handoff Direction


• Acquires B; begins using Active

Set (A,B)




Completion Message

• Handoff drop timer of pilot A

expires



Channel⇒ • A and B receive Pilot Strength

Measurement Message


Message

⇐ Forward Traffic

Channel

⇐ • A and B send Handoff Direction

Message to use B only• Stops diversity combining; begins

using Active Set (B)


Message⇒ Forward Traffic


Completion Message

• A stops transmitting on the


receiving on the Reverse Traffic

Channel

(User conversation using B) (User conversation using B)

Figure 9-3: Mobile Unit transitions through a region defined by two prevailing pilots greater

than T_ADD.

In these situations, we have assumed that system access was not limited by available traffic

resources. It is clear that the hand off process will be initiated as a result of Pilot Channel Ec/Io

with no reference to call quality. The operating parameters that are directly affected by E b/Nt are

measured by the mobile unit and the base station to be used for statistical processes only.






10 CDMA Call Example

The following is a example of a possible cellular call in a CDMA system. The example describes:

• Initial system access

• Call initiation and setup,

• Soft Handoff, and

• Call termination.

10.1 Initial System Access

When the mobile is first turns on, it must find the best base station with which to communicate.

The mobile unit tunes its receiver to a specified “primary” CDMA carrier frequency (Note that

detailed CDMA frequency planning is not addressed in this document). The mobile then scans for

available pilot signals, which are all on different time offsets of the same PN short (215

chips) code.

This acquisition process is similar to what takes place in an analog system where the mobile scans

the control channels and selects the strongest one. The scanning process is made somewhat easier

since the timing of any base station is always an exact multiple of 64 system clock cycles (chips)offset from any other base station. The mobile selects the strongest pilot sequence and establishes

frequency and time reference with this signal. If the mobile does not detect any pilot signals of

adequate strength, the unit tunes its receive to another specified CDMA carrier frequency

The mobile then demodulates the sync channel which is always transmitted on Walsh 32. The Sync

Channel provides master clock information by sending the state of the 42 bit shift register, which

generates the long (242

chips) code, 320 ms in the future. The long code, generated in conjunction

with a private user mask, is used for encryption and decryption. The mobile then starts listening to

the paging channel and waits for a page directed to its phone number.

10.2 Call Initiation and Setup

The mobile user then decides to make a call and enters the desired phone number. This initiates an

access probe. The mobile uses the access channel and attempts to contact the serving base station.

Since no traffic channel has been established, the mobile uses open loop power control. Multiple

tries are allowed at random times to avoid collisions that can occur on the access channel. Each

successive attempt is made at a higher power level. After each attempt, the mobile listens to the

paging channel for a response from the base stations. Once the access request has been received by

the base station, the base station responds with an assignment to a traffic channel (Walsh code).

The base station initiates the land link, and conversation takes place.

10.3 Soft Handoff

During the call the mobile finds another base station with pilot power received at the mobile

adequate to service the call (above the TADD threshold for that cell). The mobile unit makes a

request to its serving cell to initiate a soft Handoff with the additional cell. The base station passes

this request to the MSC. Contingent on some other factors (requested site availability, system, etc.)

the MSC will approve the request for handoff. The MSC then contacts the second base station and






gets a Walsh (traffic channel) assignment. The assignment is sent to the mobile by the first base

station. The land link is connected to both base stations. The mobile coherently combines the

signals from both base stations using the two pilot signals as coherent phase (time) references. On

the reverse link, the MSC examines the signals from each base station and the best 20 ms frame is

selected based on the Frame Error Rate.

At this point, closed-loop power control is conducted by both base stations. In this case, the mobile

will increase its power only if both stations request it. However if any one serving base station

requests a decrease, the mobile will decrease its power. As the signal from the first base station

degrades (drops below the TDROP threshold), the mobile will ask that the Soft Handoff be

terminated. The mobile sends a drop request for the first cell and the MSC then discontinues its

transmission and reception from that cell.

10.4 Call Termination

Call termination can be initiated either from the mobile or the land side. In either case the

transmissions are stopped, the Walsh code is freed, and the land line connection is broken. Themobile unit resumes monitoring the page channel of the current serving cell.

11 Basic System Engineering Issues

The properties of CDMA require that the design guidelines of conventional AMPS or GSM systems

be modified to accommodate the addition of the noise floor (Nt), the constructive benefits

associated with multipath as variables that effect system performance, as well as the non-

symmetrical relationship between the forward and reverse links. There are some basic concepts that

need to be kept in mind when engineering a CDMA system that are not applicable to the other technologies.

