Presentation Outline

CDMA Basics• Multiple Access Technology Survey• Spread Spectrum Principles• CDMA N/w Structure• Advantages of CDMA.

CDMA Details and Operations• Forward and Reverse Channel Structure • Power Control • CDMA Handset Architecture• Handoff Mechanics

Multiple Access Technologies

¤ FDMA (Example: AMPS)Frequency Division Multiple Access

• each user has a private frequency

¤ TDMA (Example: GSM)Time Division Multiple Access

• each user has a private time on a private frequency

¤ CDMA (Example: IS-95)Code Division Multiple Access

• users co-mingle in time and frequency, but each user has a private code

CDMA is a Spread-Spectrum System

¤ Traditional Technologies try to squeeze signal into minimum required bandwidth

¤ CDMA uses larger Bandwidth but uses resulting processing gain to increase capacity

CDMA (Code Division Multiple Access)

• Different Stations use the entire frequency band at same time.

• Stations are differentiated by Unique Codes.

• Intended Receiver knows the Code in advance and hence recovers the signal.

• It employs Direct Sequence Spread Spectrum Technique.

• Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level.

• It was first developed for military applications to avoid jamming.

Spread Spectrum Principles

The standard data rate of a CDMA call is 9600 bits per second, this initial data is "spread” by the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. At the receiving end, the digital codes are separated out, leaving only the original information.

CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.

CDMA Block Diagram

A/D Converter Vocoder Encoding &

Interleaving

Spreading D/ARF (QPSK)

Code

Antenna

PSTN

PSTN BSC BTS

Subscriber Unit Block Diagram

Despread

User’s code

Deinterleave& Decode Vocoder Codec

Variable Rate Vocoding & Multiplexing

¤ Vocoders compress speech, reduce bit rate, greatly increasing capacity¤ CDMA uses a superior Variable Rate Vocoder

• full rate during speech• low rates in speech pauses• increased capacity• more natural sound

¤ Voice, signaling, and user secondary data may be mixed in CDMA frames

Two types of CDMA¤ Frequency Hopping

• Each users narrow band signal hops among discrete frequencies, and the receiver follows in sequence.• Frequency Hopping Spread Spectrum(FHSS) CDMA is NOT currently used in Wireless Systems, although used by the military.

¤ Direct Sequence• Narrow band input from a user is coded (“spread”) by a user unique broadband code, then transmitted.• Broadband signal is received; receiver knows, applies user’s code, recovers users’ data. • Direct Sequence Spread Spectrum (DSSS) CDMA IS the method used in IS- 95 commercial systems.

Direct Sequence Spread Spectrum CDMA

b(t) recovered

Data b(t) at low bit rate

Pseudo-noise sequence c(t) at high bit rate(sequence is unique for eachconnection; the “code”)

SpreadingOver the

air

DeSpreading

c(t)

Transmitter Receiver

CDMA FH-SS Concept

f(N)f(N-1)

f2f1 Chip Duration

¤ Frequency “hops” based on a pseudo-random sequence¤ Chip rate >> Information Data Rate

Direct Sequence Spread Spectrum (DS-SS)

¤ Multiple users share a common broadband channel by spreading their (narrowband) data across the entire channel

¤ Code sequences are used to spread the baseband data; each logical channel uses a different code sequence

¤ The ratio of the code data rate to the baseband data bit rate is called the processing or spreading gain

¤ The process involves digital modulation, and is performed in addition to analog modulation

Example for DS-SS CDMA

Despread Signal b(t)c(t)c(t)recovers b(t)

1-1

BasebandSignal b(t)

1-1

Spread Signal b(t)c(t)

1-1

Sequence is -1,1,1,-1,-1: 5 “chips” per symbolPN SequenceSignal c(t)

1-1

Correlation of Code Sequences

¤ Each time period in the code sequence is a chip, because the input signal is being divided, or chipped, into multiple output symbols per input symbol.

¤ Also, all code sequences in a code set are of the same length¤ A code is correlated with another code based on the sum of the

products of the corresponding chips in the two codes• Avg of Sum of products = 1, => 100% correlation• Avg of Sum of products = 0, => no correlation; the term

for this is orthogonal

Examples of Correlation of Code SequencesExamples of Correlation of Code Sequences

C1(t):-1, 1, -1, 1, 1, -1, 1, -1C2(t):-1, -1, -1, -1, -1, -1, -1, -1

C1*C2 1, -1, 1, -1, -1, 1, -1, 1

Sum over the sequence length: 1 -1+1-1-1+1-1+1 = 0Average over the sequence length: 0/8 = 0

