Code Division Multiple Access (CDMA) Transmission Technology

1. Code Division Multiple Access (CDMA) Transmission Technology EE 578 Assignment #5 Mohammad Alkhodary 200806080

2. Outline • Introduction • Direct Sequence Spread Spectrum (DSSS) • Frequency Hopping Spread Spectrum (FHSS) • Time Hopping Spread Spectrum (THSS) • Hybrid Systems Basic Principle of Spread Spectrum Systems • Code Division Multiple Access • M-Sequence • Gold , Orthogonal Gold Sequence • Simulation Result

3. Introduction • Multiple-access capability is primarily achieved by means of coding. • Each user is assigned a unique code sequence that he uses to encode his information signal. • The receiver. knowing the code sequences of the user, decodes the received signal after reception and recovers the original data. • Because the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information signal, the encoding process enlarges (spreads) the spectrum modulation. (SS) • The resulting encoded signal is also called an SS signal.

4. Multiple Access • FDMA • Frequency Division Multiple Access • TDMA • Time Division Multiple Access • CDMA • Code Division Multiple Access time time FDMA TDMA CDMA time code TDMA freq freq freq

5. Spread Spectrum Advantages? • Hide a signal below the noise floor • Resistance to narrowband jamming and interference • Mitigate performance degradation due to intersymbol and narrowband interference • In conjunction with RAKE receiver, SS can provide coherent combining of different multipath components • Allow multiple users to share the same signal bandwidth • Wide bandwidth of SS signals is useful for location and timing acquisition

6. Spread Spectrum Properties • Signal occupies a bandwidth much larger than is needed for the information signal • Spread spectrum modulation is done using a spreading code independent of the data in the signal • Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code

7. Spread Spectrum System: DSSS Vs. FHSS

8. General Block Diagram Direct Sequence Spread Spectrum (DSSS) • Input • Binary data dt with symbol rate Rs=1/Ts (=Rb for BPSK) • Pseudo-noise code: pnt with chip rate Rc=1/Tc (an integer of Rs) • Spreading • Transmitted baseband signal: txb=dt* pnt

9. Spreading Processing gain

10. Despreading

11. DSSS Example C=AxB A=CxB Processing Gain

12. Multiple Access CDMA for DSSS • n users each using different orthogonal PN sequence • Modulate each users data stream • Using qPSK • Multiply by spreading code of user

13. Seven Channel CDMA Encoding and Decoding

14. CDMA Power Control

15. Frequency Hopping Spread Spectrum (FHSS) • Signal broadcast over seemingly random series of frequencies • Receiver hops between frequencies in sync with transmitter • Eavesdroppers hear unintelligible blips • Jamming on one frequency affects only a few bits

16. Basic Operation • Typically 2k carriers frequencies forming 2k channels • Channel spacing corresponds with bandwidth of input • Each channel used for fixed interval

17. FHSS System Transmitter Receiver

18. Slow and Fast Frequency Hopping • Frequency shifted every Tc seconds • Duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • Generally fast FHSS gives improved performance in noise (or jamming)

19. Fast Hopping versus Slow Hopping • Fast Hopping

20. Fast Hopping versus Slow Hopping • Slow Hopping

21. Hybrid DS/FH Spread Spectrum System

22. Maximal Length Sequence (m-sequence)

23. Gold Sequence

24. IMT-2000: Gold Sequence • Mutually Orthogonal Gold Codes (256) • Generator Polynomials:

25. Simulation Result CDMA M-Sequence performance in AWGN

26. Simulation Result CDMA Gold Sequence performance in AWGN

27. Simulation Result CDMA Orthogonal Sequence Gold performance in AWGN Chanel

28. Simulation Result CDMA M-Sequence performance in Fading Channel

29. Simulation Result CDMA Gold Sequence performance in Fading Channel

30. Simulation Result CDMA Orthogonal Sequence Gold performance in Fading Channel