Space Time Trellis Coding For Atmosphere Laser Communication

As a new communication mode, atmospheric laser communication has the advantages of large capacity, high security, without limitation of spectrum. However, atmosphere as the transmission medium in atmospheric laser communication. The atmospheric turbulence will cause optical signal intensity and phase fluctuation, which leads to the increase of bit error rate. This has become a bottleneck of limiting its development.Multiple input and multiple output(MIMO) can increase the channel capacity of the system, and an effective method to realize the advantage of MIMO technology is space time code. Common space time code have space time block code, layered space-time code and space time trellis code. Compared with the previous two, space time trellis codes have large coding gain and high transmission rate, which was introduced to atmospheric laser communication system, and used to improve the performance of communication system. However, traditional space time trellis code(STTC) due to complex and negative issues, cannot be directly used for optical communication systems. Therefore, according to intensity modulation and direct detection(IM/DD) method to study the optical space time trellis code, detailed as follows:(1) According to the characteristics of optical communication, a low-complexity optical space time trellis code is proposed. The scheme is based on the idea of delay diversity at the transmitter, and establishes the correlation of signals in time and space. The feedback interface cancellation algorithm(FICA) is used at the received, the maximum likelihood decision is used to decode the eliminate interference signal. The simulation results show that when diversity gain is the same,the decoding complexity of the proposed scheme is reduced by 95.84% and 93.75% respectively compared with the Viterbi decoding algorithm, and the deterioration of the BER performance is only 3 d B and 4.77 d B. The excellent performance of this scheme reduce the complexiy of decoding by sacrificing the performance of transmit diversity.(2) Conventional space time trellis code due to complex and negative signal, cannot be directly transmitted in IM/DD link. Therefore, using the IFFT symmetry,by adding a direct-current(DC) component, a DC component space time trellis scheme was proposed. Results show that the proposed scheme can not only improve the efficiency of data transmission, but also can effectively resist the scintillation effect of atmospheric turbulence, and the link BER performance improves with the increase of antenna numbers.(3) Although the DC component space time trellis code can improve the performance of optical communication, but its optical power efficiency is poor. In order to improve optical power efficiency, by changing the mapping method of encoding signal, make the real signal of IFFT output have odd harmonic symmetry, and combined with the clipped technology cut to negative signal. A suitable M-PSK modulation asymmetrically clipped space time trellis code scheme was proposed.The results show that the BER performance is better than that of the DC component space time trellis code, for example in the system with two transmit antennas, the performance is 3.8d B better than the latter DC component space time trellis code.