Unlocking Spectral Efficiency in Intensity Modulation and Direct Detection Systems

A number of inherently unipolar orthogonal frequency division multiplexing (OFDM) modulation schemes have been introduced recently in an attempt to improve the energy efficiency of OFDM-based intensity modulation and direct detection (IM/DD) systems. All such algorithms, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) and unipolar orthogonal frequency division multiplexing (U-OFDM), experience an inherent loss in spectral efficiency caused by the restrictions imposed on the OFDM frame structure required for the generation of a unipolar signal. The current paper presents a modified modulation approach, termed enhanced U-OFDM (eU-OFDM), which compensates the spectral efficiency loss in U-OFDM. At the same time, it still allows for the generation of an inherently unipolar modulation signal that achieves better performance in terms of both electrical power and optical power dissipation compared to the conventional state-of-the-art technique direct current (DC)-biased optical OFDM (DCO-OFDM). To the best of the authors' knowledge, the current work also presents the first experimental proof-of-concept demonstration of both U-OFDM and eU-OFDM, and clearly demonstrates the significant energy advantages that these two schemes can introduce in an optical wireless communications (OWC) system

Dimmable visible light communications based on multilayer ACO-OFDM

This paper proposes a dimmable scheme for a visible light communication (VLC) system based on multilayer asymmetrically clipped optical orthogonal frequency-division multiplexing (ACO-OFDM), which is able to support a wide dimming range for different illumination requirements. In the proposed scheme, multiple layers of ACO-OFDM occupying different subcarriers are combined so that almost all of the subcarriers can be used for data transmission. The polarities of different layers of ACO-OFDM are varied to obtain flexible time-domain waveform, which can fully exploit the dynamic range of light-emitting diodes (LEDs) and achieve better performance. The scaling factor and modulation order for each layer, as well as the dc bias, are optimized for different dimming requirements to achieve improved spectral efficiency. Simulation results demonstrate that the proposed scheme can support communication over a wide dimming range and achieve higher spectral efficiency, compared with existing methods under different dimming requirements

OFDM Systems for Optical Communication with Intensity Modulation and Direct Detection

Intensity modulation and direct detection (IM/DD) is a cost-effective optical communication strategy which finds wide applications in fiber communication, free-space optical communication, and indoor visible light communication. In IM/DD, orthogonal frequency division multiplexing (OFDM), originally employed in radio frequency communication, is considered as a strong candidate solution to combat with channel distortions. In this research, we investigate various potential OFDM forms that are suitable for IM/DD channel. We will elaborate the design principles of different OFDM transmitters and investigate different types of receivers including the proposed iterative receiver. In addition, we will analyze the spectral efficiency and decoding complexities of different OFDM systems to give a whole picture of their performance. Finally, simulation results are given to assess the detection performance of different receivers

Laser-Plasma Interactions Enabled by Emerging Technologies

An overview from the past and an outlook for the future of fundamental laser-plasma interactions research enabled by emerging laser systems