FSO-OFDM系统关键技术研究
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摘要
自由空间光通信(FSO)由于具有成本低、组网灵活、无需频率许可等优点,已成为当今信息技术的一大热点。但由于光色散以及电子器件速率的限制,使系统传输速率受限,误码性能不理想。为了充分发挥FSO的潜力,支持更高的信息传输速率和改善系统误码特性,必须采用频谱效率高、抗大气效应能力强的新型传输技术。在各种能提供高速率传输的无线解决方案中,正交频分复用(OFDM)技术最能代表自由空间光通信的发展方向。因此,本文将OFDM与FSO这两项较成熟的技术进行结合,并对FSO-OFDM系统亟待解决的理论和技术问题展开研究。
1、把OFDM技术引入到FSO系统中,给出了基于光偏振特性的基带FSO-OFDM模型和基于相干检测的频带FSO-OFDM模型,并对其信号处理过程进行了论证。
2、提出了一种新的基于时频码的FSO-OFDM系统模型,将空时编码的思想引入到OFDM系统,通过在发射端改变信号结构,增加信号冗余和在接收端进行信号合并来完成信号检测,可达到降低系统复杂度和改善系统误码特性的效果。
3、针对传统算法频偏估计范围小的缺点,在Schmidl&Cox和Minn算法的基础上,讨论了基于FH共轭对称序列的符号定时和频偏联合估计算法。该算法的定时尺度曲线仅在正确定时点处出现一个尖峰,大大提高了分辨率,从而可以更加准确的进行符号同步。同时通过合理设计训练序列的长度,可扩大频偏估计范围。
4、应用Mie散射理论,数值计算了雨粒子、雾粒子和沙尘粒子对光波传输的衰减,推导了在相应信道条件下FSO-OFDM系统的误符号率公式,并通过仿真实验验证了其正确性。结果表明,在散射信道中FSO-OFDM系统的性能主要取决于散射造成的光强度起伏方差,其次为散射造成的光强度衰减系数。
5、在对信道进行数值分析的基础上,特别研究了频率选择性衰落信道中的LMS自适应滤波的信道估计算法,该算法仅仅在学习阶段需要导频数据,而在跟踪阶段是利用反馈的判决信息进行信道估计,无需额外的导频数据信息,可以提高系统效率,并有较好的信道估计性能。
综上所述,论文对于FSO-OFDM系统实现中的若干关键问题进行了深入的研究,理论分析和仿真实验表明本文所提出的思路具有一定的创新性并能够获得较好的效果。
Due to the Free Space Optical(FSO) communication has the advantages of low cost, flexible networking, convenient installation and dispense with frequency permission, it has become a major technology in the present information technologies. But due to light dispersion and speed restriction of electronic devices the transfer rate is often limited and the performance is not satisfactory. In order to sufficient exploit the potentials of FSO to enhance the transfer rate and the bit error rate(BER) performance of the optical communication system, we have to adopt a new transmission technology which has high spectrum efficiency and high ability to combat the atmospheric scattering effect. Hence, the two mature technologies of OFDM and FSO are combined. But the FSO-OFDM system also has many theory and technology problems remain yet to be solved. In this paper, we focus on these problems.
Firstly, the OFDM technology is introduced into FSO. The baseband model based on polarization and the frequency model based on coherent detection are proposed in this paper. The signal processing of the two models are demonstrated.
Secondly, because of the baseband model can not make full use of hardware resource and the frequency model requests coherent detection, a new FSO-OFDM model based on time frequency code is proposed in order to avoid the disadvantages of the two models. This coding method is proposed by applying the space-time code to OFDM system, it can reduce the system complexity and improve the system BER performance by changing the signal structure, providing redundancy for receiving signals and combining signals at receiver.
Thirdly, because of the traditional algorithm's frequency deviation estimation range is limited, the symbol timing and frequency deviation joint estimation algorithm by adopting FH conjugate symmetrical sequence is proposed on the basic of Schmidl& Cox and Minn algorithm. The timing curve of this algorithm will present a spike pulse only at the right time, the resolution is greatly enhanced, so the symbol timing will be more accurate. The estimation of frequency deviation range will be expanded with rational training sequence length.
Fourthly, the energy attenuation caused by raindrop, fog drop and dust are calculate by applying the Mie theory. The symbol error rate formula of the FSO-OFDM system under these channel conditions is deduced. Then its correctness is verified through simulation. The results show that the FSO-OFDM system performance in scattering channel primarily depends on the variance of light intensity attenuation caused by scattering, and also depends on light intensity attenuation coefficient caused by scattering.
