# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 1 Introduction  
With the development and progress of science and technology, space laser communication technology has gradually expanded from satellites to the ground. At present, laser communication technology has been studied in the fields of lidar, the military, and underwater communication [1; 2; 3]. Space laser communication facilitates the exchange of information between two devices without any connection relationship in free-space. During laser communication, the laser transmitting end and the receiving end need to be aligned with high accuracy, and there should be no obstacles blocking the optical path between the two ends; otherwise, data transmission will fail. At the same time, point-to-point laser communication cannot complete point-to-multipoint reception. This greatly increases the difficulty of using laser communication technology in cities and in vehicle-mounted lidars. Therefore, laser communication technology is not widely used in life at present [4; 5; 6]. Compared with traditional wireless communication, laser communication does not require complex antennas and frequency applications. At the same time, it has the advantages of large transmission capacity, long transmission distance, good confidentiality, and strong anti-interference ability. Therefore, laser communication technology has broad application prospects in the field of wireless communication [7; 8; 9]. Realizing data and information transfer of free-space optical communication and information transmission of one-to-many and many-to-many communication devices is a necessary condition for the application of free-space optical communication technology. Therefore, the research on free-space optical communication networking technology has important research significance for the wider use of optical communication [10; 11; 12; 13]. In the application of a free-space optical communication system, the atmospheric turbulence effect is an important problem to consider [14], due to the changes in wind speed, the unevenness of the sun's radiation energy on the ground and other factors which cause local atmospheric density changes and produce vortexes in the atmosphere. Various vortex superpositions produce atmospheric turbulence, which greatly reduces the communication distance of free-space optical communication and damages the free-space optical communication link between the satellite and the ground, resulting in the attenuation of laser signals. Increasing the gain of the signal at this point does not necessarily improve the quality of the laser beam [15]. However, adaptive optics can effectively improve the effects of atmospheric turbulence or scattering and increase the speed of communication systems [16; 17]. Adaptive optics technology is a correction module that adds a conjugate with a distortion phase in front of the communication terminal, thereby suppressing the atmospheric turbulence effect, which is of great significance for the realization of high-rate laser communication such as satellite-ground communication, air-ground communication, and underwater optical communication [18; 19]. Free-space optical communication technology needs to load the information to be transmitted onto the laser light wave and send the data in the form of a light wave, which involves converting the data into optical signals (signal modulation) [20; 21]. At present, the modulation methods of space laser communication mainly include switch keying (OOK) modulation, pulse interval modulation (PIM), single pulse position modulation (LPPM), differential pulse position modulation (DPPM), pulse amplitude modulation (PAM), etc. [22; 23; 24; 25]. Among these modulation modes, the OOK mode has the simplest modulation principle, but its average transmission power is large. The number of timeslots included in each symbol of PIM mode is not fixed, and it has a high channel capacity, but its intersymbol interference is large, and its bit error rate is high. The LPPM mode has a fast transmission rate and low requirements for energy, but it requires high peak power.