MIMO

MIMO puts multiple antennas at both ends of a link and uses the fact that signals bounce around the environment differently to do two distinct jobs. Spatial multiplexing sends several independent data streams ("layers") on the same time and frequency, multiplying throughput. Diversity instead sends the same data over different paths to make the link more reliable in fading. Which one you get depends on channel conditions — a rich, scattering environment supports more layers; a clean line-of-sight path supports fewer.

A common misconception is that more antennas always means proportionally more speed. The number of usable layers is capped by the rank of the channel and by how decorrelated the paths are, not just the antenna count. In practice 4-layer downlink is routine for a good handset; the eye-catching 64T64R figures you see on base stations are mostly about beamforming gain and serving many users at once, not 64 streams to one phone.