Performance evaluation of spatial OFDM for peak power limited optical wireless channels

Abstract

In fulfilling the current demand for high bandwidth-intensive applications, optical wireless communication (OWC) is considered as a supplementary solution to radio frequency (RF) based systems. One type of OWC is a pixelated system which transmits data by a sequence of image frames. In the process of acquiring robustness against several optical channel impairments, a pixelated OWC system encodes data using spatial orthogonal frequency division multiplexing (OFDM) modulation. Several derivatives of spatial OFDM termed as spatial asymmetrically clipped optical OFDM (SACO-OFDM), spatial DC biased optical OFDM (SDCO-OFDM) and noise-cancelled spatial OFDM (NCS-OFDM) have been proposed in the literature. As yet, spatial OFDM has only been considered for optical wireless channels with average electrical and optical power as the limiting factor. However, because of the practical limitations of the transmitter such as a light emitting diode (LED), there is a limit to the peak power that can be transmitted. In this thesis, peak limited channels in addition to average power limited channels are considered for evaluation of spatial OFDM. For peak limited channels, this thesis proposes spatial asymmetrically clipped DC biased optical OFDM (SADO-OFDM) by combining the concepts of SACO-OFDM and SDCO-OFDM. The optimum DC bias for SADO-OFDM is determined and then the power efficiency of optimum biased SADO-OFDM is compared to that of existing SACO-OFDM, NCS-OFDM and optimum biased SDCO-OFDM. Simulation results show that NCS-OFDM performs better than SACO-OFDM, SDCO-OFDM and SADO-OFDM for average electrical and average optical power channels for all constellations sizes. Nonetheless, simulation results indicate that when peak power is considered as the limiting factor, SADO-OFDM with an optimum DC bias gives significantly better performance than the other three modulation schemes for all constellations. Results also show that the proposed SADO-OFDM as well as the existing SDCO-OFDM require higher optimum DC bias values when peak power limited channels are considered in comparison to average optical and average electrical power channels. With the use of optimum DC bias value, SADO-OFDM will be appropriate for peak power limited pixelated