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

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

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. Direct Sequence Spread Spectrum (DSSS)
General Block Diagram
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. CDMA for DSSS Multiple Access
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

20. Fast Hopping versus 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. CDMA M-Sequence performance in AWGN
Simulation Result
CDMA M-Sequence performance in AWGN

26. CDMA Gold Sequence performance in AWGN
Simulation Result
CDMA Gold Sequence performance in AWGN

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

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

29. CDMA Gold Sequence performance in Fading Channel
Simulation Result
CDMA Gold Sequence performance in Fading Channel

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