Average BER performance of FSO SIM-QAM systems in the presence of atmospheric turbulence and pointing errors

Abstract

This paper presents the exact average bit error rate (BER) analysis of the free-space optical system employing subcarrier intensity modulation (SIM) with Gray-coded quadrature amplitude modulation (QAM). The intensity fluctuations of the received optical signal are caused by the path loss, atmospheric turbulence and pointing errors. The exact closed-form analytical expressions for the average BER are derived assuming the SIM-QAM with arbitrary constellation size in the presence of the Gamma-Gamma scintillation. The simple approximate average BER expressions are also provided, considering only the dominant term in the finite summations of obtained expressions. Derived expressions are reduced to the special case when optical signal transmission is affected only by the atmospheric turbulence. Numerical results are presented in order to illustrate usefulness of the derived expressions and also to give insights into the effects of different modulation, channel and receiver parameters on the average BER performance. The results show that the misalignment between the transmitter laser and receiver detector has the strong effect on the average BER value, especially in the range of the high values of the average electrical signal-to-noise ratio.

Performance study of terrestrial multi-hop OFDM FSO communication systems with pointing errors over turbulence channels

“…Here, either the Gamma-Gamma or the Gamma distribution is used. The former has been proven that models accurately enough weak-to-strong turbulence conditions, while the latter provides a less complicated model and additionally, it has been demonstrated that is accurate enough, mainly for weak turbulence conditions, [3,9,12,15,19,23,26,27,[30][31][32][33]. It should be mentioned here that the Gamma distribution has been studied and gives very accurate results, especially, for satellites' optical up/downlinks under weak turbulence conditions, [26,27,33].…”

“…In order to investigate the scintillation's influence on the performance variations, many statistical models have been proposed. One of them is the Gamma-Gamma distribution which has been proved that is accurate enough for weak-to-strong turbulence conditions, [3,9,12,19,23,[30][31][32]. Another, accurate one, mainly for weak turbulence conditions, is the Gamma model which has the additive, important advantage that is given by a not very complicated mathematical expression compared with others distributions' expressions, [26,27,33].…”

“…(3) P r (t) = P t (t)L tot I + n(t) [9,12,13,15,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Here, either the Gamma-Gamma or the Gamma distribution is used.…”

“…building sway, strong wind, earthquakes, etc. More specifically, taking into account that the transverse radius of the optical beam at the receiver is relatively small, these minor movements of the trans-receiver system cause the pointing errors effect which will be taken into account for the estimation of system's performance, below, [9,19,25,[33][34][35][36][37].…”

“…The raw data are mapped according to the selected modulation format which in this work is the M-QAM scheme, [12,18,19]. Next, the OFDM signal, s OFDM (t), modulates the optical intensity of the transmitter's LD and its nonlinear responsivity can be modelled by nonlinear expressions or series [10,12].…”