1. The coverage provided by CDMA system is not static. As the loading on a given

base station changes, the coverage provided by that base station changes

inversely. Otherwise stated; just because you have great RF coverage doesn’t

guarantee good signal.

2. Holes in coverage may result when there is either insufficient or abundant levels

of RF. System coverage is measured as the ratio of desired signal to all other

signals and that the ratio can be unacceptable regardless of the absolute quantity.

3. CDMA systems allow for the non-symmetrical simultaneous processing of a call by multiple base stations. The energy in the forward link is summed to a greater

strength than the individual components. The reverse link employs the shotgun

effect in that multiple base stations will receive the transmitted signal and the

probability that the signal will be acceptable for at least one of them is greatly

increased.

4. Traffic engineering in a CDMA system requires that in addition to all of the

factors associated with engineering a FDMA or TDMA system, the element of






time also be introduced. PN Offset Planning for a CDMA system requires the

careful assignment of 512 available time offsets to the cells/sectors in a system.

11.1 Propagation Modeling of the Wideband CDMA RF signal

Modern RF engineering must rely on accurate and reliable EM wave propagation models. The

foundations of good propagation models are well-established theory, statistical analysis and ability

to be modified by measured data. The question to ask is: can we use existing narrow-band signal

propagation model to design wideband CDMA system?

A RF signal propagating through wireless medium arrives at the receiver distorted as a result of

different propagation paths. These paths are caused by a scattering, reflection and diffraction from

either a natural or man made structure existing over the propagation area. In addition, the received

signal reaches the receiver significantly attenuated due to the propagation loss phenomena. In

theoretical modeling of the propagation loss we can determine two separate loss mechanisms.

The first one is the signal level decay due to the dispersion of the energy in space, absorption of the

ground and foliage and effects of the ground reflection. This phenomenon defines mean power pathloss. In addition to mean power path loss, existing terrain features as well as large man made

structures impose additional variations of the signal commonly referred to as slow or long-term

fading. The statistical distribution of the long-term fading has been studied extensively and it can

be modeled as additional loss having normal zero mean normal distribution in the logarithmic

domain. For that reason the long-term fading is frequently called log normal fading.

Multipath propagation causes large signal strength variations over distances comparable with signal

wavelength. These large variations are commonly termed short term or fast fading. Due to the fast

fading, the envelope of the received signal has a statistical distribution that is often model by

Rayleigh density function [1].

Theoretical analyses described above, assumes a signal bandwidth which is relatively small in

comparison to the RF carrier frequency. In comparison to other cellular standards, IS-95 CDMA

has a considerably larger bandwidth. Study of the path loss characteristic for the wide-band signals

presented in [2] demonstrated that, provided the power spectrum density of the signal is

approximately flat, narrow-band path loss estimation are of sufficient accuracy as long as the

bandwidth of the signal is smaller than 66% of the carrier frequency. For the case of cellular IS-95

based CDMA systems this is certainly the case. In addition, due to its wide-band nature CDMA

signal has an inherent multipath fading resistance and for that reason fast fading is not as

pronounced as in the case of narrow-band signals.

Two most popular macroscopic propagation models are Lee’s Propagation Model and Hata-

Okumura Propagation Model. As it is shown in [3], Lee’s Model is valid for 1900 MHz band, too.

Although Hata-model is developed for frequencies from 150 and 1500 MHz, there is a separate

version for 1500 to 2000 MHz band called COST-231.






11.2 Link Budget

Link budget analysis examines all gains and losses present in the radio path between a transmitter

and a receiver. In order for a system to operate properly, both forward and reverse links have to

satisfy power and quality requirements. Balancing the forward and reverse link budget for a

CDMA system takes into consideration: traffic load in a particular system, various hardwarelimitations, equipment characteristics, signal quality requirements, required coverage reliability and

type of propagation environment. The link budget analysis provides the maximum allowable path

loss that can be tolerated on the radio link and determines the extent of the cell coverage radius.

Due to the different processing schemes for six channel types defined by IS-95 standard, link

budget analysis must be examined separately for each channel type. Usually, analysis starts with

reverse link calculations using the expected traffic load as a main input parameter. The result is

maximal allowable path loss. Next step combines previously calculated maximal path loss and

receiver sensitivity to obtain the appropriate power allocation for each of the forward link channels.

This is illustrated in Table 11-2, which uses pre-calculated receiver sensitivity (see Table 11-1).

Detailed explanation of all aspects of link budget is given in ‘Unit C2: Intermediate CDMA Planing

and Design Issues’.

