Code Division Multiple Access (CDMA)
Transmission Technology
Chapter 5 of Hiroshi Harada Book
Khurram Masood
200806100
Outline
• Introduction
• Type of CDMA
– Averaging systems
– Avoidance systems
• Spreading code
– M-seuence
– Gold sequence
– Ortogonal Gold sequence
• Simulation and results
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Type of Multiplexing:
1. Frequency-Division Multiple Access (FDMA).
2. Time-Division Multiple Access (TDMA).
3. Code-division Multiple-Access (CDMA)
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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)
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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Direct Sequence
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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Direct Sequence
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Direct Sequence
Pseudo-Noise Spreading
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Direct Sequence
Direct Sequence Spread Spectrum System
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Direct Sequence Spread Spectrum Example
Direct Sequence Spread Spectrum System
DSSS Example Using BPSK
Direct Sequence
Processing Gain:
is the processing gain
fc is Chipping Frequency (the bit rate of the PN
code).
fi is Information Frequency (the bit rate of the digital data).
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Direct Sequence
Advantages:
o Increased capacity
o Improved voice quality
o Eliminating the audible effects of multipath fading
o Enhanced privacy and security
o Reduced average transmitted power
o Reduced interference to other electronic devices
Disadvantages:
o Wide bandwidth per user required
o Precision code synchronization needed
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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 Example
FHSS (Transmitter)
Frequency Hopping Spread Spectrum System
(Receiver)
Slow and Fast FHSS
commonly use multiple FSK (MFSK)
have frequency shifted every T c seconds
duration of signal element is T s seconds
Slow FHSS has Tc Ts
Fast FHSS has Tc < Ts
FHSS quite resistant to noise or jamming
with fast FHSS giving better performance
Slow MFSK FHSS
Fast MFSK FHSS
Linear Feedback Shift Register
Implementation of PN Generator
•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-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 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 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 Sequences
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Orthogonal 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
m-sequence in AWGN
0
BER performance of DS-CDMA with M-sequence in AWGN
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
-1 Number of users = 7
10
BER
-2
10
-3
10
-4
10
0 2 4 6 8 10 12 14 16 18 20
Eb/N0 [dB]
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BER performance of DS CDMA with
Gold sequence in AWGN
0
BER performance of DS-CDMA with Gold sequence in AWGN
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
-1 Number of users = 7
10
BER
-2
10
-3
10
-4
10
0 2 4 6 8 10 12 14 16 18 20
Eb/N0 [dB]
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BER performance of DS CDMA with
orthogonal Gold sequence in AWGN
0
BER performance of DS-CDMA with orthogonal Gold sequence in AWGN
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
-1 Number of users = 7
10
BER
-2
10
-3
10
-4
10
0 2 4 6 8 10 12 14 16 18 20
Eb/N0 [dB]
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BER performance of DS CDMA with
m-sequence in Rayleigh fading
BER performance of DS-CDMA with M-sequence in Rayleigh fading
0
10
QPSK Rayleigh Fading theory
Number of users = 1
Number of users = 4
Number of users = 7
-1
10
BER
-2
10
-3
10
0 2 4 6 8 10 12 14 16 18 20
Eb/N0 [dB]
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BER performance of DS CDMA with orthogonal
Gold sequence in Rayleigh fading
BER performance of DS-CDMA with orthogonal Gold sequence in Rayleigh fading
0
10
QPSK Rayleigh Fading theory
Number of users = 1
Number of users = 4
Number of users = 7
-1
10
BER
-2
10
-3
10
0 2 4 6 8 10 12 14 16 18 20
Eb/N0 [dB]
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