# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 2 Theory and System Design
### Design of system  
Figure 7 is a block diagram of the hardware composition of the point-to-multipoint free-space optical communication system, which aims to transmit the data from the transmitter to the receiver in different directions through the repeater. In the laser emission system, the data in the microprocessor (MCU) are first sent to the data transmission module of FPGA, and the data transmission module sends the obtained data to the PWM modulator for modulation. The modulated signal drives the laser emission circuit. The optical communication relay system can receive laser signals in all directions. The relay controller realizes the functions of ADC sampling, data demodulation, address distribution, data re-modulation, and the control of the receiving and transmitting directions of the received signals in all directions. The modulated data drive the laser transmitting circuits in different directions according to the distributed addresses. The receiving end detects the laser signal through APD, the laser signal is sent to the PWM demodulator for demodulation through the receiving circuit, and the demodulated data are sent to MCU through the data receiver, thus completing point-to-multipoint free-space optical communication. A diagram of the operation of the relay station is shown in Figure 8, mainly composed of a direction selector and a relay controller (inside FPGA). The direction selector is mainly composed of ADC and analog switches. RX represents the sampling signal of ADC, which controls the ADC to sample the received signals (RX_1\(\rightarrow\)RX_n) in different directions by controlling the analog switch. TX represents the driving signal of the laser transmitting circuit, which drives the LD in different directions by controlling the analog switch driving signals (TX_1\(\rightarrow\)TX_n). The relay system requires fast ADC sampling. Here, AD9226 from Analog Devices (ADI) Semiconductor Company was selected to achieve a sampling frequency of 65M. The relay controller is mainly composed of a PWM demodulator, an address resolver, a PWM modulator, and a direction controller. The principles of the PWM modulator and the PWM demodulator are shown in Figures 9 and 10, respectively. The direction controller controls the analog switch to further control the sampling direction and laser emission direction of ADC. The sampling results (data) of ADC are directly sent to the PWM demodulator for data discrimination and further data demodulation. Point-to-multipoint communication involves each group of data having a specific code; the demodulated data are sent to the address parser, the transmission direction target is selected according to the different coding addresses of the data, and the data are sent to  
Figure 7: Hardware composition block diagram of a point-to-multipoint free-space optical communication system. different receiving targets through the PWM modulator, thus realizing point-to-multipoint free-space optical communication. Figure 9 is the system block diagram of the transmitting device, which is mainly composed of a PWM modulator and a laser transmitting circuit. The PWM modulator is contained inside the FPGA, which is mainly composed of a PWM pulse generator, a data converter, and an optional data selector. The laser emission circuit is composed of a voltage-controlled current source, N-MOSFET, and a laser diode (LD). The data input to the PWM modulator is data with a bit width of 8. The data converter converts the 8-bit parallel data into serial data and modulates the converted data bit by bit. If the data belong to datum 1, the PWM pulse generator will generate a narrower pulse signal and send it through the data selector. If they belong to datum 0, the PWM pulse generator will generate a wider pulse signal and send it through the data selector, and the PWM signal TX generated by the PWM modulator drives N-MOSFET.