# Hybrid Adaptive Bias OFDM-Based IM/DD Visible Light Communication System
## 1 Introduction  
With the rapid increase in wireless mobile devices, the continuous increase of wireless data traffic has brought challenges to the continuous reduction of radio frequency (RF) spectrum, which has also driven the demand for alternative technologies [1, 2]. In order to solve the contradiction between the explosive growth of data and the consumption of spectrum resources, visible light communication (VLC) has become the development direction of the next generation communication network with its huge spectrum resources, high security, low cost, and so on [3, 4, 5]. The orthogonal frequency division multiplexing (OFDM) technology has the advantages of high spectrum utilization and strong ability of resisting inter-symbol interference (ISI) [6, 7]. Hence, the OFDM technology is also an excellent choice for VLC. Considerable research has applied OFDM technology to VLC. As we all know, the VLC requires non-negative and real OFDM signals, due to intensity modulated direct detection (IM/DD) is widely used in VLC systems [8, 9, 10]. In order to meet the requirement, several optical OFDM schemes have been proposed for IM/DD VLC systems. Asymmetrically clipped optical OFDM (ACO-OFDM), where only the odd subcarriers are modulated to transmit the information data, achieves high power efficiency [11]. Pulse amplitude modulated discrete multitone (PAM-DMT) is one of clipping-based schemes, where only the imaginary parts of the subcarriers are used, while the real parts are set to zero [12]. However, due to half of the subcarrier resources not being utilized, the ACO-OFDM and PAM-DMT schemes lead to a significant loss of spectral efficiency [13]. Hence, some advanced schemes superimposing various optical OFDM components have been proposed to improve spectral efficiency, including hybrid asymmetrically clipped optical OFDM (HACO-OFDM) [14] with pulse-amplitude-modulated based hybrid optical OFDM (PHO-OFDM) [15] and adaptively biased optical OFDM (ABO-OFDM) [16]. The ACO-OFDM signals and PAM-DMT signals are transmitted simultaneously in HACO-OFDM scheme, where high spectrum efficiency is achieved compared with the ACO-OFDM and PAM-DMT schemes. However, the interference of the clipping of the bipolar signal falls on the superimposed components. The operation of mitigating the interference at the receiver leads to a significantly high receiver complexity [17]. Similarly, the PAM signals and the QAM signals are transmitted on odd and even subcarriers in the PHO-OFDM scheme, respectively. Unfortunately, the clipping noise of the PAM signals affects the demodulation of the QAM signals that are modulated on the even subcarriers, which significantly increase the complexity of the receiver. Another advanced scheme, known as ABO-OFDM, uses the odd and a part of the even subcarriers to convey the information data, which enhances the spectral efficiency compared to ACO-OFDM scheme. Nevertheless, the reserved subcarriers are not utilized, which incurs a bandwidth penalty. In this paper, a novel hybrid adaptive bias optical OFDM (HABO-OFDM) scheme is conceived to enhance spectral efficiency with power efficiency, and achieve low-complexity implementation of the receiver. Our main contributions are as follows:  
* A novel structure combining different optical OFDM components to transmit the signals simultaneously is proposed for VLC. This structure occupies all subcarriers to convey the signals, thus significantly enhancing the spectral efficiency in comparison with ABO-OFDM. * The adaptive bias is carefully designed to guarantee the non-negativity of the transmitted signal and improve power efficiency. Furthermore, since the interference caused by the adaptive bias does not disturb the estimation of the superimposed signals at the receiver, a relatively simple receiver can be employed in HABO-OFDM. * Different performances analysis of our proposed system is presented.