## 4 Construction of QD Experimental System and Digital Signal Processing  
To validate the ability of spread-spectrum technology to enhance the precision of position detection and reception sensitivity achieved by the QD, an experimental setup was constructed; this is shown in Figure 4. The setup primarily consists of a signal transmission board, laser driver board, laser emitter, optical power attenuator, zoom lens, semi-transparent half-mirror, Charge-Coupled Device (CCD) camera, QD module, and digital signal processing box. The QD module comprises the QD and a transimpedance amplifier, while the digital signal processing box includes an ADC and a field-programmable gate array (FPGA). A block diagram of the quadrant detector experimental system is shown in Figure 4. Experimental equipment parameters and models can be found in Table 3.  
Figure 4: Block diagram of QD experimental system. “BS” stands for beam splitter, which is a half-transparent and half-reflective mirror. Figure 3: The relationship between the received optical power and signal bit error rate under different spreading gains. As shown in Figure 4, the QD utilized in the experiment is the QA-4000 model from First Sensor, with a bandwidth of 50 MHz and a target surface diameter of 4 mm. The laser and QD are fixed on a three-dimensional displacement stage (with an accuracy of 0.01 mm), allowing the incident spot position on the QD surface to be adjusted via the manipulation of the lower displacement stage of the laser. The incident spot radius is adjusted using a variable-focus lens in front of the laser. The experimental system employs a laser diode with a wavelength of 850 nm as the light source. At the transmission end, the PRN signal is modulated at a frequency of 10 MHz, and an optical power attenuator is used to control the transmitted light power. A half-transparent and half-reflective mirror splits the incident light, with half of the light power directed into the camera for the fine calibration of the incident spot radius and displacement. The other half of the light power enters the QD module, generating analog optoelectronic signals. Finally, the optoelectronic signals outputted by the QD are collected using a signal-processing box. The A/D converter in the digital signal processing box has a sampling rate of 50 Mbps and an output data width of 12 bits. It converts the amplified analog optoelectronic signals into digital signals, which are then processed digitally based on an FPGA used for signal despreading, outputting the spot position, and communication data. Based on the block diagram of the QD experimental system shown in Figure 4, a desktop experimental system was constructed, as illustrated in Figure 5. In this setup, the QD module receives spatial light, and within the digital signal processing unit, a digital phase-locked loop based on FPGA is constructed for local PRNs; this is in order to acquire and track the phase of the input photoelectric signal and thus achieve signal despreading. Since the frequency of the PRN signal modulated on the beacon light is much lower than the light frequency, the phases of the photocurrent signals received by the four quadrants of the QD can be considered to be approximately the same. Therefore, only the phase of the sum signal of the four quadrants of the QD is tracked, thus enabling the local PRN to be tracked for the four input signals of the QD quadrants. Additionally, four correlators were set up to perform correlation operations between the tracked local code and the photocurrent signals of each quadrant. This enabled the PRN signals received by each quadrant to be despread. The digital delay-locked loop (DLL) consists of three main modules: a delay detector, loop filter, and local PRN code generator. The local PRN code generator is used to generate three sets of PRNs that are the same type as the input PRN but have different phases: the Punctual code, the Early code, and the Late code. The delay detector comprises a multiplier, an integrate, and dump, and an absolute value module.