大气激光通信中正交频分复用技术研究
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摘要
大气激光通信(Atmospheric Laser Communication, ALC)是一种以大气为传输媒介,利用激光作为信号载体来实现点对点、点对多点和多点对多点间语音、数据、图像信息双向传输的通信技术,它在解决目前宽带网络通信中“最后一公里”问题和应急通信等方面有着良好的应用前景。
在大气激光通信链路中,由于受到各种恶劣天气的影响,接收到的光信号能量会得到衰减。此外,大气湍流还会引起光信号强度和相位的随机起伏,导致系统误码率性能下降。因此,研究高性能的调制解调技术和信道编码技术已成为大气激光通信的重要课题之一。论文主要从以下几个方面进行了研究：
1、目前正交频分复用(OFDM)技术并没有成功地运用到大气激光通信领域中,这主要是因为大气激光通信系统采用的是光强度调制,只能传送单极性实信号。鉴于这种情况,本文采用了一种非对称限幅光正交频分复用(Asymmetrically Clipped Optical OFDM, ACO-OFDM)技术,并把它运用到大气激光通信系统中。在大气湍流信道下,对该调制方案的性能进行了仿真研究,并和传统的光强度调制技术进行了性能比较。仿真结果表明在大气湍流(湍流强度为σsc2=0.2)环境下,非对称限幅光OFDM系统性能要分别比直流偏置光OFDM系统和OOK系统提高了4dB和6dB,并且它也是目前光功率效率最高的调制技术。
2、在大气激光通信系统中,较高的峰值平均功率比(PAPR)不仅会对系统性能带来影响,还会对人眼或皮肤造成伤害。针对降低非对称限幅光OFDM系统高PAPR的问题,本文对目前已经提出的降低无线OFDM系统PAPR的方法进行了研究,结果发现所有这些方法都不能直接运用到非对称限幅光OFDM系统中,因此本文对这些方法进行了结构上的改进,并对其性能进行了研究仿真。此外,在综合现有算法优点的基础上,本文还提出了一种新的降低非对称限幅光OFDM系统PAPR的联合算法,通过仿真发现该联合算法可以获得近5dB的PAPR1性能改善,很好地降低了非对称限幅光OFDM系统的PAPR。
3、由于非对称限幅光OFDM系统采用的是强度调制／直接检测,因此它与无线OFDM系统不同,系统只需要符号同步而不需要频率同步。根据非对称限幅光OFDM符号的特点,本文提出了一种适合于非对称限幅光OFDM系统的定时同步方法。与目前存在的定时同步方法相比,该定时同步方法的性能不依赖于训练符号的选取,并且训练符号可以随机生成。并在各种信道环境下对训练符号的自相关性进行了仿真研究,仿真结果表明该定时同步算法无论在向正确定时点的收敛速度上还是定时估计方差上,都要优于其它算法,并且在定时估计方差上可以得到近5dB的性能改善,大大提高了非对称限幅光OFDM系统定时估计精度。
4、LDPC码是一种基于稀疏矩阵的线性分组码,其性能可以比Turbo码更逼近香农限。因此,本文将LDPC码作为信道编码应用到了大气激光通信系统中,并提出了将LDPC码和非对称限幅光OFDM强度调制相结合的系统方案,并在大气湍流信道中对该方案进行了仿真研究。仿真结果表明LDPC码具有优越的纠错性能,该方案对大气湍流引起的光强闪烁具有很强的抗干扰能力,可以满足大气激光通信系统的要求。此外,在相同误码率性能条件下,LDPC码还降低了平均发射光功率,这对平均发射光功率严格受限的大气激光通信无疑是至关重要的。
Atmospheric laser communication is a point to point and multipoint-to-multipoint communication technology, which transmits voice, data, image information in atmosphere by the use of laser as signal carrier. In recent years, this technology has received noticeable attention due to a variety of applications in overcoming the last-mile problem and emergency communications field.
In atmospheric laser communication links, the received light signal energy is attenuated due to a variety of bad weather, and atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. Therefore, the study of high performance modulation and channel coding technology for the atmospheric laser communication has become an important issue. All efforts are centered as follows.
1、Orthogonal Frequency Division Multiplexing (OFDM) has not been used in practical atmospheric laser communication system. This is because OFDM signals are bipolar, while in atmospheric laser communication system that use intensity modulation (IM), only unipolar signals can be transmitted. Basing on this situation, we adopt asymmetrically clipped optical OFDM technology and apply it to the atmosphere laser communication system. For the case of an atmospheric turbulence channel, we compare asymmetrically clipped optical OFDM modulation and conventional optical intensity modulation techniques, and simulation results shows that the performance of asymmetrically clipped optical OFDM system has up to 4dB and 6dB improvement than DC biased optical OFDM and OOK in atmospheric turbulence (turbulence intensityσsc2= 0.2) channel. It has recently been shown that asymmetrically clipped optical OFDM is more efficient in terms of optical power than other optical modulation techniques.
