# Experimental Characterization of RGB LED Transceiver in Low-Complexity LED-to-LED Link
## 2 Experimental Setup  
White light can be produced using either blue-phosphorus or RGB LEDs. While the former is not suitable for LED-to-LED communication because the phosphorus coating prevents the LED from receiving light, the latter can emit and detect simultaneously through three independent channels [13]. This paper utilizes common cathode RGB LEDs (Kingbright) on both ends of the communication link. The green and red sub-LEDs are employed for transmission and photodetection respectively, as recommended by the findings in [13].  
The block diagram of the experimental setup is shown in Figure 1. The transistor-based LED driver biases the green sub-LED with 15 mA while keeping the energy consumption at a minimum and allows fast on-off switching of the transmitter LED [21]. The waveform generator transmits a non-return-to-zero (NRZ) on-off-keying (OOK) modulated cyclic and pseudorandom binary sequence (PRBS) of length \(2^{20}-1\) bits at 10, 50, 100 and 200 kbps. The sequence is generated using linear feedback shift registers applying the monic polynomial:  
\[PRBS20=x^{20}+x^{3}+1 \tag{1}\]  
For testing the LED's response and noise characteristics as photodetector, the waveform generator is replaced with a constant (DC) signal to keep the transmitter LED on.  
At the receiver end with a varying channel distance \(d\) an identical RGB LED (to the transmitter) is connected to the TIA based on an operational amplifier chip (OPA380) as seen in Figure 2. The transmitter and receiver circuits are galvanically completely separated. For comparison purposes, the receiving LED is replaced with a PIN photodiode (BPW34 Vishay Semiconductors) under the same measurement conditions. Since reverse biasing the LED has only a negligible effect on improving its bandwidth and sensitivity [13] unlike PIN photodiodes, the photovoltaic mode is chosen for the detection circuit. The detected signal from the TIA feeds into an optimised matched filter, to mitigate the LED photodetector noise effect and is captured on an oscilloscope (Picoscope 6404D). Offline processing is for performance analysis.  
To achieve good synchronization between the transmitter and receiver circuits and hence a minimum BER, a preamble consisting of a sequence of '1' and '0' bits is transmitted at the begin of the transmission phase. The detecting circuit uses this preamble to detect the rising edge, know the exact bit duration and adjust the threshold. After the preamble, the detecting processor can sample at a time that roughly falls at the middle of the received bit (at the widest eye opening).  
The proposed system employs only a single 5 V power supply and consumes a maximum of 110 mW. Care has been taken to choose components which are cheap and available off-the-shelf.  
Figure 1: Block diagram of the proposed experimental setup.  
Figure 2: Transimpedance amplifier (TIA) circuit diagram.  
The low price and power consumption are attractive features for IoT device manufacturers and mass producers. The experiments take place in a well-lit lab with overhead artificial lighting placed at a distance of approximately 2 m from the testing bench. Since the ambient light is perpendicular to the transmission direction of the LED-to-LED link and the cone of acceptance of the LED is proven to be only \(18^{\circ}\) according to [12], the ambient light has no effect on the receiver LED. Furthermore, the LED photodetector sensitivity is too low to detect indirect ambient light from the ceiling lights [13; 22]. For those reasons, the ambient light is found to have a negligible effect on the experimental results presented in this paper.