Influence of the Path Delay Differences on the Channel Capacity in MIMO OWC Systems

In this work, the impact of the path delay differences in the various transmitter-receiver links on the channel capacity in an multiple-input multiple-output (MIMO) optical wireless communication (OWC) system with focus on line-of-sight (LOS) scenarios is investigated. Typically, the gain that can be achieved by using multiple transmit and receive elements in such a system is very limited, since the correlation between the different channels is extremely high. In the LOS channel, the magnitude of the transfer function is approximately constant for all frequencies. However, the different relative propagation times in the individual transmitter-receiver links lead to a varying frequency-dependent phase shift. So far, this aspect has been neglected in the most common optical free-space channel models, due to the relatively small path differences. In this paper, it is shown that even these small time delays have a significant impact on channel diversity for the highest modulation bandwidths reported in literature. With a single-sided modulation bandwidth of 1 GHz, the increase in normalized channel capacity due to the diversity caused by the different time delays is approximately 4 bps/Hz, depending on the geometry of the scenario. Therefore, accessing higher bandwidths not only increases the data throughput by shortening the symbol duration, but also enhances the potential gain from using MIMO techniques in a LOS OWC system.