# Low-Complexity Hybrid Optical OFDM with High Spectrum Efficiency for Dimming Compatible VLC System
## 1 Introduction
A popular analogous dimming approach, known as the conventional DCO-OFDM, achieves dimming control upon adjusting the DC bias, which is easily implemented. Unfortunately, the performance of DCO-OFDM may fluctuate with different dimming levels. Especially when the requested dimming level is considerably low or high, the resulting performance becomes degraded, due to the grave non-linear distortion [18]. The analogous dimming approach is further investigated in terms of advanced O-OFDM scheme, such as the asymmetrical hybrid optical-OFDM (AHO-OFDM) scheme [19]. In AHO-OFDM, either the signal of PAM-DMT or ACO-OFDM is inverted, and then compounded for transmission. By modifying the power allocated to the two components, a wide dimming range can be attained. However, the analogous dimming approach is challenging to precisely control the brightness and may further impinge upon the wavelength of the emitted light, resulting in undesired chromaticity shift [20]. Therefore, another category of the dimming approach, which is referred to as digital dimming relying on the the pulse width modulation (PWM) technique has been extensively applied. A reverse polarity O-OFDM (RPO-OFDM) was proposed to dim the transmitted signal, such as the conventional ACO-OFDM, upon invoking the PWM technique [21]. RPO-OFDM can realize a steady BER performance over a wide range of dimming levels, but has a low spectrum efficiency. To improve the spectrum efficiency, a negative HACO-OFDM (NHACO-OFDM) scheme was conceived in [22] for combining with the high level of PWM signal, which is further amalgamated with HACO-OFDM at the low level. In this regime, the various dimming requests can be satisfied upon altering the proportion of these two branches, while a high spectral efficiency can be realized. Moreover, the authors in [23] proposed a reconstructed layered ACO-OFDM (RLACO-OFDM) scheme based on LACO-OFDM, which is further combined with PWM signal to achieve various brightness levels, without introducing additional complexity. As a benefit, the PWM-biased dimming approach constitutes an implementation-friendly philosophy, where the brightness level can be well-controlled upon varying the parameter of duty cycle [24]. However, the existing digital dimming approaches are vulnerable to the varying dimming level, where a preliminary is required at the receiver side to detect the PWM signal, leading to an increasing complexity. In fact, detecting the PWM signal may be intractable especially in the scenario of multiple LEDs, since the received multi-path signals may have different dimming levels [23]. Furthermore, as mentioned above, both the HACO-OFDM and LACO-OFDM require a successive demodulation operation, suffering from an increased receiver complexity. Against this background, _a novel dimming compatible hybrid optical OFDM (DCHO-OFDM) is conceived in this paper incorporating the PWM technique to realize high-spectral-efficiency transmission under various dimming levels, where the transmitted signal can be readily recovered upon relying on a standard receiver._ To be specific,  
* The conceived DCHO-OFDM signal is beneficially composed of the unclipped ACO-OFDM signal and the down/upper-clipped unipolar PAM-DMT signal, where the time-varying bias is further facilitated to diminish the undesired non-linear distortion. In this way, the proposed DCHO-OFDM achieves a higher spectrum efficiency than the conventional schemes over a broad dimming range. * Thanks to the elaborately combined signal, the symbol transmitted by the proposed DCHO-OFDM is capable of being recovered without detecting the PWM signal, which is superior to the existing digital dimming approaches. Additionally, the BER performance of DCHO-OFDM is invulnerable to the various duty cycles of PWM, achieving a steady system performance in a wide dimming range compared to DCO-OFDM. * Moreover, the information-carried subcarriers in the proposed DCHO-OFDM scheme can avoid the corruption by the generated interference.