# Candidate Waveforms for ARoF in Beyond 5G
## 5 Comparison of Candidate Waveforms  
Table 2 summarizes the different waveform formats analyzed throughout this paper in terms of the requirements discussed in Section 2. Thus, for each of those waveforms, different requirements are compared and analyzed in further detail. * Filter granularity: Both GFDM and FBMC need longer filter lengths as they perform it by subcarrier (narrow filter bandwidth). On the other hand, F-OFDM, UFMC, and multi-CAP use shorter filter length because its granularities are per sub-band (wide filter bandwidth) [17]. Filter granularity can only be associated with multi-carrier waveforms (full band, sub-band, and subcarrier). Therefore, GFDM and FBMC present higher latency that the rest of multi-carrier waveforms. * PAPR: From Table 2, we can observe that pure multi-carrier waveforms (OFDM, FBMC, UFMC, and GFDM) are associated with high values of PAPR. On the contrary, single-carrier waveforms present very low PAPR. It is worth noting that multi-CAP and SC-FDM both provide a low PAPR because they are not pure multi-carrier waveforms. On the other hand, any multi-carrier waveform can reduce its PAPR through different techniques. Nevertheless, they all increase the complexity in the system and, furthermore, include at least one of the following impairments [53]; power increase, bandwidth expansion, or BER degradation. For this reason, these techniques are not frequently considered as desirable. * Spectral efficiency: This indicator is very important in order to achieve the bit rate requirements for 5G and beyond. High spectral efficiency is reached with a multi-carrier waveform and it increases by augmenting the modulation order. Single-carrier waveforms on the other hand provide lower spectral efficiency due to their limitations in the spectral domain. Comparing the spectral efficiency among the multi-carrier waveforms, FBMC, GFDM, and UFMC are the best because of their CP structure. In particular, FBMC and UFMC do not use CP while GFDM requires low CP overhead. * Block processing delay: GFDM and FBMC present large block processing delays, as, among other reasons, its filter lengths are long, as mentioned previously. On the other hand, the block processing for the single-carrier waveforms is low because of their simplicity. Moreover, it is important to highlight that the final delay is intrinsically related to the symbol duration and thus, the latency is proportional to the subcarrier spacing in the case of FFT-based modulation formats. * Robustness to phase noise: Single-carrier amplitude only is the best option as it does not use phase modulation. Robustness to phase noise is a pivotal point for low-cost base stations as very sophisticated and expensive devices are necessary to reduce the phase noise in hardware [24]. Pure single-carrier waveforms are inherently robust to phase noise, and they are better than the multi-carrier waveforms in this aspect [24]. However, we have to consider that this type of robustness is proportional to spacing among the subcarriers in the multi-carrier waveforms as mentioned in Section 3. * Robustness to frequency-selective channels: Multi-carrier waveforms are better than single-carrier waveforms for this factor because frequency-selective fading will affect only a few subcarriers and not the entire band. That is, with adaptive bit loading, the impact of frequency-selectivity can be normalized. * Robustness to time-selective channels: Single-carrier waveforms present better behavior than multi-carrier waveforms since the ICI inherently affects the multi-carrier waveforms [24]. Furthermore, as explained in Section 3, this robustness is proportional to the spacing among the subcarriers in the multi-carrier waveforms. GFDM, in particular, is the worst option due to it needs long symbol duration, and therefore the changes of the channel strongly affect the GFDM symbol.