Citation
Abstract
Optical communications over space channels (satellite-to-satellite or deep-space-torelay-satellite) are commonly designed as pulse-position-modulated (PPM) laser links. When coding is needed to improve the link performance, it is advantageous to use Reed-Solomon (RS) block codes over the PPM frames to obtain the largest degree of error correction. Since RS codes can correct both symbol errors and symbol erasures, a question arises as to the best way to demodulate the PPM laser fields in order to generate the input symbols for the RS decoder. The method selected for demodulating (converting the received laser field to digital symbols) will define the erasure and transmitted symbols of the laser link, and therefore will determine the word error probabilities of the system. In this paper, several possible demodulating schemes were considered, and the effect of each on RS decoding performance was computed. This computation was carried out for various optical receiver models, and required fairly lengthy numerical analysis to determine accurate word error probabilities when the RS code lengths are long. It is shown that simple threshold decisioning of pulse slots will produce performance that degrades as the background noise increases. This is caused by the generation of too many erasures for the RS decoder to handle. We propose a decision scheme, delta-max demodulation, which offers improvement over threshold decisioning by redefining the generation of an erasure.
Details
- Volume
- 42-70
- Published
- August 15, 1982
- Pages
- 47–59
- File Size
- 433.5 KB