FHMA principle and hybrid techniques for near-far

Principle of Frequency Hopping Multiple Access (FHMA)

In FHMA the carrier frequency of each user is changed pseudo-randomly over a wide band, following a PN hopping pattern unique to that user. At any instant each user occupies only one narrow channel, but over time it uses the whole band. Different users have different hopping patterns, so they rarely collide.

 freq ^
  f5  |      A           B
  f4  |  B         A
  f3  |            B          A
  f2  |      B           A
  f1  |  A                    B
      +-------------------------> time (hops)

• Transmitter: data → FSK modulator → mixer driven by a frequency synthesizer controlled by a PN generator → wideband channel.
• Receiver: a locally generated, synchronised hopping pattern "de-hops" the signal to a fixed IF before demodulation.
• Slow hopping: one or more symbols per hop (GSM). Fast hopping: several hops per symbol.
• A collision (two users on the same frequency at the same time) causes errors that are corrected by coding and interleaving.
• Advantages: resistance to narrowband jamming and frequency-selective fading, security. Example: Bluetooth hops 1600 times/s over 79 channels of 1 MHz.

Hybrid techniques that mitigate the near-far problem

1. Hybrid DS/FHMA

• The signal is first DS-spread by a PN code, then its centre frequency hops pseudo-randomly.
• A strong near user and a weak far user are rarely on the same hop frequency at the same time, so the near user cannot swamp the far user's despreading. Thus the near-far effect is avoided.
• Drawback: complex; soft handoff is hard since the base must synchronise to many hopping signals.

 d(t)->(x)c(t)->(x)->[BPF]->channel
                  ^
        [synth] <- [hop PN]

2. Time Division CDMA (TCDMA)

• Different cells use different spreading codes.
• Inside a cell, users are given time slots, so only one CDMA user per cell transmits at any time.
• Since near and far users of one cell are never received together, the near-far effect disappears. During handoff the user's code is switched to the code of the new cell.
• Drawback: needs time synchronisation; capacity per cell limited by slots.

 Cell A, code C_A: |u1|u2|u3|u1|u2|u3|
 Cell B, code C_B: |u4|u5|u6|u4|u5|u6|