Code Division Multiple Access

Prepared by: Christina Chiu Mar 24, 2004

Outline

Spread Spectrum Communications Multiple Access Technique

Direct Sequence DS-CDMA CDMA standards

CDMAOne (2G) – IS 95A, (2.5G) IS-95B CDMA 2000 and WCDMA (3G) – IMT2000

4G in the future Conclusions

Spread Spectrum Technique

Low power spectral density Rejection to jamming signal and

interference Pseudorandom sequence

Randomness and noise properties Walsh, M-sequence, Gold, Kasami, Z4 Provide signal privacy

Direct Sequence Spread Spectrum

Spreading Source signal is multiplied by a PN signal

Processing Gain:

Despreading Spread signal is multiplied by the spreading code

DataRate

ChipRate

T

T

T

TG

b

c

c

bp

1

1

System Block DiagramJammer/Noise/Interference

j(t)

BPSKModulator

BPSKMatched

Filter

Channel

PseudorandomSequence Generator

PseudorandomSequenceGenerator

SourceData

OutputData

(to detector)

b(t)

c(t)

s(t) x(t) u(t)

c(t)

y(t) rn

y(t) = j(t) + x(t)u(t) = s(t) + j(t)c(t)

rn = bn + jammer projection

s(t) = b(t)cos(wot)x(t) = s(t)c(t)

Spreading & Despreading

Polar {±1} signal representation

Spreading & Despreading

CDMA

Unique code to differentiate all users Sequence used for spreading have low

cross-correlations Allow many users to occupy all the

frequency/bandwidth allocations at that same time

Processing gain is the system capacity How many users the system can support

Demodulation (Correlator)

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Pick thelargestCross

CorrelationMetrics

receivedsignal

r(t)

c1(t)

c2(t)

ck(t)

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r1

r2

rk

outputbitsb(n)

2G: IS-95A (1995) Known as CDMAOne Chip rate at 1.25Mbps Convolutional codes, Viterbi Decoding Downlink (Base station to mobile):

Walsh code 64-bit for channel separation M-sequence 215 for cell separation

Uplink (Mobile to base station): M-sequence 241 for channel and user

separation

2G: IS-95A (1995)Standard IS-95, ANSI J-STD-008

Multiple Access CDMA

Uplink Frequency 869-894 MHz

Downlink Frequency 824-849 MHz

Channel Separation 1.25 MHz

Modulation Scheme BPSK/QPSK

Number of Channel 64

Channel Bit Rate 1.25 Mbps (chip rate)

Speech Rate 8~13 kbps

Data Rate Up to 14.4 kbps

Maximum Tx Power 600 mW

2.5G: IS-95B (1998)

Increased data rate for internet applications Up to 115 kbps (8 times that of 2G)

Support web browser format language Wireless Application Protocol (WAP)

3G Technology

Ability to receive live music, interactive web sessions, voice and data with multimedia features

Global Standard IMT-2000 CDMA 2000, proposed by TIA W-CDMA, proposed by ARIB/ETSI

Issued by ITU (International Telecommunication Union)

3G Specficiations

Excellent voice quality Data rate

144 kbps in high mobility 384 kbps in limited mobility 2 Mbps in door

Frequency Band 1885-2025 MHz Convolutional Codes Turbo Codes for high data rates

3G: CDMA2000 (2000) CDMA 1xEV-DO

peak data rate 2.4 Mbps supports mp3 transfer and video conferencing

CDMA 1xEV-DV Integrated voice and high-speed data

multimedia service up to 3.1 Mbps Channel Bandwidth:

1.25, 5, 10, 15 or 20 MHz Chip rate at 3.6864 Mbps Modulation Scheme

QPSK in downlink BPSK in uplink

3G: CDMA2000 Spreading Codes

Downlink Variable length orthogonal Walsh

sequences for channel separation M-sequences 3x215 for cell separation

(different phase shifts) Uplink

Variable length orthogonal Walsh sequences for channel separation

M-sequences 241 for user separation (different phase shifts)

3G: W-CDMA (2000)

Stands for “wideband” CDMA Channel Bandwidth:

5, 10 or 20 MHz Chip rate at 4.096 Mbps Modulation Scheme

QPSK in downlink BPSK in uplink

3G: W-CDMA Spreading Codes

Downlink Variable length orthogonal sequences for

channel separation Gold sequences 218 for cell separation

Uplink Variable length orthogonal sequences for

channel separation Gold sequences 241 for user separation

4G Technology (2010)

High data rates => 100 Mbps Multimedia data up to ~102 Mbps IP-oriented network => Internet MIMO using Multiple Antennas Spreading code

Non-binary sequences Multiple Access Technique

MC-CDMA

MC-CDMA

Conclusions

CDMA can increase the capacity of mobile system

Goals of next generation cellular Further increase data rate Reduce interference among users

Future research: Search for good non-binary sequences

Ideal 2-level autocorrelation Low cross correlations