Achieving full transmit diversity for PPM constellations with any number of Antennas via double position and symbol permutations. (c2008)

Abstract

Encoding and decoding techniques present in the literature, mainly designed for radio frequency (RF) applications, face many problems when applied in optical wireless (OW) communications. These schemes seem to have lacks in some places like diversity, rate, or delay criteria. Therefore, we present a general technique for constructing minimal-delay, fully-diverse and rate-l Space-Time (ST) block codes for Pulse Position Modulations (PPM) applied with an arbitrary number of transmit antennas and modulation positions. Introducing the novel idea of time-domain constellation extension as well as joint position and symbol permutations permitted to maintain unipolar transmissions the fact that renders the proposed code suitable for Free Space Optical (FSO) communications with direct detection at the receiver. To construct this code, it has been shown that for conveying a M -ary PPM symbol, each symbol duration must contain M + 1 time slots (or positions) rather than M. This increases the system dimensionality by I but guarantees a code with full diversity for an arbitrary number of transmit antennas and modulation positions in the PPM constellation. In parallel, an M -PPM decoder was developed to be applied with the same systems without any obstacles.