# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 2 Theory and System Design
### Theory of laser relay station networking  
Figure 4 is a schematic diagram of the structure of the laser relay station networking system we constructed. The system consists mainly of a repeater and multiple transceivers. The repeater can simultaneously realize the integration of the transceiver in any direction on the same receiving level, and the transceiver can transmit data in any direction within 360\({}^{\circ}\) centered on the repeater, while the repeater can send data to other transceivers in any direction, without considering the high-precision alignment b  
Figure 3: Principle of pulse width modulation and demodulation [34]. and the final receiving end, and also avoid the signal interference problem when multiple devices in different positions are cross-laser communication. The transceiver can be placed anywhere around the repeater, the repeater is responsible for transmitting data, and the transceiver can send data to the repeater or receive the data forwarded by the repeater. When the transceiver transmits data, it first encodes the address of the data to be transmitted, specifies the position of the receiving end, and then modulates the encoded data. The data are sent in the form of a laser signal through laser secondary light (LD), realizing the conversion from a digital signal to an optical signal. In Figure 4, TX represents the laser signal sent by the transceiver to the repeater, which contains address information and data. The laser transceiver area in the repeater contains an avalanche photodiode and laser diode, which can be used for laser transmission and laser reception, realizing the function of the integrated transceivers. In the repeater, it is necessary to scan and control the photodetectors in all directions, so as to receive the laser signals in different directions. The repeater demodulates the received laser signal, parses the address information and data information, sends the data to the transceivers in different directions according to the address information, realizes the function of address routing, and also completes the conversion of the laser signal into a digital signal. Therefore, the data demodulation and re-modulation are completed in the repeater. TX represents the laser signal sent by the repeater to the transceiver. This signal is a re-modulated optical signal. Because they specify the specific direction of the receiver, the RX signals only contain data. When the transceiver is used as a receiver, it is only responsible for receiving the laser signal sent by the repeater and demodulating it. The demodulated data are the data that the transmitter wants to send to the target receiver transmitter, enabling laser communication between the transceivers in different directions. Figure 5 shows the internal structure of the laser relay station. The data routing module is responsible for selecting the data path and specifying the receiving and transmitting paths. The data forwarding module completes the data demodulation and re modulation, realizes the forwarding of the received data, and selects the forwarding path according to the data routing module. The laser transceiver area is composed of a lens array, an avalanche photodiode and a laser diode. The avalanche photodiode and laser diode are close to each other to achieve laser reception and transmission through an optical lens. If the divergence angle of the laser diode selected is 30\({}^{\circ}\), and the photosensitive area of the avalanche photodiode is 500 \(\upmu\)m, the placement angle of the lens array can be shown in Figure 5. Each lens in the lens array is closely arranged, and 360\({}^{\circ}\) data can be transmitted and received to further complete communication with transceivers in any direction. Figure 4: Schematic diagram of the structure of the laser relay station networking system. Figure 6 shows the data address routing scheme.