From a post on weibo:
(Google translated)
" . . . MU-MIMO requires routers and mobile phones and computers to support at the same time. Routers that support MU-MIMO technology can simultaneously serve multiple such mobile phones, allowing them to simultaneously transmit data without network congestion, improving performance while reducing power consumption.
The mobile phone needs corresponding cooperation to achieve the optimal Wi-Fi transmission speed. Different from the traditional single-antenna scheme of the traditional mobile phone, the Xiaomi MIX 3 adopts a 2x2 dual antenna design, which has dual receive channel 2RX and dual transmit channel 2TX. Dual antennas transmit and connect two streams simultaneously, effectively multiplying throughput and providing faster download speeds than 1x1 single antennas. . . .
Xiaomi MIX 3 is equipped with the most advanced network connection technology such as dual-frequency GPS, 2x2 dual-channel Wi-Fi and 4x4 MIMO (LTE). The measured performance is obvious. It can be said that the current network connection performance is the strongest Xiaomi mobile phone."
But the antenna area is 72% less than previous phones, so . . . it all needs to be seriously tested in real-life situations.