2、In atmospheric laser communication system, the high peak-to-average power ratio (PAPR) will not only have an impact on system performance, but also have damage to the human eye or skin. To solve high PAPR of asymmetrically clipped optical OFDM system, some solutions that have been proposed for wireless OFDM system have been studied. But all these methods can not be directly applied to the asymmetrically clipped optical OFDM system, because the asymmetrically clipped optical OFDM system signals are real. In this paper, these methods have been adapted, and performance of these methods is analyzed. In addition, basing on the advantages of existing algorithms, a new joint algorithm is presented. Simulations results show that the joint algorithm can achieve near 5dB performance improvement, and can reduce the PAPR of asymmetrically clipped optical OFDM system efficiently.
3、In asymmetrically clipped optical OFDM, unlike RF-OFDM, no frequency synchronization is required because the signals used in asymmetrically clipped optical OFDM are intensity modulated. In this paper a new timing synchronization method designed for the characteristics of asymmetrically clipped optical OFDM is presented. Compared with existing synchronization methods, the performance of proposed timing synchronization method does not depend on the selection of the training symbols and training symbols can be randomly generated. The autocorrelation properties of the training symbol are studied in various channel conditions and simulation results show that the proposed method in terms of the convergence rate to right timing point or the variance on the timing estimation, is better than other algorithms, and the variance on the timing estimation can be close to 5dB performance improvement. Analysis and simulation results show that the new timing synchronization method is effective in asymmetrically clipped optical OFDM system.
4、Low Density Parity Check Codes are linear block codes basing on sparse matrix, and the performance of LDPC is more near Shannon limit than turbo codes. Therefore, LDPC as channel coding is applied into the atmospheric communication system, and a new program in which LDPC is combined with asymmetrically clipped optical OFDM intensity modulation is proposed. The new program is simulated in the atmospheric turbulence channel. The simulation results show that LDPC codes have excellent error correction capabilities. The above scheme has strong anti-interference ability to light intensity fluctuation caused by atmospheric turbulence and can satisfy the need of atmospheric laser communication system. Under the same BER performance, LDPC can reduce the average transmitted optical power, which is very important in atmospheric laser communication system where the average optical power is strictly limited.
在大气激光通信链路中,由于受到各种恶劣天气的影响,接收到的光信号能量会得到衰减。此外,大气湍流还会引起光信号强度和相位的随机起伏,导致系统误码率性能下降。因此,研究高性能的调制解调技术和信道编码技术已成为大气激光通信的重要课题之一。论文主要从以下几个方面进行了研究：
1、目前正交频分复用(OFDM)技术并没有成功地运用到大气激光通信领域中,这主要是因为大气激光通信系统采用的是光强度调制,只能传送单极性实信号。鉴于这种情况,本文采用了一种非对称限幅光正交频分复用(Asymmetrically Clipped Optical OFDM, ACO-OFDM)技术,并把它运用到大气激光通信系统中。在大气湍流信道下,对该调制方案的性能进行了仿真研究,并和传统的光强度调制技术进行了性能比较。仿真结果表明在大气湍流(湍流强度为σsc2=0.2)环境下,非对称限幅光OFDM系统性能要分别比直流偏置光OFDM系统和OOK系统提高了4dB和6dB,并且它也是目前光功率效率最高的调制技术。
2、在大气激光通信系统中,较高的峰值平均功率比(PAPR)不仅会对系统性能带来影响,还会对人眼或皮肤造成伤害。针对降低非对称限幅光OFDM系统高PAPR的问题,本文对目前已经提出的降低无线OFDM系统PAPR的方法进行了研究,结果发现所有这些方法都不能直接运用到非对称限幅光OFDM系统中,因此本文对这些方法进行了结构上的改进,并对其性能进行了研究仿真。此外,在综合现有算法优点的基础上,本文还提出了一种新的降低非对称限幅光OFDM系统PAPR的联合算法,通过仿真发现该联合算法可以获得近5dB的PAPR1性能改善,很好地降低了非对称限幅光OFDM系统的PAPR。
3、由于非对称限幅光OFDM系统采用的是强度调制／直接检测,因此它与无线OFDM系统不同,系统只需要符号同步而不需要频率同步。根据非对称限幅光OFDM符号的特点,本文提出了一种适合于非对称限幅光OFDM系统的定时同步方法。与目前存在的定时同步方法相比,该定时同步方法的性能不依赖于训练符号的选取,并且训练符号可以随机生成。并在各种信道环境下对训练符号的自相关性进行了仿真研究,仿真结果表明该定时同步算法无论在向正确定时点的收敛速度上还是定时估计方差上,都要优于其它算法,并且在定时估计方差上可以得到近5dB的性能改善,大大提高了非对称限幅光OFDM系统定时估计精度。
4、LDPC码是一种基于稀疏矩阵的线性分组码,其性能可以比Turbo码更逼近香农限。因此,本文将LDPC码作为信道编码应用到了大气激光通信系统中,并提出了将LDPC码和非对称限幅光OFDM强度调制相结合的系统方案,并在大气湍流信道中对该方案进行了仿真研究。仿真结果表明LDPC码具有优越的纠错性能,该方案对大气湍流引起的光强闪烁具有很强的抗干扰能力,可以满足大气激光通信系统的要求。此外,在相同误码率性能条件下,LDPC码还降低了平均发射光功率,这对平均发射光功率严格受限的大气激光通信无疑是至关重要的。
Atmospheric laser communication is a point to point and multipoint-to-multipoint communication technology, which transmits voice, data, image information in atmosphere by the use of laser as signal carrier. In recent years, this technology has received noticeable attention due to a variety of applications in overcoming the last-mile problem and emergency communications field.
