In this paper, we propose a practical adaptive coding modulation scheme to approach the capacity of free-space optical (FSO) channels with intensity modulation/direct detection based on probabilistic shaping. The encoder efficiently adapts the transmission rate to the signal-to-noise ratio, accounting for the fading induced by the atmospheric turbulence. The transponder can support an arbitrarily large number of transmission modes using a low complexity channel encoder with a small set of supported rates. Hence, it can provide a solution for FSO backhauling in terrestrial and satellite communication systems to achieve higher spectral efficiency. We propose two algorithms to determine the capacity and capacity-achieving distribution of the scheme with unipolar M-ary pulse amplitude modulation (M-PAM) signaling. Then, the signal constellation is probabilistically shaped according to the optimal distribution, and the shaped signal is channel encoded by an efficient binary forward error correction scheme. Extensive numerical results and simulations are provided to evaluate the performance. The proposed scheme yields a rate close to the tightest lower bound on the capacity of FSO channels. For instance, the coded modulator operates within 0.2 dB from the M-PAM capacity, and it outperforms uniform signaling with more than 1.7 dB, at a transmission rate of 3 bits per channel use.
翻译:在本文中,我们提出一个实用的适应性编码调制方案,以在概率成形的基础上,以强度调制/直接检测的方式,接近自由空间光学频道的能力。编码器高效地将传输率调整到信号对噪音的比例,同时考虑到大气动荡引起的消退。转发器可以使用一套小节率的低复杂频道编码器支持任意大量的传输模式。因此,它可以为地面和卫星通信系统中的FSO回航提供一个解决方案,以实现更高的光谱效率。我们建议两种算法,以确定单极M-ary脉冲调制(M-PAM)信号的系统的能力和能力实现分布。然后,信号星座根据最佳分布而形成概率,而形状的信号信号是由一套高效的二进制前误差校正方案编码的频道。提供了广泛的数字结果和模拟,以评价性能。拟议办法在FSOA3的信号传输速率中,在FSO3-PM中,在BSA型的信号传输能力上,在BSA+M的频率上运行。