# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 3 Experiment and Analysis
Each pixel was divided into two groups of 8-bit data and sent out successively. Each group of data has a fixed code to represent the address of the receiver. The repeater retransmits the data to different receiving targets according to different coded information. In Figure 12, the yellow signal represents the signal waveform at the transmitting end, the blue signal represents the signal waveform received by receiver A, the red signal represents the signal waveform received by receiver B, and the green signal represents the signal waveform received by receiver C. The modulation and demodulation of the signal are  
Figure 11: Test diagram of the free-space optical communication system. realized by a finite state machine. The data bit width transmitted by the transmitting end each time is 10 bits, where the first two bits represent the receiver address code, and the last eight bits represent the data of the picture. The repeater demodulates the signal at the transmitting end, selects the address routing, and automatically extracts the picture data and sends them to the designated receiving target. The receiving data at the receiving end are 8 bits in size. The address code of receiver A is 01, that of receiver B is 10, and that of receiver C is 11. 01_1101_110 represents the data sent by the transmitter in Figure 12, in which 01 represents the address information of receiver A. After the relay system recognizes the address information, the data 1101_1110 are sent to receiver A. At this time, the data received by receiver A do not contain address information. In the experiment, the cycle of PWM was \(T\) = 1.2 us, the pulse width time of characterization datum 1 was \(T_{1}\) = 0.4 us, and the pulse width time of characterization datum 0 was \(T_{2}\) = 0.8 us. Therefore, the transmitting end needed at least 1.2 us to send 1-bit data, and the receiving end needed at least 0.6 us to demodulate 1-bit data. Since the relay system needed to receive complete 10-bit data and send them to the receiving target through address judgment, the signal time difference between the transmitting end and the final receiving end was 11.975 us. Since the transmission power of the light source is 200 mW, the average output power of the PWM signal is:  
\[P_{\text{PWM}}=(\frac{T_{1}}{T}+\frac{T_{2}}{T})P_{\text{s}}=200\text{ mW} \tag{9}\]  
It takes 10 cycles to send 1 byte of data, and so the data transmission bandwidth is:  
\[BW_{\text{PWM}}=\frac{1}{10T}\approx 81.38\text{ KB/s} \tag{10}\]  
Reducing the pulse width of PWM can reduce the average output power. For example, \(T_{1}\) = 0.2 us, \(T_{2}\) = 0.4 us, then the average transmission power becomes 100 mW. However, during modulation, we needed to make the characteristics of datum 1 and datum 0 more distinctive--that is, make the pulse width difference between the narrow pulse and the wide pulse more obvious. Since the communication is performed in bad conditions, the too small pulse width signal will be demodulated after APD detection, which will increase the  
Figure 12: Test waveform of free-space optical communication system with point-to-multipoint. error probability, Therefore, we sacrificed the average transmission power and increased the pulse width to ensure the stability of the communication system. To verify the stability of the system, we gently moved the three receivers so that the transmission light source of the relay system was not directly aimed at the lens of the receiver. In this way, the pulse signal received by the receiver was not a signal with high energy, so the amplitude of the pulse received by the receiver could be controlled. In this way, we simulated poor communication conditions.