Contents

This presentation provides a general introduction to CDMA

-Evolution of CDMA-CDMA Concepts -Principle of spread-spectrum multiple

access-Spreading spectrum Sequences(PN)-Features of CDMA

Evolution of CDMAEvolution of CDMA

CDMA: Past, Present, and Future

The origins of spread spectrum are in military field and navigation systems

In 1949, John Pierce wrote a technical memorandum where he described a multiplexing system in which a common medium carries coded signals that need not be synchronized. This system can be classified as a time hopping spread spectrum multiple access system

In 1949, Claude Shannon and Robert Pierce introduced the basic ideas of CDMA by describing the interference averaging effect and the graceful degradation of CDMA

In 1950, De Rosa-Rogoff proposed a direct sequence spread spectrum system and introduced the processing gain equation and noise multiplexing idea

CDMA: Past, Present, and Future In 1956, Price and Green filed for the anti-multipath "RAKE" pat

ent . Signals arriving over different propagation paths can be resolved by a wideband spread spectrum signal and combined by the RAKE receiver.

In 1961 ,The near-far problem (i.e., a high interference overwhelming a weaker spread spectrum signal) was first mentioned by Magnuski

For cellular application spread spectrum was suggested by Cooper and Nettleton in 1978

During the 1980s Qualcomm investigated DS-CDMA techniques, which finally led to the commercialization of cellular spread spectrum communications in the form of the narrowband CDMA IS-95 standard in July 1993

Commercial operation of IS-95 systems started in 1996.

CDMA: Past, Present, and Future Multiuser detection (MUD) has been subject to extensive rese

arch since 1986 when Verdu formulated an optimum multiuser detection for the additive white Gaussian noise (AWGN) channel, maximum likelihood sequence estimator (MLSE)

During the 1990s ,wideband CDMA techniques with a bandwidth of 5 MHz or more have been studied intensively throughout the world, and several trial systems have been built and tested

Based on the above description, the CDMA era is divided into three periods:

1. the pioneer CDMA era 2. the narrowband CDMA era 3. the wideband CDMA era

Table 1. CDMA era Pioneer Era

1949 John Pierce: time hopping spread spectrum

1949 Claude Shannon and Robert Pierce: basic ideas of CDMA

1950 De Rosa-Rogoff: direct sequence spread spectrum

1956 Price and Green: antimultipath "RAKE" patent

1961 Magnuski: near-far problem

1970s Several developments for military field and navigation systemsNarrowband CDMA Era

1978 Cooper and Nettleton: cellular application of spread spectrum

1980s Investigation of narrowband CDMA techniques for cellular applications

1986 Formulation of optimum multiuser detection by Verdu

1993 IS-95 standardWideband CDMA Era

1995

Europe:FRAMES FMA2WCDMA

Japan: Core-AUSA :cdma2000Korea :TTA I TTA II

2000s Commercialization of wideband CDMA systems

Evolution of Mobile communication system

AMPSTACS

GSM

N-CDMA

IMT-2000

1G 2G 3G

Analog

N-BAND

Digital

W-BAND

HRPD

GSM GPRSWCDMA

IS-95Acdma2000-3x

1X-EVDV

IS-95B

cdma2000-1x

EDGE

Evolution from 2G to 3G

TD-SCDMA

CDMA Concepts - Multiple Access Techniques - Description of CDMA - Two Types of CDMA - How does CDMA work? - DSSS Spreading: Time-Domain View - Spreading from a Frequency-Domain View - CDMA Spread Spectrum Payoff - CDMA’s Nested Spreading Sequences

Multiple Access Techniques

FDMA:Frequency division multiple access Feature:each user is allocated a unique frequency

band or channel TACS AMPS

TDMA:Time Division Multiple Access Feature:Radio spectrum is divided into time

slots,and in each slot only one user is allowed to either transmit or receive.

