## 5 Experimental Verification
### Measurement of the 4 \(\times\) 4 MIMO Uplink Channel Capacity  
The MIMO channel capacity was measured using a two-dimensional bidirectional fading emulator to evaluate the performance of the four-element MIMO terminal uplink channel. Figure 16 shows the 4 \(\times\) 4 MIMO (four elements in the mobile terminal and four elements in the BS) channel capacity as a function of the incoming wave angle with the SNR as a parameter. The MIMO terminal illustrated in Figure 14 was placed at the center of the bidirectional fading emulator. The round and square symbols denote the OTA measurement results for the circular and linear arrays at the BS, respectively. The solid and broken curves indicate the Monte Carlo simulation results for the circular and linear arrays at the BS, respectively. The SNR was changed from 10 to 40 dB in 10 dB intervals [35]. The frequency was 1.95 GHz, and the XPR was set to 50 dB, which is equivalent to a vertically polarized propagation environment. \begin{table}
\begin{tabular}{c c} \hline Frequency & 1.95 GHz \\ \hline Number of MIMO antennas & 4 \\ \hline Arrangement of MIMO antenna & Quasi-liner array \\ \hline MIMO antenna element & Half-wavelength dipole \\ \hline Interval of MIMO antenna & 0.5 \(\lambda\) \\ \hline Number of BS antennas & 4 \\ \hline Angular spread of the incident wave & 1.5\({}^{\circ}\) \\ \hline Number of probes & 14 \\ \hline Traveling distance & 5000 \\ \hline Number of samplings & 5000 \\ \hline XPR & Vertical polarization only \\ \hline Method of EM analysis & Method of moments \\ \hline \end{tabular}
\end{table} 
Table 2: Measurement and analytical conditions. It can be observed from Figure 16 that the results measured via OTA testing are in good agreement with the analytical results obtained from the Monte Carlo simulations regardless of the BS arrangement and incoming wave angle. In the case of \(\phi=90^{\circ}\) in the linear array, the measured channel capacity is slightly larger than the analytical value. A possible cause of this phenomenon is the BS correlation being insufficiently close to 1 because this apparatus cannot achieve a time correlation of 1. As shown in Figure 6, when the correlation coefficient approaches 1, the MIMO channel capacity deteriorates significantly. However, it is possible to evaluate the performance of the MIMO terminal because a 0.99 BS correlation can be achieved using this bidirectional fading emulator. Similarly, with a circular array at the BS, the same phenomenon was observed when the BS correlation was 1 depending on the incoming wave angle. Figure 15: Characteristics of the BS correlation as a function of the incoming wave angle: (**a**) circular array; (**b**) linear array. In the case of a circular array at the BS, shown in Figure 15, the six BS correlations do not have the same characteristic because the correlation increases or decreases depending on the incoming wave angle. Therefore, the MIMO channel capacity does not change significantly with respect to the incoming wave angle. In the case of a liner array at the BS, the six BS correlations are inclined in the same direction depending on the incoming wave angle. Consequently, the MIMO channel capacity varies significantly with respect to the incoming wave angle compared with the case of a circular array. In particular, when the SNR is 40 dB, there is a difference of approximately 35 bits/s/Hz between the incoming wave angles of 0\({}^{\circ}\) and 90\({}^{\circ}\). Moreover, as shown in Figure 16, it can be seen that the channel capacity of the linear array is higher in the range of 0\({}^{\circ}\)-65\({}^{\circ}\) regardless of the SNR, whereas that of the circular array is higher in the range of 65\({}^{\circ}\)-90\({}^{\circ}\). Comparing Figure 15a,b, the BS correlation for the linear array is lower than that of the circular array in the range of 0\({}^{\circ}\)-65\({}^{\circ}\); thus, the channel capacity is higher.