No correlation of C and D=> C1 and C2 are orthogonal

C(t):-1, 1, -1, 1, 1, -1, 1, -1C(t):-1, 1, -1, 1, 1, -1, 1, -1

C*C 1, 1, 1, 1, 1, 1, 1, 1

Sum over the sequence length: 1 +1+1+1+1+1+1+1 = 8Average over the sequence length: 8/8 = 1

correlation of C with itself

CDMA 2ndgenerationwireless

AMPS

CNET

RC2000

NMT900

NMT450

TIATDMAIS-136

ISDN

TACS

Regulatory

standards

Wired Systems

First Generation Wireless Systems

Development of CDMA Standard

NMT-Nordic Mobile Telephone

TACS-Total Access Communication System

• Fig shows how the CDMA standard was developed using many proven technologies.

• Hence CDMA is called “ Second Generation” cellular system.

• Only radio portion of the MS and BS’s are substantially different in hardware.

• Thus the CDMA standard can be implemented at 800 MHz in one location, and then at 1900 MHz in another location.

• Since hardware's are different for these frequencies, a single mode 800 MHz handset will not function in an 1900 MHz CDMA network.

• Hence some manufacturers produce handsets that can operate on either frequency band (dual band handsets)

• Forward or Down link - Base transmitter to the mobile receiver

- all CDMA network uses a specific radio bands for signals

• Reverse link or Up link - mobile transmitter to the Base receiver

- second distinct radio frequency band for signals

• A pair of radio carrier frequencies (matching uplink & downlink) is referred to a “ frequency”

What is carrier and channel w.r.t CDMA ?

• CDMA documents consistently distinguish between both the words

• In CDMA, a carrier is divided by means of codes into 64 individual channels.

• Each channel carriers the information related to a separate & distinct conversation in digitally coded form.

• In some cases an individually coded channel carries signals related to the beginning of a conversation.

• Channel is synonymous with a carrier.

AMPS Spectrum

- MS transmitted band - 824-849 MHz

- BS transmitted band - 869-894 MHz

Radio Frequency Spectrum

PCS Spectrum

- MS transmitted band - 1850 - 1910 MHz

- BS transmitted band - 1930 - 1990 MHz

Second VLR is Optional

CDMA System

HLR VLR

VLRAuC

BTS

OMC MSCBTSBSC

BTSBSC

BTSBSC

BSSTo other MSCs

PSTN

AIR INTERFACE

IS-41 Intersystem

BSC

Tx CombinerBT0

BT1

BTn . . .

BCF

Rx Multiplier

Rx Multiplier

To MSC

Tx Ant

BSS

1st Rx ant2nd Rx Ant

BTS

CDMA Base Station Block Diagram

CDMA Cell Site Frame Layout (AT & T)

¤ Dramatically improving the telephone traffic (Erlang) capacity ¤ Dramatically improving the voice quality and eliminating the audible effects of multipath fading¤ Reducing the incidence of dropped calls due to handoff failures¤ Providing reliable transport mechanism for data communications, such as facsimile and internet traffic ¤ Reducing the number of sites needed to support any given amount of traffic ¤ Simplifying site selection ¤ Reducing deployment and operating costs because fewer cell sites are needed ¤ Reducing average transmitted power ¤ Reducing interference to other electronic devices ¤ Reducing potential health risks

CDMA Advantages

¤ Increased System Capacity¤ Simplified Frequency Reuse¤ Improved Interference Immunity¤ Lower RF Power Requirements at the Cell Site¤ Soft/Softer Handoffs¤ Variable Rate Speech Coding¤ Packetized Communications Structure¤ Cloning Protection

Benefits of CDMA to Service Providers

¤ Improved Privacy¤ Excellent Voice Quality¤ Soft and Softer Handoff to Improve Call Quality¤ Longer Battery life for Mobile Phone Units¤ Packetized Structure to Support Simultaneous Voice and Control Data¤ Increased System Capacity

Benefits of CDMA to End-User

¤ Walsh Codes: 64 are available• 64 chips long -- lasts 1/ 19200 sec• mutually orthogonal

¤ PN Short Code: one pair is used (I & Q)• 32K long -- lasts 26- 2/ 3 mS, repeats 75x in 2 sec.

Ð generated in 15- bit tapped shift register• Nearly self- orthogonal if compared out- of- sync

¤ PN Long Code: only one is used• 242 -1 chips long -- lasts 40+ days!

