Code Division Multiple Access (CDMA) Transmission Technology

1. EE578 Assignment #5 Code Division Multiple Access (CDMA) Transmission Technology Abdul-Aziz .M Al-Yami November 8th2010

2. Contents • Multiple Access Schemes • Code Division Multiple Access (CDMA) • CDMA Protocols • Direct Sequence Code Division Multiple Access (DS-CDMA) • Matlab Results

3. Multiple Access Schemes • For radio systems there are two resources: frequency and time. • Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). • Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). • In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

4. Multiple Access Schemes (cont)

5. Code Division Multiple Access (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

6. CDMA Protocols

7. Direct Sequence Code Division Multiple Access (DS-CDMA) • Characteristics: • 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.

8. Direct Sequence Code Division Multiple Access (DS-CDMA) System

9. Direct Sequence Code Division Multiple Access (DS-CDMA) System (cont) • Signal transmission consists of the following steps: • 1. A pseudo-random code is generated, different for each channel and each successive connection. • 2. The Information data modulates the pseudo-random code (the Information data is “spread”). • 3. The resulting signal modulates a carrier. • 4. The modulated carrier is amplified and broadcast. • Signal reception consists of the following steps: • 1. The carrier is received and amplified. • 2. The received signal is mixed with a local carrier to recover the spread digital signal. • 3. A pseudo-random code is generated, matching the anticipated signal. • 4. The receiver acquires the received code and phase locks its own code to it. • 5. The received signal is correlated with the generated code, extracting the Information data.

10. Parameters • AWGN sr = 256000.0; % symbol rate ml = 2; % number of modulation levels br = sr * ml; % bit rate nd = 100; % number of symbol ebn0 = 3; % Eb/No %************************** Filter initialization ************************** irfn = 21; % number of filter taps IPOINT = 8; % number of oversample alfs = 0.5; % roll off factor [xh] = hrollfcoef(irfn,IPOINT,sr,alfs,1); % T FILTER FUNCTION [xh2] = hrollfcoef(irfn,IPOINT,sr,alfs,0);

11. Transmitter

12. Data

13. After QPSK modulation

14. Spread data

15. Transmitted data

16. Receiver

17. AWGN addition

18. Despread data

19. Demodulated Data • Noe2=0

20. Direct Sequence Code Division Multiple Access (DS-CDMA) System • 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

21. GOLD CODES – FADING CHANNEL

22. ORTHOGONAL GOLD CODES – FADING CHANNEL

23. M Sequences | Gold Codes Autocorrelation

24. GOLD CODES – CROSS CORRELATION

25. DS-CDMA Performance under AWGN Environment for different sequences (1 user)

26. DS-CDMA Performance under Rayleigh Environment for different sequences (1 user)

27. DS-CDMA Performance under AWGN Environment for different sequences (5 users)

28. DS-CDMA Performance under Rayleigh Environment for different sequences (5 users)

29. 1 user AWGN Rayleigh

30. 5 users AWGN Rayleigh

31. DS-CDMA Performance under Rayleigh Environment using M-sequence

32. DS-CDMA Performance under Rayleigh Environment using Gold Sequence

33. DS-CDMA Performance under Rayleigh Environment using Orthogonal Gold Sequence

34. DS-CDMA Performance under Rayleigh Environment (Comparison) M-sequence Gold Sequence Orthogonal Gold Sequence