# A Novel OFDM Format and a Machine Learning Based Dimming Control for LiFi
## 2 Literature Review on OFDM Formats  
This section provides a review of the existing notable works on OFDM-based LiFi. This review is not performed using any evidence-based systematic review method; instead, it is performed by searching articles from reputed databases. Several keywords, including LiFi, visible light communication (VLC), optical OFDM, machine learning, and dimming, were used to search articles from IEEE, Elsevier, The Optical Society, and Springer repositories. LiFi uses infrared and visible light spectra to provide multiuser access and user mobility. In this section, the strategies of OFDM techniques used in the LiFi system are briefly studied. With an equalizer at the receiver, OFDM techniques provide a high data rate and prevent inter-symbol interference (ISI), making OFDM more attractive for LiFi [23; 24; 25; 26; 27; 28; 29] technology. OFDM was widely applied in the context of wireless radio channels to combat ISI and to provide high data rates [30; 31]. OFDM also has the potential to be useful for LiFi. Some of the popular unipolar OFDM techniques suitable for LiFi are DCO, ACO, and PAM-DMT. In DCO, both odd and even subcarriers carry data to transmit information. A DC bias is added to the bipolar signal. After that, the remaining negative part of the signal is clipped. This clipping generates a clipping noise in the DCO signal. In ACO, only odd subcarriers carry data to transmit information, and there is no additional DC bias such as DCO. The signal is clipped at zero to make it unipolar, and the clipping noise falls in the even part. In the case of power efficiency, DCO is less efficient than ACO for small constellation sizes, but for large constellations, ACO becomes inefficient. On the other hand, DCO exhibits better spectral efficiency than ACO since DCO uses both odd and even subcarriers, while ACO uses only odd subcarriers. PAM-DMT is another optical OFDM technique and is suitable for IM/DD [18]. In PAM-DMT, the pulse amplitude modulation of the subcarriers is used. In the conventional PAM-DMT, only the imaginary parts of the subcarriers carry data, and the real parts experience clipping noise. Both PAM-DMT and ACO use asymmetrical clipping and show better power efficiency compared to DCO. In ADO, a combination of ACO and DCO is used. Odd subcarriers carry ACO data, and even subcarriers carry DCO data. ADO has better spectral efficiency compared to ACO and better optical power efficiency compared to DCO. In [27], a different combination of ACO and DCO was proposed, called asymmetrical and direct current biased OFDM (AAD-OFDM). In AAD-OFDM, the spectral efficiency is improved with a wide dimming range using ACO and DCO. According to the required dimming level, the scheme of encoding is altered between ACO and DCO. Thus, the performance is improved, and the system fully utilizes the dynamic range at any dimming level. In AAD-OFDM, ACO performs well in low and high brightness levels due to half-wave symmetry. Spectral efficiency is also greater in both conditions with full dynamic range. On the other hand, DCO occupies all the subcarriers, and the spectral efficiency is also greater in the mid dimming level. Therefore, in the AAD-OFDM system, when mid dimming is required, the system is switched to DCO, and for high and low dimming requirements, it is switched to ACO [24]. The study in [28] introduces pre-distorted enhanced ADO (PEADO-OFDM), where the receiver is designed in such a way that takes care of the latency and the complexity issues. In HACO [19], a combination of ACO and PAM-DMT is used. In this system, ACO and PAM-DMT are transmitted together by sending ACO in odd subcarriers and PAM-DMT even in imaginary subcarriers. No DC bias is needed in this scheme. The clipping noise is calculated at the receiver and eliminated to retrieve PAM-DMT symbols. The advantage of this scheme is the increased data rate. Moreover, no additional DC bias is used, and the spectral efficiency is improved.