=> generated in 42- bit tapped shift register• Any short sample is nearly orthogonal with any other short sample

The Three CDMA Spreading Sequences

¤ 64 “Magic” Sequences, each 64 chips long¤ Each Walsh Code is precisely Orthogonal with respect to all other Walsh Codes

• it’s simple to generate the codes, or• they’re small enough to use from ROM

Unique Properties:Mutual Orthogonality

Walsh Codes

¤ Other CDMA sequences are generated in shift registers¤ Plain shift register: no fun, sequence = length of register¤ Tapped shift register generates a wild, self- mutating sequence 2N -1 chips long (N= register length)

• Such sequences match if compared in step ( any sequence matches itself)• Such sequences appear approximately orthogonal if compared with themselves not exactly matched in time• false correlation typically <2%

Other Sequences - Generation & Properties

¤ The short PN code consists of two PN Sequences, I and Q, each 32,768 chips long

• Generated in similar but differently- tapped 15- bit shift registers• They’re always used together, modulating the two phase axes of a QPSK modulator

The Short PN Code

¤ Generated in a 42- bit register, the PN Long code is more than 40 days long (~ 4x1013 chips) -- too big to store in ROM in a handset, so it’s generated chip- by- chip using the scheme shown above¤ Each handset codes its signal with the PN Long Code, but at a unique offset computed using its ESN (32 bits) and 10 bits set by the system

• this is called the “Public Long Code Mask”; produces unique shift• private long code masks are available for enhanced privacy

¤ Integrated over a period even as short as 64 chips, phones with different PN long code offsets will appear practically orthogonal

The Long PN Code

¤ The three spreading codes are used in different ways to create the forward and reverse links¤ A forward channel exists by having a specific Walsh Code assigned to the user, and a specific PN offset for the sector¤ A reverse channel exists because the mobile uses a specific offset of the Long PN sequence

Putting it All Together : CDMA Channels

Code Channels in the Forward Direction

Functions of the CDMA Forward Channels

¤ Pilot Channel • Each CDMA carrier has its own pilot channel. A pilot channel provides pilot signal periodically, so mobiles can identify which cell site(s) or sector(s) they are listening to.• All pilot channels share the same PN 0 (Walsh function of 64 0’s) among all CDMA sectors but are differentiated by PN sequence offsets. • There are 512 possible time pilot PN offset indices to distinguish among 512 distinct sectors in a system. Each offset index is 64 chips in length.

¤ Synchronization Channel • PN 32 is designated as the sync channel, which has a low bit rate of 1200 bps.• It is used with the pilot channel to acquire initial time synchronization which allows rapid synchronization of mobile to network. • Using the selected pilot channel as reference, the mobile coherently demodulates the sync channel, which the base station transmits continuously.• The information contained are: System ID, N/w ID, PN offset Index, System Time etc.

Details of the CDMA Forward Channels

¤ Paging Channel • When a mobile is to receive a call, it will receive a page from the base station on the assigned paging channel.• The 42-bit mask consists of 1100011001101 (13 bits), 00000 (5 bits), Paging Channel Number (PCN; 3 bits), 000000000000 (12 bits), and pilot PN sequence offset index (9 bits).• Up to seven PNs can be assigned to paging channels based on the traffic need. PN numbers 1 to 7 (i.e., W 1 to W 7 ) are reserved for these paging channels. When unused, they can be reassigned as traffic channels.

Details of the CDMA Forward Channels

Code Channels in the Reverse Direction

There are two types of CDMA Reverse Channels:

Although a sector can have up to seven paging channels, and each paging channel can have up to 32 access channels, systems today use only one paging channel per sector and only one access channel per paging channel.

Functions of the CDMA Reverse Channels

¤ Access Channel • When a mobile is ready to originate a call, it sends out a request via the access channel to the base station associated with the strongest pilot signal received by the mobile. The channel is also used when responding to a "page".• Each access frame has 96 bits (88 data plus 8 flush; 20 ms frame at 4800 bps). The long code mask consists of 110001111 (9 bits), Access Channel Number (ACN; 5 bits), Paging Channel Number (PCN; 3 bits), base_ID (16 bits), and pilot PN sequence offset of the forward CDMA channel (9 bits).• The access channel provides several messages. The origination message allows the mobile to send dialed digits. When it is accepted, a traffic channel is assigned to the mobile.• The access channel provides several messages like: Registration, Data Burst, Page Response, Origination, Authentication Challenge Response.

¤ Reverse Traffic Channel • Used by individual users during their actual calls to transmit to the BTS.

Details of the CDMA Reverse Channels

Why is Power Control Needed? ¤ Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users. The power of each individual user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.¤ Power control is implemented to overcome the near-far problem and to maximize capacity.¤ Power control is where the transmit power from each user is controlled such that the received power of each user at the base station is equal to one other.

Why Power Control ???

