## 6 Conclusions  
This paper has presented a thorough performance evaluation of digital precoding and analog beamforming schemes in a massive MIMO multi-cell deployment operating in the mmW frequency band. In the first part, a comparison among analog and digital precoding schemes has been carried out under ideal assumptions, namely, perfect and timely CSI at the transmitters, ideal phase-shifters  
Figure 10: CDF of user throughput for the ABF scheme with different values of \(P\) and \(L\). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{\(P=4\)} & \multicolumn{4}{c}{\(P=8\)} & \multicolumn{4}{c}{\(P=16\)} \\ \cline{2-13}  & \multicolumn{2}{c}{\(L=1\) dB} & \multicolumn{2}{c}{\(L=3\) dB} & \multicolumn{2}{c}{\(L=1\) dB} & \multicolumn{2}{c}{\(L=3\) dB} & \multicolumn{2}{c}{\(L=1\) dB} & \multicolumn{2}{c}{\(L=3\) dB} \\ \hline
**Thr.** & **Loss** & **Thr.** & **Loss** & **Thr.** & **Loss** & **Thr.** & **Loss** & **Thr.** & **Loss** & **Thr.** & **Loss** \\ \cline{2-13}
**[Gbps]** & [\%] & [Gbps] & [\%] & [Gbps] & [\%] & [Gbps] & [\%] & [Gbps] & [\%] & [Gbps] & [\%] \\ \hline
**Avg UE thr. LoS** & 1.775 & 5.25 & 1.518 & 18.99 & 2.693 & 6.15 & 2.082 & 27.43 & 3.180 & 8.45 & 1.941 & 44.12 \\
**Avg UE thr. NLoS** & 0.736 & 7.25 & 0.589 & 25.72 & 0.777 & 12.51 & 0.509 & 42.69 & 0.726 & 16.54 & 0.379 & 56.41 \\
**Avg UE thr.** & 1.436 & 5.52 & 1.215 & 20.10 & 2.067 & 6.89 & 1.574 & 29.08 & 2.377 & 9.23 & 1.438 & 45.11 \\
**5\%-ile UE thr. LoS** & 0.963 & 6.76 & 0.782 & 24.29 & 1.256 & 7.21 & 0.860 & 36.40 & 1.264 & 8.39 & 0.683 & 50.53 \\
**5\%-ile UE thr. NLoS** & 0.282 & 9.15 & 0.203 & 34.51 & 0.243 & 22.09 & 0.126 & 59.50 & 0.210 & 27.52 & 0.087 & 69.97 \\
**5\%-ile UE thr.** & 0.382 & 9.37 & 0.281 & 33.38 & 0.340 & 18.66 & 0.178 & 57.51 & 0.300 & 25.85 & 0.119 & 70.53 \\
**Avg cell thr.** & 23.63 & 5.64 & 19.98 & 20.20 & 33.90 & 6.95 & 25.58 & 29.79 & 38.97 & 9.50 & 23.19 & 46.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance results of the ABF scheme with combiner losses for different numbers of RF chains. and combiners in the analog scheme and lack of PAPC in digital precoding. MU schemes have been compared against ABF without spatial multiplexing of users, considered here as the baseline scheme. It was observed that by scheduling multiple users at a time, the throughput is substantially increased. Simulation results have further shown that analog beamforming schemes can reach the performance of fully digital precoders when working under LoS conditions and with a sufficient number of parallel RF chains. On the other hand, analog schemes have been shown to be more sensitive to interference and, thus, more degraded in NLoS conditions, in which increasing the number of RF chains can not provide any advantage. After the initial performance comparison, the effect of several realistic impairments has been included in the simulation setup. To this aim, the effect of having only outdated CSI for precoding at the BS has been firstly analyzed by considering a more realistic period to update the channel in all the evaluated schemes. In addition, PAPC have been set in the two considered digital precoding variants to also see the effect of this realistic constraint. Lastly, the errors introduced by both the phase shifters and combiners in the analog architecture have been modeled. From the results with outdated channel information, it can be concluded that digital precoding schemes are more sensitive to imperfections on the CSI knowledge at the BS, presenting large losses (up to 34%) when the CSI update period is increased. On the other hand, analog beamforming schemes have shown to be robust against the channel information inaccuracy, presenting a negligible reduction of performance when the channel reporting period is increased. The results with PAPC have exhibited a performance reduction in digital precoding schemes based on MRT of about 4%, which leads to a small reduction of the performance gap between digital and analog implementations.