A comparison of OFDM-based modulation schemes for OWC with clipping distortion

In this paper, the electrical power requirement and spectral efficiency of multi-carrier transmission schemes for optical wireless communications (OWC) are compared in the presence of front-end-induced double-sided signal clipping. The two existing multi-carrier modulation techniques based on orthogonal frequency division multiplexing (OFDM), direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM), are studied. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem (CLT) as attenuation of the data-carrying subcarriers and addition of zero-mean complex-valued Gaussian noise. Presented closed-form expressions for the attenuation factor and the clipping noise variance are employed in the derivation of the effective electrical signal-to-noise ratio (SNR). Using multi-level quadrature amplitude modulation (M-QAM), the electrical power requirement of optical OFDM (O-OFDM) transmission is obtained from the bit-error ratio (BER) system performance. It is shown that in a practical front-end biasing setup DCO-OFDM has a lower electrical power requirement to achieve a target BER as compared to ACO-OFDM for modulation orders with equal spectral efficiencies above 1 bit/s/Hz.