# Candidate Waveforms for ARoF in Beyond 5G
## 5 Comparison of Candidate Waveforms
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Requirement\textbackslash{}Waveform** & **OFDM** & **FBMC** & **UFMC** & **GFDM** & **MCAP** & **SC-FDM** & \
\begin{tabular}{c} **SC** \\ **amp. Only** \\ \end{tabular} &
\begin{tabular}{c} **SC amp.** \\ **and Phase** \\ \end{tabular} \\ \hline Filter granularity & FB & SR & SB & SR & SB & - & - & - \\ PAPR & H & H & H & L & L & VL & VL \\ Spectral efficiency & H & VH & VH & VH & M/H & H & M/L & M/H \\ Block processing delay & M & H & M & H & M/L & M & L & L \\ Robust. to phase noise & M & M & M & M/H & M/H & M/H & H & M/H \\ Robust. to freq-selec. chan. & H & H & H & H & M/H & M/H & M & M \\ Robust. to time-selec. chan. & M & M/H & M & M/L & M & M/H & H & H \\ OOB emissions & L & VL & VL & L & L & L & M/H & M \\ Efficient MIMO integration & H & M & H & M & L & M & L & L \\ Enable async. multi. access & No & No & No & No & Yes & Yes & Yes \\ HW complexity & M & H & H & H & M/L & M/H & L & L \\ \hline \hline \end{tabular}  
* FB \(\rightarrow\) full-band. SR \(\rightarrow\) subcarrier. SB \(\rightarrow\) sub-band. V \(\rightarrow\) very. H \(\rightarrow\) high. M \(\rightarrow\) medium. L \(\rightarrow\) low.  
\end{table} 
Table 2: Comparison between different waveform formats for mmWave ARoF. support a huge quantity of devices with low power requirements, and ultra-reliable and low latency communications (URLLC) are targeted at mission critical communications where both low latency and superior reliability must be guaranteed. Thus, the optimal solution would be to select a suitable waveform format for each scenario for both uplink and downlink. It is necessary to highlight that an additional requirement is crucial in the uplink, resulting from the fact that the user performs a multicast transmission to the base station. Thus, to enable asynchronous multiple access is a primordial requisite for the uplink and hence, waveforms allowing asynchronous multiple access are very recommendable in the uplink. For eMBB, the main requirement is the bit rate. Therefore, the spectral efficiency of the selected waveform should be very high and it should be robust to the ARoF system to achieve high modulation orders. In this sense, FBMC and UFMC are the two selected candidates since their characteristics are better adapted to those requirements. Nevertheless, FBMC implies a complex MIMO scheme due to its method to estimate and compensate the channel. In its part, UFMC allows an easier MIMO system to accomplish high antenna gain and, in consequence, high signal-to-noise ratio (SNR). Therefore, UFMC is one of the best candidates to be the heir of OFDM in the eMBB downlink. Concerning the uplink, the waveforms that allow asynchronous multiple access and high spectral efficiency are multi-CAP and SC-FDM. Indeed, multi-CAP can further achieve a higher bit rate because it can adapt the modulation order depending on the SNR in each sub-band [31]. Thus, multi-CAP could be the heir of the SC-FDM for the uplink in eMBB. Following the mMTC requirements, the used waveform should support a huge number of devices. These devices transmit reduced amount of information as indicators of the system. Therefore, the bit rate is not a critical condition in this case. In mMTC, simplicity is a very relevant requirement because power consumption is of utmost importance and most devices have limited HW and SW. Sensors are a typical example for this type of scenario. Regarding the downlink in mMTC, OFDM is a proper candidate due to its simplicity, robustness to the ARoF system and capability to have smooth MIMO integration. For the uplink, multi-CAP and SC-FDM present better resilience to the ARoF impairments than the single carrier waveform options. Nevertheless, multi-CAP demands complex channel equalizer structure and it increases complexity for an ARoF scheme. Furthermore, its less efficient MIMO integration constitutes a problem in a system where there is a huge quantity of users employing the resources. Hence, SC-FDM still being the best option for the mMTC uplink, like in LTE.