Performance Analysis of XQAM in Log-Normal Turbulence with Pointing Error and Non-zero Boresight

Abstract

In recent years, free-space optical (FSO) technology has gained fame in communication systems due to its high data rates and license-free feature. Cross quadrature amplitude modulation (X-QAM) is an optimum modulation scheme that uses odd bits per symbol and it has a low average symbol error rate (ASER) than rectangular quadrature amplitude modulation (R-QAM). In this paper, we theoretically investigate the performance of subcarrier cross quadrature amplitude modulation (SC-XQAM) corrupted by atmospheric turbulence in the presence of pointing errors (P.E) in FSO communications. We have considered boresight displacement, P.E effect, and atmospheric turbulence. To represent the atmospheric turbulence we consider Log-normal distribution that exhibits weak, moderate and strong weather effects. The P.E is employed using Rician distribution that incorporates boresight displacement. P.E is modeled using an optical beam radius and receiver aperture radius. The combination of these parameters will help enhance the average symbol error rate (ASER). Furthermore, we have derived an analytical expression that is used to develop numerical result. The ASER performance is observed against optical beam radius, P.E standard deviation and receiver aperture radius. In the end, the ASER performance is evaluated against SNR and Monte Carlo simulations are performed which validates the theoretical results.