# A Novel OFDM Format and a Machine Learning Based Dimming Control for LiFi
###### Abstract  
This paper proposes a new hybrid orthogonal frequency division multiplexing (OFDM) form termed as DC-biased pulse amplitude modulated optical OFDM (DPO-OFDM) by combining the ideas of the existing DC-biased optical OFDM (DCO-OFDM) and pulse amplitude modulated discrete multitone (PAM-DMT). The analysis indicates that the required DC-bias for DPO-OFDM-based light fidelity (LiFi) depends on the dimming level and the components of the DPO-OFDM. The bit error rate (BER) performance and dimming flexibility of the DPO-OFDM and existing OFDM schemes are evaluated using MATLAB tools. The results show that the proposed DPO-OFDM is power efficient and has a wide dimming range. Furthermore, a switching algorithm is introduced for LiFi, where the individual components of the hybrid OFDM are switched according to a target dimming level. Next, machine learning algorithms are used for the first time to find the appropriate proportions of the hybrid OFDM components. It is shown that polynomial regression of degree 4 can reliably predict the constellation size of the DCO-OFDM component of DPO-OFDM for a given constellation size of PAM-DMT. With the component switching and the machine learning algorithms, DPO-OFDM-based LiFi is power efficient at a wide dimming range. machine learning; orthogonal frequency division multiplexing; dimming; light fidelity; regression 20211020214 August 20211030 August 202110202  
pulse position modulation (PPM), and pulse amplitude modulation (PAM). However, these schemes suffer from various unwanted effects such as nonlinear signal distortion with the increase in data rates. A complex equalization process is required for overcoming such effects. To avoid this complex equalizer, as an alternative solution, multi-carrier modulation (MCM) is also applied. The most used technique of MCM in LiFi is orthogonal frequency division multiplexing (OFDM). OFDM provides high data rates and prevents inter symbol interference (ISI) by using a single tap equalizer at the receiver. Since light intensity cannot be negative, optical OFDM is unipolar. To obtain unipolar OFDM without any loss of information, there are several derivatives of OFDM, including asymmetrically clipped optical OFDM (ACO-OFDM) [12; 13], DC biased optical OFDM (DCO-OFDM) [12; 14; 15; 16], and pulse amplitude modulated discrete multitone modulation (PAM-DMT) [17; 18]. In addition, there are some hybrid forms of OFDM such as asymmetrically clipped DC-biased optical OFDM (ADO-OFDM) [12; 16], hybrid asymmetrically clipped optical OFDM (HNC-OFDM) [16], hybrid asymmetrically clipped optical OFDM (HACO-OFDM) [19], hybrid diversity combined OFDM (HDC-OFDM) [16; 20], asymmetrical hybrid optical OFDM (AHO) [21], hybrid DC biased asymmetrically clipped pulse amplitude modulated discrete multitone modulation optical OFDM (HDAP-OFDM) [22], and pulse amplitude-modulated hybrid optical OFDM (PHO-OFDM) [23]. In the rest of the paper, the terms ACO-OFDM, DCO-OFDM, AHO-OFDM, HACO-OFDM, HDAP-OFDM, and PHO-OFDM are simplified as ACO, DCO, AHO, HACO, HDAP, and PHO, respectively. In PHO, M-PAM and M-QAM were used on odd subcarriers and even subcarriers, respectively [23]. However, the dimming range of PHO-based LiFi was not studied in [23]. In HDAP, DCO was used in the lower subcarriers [22]. On the other hand, ACO was used in the higher odd subcarriers, and PAM-DMT was used in the higher even subcarriers of HDAP. The overall dimming range for HDAP was studied, but the required proportion of the elements of HDAP for a given dimming target has not been investigated before [22]. In this research, a switching algorithm for HDAP is proposed where the individual components of HDAP are switched according to a target illumination level. This paper proposes a new hybrid modulation approach termed DC-biased pulse amplitude modulated optical (DPO-OFDM), where DCO will be kept in the odd subcarriers, and PAM-DMT will be kept in the imaginary portion of the even subcarriers.