Code Division Multiple Access (CDMA)
Transmission Technology
Chapter 5 of Hiroshi Harada Book
Group 4.1
Outline
• 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
2/16
Type of Multiplexing:
1. Frequency-Division Multiple Access (FDMA).
2. Time-Division Multiple Access (TDMA).
3. Code-division Multiple-Access (CDMA)
Prepared By Ibrahim AL-OBIDA
3/16
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
Prepared By Ibrahim AL-OBIDA
4/16
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
5/16
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.
Prepared By Ibrahim AL-OBIDA
6/16
Direct Sequence
Prepared By Ibrahim AL-OBIDA
7/16
Direct Sequence
Pseudo-Noise Spreading
Prepared By Ibrahim AL-OBIDA
8/16
Direct Sequence
Direct Sequence Spread Spectrum System
Prepared By Ibrahim AL-OBIDA
9/16
Direct Sequence Spread Spectrum Example
Direct Sequence Spread Spectrum System
DSSS Example Using BPSK
Direct Sequence
Processing Gain:
=
is the processing gain
fc
fi
is Chipping Frequency (the bit rate of the PN
code).
is Information Frequency (the bit rate of the digital data).
Prepared By Ibrahim AL-OBIDA
13/16
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
Prepared By Ibrahim AL-OBIDA
14/16
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 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 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
4/13/2015
22
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
4/13/2015
23
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
4/13/2015
24
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 msequences and these are then bitwise XORed
4/13/2015
25
Gold Sequences
4/13/2015
26
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
4/13/2015
27
BER performance of DS CDMA with
m-sequence in AWGN
BER performance of DS-CDMA with M-sequence in AWGN
0
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
Number of users = 7
-1
BER
10
-2
10
-3
10
-4
10
4/13/2015
0
2
4
6
8
10
12
Eb/N0 [dB]
14
16
18
20
28
BER performance of DS CDMA with
Gold sequence in AWGN
BER performance of DS-CDMA with Gold sequence in AWGN
0
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
Number of users = 7
-1
BER
10
-2
10
-3
10
-4
10
4/13/2015
0
2
4
6
8
10
12
Eb/N0 [dB]
14
16
18
20
29
BER performance of DS CDMA with
orthogonal Gold sequence in AWGN
BER performance of DS-CDMA with orthogonal Gold sequence in AWGN
0
10
QPSK AWGN theory
Number of users = 1
Number of users = 4
Number of users = 7
-1
BER
10
-2
10
-3
10
-4
10
4/13/2015
0
2
4
6
8
10
12
Eb/N0 [dB]
14
16
18
20
30
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
BER
10
-2
10
-3
10
4/13/2015
0
2
4
6
8
10
12
Eb/N0 [dB]
14
16
18
20
31
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
BER
10
-2
10
-3
10
4/13/2015
0
2
4
6
8
10
12
Eb/N0 [dB]
14
16
18
20
32