OFDM-Based Visible Light Communications

Abstract

This chapter provides an overview of 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 details in [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 to DCO-OFDM. However, due to the spectral efficiency loss of 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 generalized 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)). IntroductionThe physical properties of commercially available light emitting diodes (LEDs) and photodiodes (PDs), which are the foremost candidates for low-cost front-end devices in optical wireless communications (OWC), allow OWC to be realised as Dobroslav Tsonev · Mohamed Sufyan Islim · Harald Haas Li-Fi Research and Development Centre, The University of Edinburgh, Edinburgh EH9 3JL, http://www.lifi-centre.com e-mail: h.haas@ed.ac.uk 1 2 Dobroslav Tsonev, Mohamed Sufyan Islim, and Harald Haas intensity modulation and direct detection (IM/DD) systems only. This is because light emitted by of-the-shelf LEDs is incoherent and so information can be reliably encoded only in the signal intensity. The phase and the amplitude of the light wave cannot be modulated or detected with LEDs and PDs. This limits the set of conventional modulation schemes that can be adopted from the field of radio frequency (RF) communications and directly translated into OWC. Signal modulation techniques such as on-off keying (OOK), pulse-position modulation (PPM), and M-ary pulse-amplitude modulation (M-PAM) are relatively straightforward to implement as they provide real signals that can be directly mapped to light intensity. However, as transmission rates increase, the limited modulation bandwidth of the front-end devices and the limited bandwidth of the OWC channel lead to intersymbol interference (ISI) in the time-domain modulation signal. Hence, a technique such as orthogonal frequency division multiplexing (OFDM) becomes more appropriate as a modulation scheme. OFDM enables simple cost-effective equalisation with single-tap equalisers in the frequency-domain. In add...

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