4 x 4 space-time codes for free space optical interconnects

Currently, free space optical interconnect systems can be severely limited by optical crosstalk that can arise due to unfocused systems and misalignments between transmitter and receiver elements. To address this limitation, space-time codes, largely developed for radio frequency channels, are adapted for use in a free space optical interconnect system. We have extended space-time coding for a 4x4 optical channel based on on-off keying that uses real intensity-based signals. These codes improve system performance by taking advantage of the optical crosstalk in a system composed of multiple transmitters and receivers. Data is encoded by space-time codes based on orthogonal designs and is split into four streams that are simultaneously transmitted using four transmitters with the same wavelength. The received signal at each of the four optical receivers is a superposition of the transmitted signals with the addition of noise. Decision metrics are calculated making use of the received signals and the optical path gains which are determined using channel training. These metrics, in conjunction with maximum likelihood decoding, decouple the individual signals transmitted from different transmitters. Use of the modified codes based on orthogonal designs allows for simple maximum likelihood decoding based on minimum Euclidean distance. Simulated results show that our system can achieve a low BER on the order of 10(-6) even in case of a substantial misalignment between the transmitter and receiver.