Maximum Spectral Efficiency With Adaptive MQAM Transmissions Over Terrestrial Coherent FSO Links

TL;DR

This paper derives the spectral efficiency limit for adaptive MQAM over terrestrial FSO links affected by turbulence and pointing errors, demonstrating that six constellation adaptive schemes nearly reach this theoretical maximum.

Contribution

It provides the first theoretical analysis of adaptive MQAM spectral efficiency over gamma-gamma turbulence channels with pointing errors, showing near-optimal performance with only six constellations.

Findings

Adaptive MQAM approaches the spectral efficiency limit within 0.12 bits/s/Hz.

Six square MQAM constellations achieve near-optimal performance across various SNRs.

Theoretical analysis guides practical adaptive modulation design for FSO links.

Abstract

Coherent free-space optical (FSO) communication is recognized as a key enabler for ultra-high-capacity fronthaul and backhaul links in next-generation wireless networks. Spectrally efficient $M$-ary quadrature amplitude modulation (MQAM) formats are well-suited for these links. However, theoretical analyses of adaptive MQAM transmissions over terrestrial FSO channels remain limited. In this letter, we first derive the spectral efficiency limit of adaptive unconstrained MQAM over gamma-gamma turbulence with pointing error. We then show that adaptive transmissions using only six square MQAM constellations performs close to the theoretical limit (within $0.10$-$0.12$ bits/s/Hz) across a wide range of signal-to-noise ratios and channel conditions.