Detection and synchronization of PPM sequences
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Summary form only given, as follows. The synchronization and detection properties of pulse-position modulation (PPM) sequences are investigated, both in the presence and absence of noise and when symbol synchronization is not present. For sequences of N Q-ary PPM symbols, a quantity d min = N/Q, somewhat analogous to the minimum distance of a block code, is introduced to describe the synchronization properties of the sequences. For large N, d mi n can be interpreted as the smallest number of symbol mismatches that can be expected between shifted versions of PPM sequences of length N. It is shown that both the correct synchronization probability and the correct symbol detection probability are bounded from above by a function f(d min ), irrespective of the synchronization or detection algorithm used. This performance ceiling is due to the absence of symbol synchronization and the randomness of the PPM sequences. A way to eliminate the performance ceiling is suggested which consists of inserting in the random data stream a synchronizable pair of PPM symbols.