Khurram MasoodKhurram Masood200806100200806100

Code Division Multiple Access (CDMA) Transmission Technology

Chapter 5 of Hiroshi Harada Book 

Prepared By Ibrahim AL-OBIDA 2/16

OutlineOutline

• Introduction• Type of CDMA

– Averaging systems– Avoidance systems

• Spreading code– M-seuence– Gold sequence– Ortogonal Gold sequence

• Simulation and results

Prepared By Ibrahim AL-OBIDA 3/16

Type of Multiplexing:Type of Multiplexing:

1. Frequency-Division Multiple Access (FDMA). 1. Frequency-Division Multiple Access (FDMA).

3. Code-division Multiple-Access (CDMA)3. Code-division Multiple-Access (CDMA)

2. Time-Division Multiple Access (TDMA). 2. Time-Division Multiple Access (TDMA).

Prepared By Ibrahim AL-OBIDA 4/16

Code Division Multiple Access (CDMA) Code Division Multiple Access (CDMA)

• CDMA: – A digital method for simultaneously transmitting signals over a shared portion

of the spectrum by coding each distinct signal with a unique code.– CDMA is a wireless communications technology that uses the principle of

spread spectrum communication.

• Advantages– Multiple access capability– Protection against multipath interference– Privacy– Interference rejection– Ant jamming capability– Low probability of interception

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Code Division Multiple Access (CDMA) Code Division Multiple Access (CDMA)

There are different ways to spread the bandwidth of the signal:

Direct sequence Frequency hopping Time hopping Chirp spread spectrum Hybrid systems

Prepared By Ibrahim AL-OBIDA 6/16

Direct SequenceDirect Sequence

Features:Features:

o All users use same frequency and may transmit simultaneously

o Narrowband message signal multiplied by wideband spreading signal, or codeword

o Each user has its own pseudo-codeword (orthogonal to others).

o Receivers detect only the desired codeword. All others appear as noise.

o Receivers must know transmitter’s codeword.

Prepared By Ibrahim AL-OBIDA 7/16

Direct SequenceDirect Sequence

Prepared By Ibrahim AL-OBIDA 8/16

Direct SequenceDirect Sequence

Pseudo-Noise Spreading

Prepared By Ibrahim AL-OBIDA 9/16

Direct SequenceDirect Sequence

Direct Sequence Spread Spectrum System

Direct Sequence Spread Spectrum ExampleDirect Sequence Spread Spectrum Example

Direct Sequence Spread Spectrum SystemDirect Sequence Spread Spectrum System

DSSS Example Using BPSKDSSS Example Using BPSK

Prepared By Ibrahim AL-OBIDA 13/16

Direct SequenceDirect Sequence

Processing Gain:

is the processing gain

is Chipping Frequency (the bit rate of the PN code).

is Information Frequency (the bit rate of the digital data).

=

fc

fi

Prepared By Ibrahim AL-OBIDA 14/16

Direct SequenceDirect Sequence

AdvantagesAdvantages::oIncreased capacityIncreased capacity

oImproved voice qualityImproved voice quality

oEliminating the audible effects of multipath fadingEliminating the audible effects of multipath fading

oEnhanced privacy and securityEnhanced privacy and security

oReduced average transmitted powerReduced average transmitted power

oReduced interference to other electronic devices Reduced interference to other electronic devices

DisadvantagesDisadvantages::oWide bandwidth per user requiredWide bandwidth per user required

oPrecision code synchronization neededPrecision code synchronization needed

Frequency Hopping Spread Spectrum (FHSS)Frequency Hopping Spread Spectrum (FHSS)

signal is broadcast over seemingly random series of frequencies

receiver hops between frequencies in sync with transmitter

jamming on one frequency affects only a few bits

Frequency Hopping ExampleFrequency Hopping Example

FHSS (Transmitter)FHSS (Transmitter)

Frequency Hopping Spread Spectrum System Frequency Hopping Spread Spectrum System (Receiver)(Receiver)

