Self jamming, FHMA and hybrid SS techniques

Self-jamming problem in CDMA

Self-jamming arises because the spreading codes of different users are not exactly orthogonal (especially in the asynchronous uplink, where codes arrive with random delays). When the receiver despreads the wanted user, the other users' signals give non-zero cross-correlation and leave residual interference. This interference comes from users of the same system, so the system "jams itself"; it grows with the number of users and limits CDMA capacity.

Operation of FHMA

In FHMA the carrier frequency of each user hops pseudo-randomly within a wide band according to a PN code. At each instant the signal is narrowband; averaged over time, it covers the whole band.

 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)

1. Data modulates a carrier (BFSK/M-FSK) to give a narrowband signal.
2. The PN generator picks one of many frequencies in the synthesizer; mixing shifts the signal to that hop frequency.
3. The receiver, with the same PN code synchronised, mixes the received signal down to a fixed IF (de-hopping), then demodulates.
4. Other users have different hopping patterns; occasional collisions are corrected by FEC and interleaving.

• Slow FH (GSM, 217 hops/s): many symbols per hop. Fast FH: several hops per symbol.

Two hybrid spread spectrum techniques

1. Hybrid DS/FHMA

• A DS-spread signal whose centre frequency hops pseudo-randomly.
• Advantage: avoids the near-far effect, since a strong near user is rarely on the same hop frequency.
• Disadvantage: complex; does not support soft handoff easily because the base station must synchronise to many hopping signals.

2. Time Division CDMA (TCDMA)

• Each cell uses a different spreading code; inside a cell, users take turns in time slots, so only one CDMA user per cell transmits at a time. On handoff, the user's code is changed to the new cell's code.
• Advantage: no near-far effect within a cell.
• Disadvantage: needs strict time synchronisation; capacity per cell limited by the number of slots.

(Other hybrids: FCDMA – sub-bands each running DS-CDMA, flexible spectrum use but lower processing gain; TDFH – hop each TDMA frame, gives diversity against fading but needs tight frame sync.)