¤ The BTS continually reduces the strength of each user’s forward baseband chip stream¤ When a particular handset sees errors on the forward link, it requests more energy¤ The complainer’s chip stream gets a quick boost; afterward, continues to diminish

Forward Power Control

¤ Three methods work in tandem to equalize all handset signal levelsat the BTS

• Reverse Open Loop: handset adjusts power up or down based on received BTS signal• Reverse Closed Loop: Is handset too strong? BTS tells up or down 1 dB 800 times/ second• Reverse Outer Loop: BSC has FER trouble hearing handset? BSC adjusts BTS setpoint

Reverse Power Control

¤ The open-loop power control in that it is purely a mobile controlled operation and does not involve the base station at all.

¤ This power control is used to compensate for slow-varying and log-normal shadowing effects where there is a correlation between the forward-link and reverse-link fades. However, since the forward and reverse links are on different frequencies, the open-loop power control is inadequate and too slow to compensate for fast Rayleigh fading.

Reverse Open Loop

¤ The closed-loop power control is used to compensate for power fluctuations due to fast Rayleigh fading.

¤ In the closed-loop power control, the base station continuously monitors the reverse link and measures the link quality.

¤ The base station sends the power-control commands to the mobile using the forward link. These power-control commands are in the form of power-control bits (PCBs).

Reverse Closed Loop

¤ The closed-loop power worked such that there exists a predetermined SNR threshold by which power-up and powerdown decisions are made.

¤ Since we are always trying to maintain an acceptable FER, and since in a mobile environment there is no one-to-one relationship between FER, the FER threshold has to be dynamically adjusted to maintain an acceptable FER. This adjustment of FER threshold (used by the inner-loop power control) is referred to as the outer loop of the closed-loop power control

¤ The outer-loop process is not defined by the IS-95 standard, and each infrastructure manufacturer is free to implement its own outer-loop algorithms.

Reverse Outer Loop

What’s in a Handset? How does it work?

¤ Every frame, handset uses combined outputs of the three traffic correlators (“rake fingers”)¤ Each finger can independently recover a particular PN offset and Walsh code¤ Fingers can be targeted on delayed multipath reflections, or even on different BTSs¤ Searcher continuously checks pilots

The Rake Receiver

ConvolutionalCode Rate

Interleaver(20mS)

64-ary WalshFunc Modulator

9.6 Kbps

PN-I

PN-Q

Cos t

Sin t

UserLong PN

RF

At Mobile Transmitter:

At Mobile Receiver:

RFViterbi Soft

DecoderDeinterleaver Descrambler

9.6 Kbps

PN-I

PN-Q

Cos t

Sin t

WalshFunctionUser

Long PN

¤ CDMA soft handoff is driven by the handset• Handset continuously checks available pilots• Handset tells system pilots it currently sees• System assigns sectors (up to 6 max.), tells handset• Handset assigns its fingers accordingly• All messages sent by dim- and- burst, no muting!• Users are totally unaware of handoff

CDMA Soft Handoff Mechanics

¤ Each BTS sector has unique PN offset & pilot¤ Handset will ask for whatever pilots it wants¤ If multiple sectors of one BTS simultaneously serve a handset, this is called Softer Handoff¤ Handset is unaware, but softer handoff occurs in BTS in a single channel element¤ Handset can even use combination soft- softer handoff on multiple BTS & sectors

Softer Handoff

¤ Handset views pilots in sets¤ Handset sends message to system whenever:

• It notices a pilot in neighbor or remaining set exceeds T_ ADD• An active set pilot drops below T_ DROP for T_ TDROP time• A candidate pilot exceeds an active by T_ COMP

¤ Handoff setup processing time usually << 1 second

Pilot Sets and Soft Handoff Parameters

What is Ec/Io

Ec Energy of the desired pilot Alone

Io Total EnergyReceived

¤ Ec / Io• “Cleanness” of the pilot• Guides soft handoff decisions• Digitally derived: ratio of good to bad energy seen by the search correlator at the desired PN offset.• Can be degraded by noise.

T_ADD

T_DROP

Pilot A in Active Set

2 Pilots in Active set

Pilot B in Active Set

Ec/Io

Distance

Source Cell A Target Cell B

(1) (2) (3) (4)

Handoff Example

¤ Key Performance Indicators and Objectives

CDMA Performance Optimization

¤ Success comes from managing resources

• Dropped Calls, Access Failures, system FER• Soft Handoff Percentage• Capacity

• Handoff: keep dynamics fast, delays shortÐ Neighbor lists well- optimized

• RF Coverage: holes vs. excessive overlap• PN Planning, optimum Search Window sizes• Per - Cell anomalies: watch parameters for clues