# Candidate Waveforms for ARoF in Beyond 5G
## 4 SoA of Waveforms Used in ARoF  
Next, in this section, we review the use of waveform formats in ARoF research and experiment. The attention of ARoF research has been devoted to merging radio frequency and optical fiber technologies, aiming to increase the capacity and mobility of the access network. In the first attempts,ARoF setups employed single carrier waveforms for their simplicity and robustness. Waveforms such as OOK and single carrier QAM were used in [33; 34; 35; 36; 37; 38], respectively. However, they are not optimal for wireless communications due to their limited spectral efficiency and the scarcity of spectrum in the wireless channel.  
Once OFDM was proposed for 4G, experiments involving ARoF also started employing it, and nowadays this waveform is still widely used since it is included in the standard for the current mobile network and for the first release of 5G [15]. ARoF setups with OFDM are found in [3; 39; 40; 41; 42; 43] as illustrative examples. However, OFDM presents several issues as already discussed above and, for that reason, advanced multi-carrier waveforms have emerged recently such as FBMC, UFMC, GFDM and multi-CAP, which can be prominent candidates beyond 5G. Accordingly, these waveforms are now used in ARoF experiments: FBMC in [44; 45]; GFDM in [46; 47; 48; 49]; and multi-CAP in [50; 51; 52], to name but a few.  
In Figure 4 we have compiled the recent usage trend of the different waveforms considered here in ARoF experiments of the last ten years. In this respect, ARoF set-ups have been organized in terms of wired/wireless experiments, real-time or offline signal processing, and featuring carriers below or above 6 GHz. Remarkably, as indicated in Figure 4, OFDM and single carrier waveforms are the most employed waveform formats for the considered period of the last ten years. Nevertheless, advanced multi-carrier waveforms are acquiring increasing relevance to mitigate ISI and multipath impairments.  
The aim of some of these experiments is to emulate the behavior of a mmWave cell over an ARoF system for 5G. To correctly emulate such a case, it is necessary to use carrier frequencies above 6 GHz, including the wireless link and implement real-time signal processing (brown label). However, from Figure 4 we observe that only OFDM and single carrier modulation formats have been used under these conditions. Therefore, for the rest of the waveform formats, research under such complete conditions, especially including real-time signal processing and the inclusion of the mmWave wireless transmission, is required to fill this gap.  
On a further note, the future 5G will work with bands below and above 6 GHz for wireless transmission. Therefore, performance evaluations for the identified candidate waveforms in these  
Figure 4: Overview about the state-of-the-art (SoA) on the waveform formats in ARoF experiments corresponding to the last ten years.  
bands are required and a comprehensive comparison thereof is of great value in identifying the best candidates for beyond 5G. In this respect, these performance evaluations should include real-time signal processing to judge on their complexity and feasibility in as system context, as well as include actual wireless transmission to evaluate them under realistic channel conditions.