In a radio network, the location of a UE changes continuously and each cell covers a limited area. The coverage-based 4G Intra-RAT handover ensures uninterrupted service provision.
After services establishes, the eNodeB delivers configuration information to the UE by using measurement control messages. The UE finishes the measurement, reports the MR, and then perform the handover under the control of the eNodeB. The following figures the connections between the UE and eNodeB.
Typical handover process: Measurement control > Measurement Report > Handover decision > Handover execution > New measurement control
Measurements
Measurement events are some of the most important steps within 4G intra-rat handover process, depending on the phase of the handover we have different types of measurements events:
As you can see, events A1 and A2 are involved in the decision to start or stop the measurements, those 2 events do not trigger handovers, just define whether the UE start or stop the measurements that will decide whether to trigger the handover or not (Events A3, A4, B1 etc.)
The usual combinations for the handover measurement and execution are:
- A2+A4 or A2+A3 mode is for inter-frequency handovers.
- The A2+A3 mode is usually for the inter-frequency handovers when the frequency bands of the source cell and target cell also partly overlap.
- Event A3 is also for Intra-Frequency Handovers, for this specific case, there is not measurement start/stop, this is because UE can measure all the time if needed. The measurement start/stop is required in other scenarios where measurement gaps are required (Measurements in different frequency than the serving cell)
Handover parameters are used to control when to report MRs and initiate handovers. The following figure, shows the basic procedure for intra frequency handover, some of the most common parameters involve from RF perspective are: TimeToTrigger, QoffserFreq, CellSpecificOffset, CellIndividualOffset etc.
Measurement Control message tells the UE what RF variables to measure (RSRP, RSRQ or both), conditions to start measurements (Event A1 or A2) etc.
When the conditions from previous message are met, the UE sends the Measurement report. Based on this report, the Source eNB decides whether to start the handover preparation or not.
Handover process
Handover preparation is mainly to prepare resources with target eNB to receive the UE after the handover.
Once the preparation finishes, the source eNB informs the UE to proceed with the Handover, UE receives information required to handover to the neighbor site.
For inter-frequency handovers, when conditions for Event A2 are met, the UE start measuring the target frequency. For this case, measurement gaps are necessary. These measurement gaps are time slots, where the UE stop receiving data from current serving cell, to measure the signal in the target frequency.
The inter-frequency handover process is the same as the intra-frequency handover process. However, the triggering processes are different.
For this type of handover, execution depends on the Event A4 or A5.
The next 2 figures, shows a detail about the 4G intra-rat handover for inter-enb X2-based handovers.
- Step 1 indicates the source eNB sends the measurement control message after services begins.
- In step 2, the UE reports the measurement report if requirements from step 1 are met.
- The source eNB then inform the target eNB of the handover preparation process.
- Source eNB sends the handover command message to UE.
- In step 11, the UE sends the handover confirm message after it successfully connects to the target cell through random accesses.
- The S1 link switches to the target side.
Common handover messages
Following figure shows the measurement control message, which specifies the parameter configuration that UE must follow to trigger the events.
When UE send the measurement report, it contains the PCI of the target cell and the measurements related to that PCI.
Usually, the first RRC Connection Reconfiguration message after measurement report is the handover command.
If the mobilityControlInfo IE exists in the RRC Connection Reconfiguration message, it is the handover command message.
The targetPhysicalCellId indicates the PCI of the target cell for 4G intra-rat handovers.
Handover feature group indicators
The UE Capability information messages indicate what LTE frequency bands are supported by the UE among other capabilities. The UE is handover only to these frequency bands.
The featureGroupIndicators IE indicates handover capabilities of the UE.
Bits 13, 14, and 25 indicate the inter-frequency handover capabilities of the UE.
The Bit 13 indicates whether the UE supports inter-frequency handovers. Bit 14 indicates whether the UE supports A4/A5 inter-frequency measurement report. Bit 25 indicates whether the UE supports inter-frequency measurement.
For more information about LTE handover, please check:
- https://www.telecomhall.net/uploads/db2683/original/3X/5/1/5169c76fff211e9a01e0454221881cf5cffb90c6.jpeg
- https://www.linkedin.com/pulse/handover-lte-techlte-world/
If you would like to learn more about RF optimization in LTE networks, please check these posts:
