## 3 ACDM-Based VLP System by Using OPRC
### System Setup  
To validate the effectiveness of the proposed OPRC-based VLP scheme, we construct a simulation model and set up a corresponding experimental platform, as illustrated in Figure 2. The evaluation is conducted on a two-dimensional plane with dimensions of 90 cm \(\times\) 77.5 cm, focusing on one-dimensional positioning. Table 1 lists the key parameters used during the simulation, which are closely approximated to those of the practical experimentation. Two LED transmitters, \(Tx_{1}\) at (\(-13.5\) cm, 77.5 cm) and \(Tx_{2}\) at (13.5 cm, 77.5 cm), are used to determine the position of a PD receiver (\(Rx\)). The PD is placed on the \(Y\)-axis (Y = 0) and is moved along the \(X\)-axis from \(-44.5\) cm to \(44.5\) cm in 5 cm intervals. Throughout this movement, the receiving plane of the PD remains parallel to the emitting planes of the LEDs.  
For VLP signal modulation, we assign two unique OPRCs, labeled \(\mathbf{S_{1}}\) and \(\mathbf{S_{2}}\), each with a length of 128, to transmitters \(Tx_{1}\) and \(Tx_{2}\). We choose on-off keying (OOK) to modulate these codewords due to its simplicity and compatibility with ACDM. During simulation, these codewords serve as VLP signals from their respective transmitters. A DC bias is applied, and random time delays are introduced to simulate asynchronous transmission. For practical experimentation, a DG1062Z (RIGOL, Suzhou, China) arbitrary waveform generator (AWG) modulates \(\mathbf{S_{1}}\) and \(\mathbf{S_{2}}\) using OOK at 500 kHz with a 1 V peak-to-peak voltage. The modulated signals are then superimposed with a 3.2 V DC bias and fed into two Osram LCW W5 SM LED light sources. Asynchronous transmission of VLP signals is achieved by successively activating the LED transmitters.  
The asynchronous VLP signals travel through the VLC channel to reach the PD receiver, a PDA100A2 (THORLABS, Shenzhen, China) photodiode. Upon reception, the light signals undergo photoelectric conversion. The resulting electrical signals are sampled by a TBS 1202B (Tektronix, Shenzhen, China) oscilloscope (OSC) and then transferred to a computer for demodulation and analysis.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Symbol** & **Values** \\ \hline Coordinates of LEDs & \(Tx_{1}\), \(Tx_{2}\) &
\begin{tabular}{c} (\(-13.5\) cm, 77.5 cm), \\ (13.5 cm, 77.5 cm) \\ \end{tabular} \\ \hline LED transmit power & \(P_{t}\) & 3 W \\ \hline Modulation index & \(\alpha\) & 0.15 \\ \hline Number of the LED transmitter & N\({}_{\text{LED}}\) & 2 \\ \hline Photodetector responsivity & \(\beta\) & 0.4 A/W \\ \hline System bandwidth & \(B\) & 500 kHz \\ \hline Physical area of \(PD\) & \(A_{r}\) & 75.4 mm\({}^{2}\) \\ \hline Lambertian order of emission & m\({}_{1}\) & 1 \\ \hline Gain of an optical filter & \(g_{f}\) & 1.5 \\ \hline Gain of an optical concentrator & \(gc\) & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Key parameters used in the simulation setup.  
Figure 2: The ACDM-VLP system: (**a**) simulation model; and (**b**) experimental platform.  
During the VLP demodulation phase, we sequentially perform cross-correlation calculations between the received VLP signals and \(\mathbf{S_{1}}\) and \(\mathbf{S_{2}}\) with various chip offsets. Using Equations (12) and (13), we determine the RSS and the signal transmission distance for each LED. By combining these measurements with the LED coordinate information, we apply the trilateration algorithm to locate the receiver.