In atmospheric laser communication links, the received light signal energy is attenuated due to a variety of bad weather, and atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. Therefore, the study of high performance modulation and channel coding technology for the atmospheric laser communication has become an important issue. All efforts are centered as follows.
1、Orthogonal Frequency Division Multiplexing (OFDM) has not been used in practical atmospheric laser communication system. This is because OFDM signals are bipolar, while in atmospheric laser communication system that use intensity modulation (IM), only unipolar signals can be transmitted. Basing on this situation, we adopt asymmetrically clipped optical OFDM technology and apply it to the atmosphere laser communication system. For the case of an atmospheric turbulence channel, we compare asymmetrically clipped optical OFDM modulation and conventional optical intensity modulation techniques, and simulation results shows that the performance of asymmetrically clipped optical OFDM system has up to 4dB and 6dB improvement than DC biased optical OFDM and OOK in atmospheric turbulence (turbulence intensityσsc2= 0.2) channel. It has recently been shown that asymmetrically clipped optical OFDM is more efficient in terms of optical power than other optical modulation techniques.
2、In atmospheric laser communication system, the high peak-to-average power ratio (PAPR) will not only have an impact on system performance, but also have damage to the human eye or skin. To solve high PAPR of asymmetrically clipped optical OFDM system, some solutions that have been proposed for wireless OFDM system have been studied. But all these methods can not be directly applied to the asymmetrically clipped optical OFDM system, because the asymmetrically clipped optical OFDM system signals are real. In this paper, these methods have been adapted, and performance of these methods is analyzed. In addition, basing on the advantages of existing algorithms, a new joint algorithm is presented. Simulations results show that the joint algorithm can achieve near 5dB performance improvement, and can reduce the PAPR of asymmetrically clipped optical OFDM system efficiently.
3、In asymmetrically clipped optical OFDM, unlike RF-OFDM, no frequency synchronization is required because the signals used in asymmetrically clipped optical OFDM are intensity modulated. In this paper a new timing synchronization method designed for the characteristics of asymmetrically clipped optical OFDM is presented. Compared with existing synchronization methods, the performance of proposed timing synchronization method does not depend on the selection of the training symbols and training symbols can be randomly generated. The autocorrelation properties of the training symbol are studied in various channel conditions and simulation results show that the proposed method in terms of the convergence rate to right timing point or the variance on the timing estimation, is better than other algorithms, and the variance on the timing estimation can be close to 5dB performance improvement. Analysis and simulation results show that the new timing synchronization method is effective in asymmetrically clipped optical OFDM system.
4、Low Density Parity Check Codes are linear block codes basing on sparse matrix, and the performance of LDPC is more near Shannon limit than turbo codes. Therefore, LDPC as channel coding is applied into the atmospheric communication system, and a new program in which LDPC is combined with asymmetrically clipped optical OFDM intensity modulation is proposed. The new program is simulated in the atmospheric turbulence channel. The simulation results show that LDPC codes have excellent error correction capabilities. The above scheme has strong anti-interference ability to light intensity fluctuation caused by atmospheric turbulence and can satisfy the need of atmospheric laser communication system. Under the same BER performance, LDPC can reduce the average transmitted optical power, which is very important in atmospheric laser communication system where the average optical power is strictly limited.
引文
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