<title>Performance enhancement of direct-detection optical communication systems impaired by pulse width inaccuracies using trellis-based pulse position modulation</title>

Abstract

M-ary pulse position modulation (PPM) has received considerable attention for direct-detection photon communications over unguided channels. The analysis generally assumes that the signaling set is orthogonal. However the orthogonality of the signaling set will be destroyed by the finite area and bandwidth of optical detectors, resulting in severe intersymbol interference. This paper presents the analysis of a trellis-based pulse position modulation (T-PPM) scheme for photon communications with non-rectangular pulses. The novelty of the scheme includes the use of a set partitioning methodology to increase the minimum distance using a simple convolutional encoder. The Viterbi algorithm is used at the receiver to separate the signaling set as part of the demodulation process. It has been shown that T-PPM will restore performance losses due to reduced peak intensity during the detection process. Furthermore, for a large range of background radiation levels and when an APD detector is used, the average number of photons per information bit for T- PPM is smaller than that of the regular PPM. Numerical examples show that for a symbol error rate of 10-3 when the received pulses extend over 4 PPM slots, the average laser energy per symbol for 256-ary T-PPM could be reduced by as much as 2 dB.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.