# Flickering-Free Distance-Independent Modulation Scheme for OCC
## 7 Conclusions  
In this work, a flickering-free distance-independent modulation scheme was proposed and validated. From the detailed analysis of its theoretical fundamentals, a procedure for selecting the appropriated switching frequencies, based on the receiver's characteristics, the emitters' sizes, and the maximum distance of the system were developed. The validity of that strategy was proved through a set of simulations detailed in Section 6. At the same time, the communication's performance when applying the modulation technique was determined with some field experiments presented in the same section. The obtained BER and success rate for the implementation with the minimum amount of consecutive frames are comparable to the outcomes of the target modulation schemes proposed in [29; 30]. Additionally, the system's throughput (\(f_{\mathrm{CAM}}*2/3\) bps) is better without the necessity of calibration procedures or overhead since the proposed modulation method is based on the analysis of the consecutive frames PCC. BER and throughput results outperform previous works, although they are not so high as conventional optical links. Nonetheless, in some cases where the BER values are under the FEC limit, the use of these error correction techniques could improve the link quality, but with a cost in the data rate that needs to be considered.  
On the other hand, the simplicity of the wake up process and the modulation method assures an overall minor complexity of the system, an improvement compared to other long-distance modulation techniques which presented significant or high complexity [25; 26; 33; 39; 40] for similar distance's ranges. Moreover, the experimental results showed a reliable 100 m link using a frequency multiplier \(\alpha\) below the maximum value calculated for the camera's characteristics. When a greater \(\alpha\) is used for the system, the switching frequencies increase, producing thinner light bands requiring fewer pixels for fulfilling the one and a half strips limitation required for proper demodulation. Therefore, longer distances can be reached. In the detailed field experiment, the maximum achievable distance is 134 m for the selected vertical resolution working with a switching frequency of 2700 Hz. Finally, the experimental results concluded that this modulation technique's main constraint is the number of lighter and darker bands extracted from each frame, since the communication's performance is directly proportional to this number. Therefore, the maximum value of \(\alpha\) should be applied to the system independently of the target distance.  
Figure 16: Communication BER for \(\alpha=75\).  
The proposed modulation technique mitigates the problem of light's condition instability during data transmission by working on the similarity of normalized images. However, more research on Signal to Noise ratio (SNR) effect over the system is required to propose a mitigation process.  
The contributions of the authors in this paper are the following: conceptualization and methodology P.C.-B., J.R., and V.G.; writing P.C.-B. and J.R.; edition V.G. and R.P.-J.; project administration R.P.-J. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by Spanish research Agency, project OSCAR reference TEC2017-84065-C3-1-R.  
This work was supported in part by Escuela Superior Politecnica del Litoral, Ecuador.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.