GSM DAMPS

CDMA:Code division multiple access Feature:In CDMA each user is assigned a unique

code sequence it uses to encode its information-bearing signal

IS-95/CDMA2000 /WCDMA/TD-SCDMA Frequency

Time

Power

FrequencyTime

Power

FrequencyTime

Power

FDMA

TDMA

CDMA

Description of CDMA

The spreading signal is a pseudo-noise code sequence that has a chip rate which is greater than the data rate of the message.

All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators.

In cdma systems,the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal.

CDMA operates by using CODING to discriminate between users. Each user has its own pseudorandom codeword which is

approximately orthogonal to all other codewords. CDMA interference comes mainly from nearby users The receiver performs a time correlation operation to d

etect only the specific desired codeword.All other codewords appear as noise due to decorrelation.

Time

Frequency

Code

Chnannel 1Chnannel 2Chnannel 3Chnannel 4

Chnannel N

Two Types of CDMA

How does CDMA work?

Sender combines data with a fast spreading

sequence, transmits spread data streamReceiver intercepts the stream, uses same

spreading sequence to extract original data

DSSS Spreading: Time-Domain View

Spreading from a Frequency-Domain View

CDMA Spread Spectrum Payoff

CDMA’s Nested Spreading Sequences

Principle of spread-spectrum multiple access

Block diagram of a DS-SS transmitter

Generation of a BPSK-modulated SS signal

- Code signal consists of a number of code bits called "chips" that can be either +1 or 1. - Chip rate of the code signal must be much higher than the rate of the information signal. - In this figure, 10 code chips per information symbol are transmitted (the code chip rate is 10 times the data rate) so the processing gain is equal to 10.

Receiver of a DS-SS signal- The receiver uses coherent demodulation to despread the SS signal, using a locally generated code sequence. - To be able to perform the despreading operation, the receiver must not only know the code sequence used to spread the signal, but the codes of the received signal and the locally generated code must also be synchronized. - This synchronization must be accomplished at the beginning of the reception and maintained until the whole signal has been received. - The code synchronization/tracking block performs this operation- After despreading a data modulated signal results, and after demodulation the original data can be recovered.

Receiver of a DS-SS signal

SpreadingModulation

Information Modulation Despreading

Information Demodulati

onb(t)

PN

C(t)

fc PN fc

A B

B PointA Point C Point

C

Interference

Block of DS-SS communication system

Spreading spectrum Sequences(PN) - Property of PN - m-sequences -Walsh Code

Property of PN

Correlation - The Rule of Sequence design The ideal Sequence: The side peak value of auto-correlation(ACF) is

zero The value of cross-correlation(CCF) is zero Ideal Sequence can eliminate co-channel interference(MAI) Unfortunately,We can’t find the ideal sequence So,our target is to find such sequence: The smaller of the side peak value of auto-

correlation and the value of cross-correlation ,the better of the sequence.

1nN

0 11 1 nN 1nN

0 11 1 nN 1nN

Auto-correlation Function

Cross-correlation Function

Definition of ACF and CCF

Periodic sequence The Periodic ACF:

The Periodic CCF:

)..,.........,(),..,.........,( 110110 NN yyyxxx

ji

N

iiy

li

N

iix

yylR

xxlR

1

0

1

0

)(

)(

li

N

iixy yxlR

1

0

)(

m-sequences

Good periodic ACF properties Bad periodic CCF properties CDMA operates by using different offset of the same m-

sequence to discriminate between users for the bad periodic CCF properties

In IS-95 and IS2000 standard ,two kinds m-sequences are used:

short code and long code The m-sequences are generated using shift register The normative ACF of m-sequence:

chips)12( 15 chips)12( 42

)(mod01

)(mod0)(

1

0 NNlNlN

xxlR li

N

iix

An Example of periodic ACF of m-sequence

length 15 m-sequence (- - - -+ - + - - + + - + + +) here , - means 1,+means -1

- - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

15- 1- 1- 1- 1- 1- 1- 1- 1- 1- 1- 1- 1- 1

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +

- - - - + - + - - + + - + + + - - - - + - + - - + + - + + +- 1- 1

m- sequence: N=15

The per i odi c ACF of m- sequence(N=15)

