Importance

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Abstract

A system for secure communication between transmitting and receiving devices in a laser communications system using pulse position modulation, has a transmitter means to modulate and a receiver means to demodulate a transmission utilizing a protocol in which a number N (N.gtoreq.1) of optical pulses represents each symbol to be transmitted. A transmission is initiated with a trigger event. Thereafter, a multi-pulse synchronization signal is transmitted as the first symbol of each transmission, the first pulse of said synchronization symbol being transmitted within a first time frame after said trigger. A buffer in said receiver receives and stores the first of said N frames of said synchronization symbol. Each subsequently received frame of said N frames of said synchronization symbol is added to data previously stored in said buffer. A frame template is calculated from the stored pulse data of said first N pulses in said buffer and from said synchronization symbol protocol. The calculated frame template is used for transmission of subsequent message symbols, which are then transmitted in N frames per symbol. The system includes an algorithm for the synchronization symbol format which pseudo-randomly positions each of said N-1 pulses after said first pulse within its frame by offsetting the start time of its frame by a predetermined amount of time, with reference to the start time of said first frame. The system also includes an algorithm for the message symbol format which pseudo-randomly positions each of said N-1 pulses after said first pulse within its frame by offsetting its pulse position from that of said first frame such that the separations of said pulses vary from frame to frame within a symbol, and said algorithmically determined position of any pulse falls within the live time of its frame.

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First Claim