Pulse position modulation for FSO systems: Capacity and channel coding

For terrestrial free space optical (FSO) systems, we investigate the use of pulse position modulation (PPM) which has the interesting property of being average-energy efficient. We first discuss the upper bound on the information transmission rate for a Gaussian channel. Next, we consider the more practical aspect of channel coding and look for a suitable solution for the case of Q-ary PPM. Instead of using a non-binary channel code, we suggest to use a simple binary convolutional code and to perform iterative soft demodulation (demapping) and channel decoding at the receiver. We show that the proposed scheme is quite efficient against demodulation errors due to the receiver noise. Moreover, we propose a simple soft-demapping method of low complexity for the general case of Q-ary PPM. The receiver complexity remains then reasonable in view of implementation in a terrestrial FSO system.