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.

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. 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

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

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

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

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

15. 1 user AWGN Rayleigh

16. 5 users AWGN Rayleigh

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

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

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

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