Avoiding spectral efficiency loss in unipolar OFDM for optical wireless communication

Unipolar orthogonal frequency division multiplexing (U-OFDM) has recently been introduced for intensity modulation and direct detection (IM/DD) systems. The scheme achieves higher power efficiency than the conventional direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) at the expense of half the spectral efficiency for the same M-ary quadrature amplitude modulation (M-QAM) order. This paper presents a modulation approach which doubles the spectral efficiency of U-OFDM and still allows it to achieve better performance in terms of both electrical power and optical power dissipation compared to DCO-OFDM. The simulation results and the theoretical analysis suggest that the performance improvement of the proposed scheme over DCO-OFDM increases with the modulation order. This trend is different from the inherently unipolar state-of-the-art techniques such as U-OFDM, asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse-amplitude-modulated discrete multitone modulation (PAM-DMT). It is typical for these schemes to exhibit a loss in the power efficiency as the spectral efficiency is increased. The novel approach is very promising for the achievement of high data rates in IM/DD systems. To the best of the authors´ knowledge, this is the first design of a strictly unipolar orthogonal frequency division multiplexing (OFDM) scheme which requires no biasing and is able to demonstrate significant energy advantage over DCO-OFDM without sacrificing spectral efficiency.