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Hybrid multiple access techniques

Short answer

Hybrid FDMA/CDMA splits the spectrum into sub-bands, each carrying an independent CDMA system, giving flexibility but lower processing gain. Hybrid DS/FHMA direct-sequence spreads the signal and then hops its center frequency pseudo-randomly, avoiding the near-far problem but requiring fast synthesizers and tight synchronization.

Explain any two hybrid multiple access techniques.

Hybrid multiple access techniques combine two basic schemes (FDMA, TDMA, CDMA, FHMA) to get the benefits of both. Two common ones:

1. Hybrid FDMA/CDMA (FCDMA)

 |<------------ total spectrum ------------>|
 | CDMA band 1 | CDMA band 2 | CDMA band 3 |
   (users by code inside each sub-band)
  • The wide spectrum is split into narrower sub-bands; each sub-band carries an independent CDMA system.
  • Sub-bands need not be adjacent, so spectrum can be used flexibly; hardware works at lower chip rate.
  • Drawback: lower processing gain per sub-band. IS-95 carriers of 1.25 MHz are an example.

2. Hybrid Direct Sequence / Frequency Hopped Multiple Access (DS/FHMA)

 data->(×PN code)->[modulator]->(×)-> hopping DS signal
                                 ^
               [PN]->[frequency synthesizer]
  • The signal is direct-sequence spread, and the centre frequency of the spread signal hops pseudo-randomly.
  • Advantage: avoids the near–far problem, since a strong nearby user seldom occupies the same hop frequency.
  • Disadvantage: complex, needs fast synthesizers and tight synchronisation.

(Others: Time Division CDMA – different codes per cell, one user per time slot within a cell; Time Division Frequency Hopping – the user hops to a new frequency each TDMA frame, as in GSM.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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