ECCM Frequency Hopping Spread-Spectrum (FHSS) example

Looking at the spectrum of the receivable signals around 7 MHz in the UKR skyes, as well as numerous and very frequent STANAG-4538 and L3Harris WHARQ waveforms, it may happen that we observe transmissions in frequency hopping mode (FHSS, or Frequency Hopping Spread-Spectrum) as shown in Figure 1.
Frequency hopping (also known as ECCM, Electronic counter-countermeasures) is the most commonly used Transmission Security (TRANSEC) technique. The frequency hopping capability provides advanced anti jam protection for communications. In HOP radio mode, the transmitter frequency changes so rapidly that it is difficult to intercept or jam the signal. For additional security, hopping data and digital voice data can be encrypted. 

Fig.1 - FHSS transmission

Me and my friend ANgazu from radiofrecuencias.es had the chance the analyze these signals and share the results. We observed transmissions which use 26 or 27 channels and occupy a bandwidth of 81 KHz, since each channel is 3 KHz wide (2700 + 300 Hz separation). Hopping rate is 8.88 sps with an hop time of ~112.5 ms (say 102 ms ON, 10.5 ms OFF).

Fig. 2 - FHSS single channel frequency occupation

Fig. 3 - FHSS timing

Like a single-channel serial tone waveform, the modulation used is 2400 Bd PSK8 for both voice and data (Fig. 4).

Fig. 4 - FHSS modulation

This waveform is fielded in AN/PRC-150(C) radios by L3Harris. Wideband hopping covers a frequency band that is bounded by a lower and upper frequency specified in multiples of 100 Hz, frequency exclusion bands may also be programmed. AN/PRC-150 narrow band hopping uses frequencies within a defined bandwidth of the center frequency (Fc) as in the Table below: notice the reported 81 KHz bandwidth  in case of  3.5 MHz <= Fc < 9.995 MHz.

Table 3.16 - L3Harris AN/PRC-150 operation manual

An important aspect of hopping is synchronization, ie all radios in a net shall use the same frequency at the same time intervall: that alignment may be accomplished with the use of GPS, but is some in cases (very very rare) it uses the manual  3x4 sync sequences as shown in Figure 5.

Fig. 5 - 3x4 sync sequences

If our guess is correct, we can assume a large employ of L3Harris equipmente in that (war) theatre.

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pseudo-random frequency hopping

Frequency Hopping Spread Spectrum (FHSS) is a method of transmitting radio signals by rapidly switching the carrier among many frequency channels using a pseudo-random sequence known to both transmitter and receiver(s). The heard packets in this sample have a duration of 2520 ms, frequency shift ~ 20 Hz and 20 data bits carried (other similar transmission have been noted with 2460 ms and 24 data bits, same shift) and span up a 2 MHz bandwidth, this recording has been on 13 MHz band.

FHSS, Frequency-Hopping spread Spectrum

Frequency-hopping Spread Spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

 

The spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:

    1) Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background.

    2) Spread-spectrum signals are difficult to intercept. A spread-spectrum signal may simply appear as an increase in the background noise to a narrowband receiver. An eavesdropper may have difficulty intercepting a transmission in real time if the pseudorandom sequence is not known.

    3) Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be used more efficiently.

Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics. Military radios use cryptographic techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.