Generalized channel estimation and data detection for MIMO multiplexing FSO parallel channels over limited space

Abstract In this paper, to increase the data rate of free-space optical (FSO) links, we consider multiple parallel FSO links arranged over a specified space (transceivers arranged over two rings with specified radius). Due to its ease of implementation, stability, and low cost, intensity modulation with direct detection (IM/DD) based on On–Off-Keying (OOK) is widely employed in practical terrestrial FSO links. For long-haul FSO links, avalanche photo diodes (APDs) are commonly used, which provide an internal gain in photo-detection, allowing larger transmission ranges, as compared with PIN photo-detector counterparts. Over the considered general case, firstly, we propose maximum likelihood (ML)-based detection methods when the receivers are aware of the instantaneous channel fading coefficients (CSI). Secondly, we propose an efficient OOK data detection method for a more practical case when the instantaneous CSI is not available at the receiver. For this state, initially, the channel coefficients are estimated by employing some pilot symbols and then, we use the channel estimated coefficients to detect OOK signals. For a specified space, due to crosstalk between parallel channels and pointing errors, performance of proposed data detection methods depends on the optimum value for beam waist. By providing Monte-Carlo simulation results, the optimum value of beam waist is obtained for various channel conditions. To evaluate the BER performance, we also provide analytical results and derive an analytical expression for the considered MIMO multiplexing scheme. The validity of our analytical results are confirmed by Monte-Carlo simulations.