Table 11-1: Receiver Sensitivity for Different CDMA Channel Types

Channel

Type

Bit Rate

[Kb/sec]

PG

[dB]

Quality

Requirement

[dB]

Noise

Figure

[dB]

RxSens

[dBm]

Pilot N/A 0 -14 8 -119

Paging 7.2 22.3 8 8 -1194.8 24.1 8 8 -121

Sync 1.8 25.3 8 8 -122

1.2 27.1 8 8 -124

Traffic-

Forward

14.4 19.3 6.5 8 -118

9.6 21.1 5 8 -121

Traffic –

Reverse

14.4 19.3 7 5 -120

9.6 21.07 6 5 -123






Table 11-2: Simplified Example of IS-95 CDMA Link Budget for In-Vehicle Coverage

Forward Link Reverse

Link Pilot Paging Sync Traffic

Reverse Link TX

Power

23.01 dBm

(0.2 Watts)

Forward Link

TX Power

34.7 dBm

(2.9 Watts)

28.7 dBm

(0.74 Watts)

25.7 dBm

(0.37 Watts)

25 dBm

(.3 Watts)

MS antenna gain

(dBd)

-2.16 -2.16 -2.16 -2.16 -2.16

Human/Head Loss -3 -3 -3 -3 -3

Cable Loss -3.0 -3.0 -3.0 -3.0 -3.0

BS antenna gain (dBd) 15.0 15.0 15.0 15.0 15.0

In vehicle loss -8 -8 -8 -8 -8

Soft Hand-off Gain 3.7 0 0 0 3.7

Fade Margin -4.3 -4.3 -4.3 -4.3 -4.3

Interference Margin

(60% loading)

-4 -8 -4 -4 -4

RX sensitivity (dBm) -123 -119 -121 -124 -121

Maximum path loss(dB)

140.24 140.24 140.24 140.24 140.24

11.3 Nominal Cell Configurations & Nominal Cell Radii Calculations

The basic cell configurations for a CDMA system is pretty much the same as for conventional

cellular with regards to radiation centerlines, building locations, etc. A set of assumed parameters

(antenna heights, etc.) can be developed for a system designed from the ground up with no specific

requirements concerning existing structures. This will provide a “cookie cutter” approach to the

initial design process that is customized on a per cell basis as the system is built. The customization

will require revisiting the Link Budget if the assumptions are changed. When the exact location and






present configuration of the cell sites are known, the Link Budget is modified to accommodate the

cell specific capabilities. The cell parameters, nominal or otherwise, can be loosely translated into a

circular cell coverage area that meets the coverage minimum criteria for a balanced path. The edge

of the coverage circle is referred to as the nominal cell radii. The nominal cell radii shown in Table

11-1 refers to the expected cell radii for an assumed loading percentage, the propagation model

type, the nominal values for the propagation model, and the signal level which corresponds to adesired area coverage reliability.

Establishing the conditions necessary for Nominal Cell Radii Calculations requires knowledge of

statistics and propagation modeling which is provided in SAFCO’s “Introduction to Statistics,

Propagation Modeling, and the WIZARD

propagation Model” course and is not provided here.

The section below does, however, provide the process for the calculation once the conditions are

established.

Procedure for Calculating the Nominal Cell Radii with Example

• Based on a required signal level for a given performance level, a nominal cell radius can be

computed. The term nominal is used because the calculation process itself assumes a

homogenous terrain type with no effective antenna height gain. However, because we have

assumed a standard deviation in our link calculations the use of a prediction model to show

the desired coverage bands will in fact illustrate the desired coverage reliability we wish to

show.

• The steps to be followed in computing a nominal cell radius for coverage purposes as well

as required coverage bands.

1. Determine the application of the prediction model to the area type. Determine the

standard deviation that can be expected for a model that is optimized for the area where

the model will be used.

2. Determine a nominal cell configuration to be used (antenna radiation centerline, antennagain) as well as path loss slope and 1 mile intercept values,

3. Compute required area and boundary coverage reliability numbers and corresponding

Fade Margin,

4. Calculate a balanced path maximum path loss for the area type, application (in-building,

in-vehicle, and outdoors), and class of mobile or portable. Ensure all factors in the

reverse link and forward link have been accurately accounted for.

5. From the balanced path calculations, ensure that the TX power from the BS (we predict

the DL) is only large enough to represent a balanced path (what you display on screen

will in fact allow the mobile unit. The TX Power (dBm) maximum for a balanced path -

maximum path loss from link budget = received signal level (dBm) for the coverage type

desired.6. Using the antenna height, slope, 1 mile intercept, and the Lee model for the area type

calculate the maximum cell size for a homogenous area of the type specified. The

propagation model will adjust the predictions as the terrain profile is traversed with

point by point adjustments.

