# Compact MIMO System Performances in Metallic Enclosures
## 2 Measurement Setup  
The measurement setup consists of two X310 USRPs, four wideband antennas, a mode stirrer, and a high-performance Intel Xeon PC with P400-quadro NVIDIA GPU. We used a brass metal enclosure with dimensions of \(h\times\ell\times w\) of 45 cm \(\times\) 37 cm \(\times\) 55 cm. The metal enclosure is a perfectly reflecting environment; when excited at 5.6 GHz, it produces a rich multipath-fading environment. Near-field MIMO BER and EVM measurements were performed using wideband antennas at a frequency of 5.6 GHz in the metal enclosure. The distance between the transmitter and receiver \(D\) was kept fixed at 50  mm, and the distances between the transmit antenna elements \(d_{t}\) and receive antenna elements \(d_{r}\) were also kept fixed at \(d_{t}=d_{r}=45\)  mm for all RF measurements. The overall MIMO measurement setups for the near-field BER and EVM measurements are shown in Figure 1, where the vertically polarized ultra-wideband (UWB) transmit and receive antennas were mounted on the metal lid of the enclosure, such that Tx and Rx antennas faced each other. This scenario depicts a confined propagation environment where Tx and Rx were deployed in the near-field, expecting direct coupling as well as rich scattering introduced by a metal enclosure. Tx and Rx UWB antennas were connected to Tx and Rx X310 USRPs. The data transfer between the host PC and the USRPs took place over a 1 gigabit Ethernet cable. Real-time streaming of the IQ data was important for EVM and end-to-end BER performance; we checked the effect of the channel in configuration. The presence of any dropped samples, i.e., overflows at the receiver, introduces inconsistent and intractable performances. In our measurements, we did not observe any underflow and overflow when the Tx and Rx ports were configured to transfer IQ data to the host PC at sampling rates of 400 ksps. Therefore, the two-channel EVM and BER values were functions of hardware impairments and the channel conditions in the enclosure. Figure 1 shows the overall measurement setup, which consisted of two host PCs, two USRPs, OctoClock-G, metal enclosure, and ultra-wideband antennas. Figure 2 shows an internal view of the metal enclosure, including a stirrer and UWB antennas attached to the metal enclosure lid. It also shows the front view, side view, and top view of the broadband MIMO antenna configurations. Holes in the metal panel (other than what were used for different Tx and Rx inter-element spacings created for convenience) were covered with metal tape for RF shielding so that the measurements were not affected by external RF signals. BER measurements were performed using the QPSK receiver implemented in Simulink. Before running the model, the USRPs were frequency calibrated. The frequency offset of 17.19 Hz was found using the frequency calibration transmitter and receiver baseband models, which are tractable by the coarse frequency compensation block in the baseband receiver. The transmit baseband models were composed of frame generations, which contained the upsampled Barker-13 code as the frame header plus payload corresponding to the ASCII equivalent of 'Hello World\(\#\#\)', where \(\#\#\#\) is the repeating sequence from 001 to 099. The generated frames were QPSK-modulated with Gray mapping. The QPSK-modulated symbols were upsampled by the transmit raise cosine filter producing a baseband rate of 400 ksps at the transmitter. The transmit USRP sample rate should match the baseband sampling rate by setting the DAC clock rate of 200 MHz with an interpolation factor of 500 or the DAC clock rate of 20 MHz and an interpolation factor of 100. In a nutshell, the USRP sampling rate was selected as:  
\[\text{Sampling Rate}=\frac{\text{DAC}(\text{ADC})\,\text{Clock Rate}}{\text{ Interpolation}\,\text{(Decimation)}} \tag{1}\]  
Over-the-air transmitted QPSK waveforms were received by the Rx USRP with a sample rate of 400 ksps for the baseband processing that took place in the MATLAB session.