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The optimum hop timing estimator (based on likelihood-ratio (LR) theory) is derived for noncoherent slow and fast frequency-hopped M-FSK intercept receivers. Such receivers have no a priori knowledge of the hopping code and thus the solution to this estimation problem differs considerably from the more commonly considered case of the friendly receiver. The implementation and performance of the LR hop timing structures are presented and compared with that of other suboptimum schemes that have been discussed in the literature.

Channelized intercept receivers with fast Fourier transform (FFT) front-end preprocessors are often used to detect frequency-hopped (FH) signals. Ideally, the FFT frequency band should coincide with the entire FH spread-spectrum bandwidth. However, in some practical implementations, hardware and/or real-time computational signal processing capabilities limit the FFT to a fraction of the total band of interest. In this case, the preprocessor will only intercept some of the received hops, but the FH signal can still be successfully detected assuming that enough hops are observed.If the receiver front end is tunable, an alternative approach is to step the FFT band within each hop such that more or all of the FH signal bandwidth is covered. The tradeoff is that more of the hops will be intercepted but the receiver will detect only part of each hop. This paper analyzes this partial-band FH detection scheme and determines the optimum performance strategy.

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