Well, TDD, which means the base station and the phones take turns transmitting on a single frequency, is still possible in 4G and 5G. How often the switch happens is configurable by the operator and in 4G can range from 1ms for phones and 4ms for base station, to 3 ms for phones and 2 ms for base station. So, just the directional switching shouldn't be the full story (and even though I used to work on base station radio software goes beyond my understanding. Too near the digital processing side, I guess :)