An Introduction to 5G

10Link Adaptation

As radio signals travel from the transmitter to the receiver, they are subjected to attenuations and phase shifts. The base station and mobile measure those attenuations and phase shifts, so as to compensate for them. If the system is using analogue beam selection, then the base station and mobile use related measurements to identify the best spatially filtered transmission and reception beams. We will address those issues in this chapter.

We will begin with the downlink, in which the processes are more transparent. The base station transmits channel state information reference signals (CSI-RSs), which the mobile uses to compute channel state information (CSI) that describes the state of the downlink radio channel. The mobile returns that information to the base station for use during scheduling, using the physical uplink shared channel (PUSCH) if it is transmitting uplink data in the same slot, and the physical uplink control channel (PUCCH) otherwise. On the uplink, the base station tells the mobile to transmit sounding reference signals (SRSs), calculates the state of the uplink channel by inspecting the signals that arrive and uses thee information internally.

In the absence of channel reciprocity the two processes are independent, but if channel reciprocity applies then shortcuts are possible. For example, the base station might use the SRS to adapt to most aspects of the downlink radio channel, but might assist that process using interference measurements ...