Channel estimation: Note, the above procedure relies on accurate knowledge of the wireless channel, H, at the base station. Any error in estimating H leads to interference for the clients and reduces their data rate. First, let us focus on TDD systems, where the uplink and downlink happen on the same frequency. Recall, from Eq. 1, the wireless channel depends on the frequency and distance. Wireless signals travel the same paths on uplink and downlink. Therefore, for a base station-client pair using TDD, the channel for the uplink and the downlink are equal modulo some hardware factors that can be calibrated for. This principle is called reciprocity. The client transmits some pilot symbols known to the base station so that the base station can estimate the uplink channel, and use reciprocity to infer the downlink channel (which is just a constant multiplication to the uplink channel). In terms of overhead, this process requires just
标签：FDD,downlink,TDD,station,Overhead,base,uplink,channel From： https://www.cnblogs.com/sq-feng/p/18439496