# Compact MIMO System Performances in Metallic Enclosures
## 1 Introduction  
Wireless communication system deployment in highly reflective and complex propagation environments, such as in a metal enclosure or a compact box environment, is extremely challenging [1, 2]. Near-field MIMO for high-data-rate communication is important for applications, such as wireless chip-to-chip communications and wireless network-on-chip (WNoC) [2, 3, 4, 5, 6, 7]. Traditionally, near-field communication was used for short-range, low-rate, and power-constraint applications using magnetically coupled loop antennas [8]. Near-field magnetic induction (NFMI) communication also uses magnetically coupled loop antennas for data transfer [9]. However, such near-field communication systems operate in media with high permittivities, such as underground [10, 11], biological masses [12], and underwater environments [13, 14]. Near-field communications in such environments are sensitive to alignment errors and offer limited data rates. This scenario represents a complex propagation environment where, in addition to LOS, there exists rich scattering as well as forward propagation through materials with high permittivity. Near-field MIMO digital communication system designs in such environments are considerably challenging and expose the benefits of MIMO signal processing [15]. There are channel modeling and characterization efforts for applications, such as wireless chip-to-chip communications; however, end-to-end near-field MIMO BER performances have not been demonstrated. Rx power measurements in near-field and path-loss measurements are rare in the literature. There is a significant gap in the assessment of the effects of near-field coupling on the performances of wireless communication systems. We checked the performance of a near-field MIMO communication system (in terms of EVM and BER measurements) in various scenarios in a stirred metal enclosure environment. The near-field MIMO communication scenarios in the presence of rich scattering may arise in different applications, such as the industry environment, animal cages [16], ICT equipment [17], kiosks [18], computer chassis [19], metal cabinets [20], andwireless inter-board communication [21; 22]. We previously presented over-the-air (OTA) testing of a QPSK receiver for the performance of baseband receiver design stages and the measurements were performed in a metal enclosure with all baseband receiver design stages [2]. In [1], we conducted near-field EVM measurements in a metal enclosure using a single transmit and a single receive antenna. We transmitted non-high throughput (nonHT) IEEE 802.11a OFDM packets for all modulation and coding techniques (MCS0-MCS7) in metal enclosures in different loading conditions. The measurements were performed with a single-channel full-duplex PlutoSDR from Analog Devices. In contrast, this work used two channels and a high-performance full-duplex X310 USRP from Ettus Research for EVM and end-to-end BER measurements in the metal enclosure in different channel conditions. Based on the application requirements X310 USRP can be interfaced with different RF cards, such as WBX and UBX-160. We used two UBX-160 per X310 USRP, which can offer an effective bandwidth of 160 MHz. We organized our paper into five sections. Section 1 highlights the near-field measurements in the metal enclosure. Section 2 discusses the X310 USRP-based 2 \(\times\) 2 near-field MIMO measurement prototype and the experimental setup, including the USRPs, metal enclosure, type of antennas, and mode stirrer. Section 3 discusses the measurement process, the important prototype development steps, and the measurement stages. Section 4 presents the experimental results of four different metal enclosure scenarios. It contains a detailed discussion about the near-field MIMO BER and EVM measurements and presents two-channel measurement data. Section 5 concludes the study and presents future extensions of the prototype and recommendations for near-field measurement scenarios.