# Adaptive Software Defined Equalization Techniques for Indoor Visible Light Communication
## 7 Discussion  
While the presented equalization algorithms definitely improved the measured parameters, there was still room for improvement. The system vastly improved the first generation of our VLC system, mentioned previously. However, QAM in this field is slowly pushed back in favor of more modern OFDM variants. Therefore, during the testing, the development of the beta OFDM system, already began (Figure 17). It was based on modified hardware components from the presented QAM system. During the development, a third USRP had to be used, since the OFDM was highly sensitive to precise synchronization. However, a limitation of gigabit Ethernet was reached as well; therefore, the whole platform would be modernized and moved to newer SDRs with the PCI-Express interface. Based on the measurements, the used amplifier suffered from significant non-linearity. Alternative models are currently being explored, and LZY-22+ from Mini Circuits is the most likely candidate for future testing. Non-linearity significantly influenced the results, and the values might be even improved by at least 30%. A number of filters could be also tested in future works, as indicated by [PERSON] et al. (2018), who tested blue and flattening filters. Their system managed to improve BER values by 15-40%. According to them, blue filters omitted about 60% of the LED transmit power, while the response-flattening filter did not influence the LED power capabilities. To test further revisions of the presented platform, a set of blue light filters from Thorlabs was acquired. Further tests are scheduled for newly developed OFDM systems. The receiver was another limiting factor of the whole platform. The commercially available Thorlabs solutions suffer from a lower quality of in-built amplifier. To overcome this issue, a set of PIN photodiodes from Hamamatsu photonics was acquired, and a new, custom PIN photodetector is planned. A test APD based photodetector is also planned. The solution from Hamamatsu seems to be the most suitable candidate, and its acquisition is a topic of future cooperation. Figure 17: Schematics of the developed beta OFDM system. A new set of adjustable lenses was also acquired from Thorlabs. The previously used lens was scratched upon the final set of measurements, so the results were scrapped. However, replacements parts are already scheduled to arrive soon. The final step will be an implementation of an FPGA based version. The platform at it stands is highly dependent on the performance of the computer, which is used for LabVIEW code execution. It is basically a tradeoff between modularity and development speed vs. platform speed. The current implementation is significantly slower than the FPGA version, but any adjustments can be quickly tested and pulled back if needed. The presented results can be transferred to car light sources, as the presented platform is very modular and offers a certain degree of portability. Further tests of modulation schemes in outdoor environments are planned, as well as compatibility with modern mobile networks. Figure 18 shows an early concept of a hybrid 4G/5G/VLC network for communication in congested environments. Mobile network base stations would handover connections to the VLC infrastructure in certain advantageous scenarios (such as road tunnels). After the vehicle passes through the congested environment, the VLC infrastructure passes the user equipment back to the mobile network. VLC technology can be also used for tarification based on the VIN of individual vehicles. [PERSON] et al. presented their own platform for vehicular VLC. Their system was built around traffic lights and provided a noise resilient solution. It was capable of communication up to 50m, which was similar to the preliminary results of our OFDM based platform. OFDM/QAM definitely offered higher transmit speeds in comparison to OOK; however, this experiment mainly tested the influence of direct sunlight. The presented method is very interesting and will definitely be a topic for future research [65].