## 5 Conclusions  
This paper proposed a VLC-based IoT system architecture, where the emitter was implemented in the digital domain while the demodulation in the receiver was accomplished in the analog domain. Two system parameters were evaluated: the filtering stage and the usage of guard bands.  
Concerning the filtering stage, two scenarios were considered: with and without inter-band interference. When a single band was modulated, i.e., no inter-band interference, it was shown that, using a raised cosine filter as the emitter pulse shaping filter with a roll-off factor of one and a Bessel or Butterworth filter in the analog receivers, the performance was degraded about 2 dB, when compared to the theoretical QAM BER performance in the presence of AWGN. The performance was even worse if the roll-off factor decreased due to the increase of the ISI, created by the filters' mismatch. By using a matched filter in the digital emitter, the system performance was improved, in particular for Bessel filters, achieving a remarkable BER performance, with a 0.5 dB Eb/No gain loss when compared with the theoretical QAM modulation BER curve in the presence of AWGN. The improved performance obtained by the matched Bessel filter was due to its constant group delay characteristic, as opposed to Butterworth filters. However, when all the bands were modulated, inter-band interference arose, thus degrading the SNR. Although the results pointed out an overall performance decrease, the matched filters presented the worst results, in particular the Butterworth filters. In scenarios where contiguous bands were modulated, the best result was achieved when using a raised cosine filter with \(\beta=1\) as \(p(t)\) and an analog Bessel filter.  
Since high spectral efficiency is not a priority in VLC-based IoT systems, the usage of guard bands was considered in this study. The results stated that, by suppressing the even bands, the BER performance was identical to the scenarios where only one band was modulated, for all \(p(t)\) and \(g(t)\) configurations. Therefore, as previously, the best performance was achieved by using a digital matched Bessel filter as \(p(t)\) together with an analog Bessel filter. The usage of guard bands proved to be effective to mitigate inter-band interference.  
In conclusion, the results showed that a VLC-based IoT system using a digital m-CAP modulator and analog receivers was feasible, achieving a BER identical to the fully digital implementation. In order to improve the system BER, guard bands were required, improving inter-band interference rejection, as well as a synchronism loop in the analog receivers. In particular, using a third-order analog Bessel filter, along with a quadrature mixer properly synchronized, the proposed architecture could achieve a BER lower than \(10^{-3}\) for an Eb/No of 6 dB.  
Conceptualization, L.R., M.F. and L.N.A.; methodology, L.R., M.F. and L.N.A.; software: L.R.; validation, M.F. and L.N.A.; formal analysis, L.R.; investigation, L.R.; data curation, L.R.; writing--original draft preparation, L.R.; writing--review and editing, M.F. and L.N.A.; supervision, M.F. and L.N.A.; project administration, L.N.A. All authors have read and agreed to the published version of the manuscript.  
This paper was raised under the frameworks of the COST Action NEWFOCUS (CA19111), supported by COST (European Cooperation in Science and Technology). This research was funded by H2020/MSCA-ITN funding program under the framework of the European Training Network on Visible Light Based Interoperability and Networking, project (VisIoN) grant agreement No 764461.