As UEs have become smarter and the air interface has become more complex, the need for more detailed communication between UE and the network has increased dramatically. The UE has become the eyes and the ears of the radio access network. Its main goal is to find the best cell and maximize performance with that cell.
To help the UE accomplish this, the eNodeB gathers several pieces of information from the UE and then adjusts its’ downlink transmissions accordingly. The UE tells the network what cells it can see and how strong it can see them, its current channel conditions, the current state of its’ memory buffer, which antennas should be transmitting in the downlink, how many different transmission streams can be supported simultaneously, acknowledgements when data is received successfully, and any other information that the network wants to know.
The measurement report is the mechanism used by the UE to tell the network whatever results have been requested. Typically, these are measurements of the surrounding cells. They can also include requested measurements of block error rate; transmit power and other UE-based parameters. The UE learns the requested information using a measurement configuration.
When a UE is in RRC-CONNECTED mode, this measurement configuration is provided to the UE by means of dedicated signaling; typically using the RRCConnectionReconfiguration message.
The measurement configuration provided to the UE includes the following parameters:
1. Measurement Objects: the objects on which the UE shall perform the measurements; i.e. frequencies and cells. In other words: who should the UE measure? These include intra- and inter- frequency neighbors, IRAT UMTS neighbors, IRAT GSM neighbors and IRAT CDMA2000 HRPD and 1xRTT neighbors.
2. Reporting Configurations: the criteria used by the UE to trigger the transmission of a measurement report and the quantities that the UE includes in the report. In other words: when should the UE send a report? This trigger can either be periodical or event-based.
3. Measurement Identities: an identifier that links one measurement object with one reporting configuration. In other words: the UE needs to keep track of the objects to be measured and their specific triggers. The measurement identity is used as a reference number in the measurement report.
4. Quantity configurations: the measurement quantities and associated filtering used for all event evaluation and related reporting per Radio Access Technology.
5. Measurement gaps: periods of time that the UE may use to perform measurements while in connected mode.
The UE maintains a single measurement object list, a single reporting configuration list and a single measurement identities list. Any measurement object can be linked to any reporting configuration of the same RAT type.
As mentioned earlier, a report can be event-triggered or periodical. An event-based measurement report will be transmitted when the criteria for any of the following events have been met:
A1: Serving Cell becomes better than a defined threshold
A2: Serving Cell becomes worse than a defined threshold
A3: Neighbor cell becomes some offset better than the primary cell
A4: Neighbor cell becomes better than a defined threshold
A5: Primary cell becomes worse than a defined threshold and a neighbor becomes better than a second threshold
A6: Neighbor cell becomes some offset better than the serving cell
B1: Inter-RAT neighbor becomes better a defined threshold
B2: Primary cell becomes worse than a defined threshold and inter-RAT neighbor becomes better than a second threshold
Periodical measurement reports are sent based on the reporting configuration. For instance, it could be configured that the UE report its’ transmit power every 2 seconds or its’ transport channel block error rate every second. This is operator-specific.
We have discussed the vehicle used by the UE to tell the network what it can see as well as other operator-configured parameters. Another piece of information that the network needs to be successful is some indication of the channel conditions at the UE. This will help the network adapt its downlink transmission to match the UE’s capability at that time. These channel quality indicators or CQI’s will be the subject of the next blog in this series.