OFDM-Based Visible Light Communications

This chapter provides an overview of the latest enhancements of the orthogonal frequency division multiplexing (OFDM)-based visible light communications. The principals of OFDM techniques for intensity modulation and direct detection (IM/DD) systems are explained in detail in Dimitrov and Haas, Principles of LED Light Communications: Towards Networked Li-Fi, (2015) [1]. A number of inherently unipolar OFDM techniques were recently proposed as power efficient alternatives to the widely deployed direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM). The unipolar orthogonal frequency division multiplexing (U-OFDM) technique achieves higher power efficiency compared with DCO-OFDM. However, due to the spectral efficiency loss of the U-OFDM technique, the power efficiency advantage over DCO-OFDM starts to decrease as the spectral efficiency increases. Multiple U-OFDM streams are superimposed in enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) to double the spectral efficiency of U-OFDM technique. For the first time, the novel eU-OFDM allows unipolar OFDM techniques to have same spectral efficiency of DCO-OFDM. In this chapter, the concept of eU-OFDM is generalised to GeneRalizEd ENhancEd UnipolaR OFDM (GREENER-OFDM), and extended to other unipolar OFDM schemes (asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse-amplitude-modulated discrete multitone modulation (PAM-DMT)).