MIMO Channel Capacity Measurements in an Outdoor-to-Indoor Environment at 6 and 37 GHz

Extreme multiple-input multiple-output (MIMO) is identified as a future technology trend for IMT-2030 6G, evolving from MIMO in 4G and massive MIMO in 5G. A large antenna array not only forms a high-gain narrow beam to compensate for path loss but also enables multiple layers or streams that increase channel capacity. The classical Shannon theorem evaluates the single-input single-output (SISO) channel capacity. Applying a channel transfer function (CTF) matrix to Shannon's theory enables the estimation of the MIMO channel capacity. This paper compares MIMO capacity and SISO capacity in a similar propagation environment based on field measurement data in 6 and 37 GHz shared bands. Measurements were in fixed wireless access (FWA) like outdoor-to-indoor scenarios in a typical North American residential neighborhood. The channel sounder consists of two transmit antennas separated by two wavelengths and a receiver antenna moving on a virtual circular array (VCA). One thousand 2x2 CTF matrices are extracted from the measurement to calculate the MIMO channel capacity and capacity gain compared with SISO channels. The measurement was completed at 136 transmitter-receiver positions in various line-of-sight (LOS) and non-LOS (NLOS) conditions and both frequencies. The average 2x2 MIMO channel capacity is 1.7 times larger than SISO in the LOS condition at 6 GHz and 1.9 in the NLOS condition with rich multipath. This MIMO channel capacity gain is 1.7 at 37 GHz in the LOS condition and 1.8 in the NLOS condition. The MIMO capacity is further compared for various receiver (mimicking a consumer premise equipment, CPE) orientations, which indicates careful selection of CPE orientation in a FWA scenario is essential to optimize MIMO capacity in LOS conditions while it is insignificant in NLOS condition. Higher frequency is more orientation dependent.