Example:






Assume the following parameters based on maintaining a balanced path with a 90%

coverage reliability for the area type and link budget parameters assumed in a relatively flat

standard suburban area type.

Parameter Nominal

Value

Minimum Received Signal Level at cell edge for a balance path (RSL) -100.9 dBm

Reference ERP Power (PTX Ref ) 100 Watts

Actual ERP Power from Link Budget (PTX ) 8 Watts

Reference Transmit Antenna Height (HTX Ref ) 150’

Actual Transmit Antenna Height from Nominal Cell Configuration (HTX) 131’

One mile intercept as referenced to 50 dBm transmit power (P1 mile) -75 dBm

Decay Slope (dB/decade) 38.4 dB/decade

The Nominal Cell Radius in miles, is given by:

⋅−

⋅−−=

ref tx

tx

ref tx

txmile

H

H

P

P P RSL PL

,,

1 log15log10 (equation 1)

Rnominal DecaySlope

PL

10= (equation 2)

Equation 11-1: Calculation of Nominal Cell Radii

Substituting these values results in a calculated nominal cell radius of 2.322 miles.

Table 11-1: Summary of Parameters used to calculate nominal cell radius, and calculated cell

radius for each area type and antenna configuration of a typical system at 50% loading.

Area Type

Service

Offering

Antenna

Height

meters

(feet)

TCE MAX

TX ERP

(dBm)

Signal

Level at

Boundary

(dBm)

Nominal Cell

Radius

miles (km)

DU IB 30 (98.4) 36.94 -99.08 1.07 (1.71)

U IB 40 (131.2) 36.44 -101.06 1.60 (2.56)

S IB 40 (131.2) 38.94 -101.06 2.33 (3.73)

R IB / IV 75 (246) 37.44 -105.06 5.80 (9.28)DU OD 30 (98.4) 36.94 -113.26 2.21(3.54)

U OD 40 (131.2) 36.44 -113.26 3.13 (5.01)

S OD 40 (131.2) 38.94 -113.26 4.85 (7.76)

R OD 75 (246) 37.44 -113.26 10.44 (16.70)






11.4 Nominal System Parameters

Figure 11-1 lists the system parameters typically used by WIZARD

.

Figure 11-1: Typical CDMA System Parameters

11.5 Coverage & Capacity Relationship

There is an inverse relationship between the coverage area of a given cell and the loading on that

cell due to a rising of the noise floor induced by the users on that site. This phenomena results in a

“breathing” and a “self regulating” communication system. The average required capacity of a

given base station has to be estimated at the time of design so as to predict both the coverage it will

provide and the interference it will introduce at a given average loading. On the system level, the

design of a network that will provide seamless coverage at 70% theoretical loading will require a

greater number of cells spaced closer together than a network designed to operate at 50%

theoretical loading.

11.5.1 Sensitivity Analysis: Effects of Loading on the System

A sensitivity analysis will provide the design engineer with an idea of the extent system

performance will change for increases and decreases in instantaneous traffic loading. This analysis

is essentially an overlay of the coverage provided at the maximum anticipated operating level

placed atop the coverage provided at an average anticipated operating levels. By performing

several iterations at various levels, an engineer will be able to determine the maximum average

loading the system can sustain and still meet the design coverage objectives.






11.5.2 Sensitivity Analysis Example

Figure 11-1 represents the reverse link voice channel coverage provided by a given cell at 5% and

80% of theoretical capacity. Notice the reduction in effective voice channel coverage as a result

of the increase in system noise due to the increase in traffic at the cell. AMPS and GSM

technologies do not experience changes in effective coverage area due to increases in traffic

demand on the system.

Figure 11-1: Comparison of Coverage due to change in traffic (5% to 80% of theoretical

capacity)

11.6 PN Offset Planning

In general, PN offset planning for a CDMA system is analogous to frequency planning in an FDMA

or TDMA system. For a given CDMA system, PN offset planning is a function of the same basic

parameter as an AMPS channel plan such as:

• Base Station Locations

• Propagation Characteristics

• Topography of the area

As discussed earlier, each base station transmits a pilot signal used for acquisition, system

synchronization, cell selection, and coherent demodulation of the traffic channels. All base stations

transmit a unique pilot signal using the same Pseudo Random Noise (PN or PRN) spreading code

(Short Code) but with different time offsets. There are a total of 512 phase offsets that are used to

uniquely describe a base station. PN offsets can be reused if there is sufficient separation between

cells using the same offset.

