Modulation and constrained coding techniques for wireless infrared communication channels

Abstract

Short-distance, point-to-point wireless infrared optical links provide a cost-effective means of high speed data transfer between portable devices. To investigate such links, a test-bench and circuits were constructed to determine the limitations of existing optoelectronics. The results of these measurements we used to formulate a signal-space channel model which is employed for the subsequent analysis of candidate bandwidth efficient modulation schemes. The modulation scheme Adaptively Biased QAM (AB-QAM) is developed based on the channel model. AB-QAM provides an asymptotic 3 dB optical SNR improvement over PAM while maintaining the same bandwidth efficiency. The use of constellation shaping is shown to further improve the average optical power efficiency of AB-QAM. This thesis proposes the use of constrained coding techniques to satisfy the non-negativity constraint of the optical channel. These coding techniques are illustrated through an example and contrasted to a baseline case. Constrained coding techniques allow greater flexibility in the choice of pulse shapes used in the channel, leading to possible optical power and bandwidth gains.