# Low-Complexity Hybrid Optical OFDM with High Spectrum Efficiency for Dimming Compatible VLC System
## 1 Introduction  
Driven by the rapid development of light emitting diodes (LED), visible light communication (VLC) constitutes a promising part of next-generation wireless communications [1; 2; 3]. Under the trend of internet of everything (IoE), the exhaustion of radio frequency (RF) band to resolve the unprecedented data escalation dilemma becomes a bottleneck of current communications [4; 5]. As a benefit, the environmentally friendly technology of VLC possesses plenty of unlicensed spectral resource at the high frequency band [6]. This part of electromagnetic-interference-free spectrum can be exploited, for the sake of supporting high-speed and high-security communications in the near future [7]. Upon relying on modulating the intensity of lights above the flicker-fusion frequency, the dual functionality of communications and illuminations can be simultaneously achieved in VLC, providing an efficient solution especially for short-range communications [8]. In order to achieve high-speed transmission, optical orthogonal frequency division multiplexing (O-OFDM) modulation schemes have been extensively explored for VLC systems [9]. Since a VLC system is based on intensity modulation and direct detection (IM/DD), the transmitted signal viaVLC link should be both real-valued and positive-valued [10]. To meet these constraints, there are two classic O-OFDM schemes in VLC, namely, asymmetrically clipped O-OFDM (ACO-OFDM) and DC-biased O-OFDM (DCO-OFDM). DCO-OFDM relies on a DC bias to guarantee the positivity, which enjoys the advantage of implementation simplicity. However, it suffers from the power inefficiency since the DC bias which does not convey any information leads to a waste of power. By contrast, with the aid of the properties of Fourier transform, ACO-OFDM directly clips the negative parts of the signal to generate a non-negative signal, substantially increasing the power efficiency, but ACO-OFDM leverages only half of the subcarriers for transmission hence, resulting in spectrum inefficiency [11]. Therefore, advanced transmission schemes have been further investigated, such as hybrid ACO-OFDM (HACO-OFDM) [12] and layered ACO-OFDM (LACO-OFDM) [13]. In HACO-OFDM regimes, for the sake of promoting the spectrum efficiency, the ACO-OFDM signal is further combined with pulse-amplitude-modulated discrete multitone (PAM-DMT), where the latter is employed to modulate the imaginary part of even-indexed subcarriers. Superior to the conventional OFDM schemes, HACO-OFDM can notably improve the spectrum efficiency while, at the same time, maintaining high power efficiency. However, on the receiver side of HACO-OFDM, additional operations including the re-generation of time-domain ACO-OFDM samples, the zero-clipping, the re-production of clipping distortion and the noise subtraction are required, leading to an increased complexity [14]. The perspective of LACO-OFDM improves the traditional ACO-OFDM upon relying on actively invoking the unexploited subcarriers for transmission in a layer-based approach. Compared to HACO-OFDM, the spectrum efficiency can be further enhanced. Unfortunately, the improvement of spectrum efficiency in LACO-OFDM is accompanied by the complexity increment due to the multi-layer transmitter and the successive interference cancellation (SIC) receiver [15]. It is worth mentioning that conventional O-OFDM schemes mainly concentrate on promoting data rate, while ignoring the various illumination requirements. As an emerging member of green communication technology, VLC is intrinsically distinct from the conventional RF system due to its dual functionality: communications and ambient lighting, and not just the former [16]. Therefore, the dimming control is envisioned to be a critical technique in VLC, for the sake of handling the lighting and energy consumption constraints [17]. There are mainly two kinds of dimming approaches extensively used in VLC, namely, analogous dimming and digital dimming.