Frequency hopped multiple access (FHMA)

Frequency Hopped Multiple Access (FHMA) is a digital multiple access technique in which the carrier frequency of each user changes (hops) pseudo-randomly over a wide band, following a PN code unique to that user. At any instant a user occupies only one narrow channel, but over time the signal is spread over the whole band. Different users hop with different codes, so they rarely collide.

 TRANSMITTER
 data->[FSK/BFSK mod]->(×)->[BPF]-> wideband channel
                        ^
            [freq. synthesizer]<-[PN code gen.]

 RECEIVER
 channel->(×)->[BPF/IF]->[FSK demod]-> data
           ^
  [freq. synthesizer]<-[PN code gen.] (synchronised)

Transmitter

1. Data modulates a narrowband carrier, usually with FSK.
2. A PN code generator gives a new code word every hop period; the frequency synthesizer converts it into a hop frequency.
3. The mixer moves the modulated signal to that frequency. The hop rate is the number of frequency changes per second.

Receiver

1. An identical PN generator, synchronised with the transmitter, drives a local synthesizer.
2. Mixing with this local signal de-hops the signal back to a fixed IF.
3. A conventional FSK demodulator recovers the data. Signals of other users, hopping on different patterns, fall outside the IF filter most of the time.

Example: a system has 8 channels f₀ … f₇ (3-bit PN code). User A's code gives the sequence f₃, f₇, f₁, f₄ …; user B's code gives f₅, f₂, f₆, f₀ …. In each hop interval they are on different frequencies, so both share the band. If both land on the same frequency in one hop (a collision), only that hop's bits are damaged, and coding/interleaving corrects them. Bluetooth is a real example: 79 channels of 1 MHz, 1600 hops/s. GSM uses slow frequency hopping (217 hops/s) for diversity.

Types: slow hopping (several symbols per hop) and fast hopping (several hops per symbol).

Advantages: resistance to narrowband interference and fading, security, and a less severe near–far problem than DS-CDMA. Disadvantages: fast frequency synthesizers are needed, and collisions cause errors.