# Tractable Approach to MmWaves Cellular Analysis with FSO Backhauling under Feedback Delay and Hardware Limitations

## TL;DR

This paper analyzes a mmWave cellular system with FSO backhauling, considering realistic channel fading, hardware impairments, and feedback delays, providing closed-form performance metrics and insights into system robustness.

## Contribution

It introduces a comprehensive analytical framework for mmWave-FSO systems accounting for hardware impairments, feedback delays, and complex fading, with novel closed-form expressions and bounds.

## Key findings

- Derived closed-form outage probability, error probability, and rate expressions.
- Analyzed the impact of HPA impairments on system performance.
- Provided high SNR asymptotic analysis for system robustness insights.

## Abstract

In this work, we investigate the performance of a millimeter waves (mmWaves) cellular system with free space optical (FSO) backhauling. MmWave channels are subject to Nakagami-m fading while the optical links experience the Double Generalized Gamma including atmospheric turbulence, path loss and the misalignment between the transmitter and the receiver aperture (also known as the pointing errors). The FSO model also takes into account the receiver detection technique which could be either heterodyne or intensity modulation and direct detection (IM/DD). Each user equipment (UE) has to be associated to one serving base station (BS) based on the received signal strength (RSS) or Channel State Information (CSI). We assume partial relay selection (PRS) with CSI based on mmWaves channels to select the BS associated with the highest received CSI. Each serving BS decodes the received signal for denoising, converts it into modulated FSO signal, and then forwards it to the data center. Thereby, each BS can be viewed as a decode-and-forward (DF) relay. In practice, the relay hardware suffers from nonlinear high power amplification (HPA) impairments which, substantially degrade the system performance. In this work, we will discuss the impacts of three common HPA impairments named respectively, soft envelope limiter (SEL), traveling wave tube amplifier (TWTA), and solid state power amplifier (SSPA). Novel closed-forms and tight upper bounds of the outage probability, the probability of error, and the achievable rate are derived. Capitalizing on these performance, we derive the high SNR asymptotes to get engineering insights into the system gain such as the diversity order.

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Source: https://tomesphere.com/paper/1905.11234