Introduction of the short code

Introduce of the long code

Walsh Code- orthogonal sequence

Walsh codes are generated by applying Hadamard transform upon 0 repeatedly. Hadamard transform is given by

11

11

22

22

2NN

NN

N

HH

HHH

0110001101010000

4H

0100

2H 01 H

most important feature: Orthogonal Walsh Sequence is Orthogonal when synchronized. ACF and CCF of Walsh Sequence are not ideal when not synchronized. Walsh function Set is self-contained. Walsh Sequence with Length n= can constitute n= Sequences orthogonal one another.

r2 r2

Features of CDMA- Multiple Access Capability - Protection Against Multipath Interference - Privacy - Interference Rejection - Anti-Jamming Capability, Especially Narrowband Jamming

- Low Probability of Interception(LPI)

Multiple Access Capability

If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between the users provided each user has a unique code that has a sufficiently low cross-correlation with the other codes.

Correlating the received signal with a code signal from a certain user will then only despread the signal of this user, while the other spread-spectrum signals will remain

spread over a large bandwidth. Within the information bandwidth the power of the desired user will be larger than the interfering power provided there are not too many interferers, and the desired signal can be extracted. At the receiver 1 only the signal of user 1 is "despread" and the data recovered.

Principle of spread-spectrum multiple access

Protection Against Multipath Interference

The signals of the different paths are all copies of the same transmitted signal but with different amplitudes, phases, delays, and arrival angles. Adding these signals at the receiver will be constructive at some of the frequencies and destructive at others. In the time domain, t

his results in a dispersed signal. If the code sequence has an ideal autocorrelation function, t

hen the correlation function is zero outside the interval [ Tc,Tc], where Tc is the chip duration. This means that if the desired signal and a version that is delayed for more than 2Tc are received, coherent demodulation will treat the delayed version as an interfering signal, putting only a small part of the power in the information bandwidth.

1

0

2 3

Tx Rx

Privacy

Privacy -- The transmitted signal can only be despread and the data recovered if the code is known to the receiver.

Interference Rejection

Cross-correlating the code signal with a narrowband signal will spread the power of the narrowband signal thereby reducing the interfering power in the information bandwidth.

The spread-spectrum signal (s) receives a narrowband interference (i). At the receiver the SS signal is "despread" while the interference signal is spread, making it appear as background noise compared to the despread signal.

Interference rejection

Anti-Jamming Capability

This is more or less the same as interference rejection except the interference is now willfully inflicted on the system. It is this property, together with the next one-LPI, that makes spread-spectrum modulation attractive for military applications.

Low Probability of Interception(LPI)

Because of its low power density, the spread-spectrum signal is difficult to detect and intercept by a hostile listener. f

S（ f）

f0Signal Frequency Before Decoding

f

S（ f）

f0Signal Frequency Before SS

Signal

Signal

Noise

ff0

Signal Frequency after SS

Signal

f

S（ f）

f0

Signal Frequency After Decoding

Signal

Noise

Signal Pulse Noise Other Noise

Features of cdma2000-1X Reverse Pilot Support: Function:Phase reference,Coherent demodulation For each cdma2000 user, either Turbo or Convolutional co

des can be used. Fast 800 Hz forward and reverse link power control.The re

verse power control subchannel can be divided into two independent power control channel,the power control rate can be 400/400bps or 200/600bps for FCH/SCH.

Double the capacity vs. IS-95-A/B Provide higher data rates more efficiently (up to 307.2 kbp

s)

Features of cdma2000-1X

Support for Quasi Orthogonal Functions (QOF) increasing available forward channels

Quick paging channel. This allows the mobile to wake up for a shorter period of time before entering sleep mode, thereby increasing the standby time of phones.

OTD(Orthogonal Transmit Diversity) and STS(Space Time Spreading) transmit diversity