Introduction

In order to provide sufficient illumination, light installations are typically equipped with multiple light emitting diodes (LEDs). This property can readily be exploited to create optical multiple-input-multiple-output (MIMO) communication systems. MIMO techniques are well-established and widely implemented in many radio frequency (RF) systems as they offer high data rates by increasing spectral efficiency [184, 185]. Off-the-shelf LEDs provide only a limited bandwidth of about 30–50 MHz for incoherent infrared (IR) light and even less for visible light. Consequently, these incoherent light sources restrict the available bandwidth of practical optical wireless communication (OWC) systems. Therefore, it is equally important to achieve high spectral efficiencies in OWC [186]. For free-space optical (FSO) transmission, the effects of MIMO have already been studied. It has been shown that spatial diversity can combat the fading effects due to scattering and scintillation caused by atmospheric turbulence [187, 188]. Ongoing research activities intend to increase the capacity of indoor OWC systems by MIMO techniques [189, 190]. However, for indoor OWC it is still not clear to what extent MIMO techniques can provide gains. This is because in indoor environments there are no fading effects caused by turbulence, especially if line-of-sight (LOS) scenarios are considered. Therefore, indoor optical wireless links are envisaged to be highly correlated, enabling only minor diversity gains. As MIMO techniques mostly rely on spatially uncorrelated channels, it is unclear whether the optical propagation channel in indoor environments can offer sufficiently low channel correlation.

By employing multiple LEDs as transmitters (Txs) and multiple photodiodes (PDs) as receivers (Rxs) the OWC system benefits from multiplexing and diversity gains to increase the throughput. In this chapter, the performances of several MIMO techniques for OWC are demonstrated assuming LOS channel conditions [191]. These conditions are considered as a worse case scenario because of the high channel correlation due to spatial symmetries and very little path gain differences. Specifically, several 4 × 4 setups with different transmitter spacings and different positions of the receiver array are considered.