# Performance analysis of RF-FSO multi-hop networks

## TL;DR

This paper analyzes the performance of multi-hop RF-FSO networks, deriving outage probability expressions and evaluating the impact of various parameters, demonstrating HARQ's effectiveness in improving reliability and energy efficiency.

## Contribution

It provides closed-form outage probability expressions for RF-FSO multi-hop networks and explores the benefits of HARQ in enhancing performance and robustness.

## Key findings

- HARQ significantly reduces required power for target outage probability.
- Parameter settings can map RF-FSO performance to RF-only or FSO-only networks.
- RF-FSO networks show high efficiency under various conditions.

## Abstract

We study the performance of multi-hop networks composed of millimeter wave (MMW)-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in the cases with and without hybrid automatic repeat request (HARQ). Taking the MMW characteristics of the RF links into account, we derive closed-form expressions for the network outage probability. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we present mappings between the performance of RF-FSO multi-hop networks and the ones using only the RF- or the FSO-based communication, in the sense that with appropriate parameter settings the same outage probability is achieved in these setups. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, the HARQ can effectively improve the outage probability/energy efficiency, and compensate the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability 10^{-4} and code rate 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to the cases with no HARQ, by 13 and 17 dB, respectively.