.In free space optical (FSO) communications systems, the signal fading caused by atmospheric turbulence is a slow fading, so adaptive transmission technology can effectively mitigate the problem of channel fading. Considering the combined effects of turbulence, attenuation, and pointing errors on the atmospheric transmission of laser signals, we proposed an adaptive M-ary phase shift keying subcarrier modulation algorithm based on power control and studied the combined adaptation of modulation size and average transmit power. Karush-Kuhn-Tucker conditions are used to solve the optimization problem of maximizing the throughput under the constraint of average transmit power, the expressions of throughput and outage probability are derived based on Meijer G-function. We finally evaluated the effects of turbulence intensity, beam waist radius, jitter standard deviation, weather conditions, and maximum modulation order on the throughput and outage probability of adaptive system.
Free space optical system design and performance analysis are highly related to channel statistics obtained by the channel parameter estimation. The accurate estimation of the channel parameters and scintillation index needs to consider the photoelectric detection noise at the receiving end. We propose a maximum likelihood (ML) method for estimating the parameters of a gamma–gamma fading channel affected by photoelectric detection noise. The Newton–Raphson method and expectation maximization (EM) algorithm are developed to compute the ML estimates of atmospheric turbulence fading parameters and variance of the detection noise. We also derive the Cramer–Rao bound for the unknown parameters. By way of the mean square estimation errors, our estimation technique performance is compared with existing methods of the estimation which ignore detection noise. Based on the measured channel and detection noise data under three weather conditions, it is verified that the proposed EM algorithm, considering the influence of detection noise, can significantly improve the estimation accuracy of atmospheric turbulence fading parameters.
In this paper, we propose a dynamic adaptive multiple input multiple output (MIMO) free space optical (FSO) system with spatial mode switching. Based on the channel conditions, the proposed system can select different MIMO transmission modes: diversity, multiplexing or hybrid mode that yields the maximum average channel capacity while satisfying the reliable average bit error rate (BER). Considering the combined effects of path loss, pointing error and atmospheric turbulence-induced fading modeled by Malaga distribution, we derive the closed-form expressions of average channel capacity and average BER of MIMO system for each transmission mode. From adaptive system performance evaluated results, we establish the signal-tonoise ratio (SNR) threshold look-up table (LUT) in different scenarios to select the optimal transmission mode. The extensive numerical results demonstrate that the proposed adaptive system has a significant improvement in average channel capacity compared with each stand-alone mode system. Finally, Monte Carlo simulation is also provided to verify the correctness of the proposed adaptive scheme and conclusions.
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