LTE power control is applied to uplink transmission only. There is no downlink power control. The objective of power control in LTE is to reduce terminal power consumption and minimize the power dynamic range in the eNodeB receiver.
The LTE uplink power control aims to keep the Power Spectral Density (PSD) (watt/Hertz) constant for a particular UE instead of controlling the absolute power level. When MCS and data rate change, transmitted bandwidth also changes leading to a respective change in the absolute power level of the UE. The power ensures that the absolute power level of the UE is adjusted in a way to keep PSD (W/Hz) unchanged.
Compared with 3G, the power control mechanism in LTE is relatively slow with a loop delay of 5 ms, and also differs in irregular steps in power adjustment.
With power control operation, the eNodeB uses path loss estimation from the open-loop mechanism to set a coarse operating point for the transmission PSD that is estimated as sufficient in the current cell propagation environment. This estimate includes input from the UE’s measurement of downlink path loss based on the power level of the downlink Reference Signal.
The overall power control mechanism in LTE includes rescheduling uplink transmission resources to different users based on an instant transmit power used by the UE across the cell. In order to assist in resource redistribution, the UE can report its available power headroom to the eNodeB. The eNodeB uses the power headroom report to predict what additional bandwidth can be used by reporting terminal. In a case where the uplink resource grant requires more transmission power than the UE has available, the UE signals to eNodeB a negative headroom report. The eNodeB will reduce the size in RB of the next uplink grant and reallocates free resources to the other UEs in the cell.