The UE reports its measurements to the eNodeB.
It is a key mechanism for:
- The Link adaptation, the selection of the modulation and coding rate.
- The e-UTRA mobility, in case of mobility in the e-UTRA coverage.
- The Inter-RAT mobility, if the UE is leaving the LTE coverage.
- The serving cell
- The e-UTRA neighbors
- The UTRAN cell
- The GERAN cell
- The CDMA2000 cell
- The UE measures the Reference Signal which is a cell specific signal.
- It depends on the Cell ID and so is different in 2 adjacent cells.
- It is sent every sub-frame
Here is the mapping of this signal on a sub-frame:
- Cell-specific reference signals shall be transmitted in all downlink sub-frames in a cell supporting non-MBSFN transmission.
- In case the sub-frame is used for transmission with MBSFN, only the first two OFDM symbols in a sub-frame can be used for transmission of cell-specific reference symbols.
The measurement quantities on the RS are:
- The Reference Signal Received Power (RSRP)
- It is defined as the linear average over the power contributions of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.
- The Reference Signal Received Quality (RSRQ)
- It is defined as the ratio N×RSRP/(e-UTRA carrier RSSI), where N is the number of RBs of the EUTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.
- The Received Signal Strength Indicator (RSSI)
- It comprises the linear average of the total received power observed only in OFDM symbols containing reference symbols, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.
The UE reports the Channel Quality Indicator (CQI) about its serving cell.
The CQI is used by the scheduler to select the most adapted MCS.
The CQI is reported:
- periodically by the PUCCH.
- aperiodically by the PUSCH.
In case multiple antennas are used, the UE can also report by means of the following channels:
- Precoding Matrix Indicator (PMI).
- Rank Indicator (RI).