# Efficient User-Serving Scheme in the User-Centric Cell-Free Massive MIMO System
## 5 Simulation Results
### Simulation Analysis  
Figure 4 shows the number of unserved users in the proposed scheme and other cases. The simulation was performed through a total of 50 realizations, and both the number of APs and the number of users were set to 100. It is shown that unserved users did not occur in the given environment when the proposed scheme was applied. In this figure, UC indicates a user-centric scheme that does not apply any user-serving scheme. When no schemes were applied, it can be seen that the number of unserved users was very high. Furthermore, it was somewhat relaxed when the scheme in [20] was applied, but it was confirmed that there were still some existing unserved users. It is also noteworthy that when the scheme in [20] was applied compared to the case of UC, although the existing unserved users disappeared, the connection with the existing users was disconnected, and new unserved users were generated. However, the unserved users disappeared when the proposed scheme was applied; therefore, it can be seen that the proposed scheme works properly. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Area & \(1\times 1\) km\({}^{2}\) \\ Number of antennas for AP & 2 or 4 antennas \\ Number of antenna for users & Single antenna \\ User antenna height & 1.65 m \\ \(\rho_{p},\rho_{d}\) & 100 mW, 200 mW \\ Length of the pilot sequence & The number of users or 60 \\ Modulation & QPSK \\ Bandwidth & 20 MB \\ Noise figure & 9 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation Parameters and Schemes. Figure 4: Realization of unserved-user for four cases. The simulation in Figure 5 shows the sum rate for each case in three different situations, and it was also implemented in an environment with 100 APs. There were three different situations as follows:  
* Imperfect CSI and orthogonal pilot per user. * Imperfect CSI and only 60 orthogonal pilot sequences in the system. * Perfect CSI and orthogonal pilot per user. If there were more than 60 users in the situation of B, the pilot allocation method in [8] was applied, and the imperfect CSI environment was implemented as the MMSE estimator in Section 2.1. As can be seen from Figure 5, when the user-serving schemes were applied, the total sum rate increased at the beginning as the number of users increased; then, it decreased after 100 users due to the low-rank problem in all situations from A to C. Generally, in the cell-free massive MIMO network model, only a small number of antennas are installed in each AP and performance degradation is inevitable when the number of users increases. However, it is confirmed that the decrease rate in the proposed scheme was smaller than the scheme in [20]; moreover, the proposed scheme achieve a higher sum rate than the previous scheme in all situations. The proposed scheme serves new users while maintaining the connection with existing users. The performance of bit error rate (BER) for the four cases is shown in Figure 6. In this simulation, the number of APs and users was set to \(L=K=100\). Since the number of users was 100, the unserved users were likely to occur in other cases except for Full CF. In this paper, we assumed that the BER of the unserved users was 0.5 to obtain effective BER performance. It is confirmed that when the proposed scheme was used, the number of unserved users was greatly reduced compared to UC, and at the same time, a slightly improved BER performance was obtained compared to the scheme of [20]. Since CB (Conjugate Beamforming) was applied to the precoding algorithm, and all four cases corresponded to a very dense environment in which the number of users was set equal to the number of APs, the overall BER performance in the simulation shown in Figure 6 was rather poor. Figure 5: Sum rate performance for four cases. The CDF performance for four cases is shown in Figure 7. In this simulation, two antennas were installed in each AP, and it was assumed that there were \(L=100\), \(K=60\) in the area.