Spread spectrum and FHSS

Spread spectrum

Spread spectrum (SS) is a modulation technique in which the signal is spread over a bandwidth much larger than the data rate needs, using a pseudo-noise (PN) code that is independent of the data. The receiver uses the same synchronized code to despread it. It gives anti-jamming capability, low probability of interception, multipath resistance and multiple access (CDMA). Processing gain PG = B_ss / R_b.

Frequency hopping spread spectrum (FHSS)

In FHSS, the carrier frequency of a narrowband signal (usually M-ary FSK) jumps randomly among a large set of channels. The hopping pattern is decided by a PN code known to both transmitter and receiver. At any instant the signal occupies only one narrow channel, but over time it covers the whole spread band.

Transmitter

data->[FSK    ]--->(x)--->[BPF]---> FH signal
      [modulator]   ^
                    |
            [Frequency synthesizer]
                    ^
             [PN code generator]

Receiver

FH in-->(x)-->[BPF]-->[FSK      ]--> data
         ^            [demodulator]
         |
 [Frequency synthesizer]
         ^
 [PN generator + sync  ]

• At the transmitter, the PN generator selects a frequency from the synthesizer; mixing shifts the FSK signal to that hop channel.
• At the receiver, an identical synchronized synthesizer "de-hops" the signal back to a fixed IF, and a normal FSK demodulator recovers the data.

Time–frequency pattern

 f
 |      ■           ■
 |  ■         ■
 |        ■             ■
 |    ■           ■
 +------------------------> t
   one hop per dwell time

Types

• Slow hopping: several symbols are sent per hop (hop rate < symbol rate).
• Fast hopping: one symbol is spread over several hops (hop rate > symbol rate); better against fading and jamming.

Features: A jammer or deep fade affects only a few hops, which coding and interleaving can correct. FHSS does not need the tight chip-level synchronization of DSSS and suffers much less from the near-far problem. It is used in Bluetooth (1600 hops/s over 79 channels) and optionally in GSM (slow frequency hopping).