Not only Wi-Fi cannot work as full-duplex, but also two or more devices cannot transmit or receive traffic simultaneously. Unlike 3G/4G, Wi-Fi uses unlicensed frequencies in the spectrum, which simply means you do not have to pay for using them. Not only frequencies used for WiFi are quite limited, but also other technologies use the same frequencies (bluetooth, zigbee, radar systems). On top of that, there is also a lot of interference on these frequencies (microwave ovens, wireless cameras). Long story short, at any time WiFi devices have to make absolutely sure that a certain frequency is "free" from other transmitions before they can even send one packet to each other. Even MU-MIMO can only allow a few devices to transmit or receive at the same time (still one-way).
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