frequency errors on fine syn- chronization performance. With the previous discussion as background, we now return our atten- tion to the case of primary interest in this chapter, namely, coarse acquisi- tion of fast or slow FH/MFSK. A basic serial search acquisition configuration for FFH/MFSK is illustrated in Figure 3.5. For the case when the received and local hop signals are misaligned by less than a single hop interval, the bottom line of Figure 3.6 is the sequence of frequencies char- acterizing the mixer difference signal. If, as before, the IF filter bandwidth is chosen narrow enough to eliminate the difference of two adjacent hop frequencies, but wide enough now to pass the entire MFSK signalling fre- quency band, then only the frequency components corresponding to the non-crosshatched areas in Figure 3.6 will pass through this filter and be available for correlation detection. Thus, we observe that in each symbol interval Ts, which is synchronous with the local FH synthesizer, the signal component of the IF filter output will consist of m = T/T bursts of sinu- soid of duration Th 2 T. These m bursts are all at the same frquency (cor- responding to the particular MFSK tone transmitted in that symbol interval) but have random phases which are independent of one another. A measure of the lack of coarse time synchronization can therefore be obtained by separately combining the energies detected in each hop inter- val at each of the M possible MFSK frequencies Ifsi,==1,2, M} and then choosing the largest of these m-fold diversity combinations. Since this selection is made only once per symbol interval, post-detection accumu- lation (over say NFFH symbols) is required, the result of which is compared with a preset threshold to determine whether or not to continue the search. NON-COHERENT DEMODULATOR IF-'s SELECT NON-COHERENT DEMODULATOR LARGEST fif-52 RECEIVED mT sec (Ts = mTh) SIGNAL FILTER NON-COHERENT DEMODULATOR FREQUENCY SYNTHESIZER FIF -ISM GENERATOR EPOCH STEP SEARCH THRESHOLD CONTROL (CLOCK) COMPARISON