hreshold. Clearly, when the input sequence f1, f2, fm has just passed through the receiver matched filter, the above sum will have its max- imum value. One hop later, each of the M correlations will have zero con- tribution to the sum since the corresponding mixer outputs do not pass through the band-pass filters. A discrete time version of the matched filter receiver of Figure 3.11 can be had by replacing each of the M band-pass non-coherent demodulators by their equivalent low-pass version using integrate-and-dump filters as in Figure 3.4. Here, a threshold decision is made at discrete time instants sep- arated by a hop time and thus the search proceeds at the hop rate. The performance of the discrete time matched filter FH acquisition sys- tem follows directly from the detailed discussions given in Chapter 1 for the analogous PN acquisition system. Thus, other than to point out the obvious parallel between PN code chip time there and hop time here, we shall leave it to the reader to make the remaining necessary associations between the parameters of the two systems. M+/IF SQUARE-LAW DELAY ENVELOPE DETECTOR THRESHOLD HOP SYNC SQUARE-LAW DELAY DEVICE DECISION ENVELOPE RECEIVED DETECTOR FH SIGNAL 1+ IF SQUARE-LAW DELAY ENVELOPE DETECTOR Figure 3.11. A matched filter type of FH acquisition system. FH Acquisition Techniques 3.1.3 Other FH Acquisition Techniques A scheme that combines the rapid search capability of passive correla- tion with the decision reliability of active correlation (over long time intervals) was suggested in [3] and later compared with the more con- ventional FH acquisition techniques in [4]. The application for which such a two-level acquisition scheme was proposed was a ground mobile radio environment where the users operate in a "push-to-talk" mode and thus the communication among them is intermittent and characterized by fre- quent and possibly long periods of silence. At the onset of a transmission, prior to message communication, the user sends a leader consisting of several repetitions 