# A Novel OFDM Format and a Machine Learning Based Dimming Control for LiFi
## 6 Dimming Control for DPO and HDAP Systems  
In this section, the dimming flexibility of different hybrid OFDM forms is discussed. Firstly, dimming control is discussed for DPO, HDAP, and others. Secondly, the dimming control of HDAP is discussed by switching the individual components. For dimming control of a LiFi transmitter, simulations using MATLAB were performed. For this, the concepts reported in [22; 27] were used. Figure 9 shows the performance of the  
Figure 8: Bit error rate (BER) versus electrical energy per bit to noise power spectral density (\(E_{b(dec)}/N_{O}\)) for dc-biased pulse amplitude modulated orthogonal frequency division multiplexing (DPO-OFDM) and hybrid DC biased asymmetrically clipped pulse amplitude modulated discrete multitone modulation optical OFDM (HDAP-OFDM) for \(N\) subcarriers in a additive white Gaussian noise channel. Figure 7: Optical energy per bit to noise power spectral density (\(E_{b(opt)}/N_{0}\)) versus bit rate/normalized bandwidth (\(R\)) for different orthogonal frequency division multiplexing (OFDM) forms considering a bit error rate (BER) of \(10^{-3}\). dimming range with respect to R of DPO, HDAP (50% DCO and 50% ACO-PAM), AHO, DCO, and ADO. For dimming flexibility, the size of constellation points is changed so that different plots are obtained. For instance, to attain a 50% dimming, 16-PAM and 4-QAM are used for PAM-DMT and DCO elements, respectively. Note that for the plots, the target BER is considered to be \(10^{-3}\). The achievable dimming range for DPO is 3-97%, which is similar to HDAP. The dimming range of both AHO and DCO is 2-98%, and that of ADO is 5-96%. For a target dimming value, all OFDM forms will carry the same optical power. This means, at a certain diming level, if the scheme contains better \(R\), it will have better optical power efficiency. Figure 9 shows that for \(R\) of 3, the dimming range of DPO is 8-92%, which is close to that of HDAP with a 7-93% dimming range. Moreover, DPO provides better \(R\) for dimming range 40-60% compared to other OFDM techniques discussed here. Next, we show the dimming control of a hybrid OFDM scheme by switching its individual components. For instance, HDAP is considered in evaluating the effectiveness of switching components. Using the concept of AAD-OFDM reported in [24], we can obtain greater spectral efficiency, as well as a wide dimming range with dimming control for the components of HDAP. In HDAP, DCO is placed in lower half subcarriers. In the upper half, odd and even imaginary parts ACO and PAM-DMT are placed, respectively. The upper portion of HDAP, which is a combination of ACO-PAM, acts just like AHO. It can be seen from Figure 9 that for very low dimming values such as 2-10% and very high dimming value 90-98% AHO, it means that ACO-PAM has better spectral efficiency than any other OFDM techniques. Again for 10-30% dimming value and 70-90% dimming value, stand-alone HDAP achieves better spectral efficiency. For mid dimming values such as 30-70%, DCO has greater spectral efficiency. Next, the BER performance is evaluated for different dimming levels. Note that the plots will vary depending on the parameters considered. In this case, we consider the concepts reported in [27]. Figure 10 shows the BER versus \(E_{b(elec)}/N_{O}\) for DPO when the dimming is varied from 10% to 50%. It can be seen that the best BER performance is achieved at 50% dimming. The performance degrades symmetrically when the dimming increases or decreases from 50%. This means dimming at 90% will lead to the same BER performance as of 10%, and dimming at 70% will lead to the same performance as that of 30%. Now, if we switch the OFDM techniques according to our required dimming level, then we can achieve better spectral efficiency with the widest  
Figure 9: Dimming Range of dc-biased pulse amplitude modulated OFDM (DPO-OFDM) and other OFDM forms where OFDM stands for orthogonal frequency division multiplexing. dimming range using the full dynamic range of LED light for HDAP elements.