Asymmetrical And Direct Current Biased Optical OFDM for Visible Light Communication With Dimming Control Namei Yin * , Caili Guo † , Yang Yang * , Pengfei Luo ‡ and Chunyan Feng † * Beijing Laboratory of Advanced Information Networks, Beijing University of Posts and Telecommunications, Beijing, China † Beijing Key Laboratory of Network System Architecture and Convergence Beijing University of Posts and Telecommunications, Beijing, China ‡ Research Department of HiSilicon, Huawei Technologies Co., Ltd, Beijing, China Email: ynm19921208@bupt.edu.cn Abstract—In visible light communication (VLC) systems, op- tical orthogonal frequency division multiplexing (O-OFDM) is an appealing modulation. Among many proposed O-OFDM schemes, both direct current biased O-OFDM (DCO-OFDM) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) are widely used. Since VLC needs to take the dual requirements of illumination and communication into account, different dimming levels are often required. For high and low dimming level scenarios, system with ACO-OFDM normally has a better spectral efficiency performance than system with DCO-OFDM due to the smaller clipping noise. However, regarding the middle dimming level scenarios, the DCO-OFDM generally outperforms the ACO-OFDM. In order to improve system spectral efficiency within the wide dimming range, an asymmetrical and direct current biased optical orthogonal fre- quency division multiplexing (AAD-OFDM) is proposed in this paper. In AAD-OFDM based system, the modulation scheme is switched between ACO-OFDM and DCO-OFDM according to the required dimming level. Therefore, by adopting the proposed AAD-OFDM, the VLC system can fully utilize the dynamic range of LED at any dimming levels thus improving the performance. Simulation results show that the proposed scheme has high spectral efficiency within a wide dimming range. I. I NTRODUCTION Driven by the rapid development of solid-state lighting tech- nology and the nearly saturated wireless spectrum, visible light communication (VLC) has gained more and more attention. Advantages like unregulated bandwidth, low cost and security make VLC an important complement to traditional radio frequency communication, especially in indoor scenarios. Recently, orthogonal frequency division multiplexing (OFD- M) is widely used in VLC due to its high spectral efficiency and effective resistance to inter symbol interference (ISI). VLC systems require real and non-negative signal formats due to the fact that it is usually based on intensity modulation with direct detection (IM/DD), i.e., the information is only modulated onto the intensity of light. Many optical orthogo- nal frequency division multiplexing (O-OFDM) schemes are This work was sponsored by Huawei Innovation Research Program (HIRP) & National Natural Science Foundation of China (No. 61271177) proposed in order to make the OFDM signal non-negative, such as direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) [1] and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) [2]. Due to the biasing addition, DCO-OFDM suffers from poor power efficiency, but it is still an appealing O-OFDM modulation scheme for its simplicity. However, ACO-OFDM can achieve higher power efficiency compared to DCO-OFDM at the expense of losing half of the subcarriers. Dimming is an important function, since it is able to extend the lifespan of an LED with less heat emission and provides convenience of illumination. Recently, many efforts have been devoted to study the dimmable O-OFDM schemes. Dimming methods can be classified as three categories, namely, analog dimming, digital dimming and spatial dimming [3]. Analog dimming is achieved by changing the forward drive current of the LED, while digital dimming is realized by adjusting the forward current duty cycle. The spatial dimming is a recently proposed dimming method and the details in [3]. Normally, digital dimming is a preferred dimming solution than analog dimming in industry since it can offer a linear relationship between the light intensity and duty cycle and cause no chromaticity shift. But a recent study [4] has shown that analog dimming can also demonstrate excellent color stability. Therefore, analog dimming is also chosen by many research groups with its simplicity. In [5], a hybrid scheme, named AHO-OFDM, combining ACO-OFDM signal and PAM-DMT signal modulating the imaginary part of the even carrier is proposed, which can support a wide dimming range with improved spectral efficiency. However, its spectral efficiency is still limited because the real part of the even subcarriers is not used. In [6], a novel dimming scheme, named DO- OFDM based on multi-layer ACO-OFDM is proposed, which offers a large dimming range and has high spectral efficiency. However, the complexity of this scheme becomes higher as the number of modulation layer increases. Since VLC needs to meet the requirements of both illumi- nation and communication, different dimming levels are often ICC2017: WS01-The 3rd Workshop on Optical Wireless Communications (OWC) 978-1-5090-1525-2/17/$31.00 ©2017 IEEE