# A Full-Duplex LED-to-LED Visible Light Communication System
## 1 Introduction  
Illumination trends have seen a move from incandescent bulbs and fluorescent lamps to LEDs due to their advantages over conventional illumination methods, such as their high power efficiency, long life time, and fast response time. Among these advantages of LEDs, their fast response characteristic makes their use possible not only for illumination, but also for wireless communication. Visible light communication (VLC), which is also called nm-wave communication, is a communication technology that uses visible light spectra from 380 to 740 nm as information carriers [1]. VLC has several advantages over conventional radio frequency (RF) wireless communication. Firstly, VLC has a huge unlicensed bandwidth of about 400 THz. Because most RF spectra below 10 GHz have been already used for many purposes, wireless communication engineers are trying to use higher frequencies over 10 GHz. Thus, it is expected that VLC may be used in the near future because the VLC spectrum is an extension of the trend of using higher frequencies. Secondly, VLC can be easily realized using the existing LEDs already deployed in buildings for lighting purposes by simply adding a micro controller. As each LED can work as an access point (AP), VLC could be one of the key technologies for Internet of Things (IoT) or massive machine-type communication (mMTC), since multiple Aps and a large bandwidth are required for IoT to connect numerous devices with one another. Thirdly, VLC uses visible light as the carrier frequency, which does not cause electro-magnetic interference (EMI) with other electronic devices in contrast to the conventional RF communication. Fourthly, high-speed communication links over 10 Gb/s can be easily set through the VLC link [2]. Thanks to its advantages, research studies on VLC have been widely conducted, including for the IEEE 802.15.7 VLC standard [3]. Comparative research of optical wireless communication (OWC) technologies is conducted in [4]. VLC based on space-division multiple access (SDMA) [5], software-defined radio (SDR) [6], a light-to-frequency converter [7], and an indoor positioning system (IPS) using VLC is studied in [8]. A full-duplex vehicular VLC system and cooperative VLC system with full-duplex relaying is studied in [9, 10]. However, in reality, VLC has not been widely commercialized yet. We think that one of the main reasons is its insufficient commercialization strategies. In [11], IPS based on VLC was suggested as the first step of a VLC commercialization method, since IPS can achieve functionality without adding additional hardware in mobile devices. However, IPS based on VLC still requires additional optical receivers, such as photodiodes or image sensors. As another commercialization strategy of VLC, we suggest LED-to-LED VLC technology. LED-to-LED VLC is a VLC technique that uses LEDs as both the transmitter and receiver [12; 13; 14; 15]. Because many devices such as smart phones or traffic lights already have built-in LED lamps, LED-to-LED VLC does not require additional optical devices for data transmission and reception in VLC links. Thus, it is expected that technologies such as IPS and mMTC could be easily realized through an LED-to-LED VLC scheme due to their low-complexity, low cost, and ease of implementation. We believe that LED-to-LED VLC will contribute to the wide spread of VLC. To our knowledge, there have been only a few research studies on LED-to-LED VLC so far. In [12], a LED-to-LED VLC system using colored LEDs is proposed and a related networking protocol was shown. In [13], various usage cases of VLC are introduced, such as smartphone-to-LED, LED-to-smartphone, and LED-to-LED. In [14], a LED-to-LED VLC with software-based synchronization using microcontroller is introduced. In [15], the link performance of a half-duplex LED-to-LED VLC is shown by our research team. The frequency response of the receiver LED depending on the types of receiver circuits is also shown.