Power control and active channel selection in an LPI FH system for HF communications

To improve the performance of frequency-hopping systems on interference-limited HF channels, so-called adaptive frequency-hopping (AFH), which uses an adaptively selected pool of the “best” hopping-frequencies for communication, has been proposed. We extend the adaptivity of the AFH scheme by adjusting the transmitted power on each channel individually and by adaptively changing the number N_a of active channels that are selected to the pool. Fewer active channels (up to a certain point) give improved communication performance since the used channels, on the average, are less interfered. However, by decreasing N_a, the protection against hostile detection is decreased. This trade-off between communication and LPI (low probability of intercept) performance with respect to N_a is shown. Our analysis shows that the codeword error rate is minimized when about 20% of the channels are selected to the active pool, and that the LPI protection against the two tested hostile detectors, as expected, improves for larger N_a. Generally, the hostile detectors require less received signal-to-interference ratio than the legal AFH receiver to obtain acceptable performance. For the parameters we have chosen in our duel simulation, the results indicate that the LPI performance seems to be more sensitive to the choice of active channel pool size than the communication performance