Hybrid spread spectrum and FHMA types

Hybrid spread spectrum techniques combine two basic multiple access methods to get the advantages of both.

Hybrid spread spectrum multiple access techniques

1. Hybrid FDMA/CDMA (FCDMA)

• The available wideband spectrum is divided into several narrower sub-bands (FDMA); inside each sub-band, a separate CDMA system with its own PN codes serves a group of users.
• Advantages: the sub-bands need not be contiguous, so spectrum can be allocated flexibly; each sub-band has lower bandwidth, so cheaper hardware and lower chip rate; sub-bands can be switched on/off to avoid interference.
• Disadvantage: processing gain in each sub-band is lower than full-band CDMA, and frequency-selective fading is less averaged out.

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

• The signal is first spread by a DS code, then the centre frequency of the DS signal hops pseudo-randomly. Each user has a unique DS code and hopping pattern.
• Advantages: avoids the near–far problem of pure DS-CDMA, since a near user rarely shares the same hop frequency; combines anti-jamming of FH with multipath resistance of DS.
• Disadvantage: complex transmitter and receiver; needs fast, accurate synthesizers and code synchronisation.

3. Time Division CDMA (TCDMA)

• Different spreading codes are assigned to different cells, and inside a cell only one user transmits in a given time slot (TDMA). So a user meets interference only from other cells.
• Advantages: no near–far problem within a cell; easier power control. Disadvantage: needs time synchronisation and has TDMA overhead.

4. Time Division Frequency Hopping (TDFH)

• A user hops to a new frequency at the start of each new TDMA frame (as in GSM slow frequency hopping).
• Advantages: avoids severe fades or co-channel interference on one channel for long; gives frequency diversity. Disadvantage: needs synchronised hopping across frames and more complex synthesizers.

Operation of FHMA

Frequency Hopped Multiple Access (FHMA) lets many users share a wide band by making each user's narrowband carrier hop pseudo-randomly among many channels. At any instant a user uses one channel, and different users follow different hop patterns.

 TRANSMITTER
 data->[FSK modulator]->(×)->[BPF]-> antenna
                         ^
       [PN code gen.]->[frequency synthesizer]

 RECEIVER
 antenna->(×)->[BPF at IF]->[FSK demodulator]-> data
           ^
       [PN code gen.]->[frequency synthesizer]
       (same code, synchronised to Tx)

1. Data modulates the carrier (usually FSK).
2. A PN code generator drives a frequency synthesizer, which sets a new carrier frequency every hop period.
3. The transmitted signal thus occupies a different channel in each hop.
4. The receiver, using the same PN code in synchronism, de-hops the signal to a fixed IF and demodulates it.
5. When two users hop to the same channel at the same time (collision), errors occur in that hop only; channel coding and interleaving correct them.

Example: with 8 channels, user A may follow f₂, f₆, f₁, f₇ … and user B f₅, f₀, f₃, f₄ …; they share the band without colliding in these hops.

Types of FHMA

• Slow FHMA: several symbols per hop (hop rate < symbol rate), e.g. GSM SFH at 217 hops/s.
• Fast FHMA: several hops per symbol (hop rate > symbol rate); better diversity and anti-jamming, more complex synthesizer.