CDMA features and hybrid spread spectrum

Features of CDMA

CDMA (Code Division Multiple Access) lets all users share the same carrier at the same time; each user is separated by a unique spreading (PN) code. Important features:

• Universal frequency reuse – every cell uses the same frequency (reuse factor N = 1), so no frequency planning is needed.
• Soft capacity – there is no hard limit on users; adding a user only raises the noise floor slightly (graceful degradation).
• Soft handoff – the mobile can talk to two or more base stations at once, giving fewer dropped calls.
• Multipath resistance – a RAKE receiver combines delayed copies of the signal (multipath diversity).
• Power control is essential – needed to solve the near-far problem.
• Privacy and anti-jamming – the signal looks like noise without the code.
• Voice activity gain – during silence the user transmits less, raising capacity.

Hybrid spread spectrum techniques

Hybrid techniques combine spread spectrum with FDMA, TDMA or frequency hopping to get the benefits of each.

1. Hybrid FDMA/CDMA (FCDMA) – The available wide band is divided into a number of narrower sub-bands. Each sub-band is a small DS-CDMA system with its own processing gain. A user is given a sub-band and a code. The sub-bands need not be contiguous, and different users can be given different bandwidths according to their needs.
   • Advantage: flexible use of fragmented spectrum; smaller sub-bands need lower chip rates.
   • Disadvantage: lower processing gain per sub-band than one full-band CDMA system.
2. Hybrid Direct Sequence / Frequency Hopping (DS/FHMA) – A DS-modulated signal (narrow PN-spread signal) is made to hop its centre frequency in a pseudo-random way. In each hop only a small part of the band is occupied, so a nearby strong user rarely sits on the same frequency at the same time.
   • Advantage: avoids the near-far effect.
   • Disadvantage: complex transmitter/receiver; not suited to soft handoff, because the base station cannot easily stay synchronised to many hopping signals.
3. Time Division CDMA (TCDMA) – Each cell uses a different spreading code. Inside a cell, users are given different time slots, so at any instant only one CDMA user transmits per cell. During handoff the user's code is changed to the code of the new cell.
   • Advantage: avoids the near-far effect (no two users of the same cell are received together).
   • Disadvantage: needs time synchronisation; capacity per cell is limited by slots.
4. Time Division Frequency Hopping (TDFH) – The subscriber hops to a new frequency at the start of each new TDMA frame. This gives frequency diversity against deep fades and spreads co-channel interference. GSM uses this (slow frequency hopping, 217 hops/s).
   • Advantage: avoids long deep fades and bursts of co-channel interference.
   • Disadvantage: needs a central hopping plan and tight frame synchronisation.

 DS/FHMA idea:
  freq
   ^   [DS]            [DS]
   |         [DS]
   |  [DS]                   [DS]
   +----------------------------> time
   each block = DS-spread burst at
   a pseudo-randomly chosen carrier