# Compact MIMO System Performances in Metallic Enclosures
## 4 Experimental Results and Discussions
### Near-Field MIMO BER and EVM Measurements in Empty and Loaded Enclosures  
In the previous section, BER and EVM were measured in the presence of interference in an empty metal enclosure. Here, the BER performance was checked in an empty enclosure when the enclosure was loaded with losses in the RF absorber cones. Here, the BER performance of the near-field MIMO communication link was measured as a function of transmit gain, enclosure losses, and interference level. Figure 9 shows the measured BER of channel 1 and channel 2 in an empty metal enclosure at different interference gain values. It can be seen that there were BER values at lower Tx gain values as the interference was dominant at these values for all MIMO interference values. For the interferer Tx gain values, the BER of the two channels remained consistent, i.e., channel 2 had a higher BER  
Figure 8: Measured BER of USRP channel 1 and USRP channel 2 in the presence of different MIMO interference levels.  
Figure 7: Measured EVM of channel 1 and channel 2 in the presence of different levels of MIMO interference. Tx gain of the USRP is 10 dB.  
than channel 1. The BER of both channels started to degrade as the interference gain level increased. The BER curve for channel 1 flattened for a Tx gain of 70 dB.  
Figure 9c,d shows the EVM measurement of SDR channel 1 and SDR channel 2. It can be seen that the EVM measurements of channel 1 and channel 2 for all Tx gain values at the interference gain of 55 dB were less than the EVM measurements of channel 1 and channel 2 at the interference gain of 60 dB. The channel 2 EVM degraded at a high interference B210 USRP gain of 60 dB. The channel 1 EVM degraded less than channel 2 at all Tx gain values. The channel 1 EVM at the Tx gain of 10 dB degraded from 20.7% to 30.2%, which is a 45.89% increase in the EVM. Figure 10 shows the BER values as channel 1 and channel 2 when the metal enclosure was loaded with ten RF absorber cones. It can be seen that the measured BER degraded with the increase in the MIMO interference power. However, it required a higher MIMO interference power to obtain the same BER values of channel 1 and channel 2. Figure 11 shows the EVM measurements of SDR channel 1 and SDR channel 2, when the empty chamber was loaded with ten pieces of RF absorbers and the AGC gain was set to 50. It can be seen that both SDR channels have comparatively stable EVM values. This is because of the absorption effect of the absorbers and stabilized signal at the AGC output. In the next experiment, the effect of change in the AGC gain is demonstrated on EVM measurements of two USRP channels. EVM measurements were recorded by keeping the same environment and USRP configuration parameters and only changing the AGC gain from 50 to 60 dB.  
Figure 9: Measured MIMO BER of channel 1 and channel 2 in the presence of the MIMO interference when the metal enclosure was empty.  
Figure 10: Measured MIMO BER of channel 1 and channel 2 in the presence of the MIMO interference when the metal enclosure was loaded with ten pieces of RF absorber cones.  
In Figure 11, the measured EVM values of SDR channel 2 at Tx gain of 2 dB for all the interference levels are 86.1%, 86.4%, 86.2%, 86.8%, 85.9%, and 85.7% respectively.