Adaptive Subcarrier PSK Intensity Modulation in Free Space Optical Systems

TL;DR

This paper introduces an adaptive modulation technique for free space optical systems using subcarrier PSK, which dynamically adjusts to atmospheric turbulence to optimize spectral efficiency without increasing power or compromising BER.

Contribution

It presents a novel adaptive S-PSK modulation method for FSO systems that enhances spectral efficiency under turbulence conditions, including its application to MIMO configurations.

Findings

Significant spectral efficiency gains in moderate-to-strong turbulence.

Adaptive modulation maintains BER requirements without extra power.

Enhanced performance with MIMO configurations as the number of apertures increases.

Abstract

We propose an adaptive transmission technique for free space optical (FSO) systems, operating in atmospheric turbulence and employing subcarrier phase shift keying (S-PSK) intensity modulation. Exploiting the constant envelope characteristics of S-PSK, the proposed technique offers efficient utilization of the FSO channel capacity by adapting the modulation order of S-PSK, according to the instantaneous state of turbulence induced fading and a pre-defined bit error rate (BER) requirement. Novel expressions for the spectral efficiency and average BER of the proposed adaptive FSO system are presented and performance investigations under various turbulence conditions and target BER requirements are carried out. Numerical results indicate that significant spectral efficiency gains are offered without increasing the transmitted average optical power or sacrificing BER requirements, in moderate-to-strong turbulence conditions. Furthermore, the proposed variable rate transmission technique is applied to multiple input multiple output (MIMO) FSO systems, providing additional improvement in the achieved spectral efficiency as the number of the transmit and/or receive apertures increases.