# Channel Characteristics and Link Adaption for Visible Light Communication in an Industrial Scenario
## 4 Link Adaption Method to Optimize the Multipath Interference
``` Step 1: Adaptive LED selection method  Descending order of power \([P_{Sort}]\gets H_{1\times N}\)  Cumulative sum power \(P_{cum}\gets cumsum(P_{sort})\)  Pick the index that meets power requirements \(P_{LED}=P_{cum}(0.95\times\sum_{r}P_{cum})\)  Select the optimal index that meets the maximum SNR \(P_{cumopt}\gets cumsum(P_{LED})\) \(P_{r}^{2}=max(\frac{P_{LED}^{2}}{1 len(P_{cumopt})})\) Step 2: Optimization for co-channel interference  Calculate the Relative delay of LOS components: \(Delay_{rev}\)=\(max(\tau_{LOS})-\tau_{LOS}\);  Add the Relative delay for CIRs from LEDs: \(Delay_{1\times N}^{new}\)=\(Delay_{1\times N}^{origin}\)+\(Delay_{rev}\);  Output SNR based on Equation (11) and CIRFurthermore, to evaluate the system performance of DCO-OFDM in the IIOT multipath channel before and after the optimization, the simulation platform is built in MATLAB based on the previous parameters. Figure 8 gives the BER results with 64 QAM orders in the cases of the CIRs in Figure 7. It is found that the larger the SNR for 64 QAM in the DCO-OFDM system, the DCO-OFDM system working in Figure 7b shows a better BER performance than the DCO-OFDM system. Figure 8: The BER performance of the DCO-OFDM system in the IIoT channel. Figure 6: (**a**) Comparison of RMS DS before and after optimization; (**b**) comparison of SNR before and after optimization. Figure 7: (**a**) The CIR before optimization; (**b**) the CIR after optimization. performance than that in Figure 7a. This indicates that the system performance of DCO-OFDM is significantly improved after the multipath interference optimization. The finding can also be found in the [25]. This is because the system performance is dominated by the frequency-selective nature of the channel (BER due to the channel fades). As the CIR demonstrated in Figure 7, the multipath before the optimization is more than those after optimization. When the transmitted data are convolved with the CIR (\(h_{VLC}(t)\)) by using the (1), they are changed by the multipath components with the higher power. The correlation of the training sequence as a time synchronization will be destroyed, resulting in the inability to execute accurate coarse and fine synchronization. Therefore, the multipath components will interfere with the demodulation constellation diagram, resulting in the deterioration of BER. Furthermore, the minimum value of the BER is approximately equal to zero at the SNR of 17 dB, which is sufficient for reliable communication. ## 5 Conclusions

In this paper, the VLC channel in the IIoT scenario is investigated based on the ray-tracing simulation. The large-scale fading and multipath-related characteristics are analyzed and modeled. The modeling results indicate that the OPL, the RMS DS, and AS are proportional to the propagation distance under the single TX. Moreover, due to the complexity of the scenario, it can also be found that the degree of the time domain and spatial domain dispersion in the IIoT scenario is higher than that in the indoor scenarios (conference room and office). Furthermore, the VLC channel characteristics are significantly affected by the density of objects and the height of the user. The differences for the OPL and RMS DS are, respectively, 1.5 dB and 1.15 ns at the RX with the various objects' densities surrounding it. Moreover, the occlusion of the surrounding objects makes OPL larger and RMS DS smaller when the height of users is lower than surrounding objects. In addition, motivated by the fact that channel characteristics vary significantly based on the user location and multipath interference to be solved, an adaptive LED selection combined with the delay adaptation technique is further proposed. Then, to verify the performance of the proposed method, four parameters are simulated, including the SNR, RMS DS, CIRs, and the BER performance of the DCO-OFDM system. It can be found that there is a significant decrease in the RMS DS and the BER of the DCO-OFDM system after the optimization.