Relation Between DRD for Service Steering and DRD for Load Balancing
"Inter-frequency DRD for service steering" is called "DRD for service steering" for short in this section.
"Inter-frequency DRD for load balancing" is called "DRD for load balancing" for short in this section.
When both DRD for service steering and DRD for load balancing are enabled, the general principles of inter-frequency DRD are as follows:
DRD for service steering takes precedence over DRD for load balancing, that is, preferably considers service priorities.
To services of the same service priority, load balancing applies.
For example, Universal Terrestrial Radio Access Network (UTRAN) f1, UTRAN f2, UTRAN f3, and UTRAN f4 in Figure 6-16 are inter-frequency cells with the same coverage. The service priorities of real-time R99 services in these cells are listed in the following table.
Cell Value of Service Priority of R99 Real-Time Service UTRAN f1 3
UTRAN f2 2 UTRAN f3 1 UTRAN f4 1
According to the principles of inter-frequency DRD, the RAB DRD of a real-time R99 service will select UTRAN f3 to make a CAC decision, as shown in Figure 6-16.
Figure 6-16 Example of inter-frequency DRD
Inter-Frequency DRD Procedure
If the UE requests a service in an area covered by multiple frequencies, the RNC selects a suitable cell for access based on the service priority in each candidate cell and the service type.
In addition, during cell selection, the RNC checks whether DRD for service steering and DRD for load balancing are enabled. Figure 6-17 shows the procedure.
Issue Error! Unknown Error! Unknown document property 21 Figure 6-17 Inter-frequency DRD procedure
Receive a service request
Is DRD for service steering enabled?
Is DRD for load balancing enabled?
Is DRD for load balancing enabled?
Determine candidate cells
CAC successful?
Initiate a blind handover
Access the current cell
CAC successful?
Initiate a blind handover
Determine a target cell in order
The procedure of inter-frequency DRD is as follows:
If DRD for service steering is enabled but DRD for load balancing is disabled, as shown in A in Figure 6-17, the inter-frequency DRD procedure is the procedure of DRD for service steering. For details, see Inter-Frequency DRD for Service Steering.
If DRD for load balancing is enabled but DRD for service steering is disabled, as shown in B in Figure 6-17, the inter-frequency DRD procedure is the procedure of DRD for load balancing. For details, see Inter-Frequency DRD for Load Balancing.
If both DRD for load balancing and DRD for service steering are disabled:
1. The UE attempts to access the current cell when its service priority is not 0. If the service priority of the current cell is 0, the UE attempts to access another candidate cell whose service priority is not 0.
2. The CAC algorithm makes an admission decision based on the cell status.
− If the admission attempt is successful, the RNC admits the service request.
− If the admission attempt fails, the UE attempts to access another candidate cell randomly.
3. If any request for access to a candidate cell is rejected, then:
− If the service request is an HSPA one, the HSPA request falls back to a DCH one.
Then, the algorithm goes back to step 1 to retry admission based on R99 service priorities.
− If the service request is a DCH one, the RNC initiates an inter-RAT DRD.
If both DRD for load balancing and DRD for service steering are enabled:
1. The RNC determines the candidate cells to which blind handovers can be performed. A candidate cell must meet the following conditions:
− The candidate cell supports the requested service.
− The frequency of the candidate cell is within the band supported by the UE.
− The quality of the candidate cell meets the requirements of inter-frequency DRD.
2. The RNC selects a target cell from the candidate cells for UE access.
Based on the relation between DRD for service steering and DRD for load balancing:
− The RNC preferably selects the cell with the highest service priority.
− If there are multiple cells with the highest service priority, load balancing applies to these cells. In this case, the RNC follows the same DRD logic as described in Inter-Frequency DRD for Load Balancing.
3. The CAC algorithm makes an admission decision based on the resource status of the cell.
− If the admission attempt is successful, the RNC initiates an inter-frequency blind handover to the cell.
− If the admission attempt fails, the RNC removes the cell from the candidate cells and then checks whether all candidate cells are tried.
a. If there is any candidate cell not tried, the algorithm goes back to step 2 to try this cell.
b. If all candidate cells haven been tried, then:
− If the service request is an HSPA one, the HSPA request falls back to a DCH one. Then, the algorithm goes back to step 1 to retry admission based on R99 service priorities.
− If the service request is a DCH one, the RNC initiates an inter-RAT DRD.
For details about the CAC procedure, see 7 "Call Admission Control Algorithm."
For details about inter-RAT DRD, see 6.4.5 "Inter-RAT DRD."