Performance Analysis of FSO Communications With Diversity Methods: Add More Relays or More Apertures?

Abstract

This paper targets the performance analysis of Multiple-Input-Multiple-Output (MIMO) point-to-point, singleaperture relay-assisted and MIMO relay-assisted Free-Space Optical (FSO) communications. The objective of the paper is to compare the different FSO diversity methods via an outage probability analysis over gamma-gamma turbulence channels in the case of independent fading among the different apertures of the communicating nodes. Through a well-tailored calculation method, we approximate the gamma-gamma distribution and the gamma-gamma sum-distribution by appropriate gamma distributions. This method resulted in simple closed-form outage probability expressions that are accurate for average-to-large values of the signal-to-noise ratio (SNR) and in asymptotic expressions that are useful for evaluating the diversity gains and coding gains. In particular, we derived the diversity gains of MIMO repetition coding (RC), MIMO transmit laser selection (TLS), MIMO all-active relaying (AR) and MIMO selectiverelaying (SR). We prove that MIMO-RC and MIMO-TLS, on one hand, and MIMO-AR and MIMO-SR, on the second hand,achieve the same diversity gain and we evaluate the coding gain of TLS with respect to RC and of SR with respect to AR. By comparing the MIMO and cooperative methods with point receivers, we reach the central result that it is always better, from a diversity gain point of view, to add more apertures to the source and destination rather than adding more relays in their vicinity despite the fact that the fading variance along FSO links decreases with the distance. For receivers that implement aperture averaging, we derive conditions under which MIMO techniques are unconditionally better than cooperative techniques with any network topology.

Optimal Relay Selection for the Parallel Hybrid RF/FSO Relay Channel: Non-Buffer-Aided and Buffer-Aided Designs

“…More in detail, we employ the protocols in [29] and [30] for the following two scenarios: i) FSO only: Relay selection for the FSO links without RF links as backups [8], [9] and ii) Independent RF/FSO relay selection: Relay selection and data transmission are performed independently for the RF and FSO links. In the non-BA benchmark schemes, we assume that for the RF links, each time slot is divided into two sub-time slots of equal length for S − R m and R m − D RF transmission.…”

Developing Cost-Effective and High-Speed 40 Gbps FSO Systems Incorporating Wavelength and Spatial Diversity Techniques

“…The use of different modulation schemes can also alter the system performance as it has been reported that higher modulation schemes like binary phase-shift keying (BPSK) are more immune to channel adversities than the conventional on-off keying (OOK) schemes [19,21]. The diversity in the channel brings in the concept of multiple input multiple output (MIMO), which uses multiple transmitters/receivers to improve the transmission reliability [24,25]. The adoption of diversity schemes not only proved to be a major cost-effective factor in improving the link reliability in FSO systems but also helps improve bandwidth capacity, quality factor (Q), and reduced latency [26][27][28].…”

Secrecy Analysis for Multi-Relaying RF-FSO Systems With a Multi-Aperture Destination

“…Milosevic et al in [12] studied the average BER for a FSO system with a multi-aperture receiver over Málaga (M)-distributed fading channel with pointing errors. The transmission laser selection and repetition coding schemes for a MIMO FSO system were evaluated in [13]. To reduce the impacts of turbulence-induced correlated channel, a polar-coded scheme with and without spatially correlated fading was proposed in [14].…”