Sequential Detection of Unknown Frequency-Hopped Waveforms.

Abstract

The channelized receiver which is optimal for the detection of unknown non-coherent frequency-hopped waveforms bases its decision on a fixed-length block of input data. In this paper we present a sequential method of interception according to which whenever a new data element is collected, a decision is made as to the presence or non-presence of a frequencyhopped waveform. If that decision was indeterminate, another data element is collected. An optimal sequential test is derived, under the assumption that the waveform signal-to-noise ratio (S/N) is known. It is shown that this sequential test requires less data, on average, than the fixed length method to make a decision with the same reliability. Also derived is a truncated sequential method where a decision is forced, if still indeterminate, after some set amount of data is collected. The truncated test is shown to improve the number of samples needed for a decision when the input signal-to-noise ratio defers greatly from that assumed in the derivation of the test. Furthermore it is shown, that the truncated test yields a limited degree of robustness when the input S/N defers slightly from that assumed. A detailed analysis of the performance of these tests is conducted from which a method for finding an optimal truncation point follows. Numerical results which are based on this analysis as well as on simulation of the interceptor's performance are presented to prove the claims made above.