Multi-beam free-space optical link using space-time coding

All rights reserved. In this chapter we will study a FSO MISO system. Section 2 describes the components of single beam and multi-beam FSO communication links. Because spatial correlation among the propagating beams in a MISO link plays a key role in the system performance, this correlation structure must quantified as a function of turbulence strength, beam separation, and propagation distance. We present estimations of the spatial correlation obtained through numerical simulations for a MISO in Section 3. Properly designed multi-beam FSO links can deliver significant improvement in scintillation reduction which manifests in lower bit-error rates. In Section 4 we describe two space-time coding schemes using On-Off Keying (OOK) modulation. The first scheme is based on a spatial repetition code and the second scheme is based on a size-four rate-one ST block code. The latter has been adapted from an orthogonal space-time code proposed for wireless radio-frequency (RF) communications. Adaptation of such codes is mandated by the non-negativity of signals in optical communications. In Section 5 we show that both coding schemes have excellent performance in moderate and stronger turbulence conditions. The simpler repetition scheme, however, shows better performance. This difference is explained at the end of Section 5.