ADAPTIVE FREQUENCY-HOPPING IN HF COMMUNICATIONS

Frequency hopping (FH) has been proposed as a technique to combat mult ipath fading and interference problems in HF links. By means of powerf ul coding techniques, these systems are able to restore the transmitte d information also under adverse transmission conditions when several of the frequencies in use are not available. Although efficient agains t jamming and interference of short duration, these schemes may be sub stantially improved in a typical HF environment with slow fading and a large number of narrow-band, slowly varying interferers. A straightfo rward adaptive frequency hopping (AFH) scheme for duplex links is prop osed and analysed. In such a link, link quality measurements are fed b ack 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. The tradeoff between the amount of feedback information and link throughput as well as the impact of feedback reli ability is investigated. Results show that an AFH system has the poten tial of surviving in extremely congested environments at very low tran smitter power levels. Both the antijam and the LPI properties of these systems are very promising.