# Experimental Demonstration of 3 \(\times\) 3 MIMO LED-to-LED Communication Using RGB Colors
## 1 Introduction  
Since the advent of the first-generation (IG) wireless mobile communication, demand for a high-speed communication system has been increasing exponentially. In response to this demand, the fifth-generation (5G) communication is currently being deployed to provide lower latency, higher data rate, higher security, massive connectivity, and ubiquitous wireless communication services. One of the major differences between 5G and the previous generations of the communication systems is that the 5G communication system exploits a millimeter-wave (mm-wave) as an information carrier because the current 3 kHz to 30 GHz radiofrequency (RF) band does not have enough bandwidth to support complete 5G communication services. As an extension of this trend, one can expect that the mm-wave band will be exhausted in the near future and a higher frequency spectrum will be required. Optical wireless communication (OWC) is one of the very promising candidates for future indoor wireless communication that transmits information via nanometer-waves such as infrared (IR), visible light (VL), and ultraviolet (UV). Typically, the OWC system uses light-emitting diodes (LEDs) or laser diodes (LDs) for transmitters, and photodiodes (PDs) or image sensors (ISs) for receivers. OWC has several advantages compared to its RF counterpart. Firstly, OWC has an enormous bandwidth. The near-IR to near-UVwaveband ranges from 214.3 to 1000 THz, which is approximately 2500 times larger than the RF (3 kHz-300 GHz) region. Secondly, the infrastructure for the OWC system is already well-prepared. Since OWC uses LEDs for its transmitter, LEDs placed in buildings, traffic lights, or streetlights can be employed as optical signal transmitters. Thus, OWC can realize ubiquitous wireless communication at low cost. Thirdly, optical waves cannot pass through opaque objects. Therefore, the optical signal can be easily confined indoors, ensuring high security and high-frequency/wavelength reuse communication. Due to its advantages, extensive research has been performed on OWC. In [1], the authors outline the requirements of the 5G, B5G, and internet of things (IoT) techniques, and show that OWC can meet these requirements. Depending on the type of transmitter and receiver, communication distance, and exploited waveband in the system, OWC can be subcategorized into visible light communication (VLC), light-fidelity (Li-Fi), free-space optical communication (FSOC), optical camera communication (OCC), etc. [2]. Each of the OWC sub-techniques are shown in Figure 1. Typically, VLC uses the VL spectrum for the downlink (DL) and uplink (UL) communication, LED/LD for its transmitter, and PD/IS (photodiode or image sensor) for its receiver. For high-speed communication, the multiple-input multiple-output (MIMO) technique can be applied to the VLC system. However, the realization of the MIMO VLC system is quite challenging due to highly correlated channel conditions [3; 4; 5]. Similar to VLC, Li-Fi uses LED/LD for its transmitter and PD/IS for its receiver. The MIMO technique is applicable in the Li-Fi system, yet challenging due to the highly correlated channel condition [6]. The difference between Li-Fi and VLC is that (i) Li-Fi uses the VL spectrum for the DL and IR/VL/UV for the UL, on the other hand, VLC uses the VL spectrum for both DL and UL, and (ii) VLC includes both point-to-point (P2P) and point-to-multipoint (P2 MP) communication, while Li-Fi includes only P2 MP communication. FSOC uses LD and PD for the transmitter and receiver and transmits data via the IR/VL/UV spectrum. The FSOC communication scheme aims only for P2P communication. MIMO is applicable to the FSOC system as atmospheric turbulence and scintillation by long-distance transmission ensure a low correlation of each channel [7]. OCC uses the IR/VL waveband and LED and IS for its transmitter and receiver. Since optical cameras are composed of millions of pixels, optical signals from different directions can be separated into images [8].