# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 2 Theory and System Design  
* Theory of optical communication based on pulse width modulation. When modulating and demodulating data, one needs to consider the symbol length of the data. If binary data are represented by M bits, this can represent 2M different types of data. If the M is larger, the probability of errors in data modulation and demodulation is greater, and the modulation and demodulation circuits are more complex. In order to build a free-space optical communication system more easily, we choose M = 1 to realize space optical communication. When M = 1, the symbol structure diagram of several free optical communication modulation methods is shown in Figure 1.  
For M = 1, the OOK method only characterizes datum 1 or datum 0 according to whether a pulse signal appears. The number of time slots in PIM mode is not fixed, and it is characterized by datum 1 or datum 0 according to the number of time slots. The LPPM method is characterized by datum 1 or datum 0 according to the different locations where a single pulse appears. The DPPM method is similar to the LPPM method, and is also characterized by datum 1 or datum 0 according to the different positions where a single pulse appears, the difference being that when the pulse signal in the DPMM method appears, it represents the end of a datum--that is, the rest of the time slots are all removed, so the number of time slots in the DPPM method is not fixed. The PAM method is characterized by datum 1 or datum 0 according to the amplitude of the pulse. The PWM method is characterized by datum 1 or datum 0 depending on the pulse width [38; 39; 40]. Further analysis of the performance of various modulation methods, assuming the transmission power of the light source, the transmission bandwidth of OOK, and the channel capacity of OOK, is performed according to the principle of optical communication modulation [41; 42; 43; 44; 45; 46], and the performance comparison histogram of the average transmission power, transmission bandwidth and channel capacity of various modulation modes when M = 1 is shown in Figure 2. For the data with symbol length M, the average symbol length, average transmit power, transmission bandwidth, channel capacity, and signal synchronization of the above optical communication modulation methods are further summarized according to Figure 2, as shown in Table 1.  
Figure 1: Symbol structure of several modulation modes of optical communication when M = 1 [37]. Figure 2: Performance histogram of several optical communication modulation methods when M = 1 [47; 48]. (**a**) Average power. (**b**) Transmission bandwidth. (**c**) Channel capacity. When M = 1, the PWM mode requires a lower average power than other modulation methods, although the transmission bandwidth of the PWM mode is large and the channel capacity is low, while the system studied in this paper is mainly used to verify whether the proposed relay scheme is feasible, and initially realize the basic free-space optical communication, so that the transmission bandwidth and channel capacity of PWM mode meet the communication requirements. Compared with the other five modulation methods mentioned, the PWM method only needs to perform slot synchronization, without frame synchronization (symbol synchronization)--that is, each datum to be demodulated does not need to wait for timing, and only needs to wait for the rising edge of the detection signal, meaning that the PWM method will greatly reduce the modulation and demodulation difficulty of the communication system, but also reduce the error probability of data transmission, and achieve more reliable and stable communication compared with other modulation methods that need frame synchronization.