Signal Shaping and Modulation for Optical Wireless Communication

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 (M-PPM) and multi-level pulse amplitude modulation (M-PAM), and to multi-carrier transmission realized through multi-level quadrature amplitude modulation (M-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 band- width 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.