Sequential detection of unknown frequency-hopped waveforms

The channelized receiver, which is optimal for the detection of unknown noncoherent frequency-hopped waveforms, bases its decisions on a fixed-length block of input data. A sequential method of interception is presented according to which whenever a new data element is collected, a decision is made as to the presence or nonpresence of a frequency-hopped waveform. If that decision is indeterminate, another data element is collected. An optimal sequential test is derived, under the assumption that the waveform signal-to-noise ratio (SNR) 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. A truncated sequential test is also derived where a decision is forced, if still indeterminate, after some fixed amount of data is collected. The truncated test is shown to improve the number of samples needed for a decision when the input SNR differs 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 SNR differs from that assumed.