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FHMA principle and near-far effect in CDMA

Short answer

FHMA spreads the signal by hopping the carrier frequency pseudo-randomly according to a PN code; the transmitter mixes data (e.g., FSK) with a hopping synthesizer output, and the receiver de-hops with the synchronized code. The near-far effect in CDMA is when a nearby mobile's much stronger signal swamps a distant one, reducing capacity. It is solved primarily by fast power control (open-loop, closed-loop, outer-loop), and aided by hybrid techniques like TCDMA and DS/FHMA.

Explain the principle of FHMA. What do you mean by near-far effect in CDMA? How is it solved? Explain.

Principle of FHMA

Frequency Hopping Multiple Access is a spread spectrum method in which the carrier frequency of each user hops pseudo-randomly among many narrow channels inside a wide band. Each user follows its own PN hopping pattern; the receiver uses the same, synchronised pattern to de-hop the signal.

 Transmitter
 data->[FSK modulator]->(x)->[BPF]->[PA]-> antenna
                         ^
                 [Frequency synthesizer]
                         ^
                 [PN code generator]

 Receiver
 antenna->[BPF]->(x)->[IF filter]->[FSK demod]-> data
                  ^
          [Frequency synthesizer]
                  ^
          [PN gen] (synchronised to Tx)
  • At any instant the signal is narrowband; over time it occupies the whole band.
  • Slow FH: several symbols per hop (GSM); fast FH: several hops per symbol.
  • Collisions between users are rare and are corrected by coding and interleaving.

Near-far effect in CDMA

In the CDMA uplink all mobiles share the same frequency. Path loss varies a lot with distance, so a near mobile arrives at the base station with much more power than a far mobile (often 60–80 dB difference without control). The receiver sees other users as noise, so the strong near signal swamps the weak far signal and the far user's call fails. Capacity drops sharply.

How it is solved

  1. Power control – the main solution: every mobile's power is adjusted so all signals reach the base station at about the same level.
    • Open-loop: mobile estimates path loss from the received forward signal and sets its power (fast, rough).
    • Closed-loop: base station measures received Eb/N0 and sends power up/down bits (IS-95: 800 bps, ±1 dB steps).
    • Outer-loop: base station changes the target Eb/N0 to keep frame error rate at the desired level.
  2. Hybrid DS/FHMA – near and far users seldom share a hop frequency.
  3. TCDMA – only one user per cell transmits at a time.
  4. Multi-user detection / successive interference cancellation – strong signals are detected and subtracted first.

✓ Answer checked by Claude Opus 5.5 (high effort)

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