Spatial OFDM for pixelated MIMO optical wireless communication systems

Abstract

Recently, the demand for high speed wireless multimedia services has grown to an unprecedented level. Multiple-input multiple-output (MIMO) optical wireless communication (OWC) can be an excellent supplemental technology to radio frequency (RF) links to achieve high transmission rates. MIMO OWC has the potential for many applications including secure data communications, mobile advertisements, data exchange in dense high-contention scenarios, and in intelligent transportation systems (ITS). One form of MIMO OWC is a pixelated imaging system which transmits information via a series of pixelated image frames. Such systems have the potential to provide high transmission rates by exploiting spatial diversity at a large scale. For pixelated OWC, a liquid crystal display (LCD) screen or a pixelated grid of light-emitting diodes (LEDs) can be used as a transmitter, while a camera or an imaging lens along with an array of photodiodes can be used as a receiver. In order to be resilient to spatial distortions, pixelated systems can encode data using spatial orthogonal frequency division multiplexing (spatial OFDM) which is an extension of the concept of conventional OFDM to the 2-D spatial domain. Spatial DC biased optical OFDM (SDCO-OFDM) has been described in the literature for pixelated systems. Due to the attractive features mentioned above, spatial OFDM based pixelated systems can be promising candidates for next-generation OWC. However, the technology is still in its infancy and faces a number of challenges including power constraints, linear misalignment error, defocus blur and illumination fall-off known as vignetting. In this research, a novel modulation technique termed spatial asymmetrically clipped optical OFDM (SACO-OFDM) is presented for pixelated OWC. Misalignment, defocus, and vignetting are then modelled and their effects on spatial OFDM are analysed. Different signal processing techniques are applied to reduce the effects of these impairments. The joint impact of misalignment, defocus and vignetting on SACO-OFDM and SDCO-OFDM are then presented analytically and through simulations. The optimum choice of DC bias for SDCO-OFDM for a pixelated system is also presented. For the cases considered and for a given data rate, the proposed SACO-OFDM is shown to be more optically power- efficient than the existing SDCO-OFDM.