Detection Alternatives For Pulse Position Modulation (PPM) Optical Communication

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Abstract
The alternative detection strategies for optical communication with pulse position modulation (PPM) are analyzed and compared both for systems with and without error correcting codes. If no error encoding is used, the detection alternatives include the threshold and maximum count M-ary strategies, as well as a method of bit decoding on parallel binary channels. M-ary maximum count offers the best performance as it is a maximum-a-posteriori (MAP) strategy, but threshold may be much easier to implement at high data rates. For the particular choice of metric analyzed, the binary scheme is competitive only for low values of PPM word size. For PPM communication with error correcting codes, the decoder has the additional option for hard or soft decisioning. Each of the unencoded detection schemes can interface directly with a hard decision decoder. The hard decision coding performance can be characterized in terms of input/output bit error probability for a given code. Coding gain is defined to reflect the optical power savings by incorporating the code, and example hard decision coding gains are evaluated for M-ary Reed-Solomon and binary BCH codes with an APD receiver. Soft decision decoder performance is dependent on both the input error and erasure probabilities from the channel. Soft decisions strategies offer additional coding gain, and the performance of rate 1/2 Reed-Solomon codes is compared for A-Max and threshold soft decision strategies with an APD receiver.© (1986) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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