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Wi-Fi Pro Lab › Modulation and waveforms

FHSS: frequency-hopping spread spectrum

A narrow signal that never stays still. FHSS was one of the two radios in the original 802.11 standard, and the idea is still very much alive next to Wi-Fi in the same band.

79hop channels of 1 MHz
1 and 2 Mbit/sthe original FHSS rates
about 28%of hops hit by a 22 MHz network
1997802.11 original standard

1. DefinitionWhat is frequency-hopping spread spectrum?

In FHSS the transmitter sends on one narrow channel for a short dwell, then jumps to another, following a pseudo-random hop pattern that the receiver also knows. The signal is spread over the whole band across time, not made wider at any one instant.

Status of FHSS in 802.11. FHSS belongs to the 1997 original. It no longer appears at all in the current IEEE Std 802.11-2020, so this is a historic radio. It matters because Bluetooth, which shares 2.4 GHz with Wi-Fi, still hops.

2. PurposeWhy hop?

  • A bad channel hurts only briefly. If one channel is noisy, only the hops that land on it are affected, and the lost packet is retried on the next hop.
  • Low interference to others. The signal is only on any one channel for a short time.
  • Legal in licence-exempt bands. Spread-spectrum rules allow hopping systems.

3. MechanismHopping across 79 channels

The model shows each hop as a dot. A 22 MHz-wide Wi-Fi network sits across the band as the shaded area: every hop that lands inside it is red. Try moving the other network, then turn on adaptive hopping, where the hop pattern skips channels known to be busy.

5. MathematicsHow much is lost?

  • Probability that a hop is hit = busy channels / total channels = 22 / 79 = 27.8% for a 22 MHz-wide network.
  • Because the hit hops are retried on the next hop, a lost packet costs one dwell of airtime, not a failed connection.
  • Adaptive hopping removes the busy channels from the pattern, so the hit probability falls to zero for an interferer that stays still, at the cost of using fewer channels.

6. Real-world behaviourWhere hopping shows up today

  • Bluetooth. Classic Bluetooth hops across 79 channels of 1 MHz and uses adaptive hopping to avoid channels where Wi-Fi is active.
  • Coexistence. A Wi-Fi network and a Bluetooth device in the same room each see the other as interference, and a strong Wi-Fi network can make some Bluetooth hops unusable.
  • Legacy gear. Very old FHSS Wi-Fi equipment is no longer made, but it explains some old interoperability behaviour.

Practical validationSeeing hopping

  • Hopping is invisible in a Wi-Fi capture. Use a spectrum analyser: its waterfall view shows short bursts that jump across the band instead of one steady block.
  • Compare it with an ordinary Wi-Fi signal, which sits in one fixed block of the band, to see what hopping looks like.
  • Question to answer: how would you tell a hopping interferer from a continuous one on a duty-cycle chart?

Check your understandingQuick quiz

ReferencesSources and further reading

FHSS was defined in the 1997 original standard; the IEEE Std 802.11-2020 text contains no FHSS clause. The 79 x 1 MHz channel figure and the hit-probability example describe the historic radio and the same structure used by classic Bluetooth.