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FHSS working principle

Short answer

FHSS divides the total bandwidth into many hop channels. A pseudorandom noise generator selects a channel each hop period, and a frequency synthesizer shifts the carrier accordingly. The transmitter hops rapidly among frequencies in a pattern known to the synchronized receiver, which demodulates the data. Narrowband interference affects only occasional hops, and without the code interception is difficult.

Explain the mechanism of frequency hopping spread spectrum (FHSS).

Frequency hopping spread spectrum (FHSS) transmits a signal over a number of different frequencies, switching rapidly from one carrier to another in a pseudorandom pattern known to both transmitter and receiver.

Mechanism

  1. The total bandwidth Bss is divided into 2^k channels (hop frequencies), each wide enough for the data signal.
  2. A pseudorandom number (PN) generator produces a k-bit pattern for every hop period (Th). Each pattern selects one frequency from the table.
  3. The frequency synthesizer generates a carrier at the selected frequency.
  4. The modulator (FSK or BPSK) puts the data on this carrier. The carrier stays at that frequency for Th and then hops to the next.
  5. The receiver has the same PN generator (same seed, synchronised in time), so it hops to the same frequencies and demodulates the data.
 Freq
  f8 |            #
  f7 |                  #
  f6 |      #
  f5 |
  f4 |   #
  f3 |                        #
  f2 |         #
  f1 |#
     +------------------------> time
      T  T  T  T  T  T  T  T  (hop period Th)

Pattern here: f1, f4, f6, f2, f8, f7, f3 ... The order looks random but repeats after the PN cycle.

Types

  • Slow hopping: several bits per hop.
  • Fast hopping: several hops per bit, better against jamming.

Properties

  • Bandwidth used is 2^k times the signal bandwidth.
  • Narrow-band interference or jamming affects only some hops, and error correction repairs them.
  • Hard to intercept without the PN code.
  • Several users can use different hop sequences in the same band.

Example: Bluetooth uses 79 channels of 1 MHz in the 2.4 GHz band and hops 1600 times per second.

✓ Answer checked by Claude Sonnet 5.5

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