Adaptive frequency hopping in HF communications

Frequency hopping (FH) has been proposed as a technique to combat multipath fading and interference problems in HF links. By means of powerful coding techniques, these systems are able to restore the transmitted information also under adverse transmission conditions when several of the frequencies in use are not available. Although efficient against jamming and interference of short duration, these schemes may be substantially improved in a typical HF environment with slow fading and a large number of narrow-band, slowly varying interferers. A straightforward adaptive frequency hopping (AFH) scheme for duplex links is proposed and analysed. In such a link, link quality measurements are fed back to the transmitter such that `bad´ frequencies may be avoided. The system is analysed based on an extension of the HF interference model presented by Gott et al. (1991). The tradeoff between the amount of feedback information and link throughput as well as the impact of feedback reliability is investigated. Results show that an AFH system has the potential of surviving in extremely congested environments at very low transmitter power levels. Both the antijam and the LPI properties of these systems are very promising