Handover refers to the process by which a UE, while in RRC connected state, changes its service from one cell to another. Handover can generally be divided into intra-frequency handover, inter-frequency handover, and inter-system handover. Intra-frequency/inter-frequency handover refers to cell switching at different frequencies, while inter-system handover refers to handover between different systems (such as 4G communication systems and 5G communication systems).
This article mainly explains the handover between different systems. Interoperability generally refers to the handover between different systems, such as the handover from 5G to 4G, and the handover from 4G to 5G.
Example illustration:
The system measures the RSRP and RSRQ of the serving cell and neighboring cells, and reports the measurement results based on the previously distributed measurement objects and configurations from the network to trigger systemic events.
1. Measurement Report
Report the measurement measID as 13. Inform the current RSRP and RSRQ measurement results of 5G PCI 326, and the RSRP and RSRQ results of 4G neighboring cell PCI 256.
2. mobilityFromNRCommand (Toggle command)
The network issues a handover command, informing the user that the cell to be handed over to is 256. The command also carries various information about the cell, such as uplink and downlink frequencies, bandwidth, random access configuration, and radio resource configuration.
3. rrcConnecTIonReconfiguration
After switching to the LTE network, reconfiguration is performed in target cell 256. After the reconfiguration is completed, MIB and SIB information will be received, TA will be updated, etc. At this point, it can be basically determined that the current inter-system handover is underway (interoperability in the test case).
4. Now let's look back at the terminal measurement report information measId=13. measId 13 is composed of the measurement object ID 10 and the measurement report configuration ID 5.
5. In a reconfiguration signaling message issued by the network, confirm the measurement object 10 and the measurement configuration 5.measObject 10, and inform the measurement object of its frequency and bandwidth information.
The reportConfigId 5 configuration specifies the reportType as the triggering event (and periodic events), the event ID as the B1 event, the threshold value of B1, and other information.
In step 1, the information in the terminal measurementReport is only reported if it meets the requirements of the measurement object and measurement report. You can verify whether the information on both sides is consistent.
Since the specific command for measuring the Samsung chip platform could not be found, further analysis based on the measured RSRP and RSRQ is temporarily impossible. However, this does not affect the determination that this operation was a switch.
Additional notes:
NR event descriptions (see TS 38.331 for details): Category A events are events within the same system, while Category B events are events between different systems. Therefore, in the above 5G-4G handover, the measurement configuration reported by the terminal must include Category B events.
1.A1 Event
When the quality of the service area exceeds the absolute threshold, it is generally used to shut down the measurement.
2.A2 Event
If the quality of a serving cell falls below the absolute threshold, measurements between certain cells will be initiated because handover operations may occur after this event.
3.A3 Event
The quality of the neighboring cell is higher than that of the serving cell. This is generally used to enable measurement and can be used for inter-system cell handover.
4.A4 Event
The quality of neighboring cells is higher than the absolute threshold.
5.A5 Event
When the quality of the serving cell is below the absolute threshold and the quality of the neighboring cell is above the absolute threshold, it can generally be used for inter-system cell handover.
6.A6 Event
The serving cell quality is better than the secondary cell quality. This is generally used in carrier aggregation. The eNodeB automatically selects the better SCC cell to provide a high-quality Scell.
7.B1 Event
The quality of neighboring cells in a different system is higher than the absolute threshold.
8.B2 Event
The quality of the serviced community is below the absolute threshold, while the quality of neighboring communities is above the absolute threshold.
2. Reselect (4G-5G)
Reselection is the process by which a UE, while idle (not performing data or voice services; it is recommended to disable data services during testing), selects a new cell from one cell to another. Since 4G-5G reselection configurations are relatively rare in current networks, environments where a UE can reselect from a 4G network to a 5G network are uncommon. This article will primarily discuss 4G-5G reselection briefly.
Example illustration:
1. Confirm the NR information in SIB24, such as 5G frequency information (CarrierFreq-r15 627264), measurement timer configuration information, subcarrier spacing (subcarrierSpacingSSB), cellReselectionPriority, threshold value (threshX), minimum access level value (q-RxLevMin), measurement duration satisfies time t-ReselectionNR, etc.
The current 5G cell reselection priority is cellReselectionPriority = 7, which is higher than the reselection priority of the currently serving 4G cell (cellReselectionPriority = 6 in the SIB3 message).
SIB3 cellReselectionPriority=6
At this point, it is not necessary to satisfy Srxlev > SIntraSearchP and Squal > SIntraSearchQ to perform 5G measurements (frequency points issued by SIB24).
Therefore, the cells measured by 5G satisfy Squal > ThreshX, HighQ, and time >= t-ReselectionNR-r15. This means they can reselect from 5G cells to 4G cells.
From SIB24, we can obtain the values of Qqualmin = -28, Qoffsettemp = 0 (undefined), and ThreshX, HighQ = 1.
Qqualmeas, on the other hand, is the result of real-time measurement.
The 5G cells detected were 514, 341, 112, and 365.
Taking 514 residential area as an example, according to the formula:
Squal=Qqualmeas – Qqualmin – Qoffsettemp > ThreshX,HighQ=-15.00–(-28)–0>1
Calculations showed that all four cells met the first condition, while the second condition was that the time interval > t-ReselectionNR-r15. Therefore, the 5G cell information was measured again after a 1-second interval.
The second measurement detected 7 5G cells.
If the condition >= t-ReselectionNR-r15 is met, then cells 514, 341, 112, and 365 must be measured in both the first and second measurements. These must be confirmed separately.
Squal=Qqualmeas – Qqualmin – Qoffsettemp>ThreshX, HighQ
All four cells meet both conditions, so they are sorted according to RSRQ:
PCI 341: RSRQ = -13.35
PCI 365: RSRQ=-13.41>
PCI 541:RSRQ = -15>
PCI 112: RSRQ = -17.41
Reselect to 5G cell 341. Start receiving MIB, SIB and other information of this cell and register for 5G cell.
The re-election of neighborhoods 4 and 5 has been completed.
If you want to ask why RSRP is not used for sorting or for condition determination, this is specified in protocol TS36.304[5].
3. Redirection (4G-5G)
Like handover, redirection is performed in connected state using RRC, redirecting from one cell to another. However, redirection cannot carry services. Because both are performed in connected state, the terminal's choice between redirection and handover depends on whether the network has an N26 interface configured and whether both modes are enabled on the network side.
There are two ways to redirect
1. Measurement Redirection
Based on measurement redirection, the terminal measures neighboring cell information of different systems, and reports the measurement results (consistent with handover) after meeting the conditions. After receiving the results, the network sends RRRCRelease (indicating the frequency point information, etc., for redirection).
2. Not based on measurement redirection
Non-measurement-based redirection, also known as blind redirection, does not measure information about neighboring cells in different systems. Instead, the network directly sends an RRCRelease, carrying information about cells in different systems, instructing the terminal to redirect from the serving cell to that cell.
Example illustration:
1. Measurement Report
Based on measurement redirection, information about the 5G cell, such as PCI, RSRP, and RSRQ, is reported. For details on MeasID, please refer to the handover section.
2. RRRCrease
When redirecting from the LTE network back to the NR network, the RRC Connection Release carries redirection information and indicates the 5G frequency point information to be redirected.
