Optical OFDM for visible light communications

Visible light communication (VLC) has become a promising complement to its radio-frequency (RF) counterpart. In VLC systems, orthogonal frequency division multiplexing (OFDM) has drawn much attention due to its high data rate, simple equalization and robustness to the inter-symbol interference (ISI). In this paper, we present a comparative performance evaluation of several classical optical OFDM schemes, including DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone (PAM-DMT), unipolar OFDM (U-OFDM) and Flip OFDM. Since DCO-OFDM suffers from energy efficiency loss due to addition of DC-bias, whilst ACO-OFDM, PAM-DMT and U-OFDM/Flip OFDM are spectrally inefficient due to their unique frame structures, the state-of-the-art energy- and spectrum- efficient optical OFDM schemes are investigated, including asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), hybrid ACO-OFDM (HACO-OFDM), asymmetrically clipped absolute value optical OFDM (AAO-OFDM), the spectral and energy efficient OFDM (SEE-OFDM), layered ACO-OFDM (LACO-OFDM), enhanced U-OFDM (eU-OFDM), optical OFDM with index modulation (O-OFDM-IM) and optical dual-mode index modulation aided OFDM (DM-OFDM). In this paper, their principles are firstly illustrated, then performance comparisons are conducted for those optical OFDM schemes in terms of spectral efficiency, energy efficiency and computational complexity. Since light emitting diodes (LEDs) need to support both the illumination and communication simultaneously, dimming control should be considered in VLC systems. Therefore, dimmable optical OFDM for practical VLC systems incorporating illumination is also addressed.