MIMO
MIMO (Multiple Input, Multiple Output) uses slightly different signals between multiple sending and receiving antennas to increase throughput. Introduced in Wi-Fi 4 (11n), it turned an RF liability into an advantage. Indoor Wi-Fi results in a lot of reflections; very little signal goes directly to the receiving antenna and much of it bounces off of the walls, ceiling, and etc. The bounced signal arrives at a later time because it followed a different path, so this is called multi-path.
The problem is that this distorts signal. It is exactly analogous to trying to talk in an echoing chamber. In an amazing feat of engineering, Wi-Fi standards have been developed to take advantage of this by using multiple antennas. Advanced signal processing, which is part of every FortiAP, can take advantage of multi-path so that different transmit antennas can align with different receive antennas to each carry a different data stream. Two antennas on each end have the potential to double the data rate, three to triple it, etc.
The nomenclature 'Tx X Rx : SS' where Tx is the number of transit antennas, Rx is the number of receivers and the S is the number of spatial streams, or simply streams. There must be at least enough antennas to support the streams, but there can be more antennas than streams. Wi-Fi 4 (11n) MIMO was AP to one client only, and earlier APs often supported 3 antennas but only 2 streams.
Client capabilities always matter with Wi-Fi. While it is pretty reasonable to get 3 streams to a laptop, smart phones just do not have the space and battery power for enough antennas for effective multipath. Smart phones are generally not capable of simple MIMO and have a much lower capacity than laptops.
MU-MIMO (Multi-User MIMO), is the next evolution of the technology. MU-MIMO allows the AP transmit streams to be divided up among multiple clients. With MU-MIMO, an AP can transmit to one client with one or some antennas, and use other antennas to simultaneously transmit to another client. Fortinet's Wi-Fi 6 FortiAPs include support for both Uplink MU-MIMO as well as Downlink MU-MIMO.
