Khurram Masood 200806100

1. KhurramMasood200806100 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 Prepared By Ibrahim AL-OBIDA

3. 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

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 Prepared By Ibrahim AL-OBIDA

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 Prepared By Ibrahim AL-OBIDA

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 transmitter’s codeword. Prepared By Ibrahim AL-OBIDA

7. Direct Sequence Prepared By Ibrahim AL-OBIDA

8. Direct Sequence Pseudo-Noise Spreading Prepared By Ibrahim AL-OBIDA

9. Direct Sequence Direct Sequence Spread Spectrum System Prepared By Ibrahim AL-OBIDA

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 fc is Chipping Frequency (the bit rate of the PN code). fi is Information Frequency (the bit rate of the digital data). Prepared By Ibrahim AL-OBIDA

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 Prepared By Ibrahim AL-OBIDA

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 < 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 N=2n-1; • LFSR can always give a period N sequence -> 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