Code Division Multiple Access
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The document provides an overview of Code Division Multiple Access (CDMA), detailing its background, characteristics, and technical features such as spread spectrum techniques and voice compression methods. It highlights advantages such as higher spectral efficiency, resistance to interference, and improved privacy, alongside a discussion on CDMA's channel structure and power control mechanisms. The future trends and conclusion emphasize the continued relevance and progress in CDMA technology.
Code Division Multiple Access
- 1. CDMA (Code Division Multiple Access) Mr. Vishal Pawar Assistant Professor MITSOT
- 2. Contents • CDMA Overview • Spread Spectrum • CDMA Characteristics • Advantage/Disadvantage • Future Trend & Conclusion
- 3. CDMA Overview • Background – Multiple users per channel based on spreading codes • using direct sequence spread spectrum(DSSS) technique – Developed by Qualcomm as IS-95 • Special soft handoff capability • “Narrowband CDMA” : 1.25MHz spread bandwidth • “Broadband CDMA” : 10MHz spread bandwidth
- 4. CDMA Overview - cont.
- 5. Spread Spectrum • Spread the radio signal over a wide frequency range by modulating – Increasing the frequency of the discrete time signal – Total energy under the power spectrum remains the same • Directly spreading and despreading the baseband data by a PN(pseudonoise) sequence – PN sequence is identical for both spreading and despreading – Receiver’s correlator distinguishes sender’s signal • More resistant to multipath effects and more tolerant of interference
- 6. Types of Techniques used for SS • Direct Sequence SS – all users share a huge frequency band. - general method • Frequency Hopping SS – transmitter changes its carrier frequency often – frequency is constant in each time chip • Time Hopping SS – message is transmitted only one time slot • Chirp SS • Hybrid Method SS
- 7. Spread Spectrum - cont. • Processing Gain : – degree of interference rejection – For high values of Gp, information can be transmitted at power levels below ambient noise – Commercially available SS systems typically implement processing gains in the 10-100 rage – Purpose is media access via code division rather than lowering error rates in presence of noise – If Gp is low, more difficult to distinguish between individual. )log(10 s ss p B B G =
- 9. CDMA Characteristics • Modulation & coding Feature – data rate - 9.6 kbps – filtered bandwidth - 1.25MHz – Coding - Convolution encoding – Interleaving 20 ms span
- 10. CDMA Rake Receiver • multipath diversity combining to overcome multipath fade
- 11. CDMA Voice Compression • Voice compression technique – vocoder : encoding scheme applied only to voice signal • Analog voice signal → digital signal • PCM 64kbps data → 8.6, 4.0, 2.0, 0.8kbps • Selective transmission – channel vocoder – Linear Prediction Coding – Vector quantization
- 12. CDMA Power Control – Make all reverse-direction signal in a cell arrive at the base station with same strength – near-far problem – variable bit rate signal – open loop power control • Terminal adjusts its transmitter power as a function of the power it measures in the received forward direction signal – closed loop power control • perform power control adjustment under control of the base station
- 13. CDMA Logical Channel Structure • Forward Link – Pilot, Sync, Paging, Traffic Channel – If less than 7 Paging Channels are used, each unused Walsh code becomes a Traffic Channel Pilot Sync Paging Paging Traffic Traffic Traffic W=0 W=32 W=1 W=8 W=9 W=10 W=63
- 14. CDMA Channel Structure - cont. – Pilot Channel • MH synchronize to the pilot channel’s SPN code – Sync Channel • Protocol information, System ID, Network ID, Pilot PN offset, System Time, Long PN Code, etc. • MH synchronize to Network System – Paging Channel • Overhead message • Personal Station Directed Message – Traffic Channel
- 15. CDMA Channel Structure - cont. • Reverse Link – Access Channel • communicate from MH to BS • response to paging channel message – Traffic Channel Access Access Traffic Traffic
- 16. Main Advantage of CDMA • Higher Spectral Efficiency - soft capacity limit • Interference Resistance • Soft Handoff • Improved Privacy • Time Diversity in Multi-path Channel • Longer Battery Life • Cell Design Flexibility • Better Voice Quality - EVRC
- 17. Thank you