Good points made by @alemabrahao ... When an AP has multiple antennas, for modern wifi these are most often used for transmitting more data to the same client device simultaneously (the concept of spatial streams) - it's a bit like connecting multiple wires, for a wired connection - you can send more data at the same time. As I mentioned before, this helps increase the connected data rate, so you send what you need to send in less time, leaving more airtime for others. Note that, in order to take advantage of each spatial stream, the client device also needs the corresponding number of antennas. You'll find that higher end laptops and tablets will likely support more spatial streams than a smartphone, for example - and this is one reason why they can connect at higher data rates. Bear in mind that, if you 'speed up' the wifi for some users, who are capable of higher data rates, not only do you improve their experience, but there's more airtime left for everyone else - their user experience improves too.
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