Efficient OFDM Signaling Schemes For Visible Light Communication Systems

Abstract

<p>Solid-state LED lighting is a promising technology to improve the energy efficiency of general illumination. The inherent modulation bandwidth of these devices can be exploited to provide a dual role as a communication device. This method of communication is termed visible light communications (VLC).</p> <p>Due to dispersive nature of the VLC channel, orthogonal frequency division multiplexing (OFDM) has been proposed to allow multi-user communication while mitigating the effects of inter-symbol interference (ISI). However, OFDM is in general not compatible with intensity modulation and direct detection (IM/DD) channels since it has both positive and negative amplitudes. Various techniques have been proposed that provide compatible optical OFDM signaling such as DC-biased OFDM, asymmetrically clipped optical OFDM (ACO-OFDM), and pulse amplitude modulated discrete multi-tone (PAM-DMT).</p> <p>This thesis develops spectrally factorized optical OFDM (SFO-OFDM) as a framework to implement OFDM on optical intensity channels. The drawbacks of conventional methods are mitigated in SFO-OFDM. Contrary to ACO-OFDM and PAM-DMT, the proposed technique uses all the available bandwidth for data modulation and does not require reserved subcarriers. Simulation results verify that SFO-OFDM has gain both in optical power efficiency and peak-to-average power ratio compared to conventional optical OFDM schemes.</p> <p>Furthermore in this thesis, a new receiver design for ACO-OFDM and PAM-DMT is proposed. Unlike conventional receivers that ignore the structure of the transmitted signal, the new detector exploits this structure to improve the optical power efficiency. By observing the time domain samples, a simple pairwise maximum likelihood detector is developed and used to remove half of the noise power. It is also shown by simulation results that employing the proposed detector design leads to a significant gain in optical power efficiency.</p>