# Experimental Characterization of RGB LED Transceiver in Low-Complexity LED-to-LED Link
## 1 Introduction  
The Internet of Things (IoT) requires that more and more devices be connected to each other. These devices are not just limited to computers and smartphones, but also include common household appliances such as washing machines, coffee machines, refrigerators as well as personal clothes and toys [1]. It is common nowadays to have tens or even hundreds of devices connected to the internet in one household. The connectivity for those smart appliances and devices is putting a great pressure on the spectrum of the radio frequency (RF) communication, which is already overcrowded [2]. Many researchers are therefore focused on finding out-of-the-box solutions for this so-called'spectrum crunch' by proposing smaller cell-sizes, where frequency reuse can be employed. In these nano and femto-cells, the short-range and low-bandwidth communications between IoT devices can be achieved without overstressing the RF spectrum [3]. It remains a challenge however to find the appropriate low-power, low-complexity and low-cost communication technology to serve the needs for IoT devices. wireless fidelity (Wi-Fi) and Bluetooth low energy (BLE) are the \"go-to\" option for many of those smaller devices. While Wi-Fi operates on relatively high power and has an extended range, BLE is favoured as standard in many IoT applications because of its relatively high data rate (up to 2 Mbps) and its range extending to 200 m [4]. However, both Wi-Fi and Bluetooth are prone to eavesdropping and can cause interference with other RF technologies [5]. They are therefore not used in RF sensitive environments such as in some hospitals and control rooms. Similarly, many users refuse to rely on them for transmission of secure data such as during contactless payment or personal identification because they are easy to hack. These drawbacks provided a rich ground for the use of visible light communication (VLC) in IoT devices especially in indoor environments [6]. As a complementary technology to RF, VLC is gaining popularity due to its low energy consumption and inherent security against eavesdropping [6]. VLC signals cannot penetrate walls and obstacles and are therefore confined within a room, safe from hacking by outsiders not in line of sight with the transmitter. Light emitting diodes (LEDs) are at the core of contemporary energy efficient lighting, which at the same time, can be deployed as VLC transceivers, a poignant step towards green communications. Due to its low power consumption, its small cell size and its immunity to interference from RF, VLC is considered a major player in the era of 5G and IoT [7]. The research in visible light communication is currently divided into two paths: on the one hand, researchers are pushing the bandwidth, range, and speed limits of VLC to compete with and complement high-end RF communication technologies. On this path, VLC is used for high-speed data streaming, internet connection and telephony. For instance, the company PureLifi provides a VLC solution to equip homes and offices with high-speed internet and connectivity over visible light [8]. Researchers from Fraunhofer Heinrich Hertz Institute proposed a 500 Mbps bidirectional VLC system using orthogonal frequency division multiplexing (OFDM) as modulation technique [9]. A 5 m free-space link based on a blue laser diode is proposed in [10], which can provide up to 9-Gbps quadrature amplitude modulation (QAM)-OFDM communication over visible light. Those high-speed solutions mark VLC as a high-speed communication technology and enable it to compete with other RF technologies in this rank. However, they are expensive and complex and are therefore considered an overkill for simple IoT devices. On the other hand, the other path considers employing VLC in applications that require low cost of implementation, wide availability, and easy commercial deployment. These applications need only infrequent data transmission, low data throughput and low communication speeds over a short range [11].