# Candidate Waveforms for ARoF in Beyond 5G
## 6 Conclusions  
In this paper, we have briefly described the prospective waveform candidates for 5G and beyond. In this regard, we have presented the main key requirements determining the performance of the waveforms in an ARoF system. Next, we have presented the current SoA of waveforms used in ARoF experiments, indicating the trend in the last ten years. In this SoA, and to the best of our knowledge, a deep comparison involving all waveform candidates for 5G with regard to ARoF was missing. In this work, we present such an SoA in terms of the requirements for ARoF based systems. Even so,it remains difficult to select a potential best candidate for all situations. For that reason, IMT-2020 defined three types of scenarios [55].  
Furthermore, in this paper, we have concluded the best waveform candidates for eMBB, mMTC, and URLLC for both downlink and uplink, respectively. The waveforms have to enable asynchronous multiple access in the uplink. Concerning eMBB, we conclude UFMC is one of the best solutions to achieve the highest bit rate in the downlink. On the other side, multi-CAP could be a very considerable successor of SC-FDM in the eMBB uplink. Regarding mMTC, we have remarked that simplicity is a considerable factor due to the SW and HW limitations of the devices in this type of scenario. Therefore, we have determined that OFDM and SC-FDM are the best waveforms because of their low complexity in downlink and uplink, respectively. Finally, for URLLC, pure SC waveforms could be the best candidates both for uplink and downlink to minimize latency. In particular, SC amplitude and phase modulation is one of the best options because it provides very low latency and flexibility to adapt the modulation order depending on the channel conditions.  
At this point, it is important to highlight that the waveforms of the comparison presented are under different conditions. Therefore, an experimental comparison between all these modulation formats in an ARoF set-up with wireless transmission would be required to perform a fully comprehensive performance analysis. This experiment should be divided into three steps referring to each scenario. Each waveform should be adapted to achieve the best result in terms of the requirements of each scenario. In other words, the parameters of each waveform are modified to achieve the maximum bit rate for eMBB, the massive device support for mMTC and the lowest latency for URLLC. Therefore, the best candidate could be identified under realistic conditions and with an equal playing field to select the most promising waveforms for the mobile communication standards beyond 5G.  
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This research was funded by the European Commission through H2020 ITN 5G STEP FWD (grant agreement 722429) and H2020 blueSPACE (grant agreement 762055) projects.  
The authors declare no conflict of interest.