偏振FSO-OFDM系统中的LMS信道估计

LMS Channel Estimation for Polarimetric FSO-OFDM System

本文首先设计了一种基于光偏振的基带FSO-OFDM系统模型,阐述了其发射和接收部分光偏振调制和解调工作原理,介绍了大气散射信道模型。由于大气散射信道为频域选择性衰落信道,在此信道中,每个子载波的信道冲激响应在相干时间内的值保持相关性,因此本文利用同一子载波中距离小于相干时间的连续若干个OFDM符号信道特性之间的相关性,采用LMS算法对未来符号位置迭代的初始值进行预测,再进行迭代得到信道冲击响应值。在本系统中,水平和垂直线偏振光经过的光路相同,因此信道特性基本一致,最后再通过对两路信道估计值进行加权,得到更加准确的信道预测值。通过仿真进行验证,结果表明,在散射信道中(最小均方)LMS算法可以很好的对信道进行跟踪,同时系统误码率可得到大约2dB的改善。

The FSO-OFDM （Free Space Optical Communication-Orthogonal Frequency Division Multiplexing） model based on light polarization is designed. We expatiated on the principles of the transceivers and the scatter channel. As the scatter channel is a frequency selective fading channel, each subchannel impulse response keeps correlation during coherent time. The correlation of the same subchannel impulse response is utilized during seriate OFDM symbols and Linear Mean Squared （LMS） algorithm is adopted to estimate the iterative initial value of the future symbols, then to get the channel impulse response. The horizontal and vertical polarization light paths are the same, so the characteristics of atmosphere channel are the same. At last, the more accurate channel impulse response is gotten after weighting the two estimative channel impulse responses. The simulation results indicate that the LMS algorithm can track the channel and get 2dB improvement of system BER.