Summary: Algorithm for Rapid Acquisition of a PPM Optical Signal
Data frames can be synchronized with part of the synchronization sequence.
NASA's Jet Propulsion Laboratory, Pasadena, California
An algorithm for rapid acquisition of a pulse-position-modulation (PPM) optical data signal
implements a pattern-matching technique for synchronization of receiver timing with the temporal
boundaries of data frames. Synchronization is necessary because in PPM, information is
conveyed by the time slot during which a pulse is detected. Fast acquisition of a signal depends
on detection of pulses in noise, and on correct estimation of the times of detected pulses. To
facilitate synchronization at the receiver, a transmitter periodically inserts a prescribed sequence
of pulses -- the synchronization sequence or word -- into the transmitted data stream. Older
PPM-signal-acquisition algorithms are based on correlations and depend on reception of the full
synchronization word (128 bytes long in some applications). The present algorithm is more
computationally efficient and is capable of achieving synchronization with part of the
synchronization word -- typically with as few as 2 to 6 bytes.
The algorithm involves, among other things, selective elimination of unlikely starting points for a
sequence of received pulses detected in the presence of noise. It is assumed that the pulse-
detection threshold of the receiver is set at a level that corresponds to a specified bit-error
probability. The receiver is switched on at a random time, and a search for the synchronization
sequence is started. The objective of the algorithm is to uniquely identify the temporal location of
a received sub-sequence of the synchronization sequence.