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Abstract
Probabilistic shaping (PS) is a promising technique to approach the Shannon limit. In this paper, we design a practical coded modulation scheme based on PS to improve the capacity of coherent free-space optical (FSO) links with quadrature amplitude modulation (QAM), where the fading channel follows the Gamma-Gamma distribution. The aim of this paper is to optimize the probability mass function (PMF) of the QAM signal points to achieve the maximum channel capacity. Due to the complexity of the objective function, the heuristic algorithm was employed to solve the optimization problem. To the best of the authors’ knowledge, the closed-form pairwise error probability (PEP) is first derived with the non-uniform signals under the turbulence channel. In addition, we measure the average symbol error rate (SER) and post-FEC bit error rate (BER) by the Monte Carlo simulation method. The numerical simulation results of both capacity and BER show that the proposed PS scheme is better than the uniform distribution. The post-FEC BER results show that the proposed PS scheme provides significant gains compared with the uniform scheme.
Highlights
Free-space optical (FSO) communication has attracted more and more attention due to its large capacity, high security, high data rates and license-free operation [1,2,3]
We can see that the uniform distribution binary sequence is divided into two parts, one of which is fed to the constant composition distribution matching (CCDM)
The probabilistic shaping (PS) technique is investigated for a coherent FSO communication system
Summary
Free-space optical (FSO) communication has attracted more and more attention due to its large capacity, high security, high data rates and license-free operation [1,2,3]. In order to confront this trend, the FSO communication system needs to continuously increase its capacity to provide higher performance. Compared with the intensity modulation and direct detection (IM/DD) systems, the coherent free space optical communication provides impressive performance enhancements, for instance, the quadrature amplitude modulation (QAM), even though their implementation is more complex. The gap between the capacity of an FSO system and Shannon limit still exists. The emergence of PS provides enhanced flexibility for FSO systems, without increasing their complexity. PS changes the probabilities of the points based on off-the-shelf constellations, incurring no additional complexity in system design and implementation [8,9,10]
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