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Khurram MasoodKhurram Masood200806100200806100
Code Division Multiple Access (CDMA) Transmission Technology
Chapter 5 of Hiroshi Harada Book
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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
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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).
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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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Prepared By Ibrahim AL-OBIDA 5/16
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
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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.
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Prepared By Ibrahim AL-OBIDA 7/16
Direct SequenceDirect Sequence
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Prepared By Ibrahim AL-OBIDA 8/16
Direct SequenceDirect Sequence
Pseudo-Noise Spreading
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Prepared By Ibrahim AL-OBIDA 9/16
Direct SequenceDirect Sequence
Direct Sequence Spread Spectrum System
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Direct Sequence Spread Spectrum ExampleDirect Sequence Spread Spectrum Example
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Direct Sequence Spread Spectrum SystemDirect Sequence Spread Spectrum System
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DSSS Example Using BPSKDSSS Example Using BPSK
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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
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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
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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
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Frequency Hopping ExampleFrequency Hopping Example
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FHSS (Transmitter)FHSS (Transmitter)
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Frequency Hopping Spread Spectrum System Frequency Hopping Spread Spectrum System (Receiver)(Receiver)
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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
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Slow MFSK FHSSSlow MFSK FHSS
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Fast MFSK FHSSFast MFSK FHSS
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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
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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
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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
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BER performance of DS CDMA with BER performance of DS CDMA with orthogonal Gold sequence in AWGNorthogonal Gold 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 orthogonal Gold sequence in AWGN
QPSK AWGN theory
Number of users = 1Number of users = 4
Number of users = 7
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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
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BER performance of DS CDMA with orthogonal BER performance of DS CDMA with orthogonal Gold sequence in Rayleigh fadingGold 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 orthogonal Gold sequence in Rayleigh fading
QPSK Rayleigh Fading theory
Number of users = 1Number of users = 4
Number of users = 7