# Experimental Characterization of RGB LED Transceiver in Low-Complexity LED-to-LED Link
## 1 Introduction
The system proposed in this paper follows that low-cost-of-implementation path. A traditional VLC system transmitter consists of an LED controlled by a driver circuit to transmit modulated light at a higher flickering rate than the human eye can perceive. The most commonly used photodetector circuit is usually equipped with a PIN or avalanche photodiode followed by a transimpedance amplifier (TIA) [7]. Such VLC system configuration however has the limitation of having a single directional, simplex channel nature. If a bidirectional or duplex system is needed, another transmitter-receiver pair must be implemented, which doubles the cost, space and complexity of the system. This sets VLC back in comparison to other RF technologies such as Bluetooth or Wi-Fi. Those latter technologies possess a single antenna on each end of the communication system acting as both a transmitter and receiver for RF signals. LED-to-LED communication is a subcategory of VLC that employs the same LED for transmitting and receiving signals, decreasing thereby the cost, complexity, energy and space requirements even further. It reduces the cost of the system by replacing the costlier photodetector and upgrades the simplex channel of the VLC system to time-multiplexed half-duplex channel, hence halving the total number of transceiver devices [12]. The structure of a PN junction allows the LED to sense an incoming beam of light with zero or reverse bias [13]. Although its sensitivity is low compared to the PIN photodiode (due to the missing intrinsic layer), it can be employed in simple low-data-rate, short-distance and low-cost IoT devices [14; 15]. [PERSON] et al., demonstrated the potential of using the LED as photosensor for ambient light by making use of its junction capacitor [12]. Employing such method, Disney Research Lab, Zurich, Switzerland, achieved a toy-to-toy communication channel at 1 kbps over small distances reaching 5 cm [8; 9]. [PERSON] et al., proposed a similar system with data rates up to 14 kbps, reaching a distance of 20 cm based on the same principle of operation [16]. However, depending on the internal capacitor of the LED for photodetection is not a reliable solution. The capacitance varies according to the make and model of the LED. Furthermore, the long discharging time results in slow communication. [PERSON] et al., showed the LED's ability as photodetector where the sub-LEDs of an RGB LED can have a bandwidth up to 6 MHz when reverse biased [13]. The same study also concluded that such LED could detect light in photovoltaic mode (with no bias) and when slightly forward biased up to 2V. Furthermore, the experimental results proved that the red LED has the highest responsivity out of all three sub-LEDs to the light emitted from the green sub-LED. Hence the combination of red-to-green has been identified as the best-case scenario for a successful LED-to-LED link and is therefore adopted in this paper. A 100 Mbps LED-to-LED communication system was designed by [PERSON] et al., [17] and [PERSON] et al., [18]. The high data rate was achieved through design optimization of a receiver circuit and the careful choice of high-quality communication LEDs along with complex modulation and wavelength division multiplexing techniques. The proposed systems in [17] and [18] are based on expensive and specifically designed single-colour LEDs, as well as complicated optics and heavy offline processing. Such a solution is too complex for IoT devices. Many researchers have already proposed VLC systems with RGB LEDs as transmitters and light sources, but not as photodetectors. For instance, [PERSON] et al., proposed an indoor VLC system employing an array of RGB LEDs as transmitters on the ceiling, which has a range of 2 m and can provide data rates up to 19.2 kbps [19]. To the best of the authors' knowledge, the gap in the field of LED-to-LED communication mentioned in literature is threefold: Firstly, only single-coloured (mostly red and amber) LEDs are employed.