Frequency Hop Spreading (FHSS)

The carrier frequency switches among many distinct channels according to a pseudo-noise (PN) code known only to the transmitter and receiver. This sequence appears random to an interceptor but is deterministic to the synchronized pair. Without the correct PN code and seed, an adversary cannot predict the next hop, making follower jamming extremely difficult. The hopping pattern is typically generated by a linear feedback shift register (LFSR) or a cryptographic algorithm, ensuring a long period before repetition.

The hop rate—the speed at which the carrier frequency changes—is a critical parameter defining FHSS resilience. Systems are categorized as:

• Slow Frequency Hopping (SFH): The hop rate is slower than the symbol rate, meaning multiple symbols are transmitted per hop. Vulnerable to slow sweep jamming.
• Fast Frequency Hopping (FFH): The hop rate exceeds the symbol rate, meaning a single symbol is spread across multiple hops. This provides superior resistance to barrage jamming and repeater jamming by forcing the jammer to cover the entire spread bandwidth instantaneously.

Processing gain quantifies the system's immunity to interference. It is the ratio of the spread bandwidth to the original information bandwidth, expressed in decibels. A higher processing gain directly increases the jamming margin—the maximum tolerable jamming-to-signal ratio (JSR) before the link fails. For FHSS, the processing gain is approximately 10 * log10(N), where N is the number of available hop channels. This gain forces a jammer to spread its power thin, reducing its effective power spectral density at any single receiver channel.

Robust synchronization is the most critical vulnerability of FHSS. The receiver must align its PN code generator precisely in time with the incoming signal to de-hop correctly. This is typically achieved through a two-stage process:

• Initial Acquisition: A known preamble or synchronization header is transmitted on a fixed set of frequencies to allow the receiver to coarsely align its clock.
• Tracking: A delay-locked loop (DLL) or tau-dither loop continuously maintains fine alignment, compensating for clock drift and Doppler shift. Without perfect sync, the signal appears as noise.

Dwell time is the duration the transmitter remains on a single frequency before hopping. This interval must be long enough to transmit meaningful data but short enough to avoid interception or jamming. The dwell time directly impacts coherence—the ability of the receiver's local oscillator to maintain phase stability. In coherent FHSS, phase continuity is maintained across hops, enabling coherent demodulation. Non-coherent systems, using modulation like M-ary FSK, are simpler but less power-efficient, as they discard phase information between hops.

An intelligent evolution of basic FHSS, AFH dynamically modifies the hop sequence based on real-time channel quality metrics. The transceiver maintains a channel map identifying frequencies suffering from persistent interference or static jamming. These 'bad' channels are excised from the hopping pattern, forcing the system to hop only within a subset of clean spectrum. This is a core Electronic Counter-Countermeasure (ECCM) technique, effectively neutralizing partial-band jamming and avoiding coexistence interference in crowded ISM bands like Bluetooth.