Anti-jam by fast FH NCFSK-myths and realities

Abstract

A number of misconceptions and ill-suited approaches concerning fast frequency-hopped (FH) M-ary NCFSK have risen that have led to the concealment of the impressive performance enhancement capability of some diversity combining methods in the presence of strong intelligent jamming, including partial-band noise (PBN) and multiple tone (MT). Contrary to the conventional wisdom, practical fast FH for anti-jam (AJ) applications uses a constant hop rate whereas the data rate is backed off as needed to provide diversity capability. Then, there is no optimum diversity and the energy per hop is a better measure than the traditional energy per bit used in nonjammed systems. It is then possible to operate under much higher jamming levels than with just the basic FH and standard error-correction techniques alone. These diversity combining techniques are relatively easy to implement, and have been implemented, in real time. Processing gain is an inappropriate measure of performance when applied to non-Gaussian jamming. The perception of a noncoherent combining loss for diversity combining is shown not only to be false, but, for the types of jamming considered, diversity combining actually performs better than coherent combining. Side information is shown to be of little or no value for the types of jamming considered. For large jamming levels, PBN jamming generally causes more degradation in performance than MT jamming. >