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Tuesday, June 5, 2012

Radio Quality Measurements in LTE

There is a set of radio quality measurements specified by 3GPP. In particular, the definitions can be found in 3GPP 36.214 “E-UTRA Physical Layer Measurements.” These measurements are split into E-UTRAN measurements that are provided by the eNB and UE measurements reported by the handset. Especially for the E-UTRAN measurements, the 3GPP standards must be seen as an option and there is no guarantee that eNB vendors will implement them. In any case there is room for proprietary implementations, because there are no standardized measurement reports defined for the S1 interface.

It is expected that in most cases the E-UTRAN measurement results will be sent to the OMC via O&M interfaces using a proprietary protocol. Also, the binning of E-UTRAN measurements is – in contrast to the 3G UTRAN standards – not defined by 3GPP. The radio quality measurements can be split into UL and DL measurements. Looking at the UL measurements illustrated in below, the only measurement sent by the UE using a RRC measurement report is the UE Tx power, the power used by the handset to send the physical UL signal toward the eNB.

On the eNB side the following parameters can be measured by the base station:

·         Received Interference Power (RIP): This is the UL noise floor for a set of UL resource blocks.
·         Thermal noise power: This is the UL noise for the entire UL frequency bandwidth of the receiving cell without the signals received from LTE handsets.
·         Timing advance: This is roughly the time it takes for the radio signal to travel from the UE to the eNB’s receiver across the radio interface. Thus, it is equivalent to the distance between the UE and the cell’s antenna.

An air interface tester can provide in addition:

·         Channel baseband power: This is a measurement for the change in power amplitude of a particular physical channel in the time domain and can only be provided by an air interface tester.

·         I/Q constellation diagrams: These are used to check the quality of the modulated symbols of the received radio signals and can only be provided by an air interface tester. The measurement principle of an I/Q constellation diagram is to compare the received symbol pattern and shape to the ideal constellation points (i.e., the expected shape and pattern of the symbols). Any differences are visualized in real time in the changing pattern of the picture.

·         Error vector magnitude: This is a measurement related to the points in the I/Q constellation diagram. Basically the Error Vector Magnitude (EVM) is the metric that indicates how far the points are from the ideal locations. This measurement is only available from air interface testers.

Both I/Q constellation diagrams and EVM measurements are available for UL and DL physical channels and a separate measurement for each type of physical channel in the cell. To measure the DL quality the air interface tester must be used as a kind of drive test device and cannot remain connected to the CPRI of the eNB.

Also for the DL quality, a set of measurement tasks is performed by the UE (see the illustration below). Drive test equipment will perform the same measurement jobs but store the measurement results in its log files with higher granularity (typical time granularity for reference signal measurements: 1 second) and correlated with the true GPS location of the geographic measurement point. Regular subscriber handsets will send RRC measurement reports with reference signal measurements only in event-triggered mode, which means only if a predefined threshold is exceeded. Depending on the radio access capabilities of the handset and the availability of network coverage, the UE is able not just to receive reference signals from LTE cells, but also to listen to 3G UMTS and GSM cells on demand.

Hence, in the E UTRAN it is possible to monitor RRC measurements that contain the following parameters:

·         RSRP of LTE cells: This is the absolute signal strength of the LTE reference signal related in dBm (absolute signal strength compared to a reference level of 1mW). RSRP is the LTE equivalent of 3G UMTS RSCP.
·         RSRQ of LTE cells: This is the DL signal-to-interference ratio in dB measured on the LTE reference signals. RSRQ is the LTE equivalent of 3G UMTS Ec/N 0.
·         P-CPICH Ec/N0: This is the Primary CPICH chip energy over noise, the signal-to-interference ratio measured on the DL reference channel of the 3G UMTS cell.
·         P-CPICH RSCP: This is the absolute signal strength of the Primary CPICH (in dBm) measured on the UE’s receiver antenna and compared to a reference level of 1mW.
·         UTRA RSSI: The RSSI is the DL noise level measured on the UE’s radio receiver antenna for the entire bandwidth of the UTRAN or E-UTRAN cell.
·         GSM Carrier RSSI: This is the RSSI (in dBm) for the signal level of the GSM cells measured on the UE’s receiver antenna and compared to a reference level of 1mW.

Furthermore, 3GPP also specifies the measurement capabilities for the UMTS TD-SCDMA standard that was launched in China. Here the reference signal is the RSCP of the Primary Common Control Physical Channel (P-CCPCH). In those regions of China that have TD-SCDMA coverage the P-CCPCH RSCP will be reported instead of the P-CPICH Ec/N 0 and RSCP that are defined for the more global UMTS FDD standard.

On the network side the eNB is equipped with the capability to measure and report the DL reference signal transmission power, which is the average absolute power level of all DL resource blocks that are used to send reference signals. This value is equivalent to the P-CPICH Tx power that was not measured in 3G UMTS cells, but configured as a fix value during cell setup or reset.

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