Hop timing estimation for noncoherent frequency-hopped M-FSK intercept receivers

The optimum hop timing estimator, based on classical 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 previously documented suboptimum schemes. In particular, when compared to the multiple-hop autocorrelation (MHAC) approach and the "ping-pong" approach, the average-likelihood ratio (ALR) and maximum-likelihood ratio (MLR) optimum estimators offer an improvement in performance of several dB in input SNR. Robustness issues for the optimal estimator are examined and alternative configurations are proposed that offer enhanced insensitivity to unknown frequency offsets.<>