Title: Khurram Masood 200806100
1Khurram Masood200806100
Code Division Multiple Access (CDMA) Transmission
Technology Chapter 5 of Hiroshi Harada BookÂ
2Outline
- Introduction
- Type of CDMA
- Averaging systems
- Avoidance systems
- Spreading code
- M-seuence
- Gold sequence
- Ortogonal Gold sequence
- Simulation and results
3Type of Multiplexing
1. Frequency-Division Multiple Access (FDMA).
2. Time-Division Multiple Access (TDMA).
3. Code-division Multiple-Access (CDMA)
4Code 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
5Code 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
6Direct 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.
7Direct Sequence
8Direct Sequence
Pseudo-Noise Spreading
9Direct Sequence
Direct Sequence Spread Spectrum System
10Direct Sequence Spread Spectrum Example
11Direct Sequence Spread Spectrum System
12DSSS Example Using BPSK
13Direct 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
14Direct 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
15Frequency 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
16Frequency Hopping Example
17FHSS (Transmitter)
18Frequency Hopping Spread Spectrum System
(Receiver)
19Slow 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
20Slow MFSK FHSS
21Fast MFSK FHSS
22Linear 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
23Properties 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
24Advantages 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
25Gold 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
26Gold Sequences
27Orthogonal 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
28BER performance of DS CDMA with m-sequence in
AWGN
29BER performance of DS CDMA with Gold sequence in
AWGN
30BER performance of DS CDMA with orthogonal Gold
sequence in AWGN
31BER performance of DS CDMA with m-sequence in
Rayleigh fading
32BER performance of DS CDMA with orthogonal Gold
sequence in Rayleigh fading