TURBO CODES FOR PULSE POSITION MODULATION: APPLYING BCJR ALGORITHM ON PPM SIGNALS

In this paper, we consider the problem of turbo encoding/decoding for Pulse Position Modulation (PPM) that constitutes a very popular modulation scheme for Free-Space Optical (FSO) communications and unipolar ultra-wideband (UWB) communications. At the transmitter side, a simple mapping strategy is applied where the systematic information bit and the parity bit determine the PPM slot in which the signal is to be transmitted. At the receiver side, a convenient demodulation scheme, which is based on the soft decisions generated in all PPM slots, is proposed. This demodulation scheme that is adapted to the multidimensional PPM signal sets is inspired from the turbo coding techniques that are often applied with the two-dimensional Quadrature-Amplitude Modulated (QAM) signals. The soft output of the PPM signal demodulator is then passed to the turbo decoders that undergo iterative decoding based on the BCJR or the LOG MAP BCJR algorithm.

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