Slow and Fast FHSSSlow and Fast FHSS

commonly use multiple FSK (MFSK)have frequency shifted every Tc seconds

duration of signal element is Ts seconds

Slow FHSS has Tc Ts

Fast FHSS has Tc < Ts

FHSS quite resistant to noise or jammingwith fast FHSS giving better performance

Slow MFSK FHSSSlow MFSK FHSS

Fast MFSK FHSSFast MFSK FHSS

Linear Feedback Shift RegisterLinear Feedback Shift RegisterImplementation of PN GeneratorImplementation of PN Generator

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•Output is periodic with max-period N=2n-1;•LFSR can always give a period N sequence -> resulting in m-sequences.•Different Ai allow generation of different m-sequences

Properties of M-SequencesProperties of M-Sequences

• Property 1:– Has 2n-1 ones and 2n-1-1 zeros

• Property 2: – For a window of length n slid along output for N (=2n-1) shifts, each n-tuple appears once,

except for the all zeros Sequence• Property 3:

– Sequence contains one run of ones of length n

– One run of zeros of length n-1– One run of ones and one run of zeros of length n-2– Two runs of ones and two runs of zeros of length n-3– 2n-3 runs of ones and 2n-3 runs of zeros of length 1

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Advantages of Cross CorrelationAdvantages of Cross Correlation

• The cross correlation between an m-sequence and noise is low– This property is useful to the receiver in filtering out Noise

• The cross correlation between two different msequences is low– This property is useful for CDMA applications– Enables a receiver to discriminate among spread spectrum signals

generated by different m-sequences

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Gold SequencesGold Sequences

• Gold sequences constructed by the XOR of two m-sequences with the same clocking

• Codes have well-defined cross correlation Properties• Only simple circuitry needed to generate large number of

unique codes• In following example two shift registers generate the two m-

sequences and these are then bitwise XORed

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Gold SequencesGold Sequences

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Orthogonal CodesOrthogonal Codes

• Orthogonal codes– All pairwise cross correlations are zero– Fixed- and variable-length codes used in CDMA Systems– For CDMA application, each mobile user uses one

sequence in the set as a spreading code– Provides zero cross correlation among all users

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BER performance of DS CDMA with BER performance of DS CDMA with m-sequence in AWGNm-sequence in AWGN

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0 2 4 6 8 10 12 14 16 18 2010

-4

10-3

10-2

10-1

100

Eb/N

0 [dB]

BE

RBER performance of DS-CDMA with M-sequence in AWGN

QPSK AWGN theory

Number of users = 1Number of users = 4

Number of users = 7

BER performance of DS CDMA with BER performance of DS CDMA with Gold sequence in AWGNGold sequence in AWGN

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0 2 4 6 8 10 12 14 16 18 2010

-4

10-3

10-2

10-1

100

Eb/N

0 [dB]

BE

RBER performance of DS-CDMA with Gold sequence in AWGN

QPSK AWGN theory

Number of users = 1Number of users = 4

Number of users = 7

BER performance of DS CDMA with BER performance of DS CDMA with orthogonal Gold sequence in AWGNorthogonal Gold sequence in AWGN

04/21/23 30

0 2 4 6 8 10 12 14 16 18 2010

-4

10-3

10-2

10-1

100

Eb/N

0 [dB]

BE

RBER performance of DS-CDMA with orthogonal Gold sequence in AWGN

QPSK AWGN theory

Number of users = 1Number of users = 4

Number of users = 7

BER performance of DS CDMA with BER performance of DS CDMA with m-sequence in Rayleigh fadingm-sequence in Rayleigh fading

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0 2 4 6 8 10 12 14 16 18 2010

-3

10-2

10-1

100

Eb/N

0 [dB]

BE

RBER performance of DS-CDMA with M-sequence in Rayleigh fading

QPSK Rayleigh Fading theory

Number of users = 1Number of users = 4

Number of users = 7

BER performance of DS CDMA with orthogonal BER performance of DS CDMA with orthogonal Gold sequence in Rayleigh fadingGold sequence in Rayleigh fading

04/21/23 32

0 2 4 6 8 10 12 14 16 18 2010

-3

10-2

10-1

100

Eb/N

0 [dB]

BE

RBER performance of DS-CDMA with orthogonal Gold sequence in Rayleigh fading

QPSK Rayleigh Fading theory

Number of users = 1Number of users = 4

Number of users = 7