In this paper, a signal shaping framework for optical wireless communication (OWC) is proposed. The framework is tailored to the single-carrier pulse modulation techniques, such as multi-level pulse position modulation (<i>M</i>-PPM) and multi-level pulse amplitude modulation (<i>M</i>-PAM), and to multi-carrier transmission realized through multi-level quadrature amplitude modulation (<i>M</i>-QAM) with orthogonal frequency division multiplexing (OFDM). Optical OFDM (O-OFDM) transmission is generally accomplished via direct-current-biased optical OFDM (DCO-OFDM) or asymmetrically clipped optical OFDM (ACO-OFDM). Through scaling and DC-biasing the transmitted signal is optimally conditioned in accord with the optical power constraints of the transmitter front-end, i.e., minimum, average and maximum radiated optical power. The OWC systems are compared in a novel fashion in terms of electrical signal-to-noise ratio (SNR) requirement and spectral efficiency as the signal bandwidth exceeds the coherence bandwidth of the optical wireless channel. In order to counter the channel effect at high data rates, computationally feasible equalization techniques such as linear feed-forward equalization (FFE) and nonlinear decision-feedback equalization (DFE) are employed for single-carrier transmission, while multi-carrier transmission combines bit and power loading with single-tap equalization. It is shown that DCO-OFDM has the highest spectral efficiency for a given electrical SNR at high data rates when the additional direct current (DC) bias power is neglected. When the DC bias power is counted towards the signal power, DCO-OFDM outperforms PAM with FFE, and it approaches the performance of the more computationally intensive PAM with DFE.
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