## 1 Introduction  
Growing demands for high data rates and low latency communication systems, particularly in indoor and in-building environments, contribute to the significant consideration of the usage of 60 GHz and above unlicensed frequency bands in the United States [1]. Recently, optical spectrum research focused on indoor wireless communication [2]. Significant interests of Terahertz (THz) or above level communications on 6G development show the possible opportunities of the optical spectrum in the perspectives of wireless networks, interactive communications, mobility service, internet of things, and even bio-tissues [3]. Especially in the case of mobility service for in-building, its communication systems have to consider VLC for high data rates instead of current wireless systems such as NB-IoT or LoRaWAN [4]. Since the optical spectrum is in the frequency range of at least 300 GHz, multicarrier OFDM modulation can be the possible candidate for the robust modulation scheme in that spectrum for indoor visible light communications [5]. OFDM has the advantages of a high data rate and bandwidth efficiency. It also provides a scheme to mitigate intersymbol interference caused by multipath propagation. However, in VLC, the LEDs' nonlinear behavior can severely affect OFDM's performance due to its high Peak-to-Average Power Ratio. For LEDs, the power amplifier operates up to the saturation for maximum power, and it may cause unwanted nonlinear distortion in amplitude and phase in this power operation. Signal clipping issue in the amplifier is another critical matter for OFDM [6]. The back-off to the average input power ensures that the amplifier avoids saturation, but the problem still exists that the back-off may deteriorate power efficiency. Linearization through predistortion is another method to compensate for the PA nonlinear distortion. Since the LED is the primary source of nonlinearity, the baseband OFDM signal in VLC systems is modulated with the instantaneous power on the optical carrier called intensity modulation. Asymmetrically Clipped Optical OFDM (ACO-OFDM) and DCO-OFDM are two primary forms of OFDM with intensity modulation. In order to produce a positive signal, the bipolar OFDM signal in DCO-OFDM is superimposed on a bias point. On the other hand, the OFDM signal in ACO-OFDM is unipolar modulated by only the odd sub-carriers, and the unipolar modulation suppresses the signal at zero levels [7]. In this paper, we select DCO-OFDM for spectral efficiency. For the investigation of the nonlinear distortion, various bias points are considered. A power back-off scheme is a possible option for the OFDM signal to set the distortion levels with LED operation near the bias point in a quasilinear segment of the LED characteristic. After sampling the LED transfer function, the predistortion method consists of the table format of the inverse of the characteristic function and compensates for the LED nonlinearity [8]. However, this method has data measurement issues in that the system must directly identify the LED data characterization. An adaptive normalized least mean square (NLMS) algorithm can be another technique to estimate correct LED bias data [9]. To compensate for the nonlinearity, it directly predicts the distortion levels given the environmental changes instead of using the fixed values of the existing Memory Look-Up-Table. High complexity in the algorithm becomes one of the remaining issues in practicality. In other words, a simple predistortion approach is necessary to resolve the distortion matters with preserving practical usability. Especially in the case of mobility services, indoor mobility transportation becomes a possible scenario due to the LED light in the building. As shown in Figure 1, using LED lights in the hallway, the building or infrastructure can broadcast the information and data specifically designed for mobility services, indoor navigation maps, announcements, and over-the-air updates.