Analysis of space diversity method in modulating retro-reflector optical communication

Overview: Free space optical communications (FSO) has become a hot research topic in the field of optical communications. Compared with the traditional optical fiber communication, the main characteristic of wireless optical communication is to get rid of the optical fiber signal, which can be used in some special occasions. Generally speaking, the advantages of FSO are as follows: the frequency of the optical signal is high enough to avoid interference by the surrounding electromagnetic waves; Enough capacity to transmit at speeds up to a gigabit per second; It has a separate communication band and does not occupy the existing band. The traditional FSO needs to load the laser transmitting/receiving system at both ends, and the complex tracking system needs to be built, which increases the weight, volume, power consumption and technical complexity of the system. Thus they become an important factor restricting the application of FSO. The modulating retro-reflector (MRR) system can eliminate the laser transmitter and the tracking system of a terminal in the communication link. As a consequence; it greatly reduces the weight, volume and power consumption of one end of the link and effectively solves the application limitation of FSO. However, the MRR system is more vulnerable to the atmospheric turbulence than the FSO system. Although so far, there are many literatures on efficiently reducing the atmospheric turbulence, most of them are based on the traditional FSO communication system. In this paper, we build the MRR laser atmospheric propagation model under weak turbulence by using the power spectrum inversion method. Scintillation index is a physical quantity describing the degree of turbulence in the atmosphere. The numerical analyses show that under the same condition and for the diversity of the reverse modulation echo reflection link at a propagation distance of 1000 m, the scintillation index increases continuously as the distance increases, and decreases as the number of sub-sets increases. When the number of modulators is four, the scintillation index is similar to that of the ring-shaped reverse modulator. The more diverse is, the greater the cost is. Therefore, within a kilometer range, it is an ideal solution to select four inverse modulators. With simulation of the four parts diversity model, it is found that with the increase of the distance between antennas, the amplitude of the scintillation index decreases. It is also suggested that increasing the aperture of the reverse-end-modulated end-diversity antenna can also reduce the system scintillation index.