Fifthly,the LMS self-adapting filter channel estimation algorithm based on time domain training sequence is putted forward. As this algorithm utilizing the feedback signals to estimate in frequency selective fading channel, so the pilot signal is unnecessary, and present higher system efficiency and better channel estimated quality.
This paper makes a further research on the several key problems about the FSO-OFDM system listing as forward. And bring forward some new algorithms which achieves perfect effect testified both by theory analysis and by simulation experimentation.
1、把OFDM技术引入到FSO系统中,给出了基于光偏振特性的基带FSO-OFDM模型和基于相干检测的频带FSO-OFDM模型,并对其信号处理过程进行了论证。
2、提出了一种新的基于时频码的FSO-OFDM系统模型,将空时编码的思想引入到OFDM系统,通过在发射端改变信号结构,增加信号冗余和在接收端进行信号合并来完成信号检测,可达到降低系统复杂度和改善系统误码特性的效果。
3、针对传统算法频偏估计范围小的缺点,在Schmidl&Cox和Minn算法的基础上,讨论了基于FH共轭对称序列的符号定时和频偏联合估计算法。该算法的定时尺度曲线仅在正确定时点处出现一个尖峰,大大提高了分辨率,从而可以更加准确的进行符号同步。同时通过合理设计训练序列的长度,可扩大频偏估计范围。
4、应用Mie散射理论,数值计算了雨粒子、雾粒子和沙尘粒子对光波传输的衰减,推导了在相应信道条件下FSO-OFDM系统的误符号率公式,并通过仿真实验验证了其正确性。结果表明,在散射信道中FSO-OFDM系统的性能主要取决于散射造成的光强度起伏方差,其次为散射造成的光强度衰减系数。
5、在对信道进行数值分析的基础上,特别研究了频率选择性衰落信道中的LMS自适应滤波的信道估计算法,该算法仅仅在学习阶段需要导频数据,而在跟踪阶段是利用反馈的判决信息进行信道估计,无需额外的导频数据信息,可以提高系统效率,并有较好的信道估计性能。
综上所述,论文对于FSO-OFDM系统实现中的若干关键问题进行了深入的研究,理论分析和仿真实验表明本文所提出的思路具有一定的创新性并能够获得较好的效果。
Due to the Free Space Optical(FSO) communication has the advantages of low cost, flexible networking, convenient installation and dispense with frequency permission, it has become a major technology in the present information technologies. But due to light dispersion and speed restriction of electronic devices the transfer rate is often limited and the performance is not satisfactory. In order to sufficient exploit the potentials of FSO to enhance the transfer rate and the bit error rate(BER) performance of the optical communication system, we have to adopt a new transmission technology which has high spectrum efficiency and high ability to combat the atmospheric scattering effect. Hence, the two mature technologies of OFDM and FSO are combined. But the FSO-OFDM system also has many theory and technology problems remain yet to be solved. In this paper, we focus on these problems.
Firstly, the OFDM technology is introduced into FSO. The baseband model based on polarization and the frequency model based on coherent detection are proposed in this paper. The signal processing of the two models are demonstrated.
Secondly, because of the baseband model can not make full use of hardware resource and the frequency model requests coherent detection, a new FSO-OFDM model based on time frequency code is proposed in order to avoid the disadvantages of the two models. This coding method is proposed by applying the space-time code to OFDM system, it can reduce the system complexity and improve the system BER performance by changing the signal structure, providing redundancy for receiving signals and combining signals at receiver.
Thirdly, because of the traditional algorithm's frequency deviation estimation range is limited, the symbol timing and frequency deviation joint estimation algorithm by adopting FH conjugate symmetrical sequence is proposed on the basic of Schmidl& Cox and Minn algorithm. The timing curve of this algorithm will present a spike pulse only at the right time, the resolution is greatly enhanced, so the symbol timing will be more accurate. The estimation of frequency deviation range will be expanded with rational training sequence length.
Fourthly, the energy attenuation caused by raindrop, fog drop and dust are calculate by applying the Mie theory. The symbol error rate formula of the FSO-OFDM system under these channel conditions is deduced. Then its correctness is verified through simulation. The results show that the FSO-OFDM system performance in scattering channel primarily depends on the variance of light intensity attenuation caused by scattering, and also depends on light intensity attenuation coefficient caused by scattering.
Fifthly,the LMS self-adapting filter channel estimation algorithm based on time domain training sequence is putted forward. As this algorithm utilizing the feedback signals to estimate in frequency selective fading channel, so the pilot signal is unnecessary, and present higher system efficiency and better channel estimated quality.
This paper makes a further research on the several key problems about the FSO-OFDM system listing as forward. And bring forward some new algorithms which achieves perfect effect testified both by theory analysis and by simulation experimentation.
引文
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