PhD Defense of Ali Waqar AZIM

The radio-frequency (RF) spectrum is dwindling due to an increase in the demand of wireless applications. Hence, there is substantial demand of alternative spectrum regions for communications to recline the conventional RF spectrum. In this context, optical wireless communications (OWC) is explored as a likely candidate to supplement the RF communications. The current work focuses on impairment mitigation techniques and energy-efficient modulation approaches for OWC. Firstly, the peak-to-average power ratio (PAPR) for optical-orthogonal frequency division multiplexing (O-OFDM) is studied and two decision-directed PAPR reduction techniques are presented. The complexity of these methods is comparatively lower than other PAPR reduction methods, whilst, their performance is likewise analogous with other alternatives. Secondly, an optical-single-carrier frequency division multiple access (O-SCFDMA) approach, referred to as Hermitian symmetry free O-SCFDMA (HSFO-SCFDMA) is pro- posed, that averts Hermitian symmetry which is normally expected for other O-OFDM and O-SCFDMA approaches. The simulation results affirm that HSFO-SCFDMA is more efficient than any other O- SCFDMA approach for OWC and manifests the lowest PAPR among the other alternatives. Thirdly, the performance of precoding is analyzed for layered asymmetrically clipped O-OFDM (LACO-OFDM). Two types of precoding are employed, i.e., Fourier transform precoding and Hartley transform precoding. It is established that Hartley transform precoded LACO-OFDM exhibits the lowest PAPR and is less vulnerable to non-linear distortion introduced by the light emitting diodes (LEDs), howbeit, the signal-to-noise ratio (SNR) increase (both electrical and optical) owing to the layered structure is higher than conventional LACO-OFDM.