# Beam Grouping Based RS Resource Reuse and De-Contamination in Large Scale MIMO Systems
## 4 Proposed BGRR Scheme for Reduction of Overhead of RS  
The proposed BGRR scheme divides a cell into several sectors adaptively depending on the parameters, i.e., UE distribution, the number of UEs, the size of angles for the sectors, etc. Then, the RS resources can be reused in a cell for independent sectors. Consequently, the required RS resources can be reduced in the system, while the performance depends heavily on the virtual sector separation. Two BGRR scenarios are considered with the proposed scheme. The first scenario is a sparse UE distribution (SUD) scenario. In this scenario, we can co-schedule and serve all UEs in the cell by using the proposed BGRR scheme. How to divide a cell into several groups is one of the important questions. First, it is considered that all of the groups have the same number of UEs. Second, it is considered that all of the groups have the same size of sector angles. Figure 3 conceptually shows the two cases of sparse UE scenario, when the number of group, \(v_{BGRR}=4\). The parameters for the figures are \(N_{t}=100\), \(K=12\). As shown in Figure 3a, in the given 120 degree cell, all of the groups have the same UEs, i.e., three UEs. It should also be noted that the angles of sectors/groups are different. As shown in Figure 3b, all of the groups have the same angle, 30 degrees, but the number of UEs in each angle is different, i.e., 5, 2, 4, 1. The second scenario is a dense UE distribution (DUD) scenario. If there are too many UEs in a cell, it is impossible to co-schedule and serve all of the UEs in the cell. In this case, we can choose part of the UEs that have favorable channel conditions for signal transmission/reception. Figure 4 shows the DUD scenario. Since there are a lot of UEs, we only choose three that have favorable channel conditions in each group, and angles are also decided with even 30 degree angles. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(\chi\)**(\%)** & **25\%** & **50\%** & **65\%** & **80\%** \\ \hline \(N_{t}(WCR)\) & 82 & 273 & 387 & 502 \\ \(N_{t}(WOCR)\) & 45 & 150 & 213 & 276 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Maximum number of allowable transmitter (TX) antennas in a given overhead of reference signal (RS), \(\chi\)(%). Figure 3: Sparse UE distribution (SUD) scenario, (**a**) the number of UEs; (**b**) the size of angles. Figure 5 shows the reduction of overhead of RS by the proposed BGRR scheme. Assuming each group has the same number of UEs, the expected overhead of RS with the proposed scheme can be derived as follows:  
\[\chi_{WCR_{BGRR}}(\%) = (\eta_{CRS}+(K+K\cdot\chi_{DM-RS})/v_{BGRR})/\eta_{RB_{tot}}\times 100, \tag{9}\] \[\chi_{WOCK_{BGRR}}(\%) = (\eta_{CRS}+(N_{t}+K\cdot\chi_{DM-RS})/v_{BGRR})/\eta_{RB_{tot}} \times 100. \tag{10}\]  
Here, we assume that the CRS is irreducible for backward compatibility, cell search and initial acquisition, etc. All of the other RSs are reducible by the reuse of resource with the BGRR scheme. As shown in Figure 5, the overhead of RS can be significantly reduced with the proposed scheme. Note that the overhead of RS is less than 35%, which is definitely acceptable as a system operation point of view, even with \(N_{t}=600\). Figure 4: Dense user equipment (UE) distribution (SUD) scenario. Figure 5: RS overhead (%) versus the number of TX antennas, \(N_{t}\) for 5 ms coherence time, \(v_{BGRR}=10\). In the case of the same size of angles, (9) and (10) can be expressed as follows:  
\[\chi_{W\text{C}R_{BGRR}}(\%) = (\eta_{CRS}+K_{g_{max}}+K_{g_{max}}\cdot\chi_{DM-RS})/\eta_{RB_{tot} }\times 100, \tag{11}\] \[\chi_{W\text{OCR}R_{BGRR}}(\%) = (\eta_{CRS}+N_{t}+K_{g_{max}}\cdot\chi_{DM-RS})/\eta_{RB_{tot}} \times 100, \tag{12}\]  
where \(K_{g_{max}}\) is the maximum number of UEs in the fixed angle groups. Note that the number of UEs in each group is different and therefore the RS resource is dependent upon the maximum number of UEs for the case of the same size of angles. Now, let us show how the proposed scheme can be realized.