- 2076 Bhadra · 5+4 marks
Multiple access and near-far effect mitigation
Short answer
Multiple access lets many users share a limited spectrum by dividing frequency, time, code, or space. In CDMA the near-far effect occurs when a nearby mobile's strong signal swamps a far user's weak signal. Mitigation includes power control, hybrid DS/FH, TCDMA, multi-user detection, and low cross-correlation codes.
Explain the term multiple access. What do you mean by near-far effect in CDMA and state all possible near-far effect mitigation techniques.
Multiple access
Multiple access is the technique that lets many users share a limited radio spectrum (a common channel) at the same time without serious interference. The spectrum is divided in frequency, time, code or space, and each user gets a share.
| Technique | Resource divided | Example |
|---|---|---|
| FDMA | Frequency bands | AMPS |
| TDMA | Time slots on a carrier | GSM, IS-136 |
| CDMA | Unique codes, same band and time | IS-95, WCDMA |
| SDMA | Space (antenna beams) | Smart antennas |
| PRMA | Packets (random access) | Data networks |
Multiple access is combined with a duplexing method: FDD (separate frequencies for uplink/downlink) or TDD (separate time slots). Narrowband systems (FDMA, TDMA) give each user a small part of the band; wideband systems (CDMA) let all users use the whole band.
freq|##|##|##| FDMA time| | | | TDMA
| | | | |slots
CDMA: all users fill whole freq-time plane,
separated by codes
Near-far effect in CDMA
In the uplink, all mobiles transmit on the same frequency. A mobile near the base station is received with much higher power than a mobile far away (path loss differs by tens of dB). Because CDMA receivers treat other users as noise, the strong nearby signal raises the interference level so much that the weak far user cannot be despread. This is the near-far problem.
Mitigation techniques
- Power control (main method): each mobile adjusts power so that all arrive at the base station with nearly equal power.
- Open-loop: mobile sets power from received forward signal strength.
- Closed-loop: base station sends up/down commands (800 times/s in IS-95, 1 dB steps).
- Outer-loop: adjusts target Eb/N0 based on frame error rate.
- Hybrid DS/FH (DS/FHMA) – a near user rarely shares the same hop frequency.
- Time division CDMA (TCDMA) – only one user per cell transmits in a slot.
- Multi-user detection / interference cancellation – the receiver detects strong users first and subtracts them.
- Codes with low cross-correlation and higher processing gain.
✓ Answer checked by Claude Opus 5.5 (high effort)
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