# Experimental Demonstration of 3 \(\times\) 3 MIMO LED-to-LED Communication Using RGB Colors
## 1 Introduction
Therefore, OCC is considered one of the most ideal candidates for optical MIMO. Further information on each OWC subcategory is well-described in [9; 10; 11; 12]. Figure 1: Subcategories of optical wireless communication. The LED-to-LED technique is a communication scheme employing LEDs for both transmitters and receivers, and transmits the information via the IR/VL/UV spectrum. Our previous work shows that LEDs can be utilized not only as optical transmitters but also as optical receivers because the basic physical structures of LEDs and PDs are the same. The LED-to-LED communication system has an interesting characteristic whereby receiver LEDs can generate photocurrents only when they receive light with shorter wavelengths than the wavelength they were designed to as a transmitter. This property allows LED-to-LED communication to form a unique channel matrix, ensuring sufficiently low correlated channels. Thereby, unlike indoor VLC or Li-Fi, the MIMO scheme is applicable in LED-to-LED communication. In wireless communications, the importance of ultra-dense networks (UDN) and IoT have been increasing to support higher network capacity and massive connectivity services. However, there are difficulties in realizing UDN and IoT. Since small cells such as picocells and femtocells need to be deployed, realizing UDN is expected to have high costs. The implementation of IoT will induce tremendous data traffic increases as a large number of end-user smart devices, cars, industrial utilities, and sensors will be connected to each other, consuming vast frequency bandwidth even for simple control signal transmission. We believe that LED-to-LED communication can be a complementary solution for a UDN or IoT application. As LED-to-LED communication schemes can be easily setup by using already-deployed LEDs in the indoors, streets, vehicles, and smart devices, UDN can be realized without further investment to develop densely located infrastructures. Furthermore, the use of vast IR/VL wavebands can relieve the communication burden of sending billions of simple control signals generated by massive device connections. We are also expecting that LED-to-LED communication can be used as a step for the OWC commercialization strategies. To our knowledge, the studies on LED-to-LED communication are not comprehensively conducted. The interchangeability between solid-state light emission and detection was widely known by [PERSON] in the 1970s, and a subsequent study on LED-to-LED communication began in 2003 [13; 14]. In [15], the characterization of single-color power LEDs as photodetectors was experimentally measured and a fair comparison between LEDs and silicon a photodetector with respect to their spectral, temporal, and spatial properties was made. In [16], a LED-to-LED ad-hoc network was suggested together with a software-based physical layer and medium access control layer for sensor networks, smart and connected consumer devices, and IoT. In [17], the performance dependency of the LED-to-LED communication system depending on the color of the transmitter and receiver LEDs was studied. In [18], the optimization method choosing the colors of the transmitter and receiver LEDs was shown to maximize the channel capacity according to [PERSON]'s channel capacity law. However, the LED-to-LED-based MIMO techniques are not studied yet. Therefore, in this paper, we propose a LED-to-LED communication system as another subcategory of OWC and experimentally demonstrate 3 \(\times\) 3 MIMO LED-to-LED communication using RGB colors. In the following sections, after consideration of various MIMO and modulation schemes, we show the mathematical model of our system. In Section 3, the characteristics of the transmitter LEDs, receiver LEDs, and LED-to-LED channels are measured. Furthermore, we compare the bit error rate (BER) performance of the MIMO LED-to-LED communication system and single-output (SISO) LED-to-LED communication. Finally, we summarize the paper with a conclusion.