To efficiently track pilot signals, the mobile station categorizes the received signals into four sets:

active set, candidate set, neighboring set, and the remaining set. The active set contains pilot PN

offsets associated with the current base station(s) (or sectors) supporting an on-going call. The






candidate set contains the pilot PN offsets associated with all base stations (sectors) likely to be

candidates for soft Handoff. The neighbor set contains all pilot PN offsets for base stations close to

the mobile station. The remaining set contains all pilot PN offsets not included in the other three

sets.

PN offsets are selected based upon the relative time delay (signal travel time at the speed of light) between sites and exact served areas of those sites. The development of a PN offset plan depends

upon exact information on final site locations. There are 512 PN offsets available to allocate to

cells / sectors. Each PN offset is 64 chips. This ‘separation’ between pilots may be increased by

parameter PN-increment (i.e. if PN-increment is 2, separation between pilots is 128 chips and the

total number of pilots is 256).

Basic PN Offset Planning Strategy

In the mobile radio environment the signal transmitted from a BS and arriving at a mobile unit will

be from different paths as a result of the multipath reflection phenomenon. Since each path has a

different path length, the time of arrival for each path is different. This means that, for an impulse

transmitted from the BS, by the time the impulse is received at the MS it is no longer an impulse but

rather a pulse with a spread width which is referred to as the delay spread . Measured data indicates

that the mean delay spread value is different for different kinds of environments. This fact is

intuitive because of the increasing amount of multipath reflectors that are present in different

environments. The table below illustrates some representative numbers:

Table 11-1: Typical Delay Spread Values for Different Environment Types

EnvironmentExpected Range of DelaySpread (micro-seconds)

Heavy Mountains 1 - 100

Dense Urban 6 - 10

Urban 4 - 6

Suburban 2 - 4

Rural .2 - 2

In the above table a delay spread value of 6 microseconds means that a very narrow pulse (i.e. .1 µ

seconds) is transmitted, that the effective pulse width of the received signal is 6µ seconds. The

delay spread number normally, in most situations, refers to the width where the received signal

energy drops to 10 dB below the peak value of energy received. In practice, a single transmitted

pulse will result in a delay spread number which is extremely large, however, only a fraction of thetime is energy received which is usable, and this usable energy is normally defined to be within 10

dB of the peak.

The actual distribution of received pulses versus time will in most cases be a function of the

environment. In some regions an exponential decay versus time is appropriate, in others, a normal

distribution versus time may be appropriate. In the PN offset planning algorithm it is assumed that






the delay spread is symmetric about the center of the specified delay spread number. This means

that the delay spread distribution is more normal than exponential.

11.7 PN Interference

Since all pilot signals in a system are time-shifted versions of the same bit-sequence (short code), a pilot from any sector can appear to belong to any other sector. When receiver can not distinguish

pilots from different sectors, demodulation is erroneous and it is known as PN interference. There

are three types of PN interference:

• Co-PN interference – if there is no enough space separation (signal attenuation) between

cells that reuse PN offset.

• Adjacent PN offset interference – if there is no enough separation (signal attenuation)

between cells that have adjacent PN offsets (i.e. serving site has PN offset 100 and interferer

has a PN offset 101)

• Handoff confusion – interference to a neighbor set pilot (i.e. due to time delay, strong pilot

appear to be a strong neighbor list pilot: unnecessary handoff occurs)

11.8 Nominal Assignment of PN (RAKE) Search Window

The mobile station searches for Pilot code offsets which arrive inside of some nominal time frame

known as the PN or RAKE Search Window. The time frame assigned is determined by the time

dispersion of multipath and desirable speed of tracking the pilot quality. Each type of pilot set

(active, neighbor and remaining) has specific settings for RAKE Search Window size. The size of

Search Window is usually expressed in chips (i.e. 8, 10, 14, 20, …).

Generally, if RAKE Search Window is too small, multipath components will not be received (and

post-processed). In the other hand, if the RAKE Search Window is too big, receiver might be

confused by strong components of nearby pilots. This issue will be covered in ‘Intermediate

CDMA Planning and Design’ class.






REFERENCES:

[1] Rappaport, T.S., Wireless Communications, Principles and Practices, Prentice Hall, 1996.

[2] Lee, W. C. Y., Overview of the Cellular CDMA, IEEE Transactions on Vehicular Technology,

Vol. 40, No.2, May 1991.

[3] Evans, G., Joslin, B., Vinson, L. and Foose, B., Optimization and Application of the W. C. Y.

Lee Propagation Model in the 1900 MHz Frequency Band, in proceedings of IEEE 47th

Annual

International Vehicular Technology Conference, Phoenix, AZ, May 1997.

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