Study On Orthogonal Frequency Division Multiplexing Techniques For Atmospheric Laser Communication

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.