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Khurram Masood 200806100

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Code Division Multiple Access (CDMA) Transmission Technology Chapter 5 of Hiroshi Harada Book Khurram Masood 200806100 * * * * * * * * * * One technique with direct ... – PowerPoint PPT presentation

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Title: Khurram Masood 200806100


1
Khurram Masood200806100
Code Division Multiple Access (CDMA) Transmission
Technology Chapter 5 of Hiroshi Harada Book 
2
Outline
  • Introduction
  • Type of CDMA
  • Averaging systems
  • Avoidance systems
  • Spreading code
  • M-seuence
  • Gold sequence
  • Ortogonal Gold sequence
  • Simulation and results

3
Type of Multiplexing
1. Frequency-Division Multiple Access (FDMA).
2. Time-Division Multiple Access (TDMA).
3. Code-division Multiple-Access (CDMA)
4
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

5
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

6
Direct Sequence
Features
  • All users use same frequency and may transmit
    simultaneously
  • Narrowband message signal multiplied by wideband
    spreading signal, or codeword
  • Each user has its own pseudo-codeword (orthogonal
    to others).
  • Receivers detect only the desired codeword. All
    others appear as noise.
  • Receivers must know transmitters codeword.

7
Direct Sequence
8
Direct Sequence
Pseudo-Noise Spreading
9
Direct Sequence
Direct Sequence Spread Spectrum System
10
Direct Sequence Spread Spectrum Example
11
Direct Sequence Spread Spectrum System
12
DSSS Example Using BPSK
13
Direct Sequence
Processing Gain

is the processing gain
is Chipping Frequency (the bit rate of the PN
code).
fc
is Information Frequency (the bit rate of the
digital data).
fi
14
Direct Sequence
  • Advantages
  • Increased capacity
  • Improved voice quality
  • Eliminating the audible effects of multipath
    fading
  • Enhanced privacy and security
  • Reduced average transmitted power
  • Reduced interference to other electronic devices
  • Disadvantages
  • Wide bandwidth per user required
  • Precision code synchronization needed

15
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

16
Frequency Hopping Example
17
FHSS (Transmitter)
18
Frequency Hopping Spread Spectrum System
(Receiver)
19
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 lt Ts
  • FHSS quite resistant to noise or jamming
  • with fast FHSS giving better performance

20
Slow MFSK FHSS
21
Fast MFSK FHSS
22
Linear Feedback Shift RegisterImplementation of
PN Generator
  • Output is periodic with max-period N2n-1
  • LFSR can always give a period N sequence -gt
    resulting in m-sequences.
  • Different Ai allow generation of different
    m-sequences

23
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

24
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

25
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

26
Gold Sequences
27
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

28
BER performance of DS CDMA with m-sequence in
AWGN
29
BER performance of DS CDMA with Gold sequence in
AWGN
30
BER performance of DS CDMA with orthogonal Gold
sequence in AWGN
31
BER performance of DS CDMA with m-sequence in
Rayleigh fading
32
BER performance of DS CDMA with orthogonal Gold
sequence in Rayleigh fading
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