Khurram Masood 200806100

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Khurram Masood200806100
Code Division Multiple Access (CDMA) Transmission
Technology Chapter 5 of Hiroshi Harada Book 
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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

• 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.

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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
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Direct Sequence Spread Spectrum System
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DSSS Example Using BPSK
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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
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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

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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

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Frequency Hopping Example
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FHSS (Transmitter)
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Frequency Hopping Spread Spectrum System
(Receiver)
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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

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Slow MFSK FHSS
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Fast MFSK FHSS
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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

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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
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BER performance of DS CDMA with Gold sequence in
AWGN
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BER performance of DS CDMA with orthogonal Gold
sequence in AWGN
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BER performance of DS CDMA with m-sequence in
Rayleigh fading
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BER performance of DS CDMA with orthogonal Gold
sequence in Rayleigh fading