If the UE performs inter-RAT handover for CS domain services, the flow for CS domain handover from WCDMA to GSM is followed.
Description to Typical Handover Flow from WCDMA to GSM
The typical handover flow includes stages as below:
Measurement control > measurement report > handover judgment > handover implementation.
During the measurement control stage, the network informs UE of
parameters to be measured by sending the measurement control message.
During the measurement report stage, the UE sends the measurement
control message to the network.
During the handover judgment stage, the network decides to handover
according measurement report.
During handover implementation, the UE and network follow the signaling
flow and respond according to signaling.
When dual-mode UE moves at the edge of WCDMA system and might perform inter-RAT handover, the WCDMA RNC informs UE of starting inter-RAT measurement. After the UE performs inter-frequency measurement and reports measurement result, the RNC judges whether to start signaling flow for inter-frequency handover according to measurement result. The WCDMA system uses code division multiple access (CDMA) technology for access, so the connected UE in all time works with a specified frequency. When the dual-mode UE needs to perform inter-RAT measurement and keeps a conversation, it and the WCDMA system might start compression mode (if the UE has a transceiver, the starting compression mode is compulsory. If the UE has two transceivers, the UE can test GSM cells without starting compression mode).
Flows of Handover from WCDMA to GSM
Figure 1.1 Signaling flow for handover from WCDMA to GSM
UE
NodeB
RNC
3G MSC
2G MSC
BSS
Measurement Report(2D)
RL Recf g Prep
RL Recf g Ready
Physi cal Channel Reconf i gurati on
RL Recf g Commit
Physi cal Channel Reconf i gurat i on cmp
Measurement Control (I nt erRAT)
Measurement Report(GSM)
Rel ocati on Requi red
Rel ocati on Command
Prepare Handover
Handover Request
Handover Request ACK
Prepare Handover RSP
Handover From UTRAN CMD
Handover Compl ete
I U rel ease command
I U rel ease compl ete
RRC Con Rel ease Req
RRC Con Rel ease Cmp
RL Del Req
RL Del Rsp
7.8 shows the tracing signaling of handover from WCDMA to GSM
Figure 1.2 Tracing signaling of handover from WCDMA to GSM
Signaling Flow at UTRAN Side
The signaling flow at UTRAN side proceeds as below:
When the UE moves outwards at the edge of a cell in the WCDMA network
and the conditions for report 2D event meet the RNC configuration, the UE sends a measurement report of occurrence of 2D event. This report indicates that the signals at the serving frequency in the WCDMA network are weak and other frequencies or signals of other systems are required.
The RNC starts compression mode to perform inter-frequency and inter-
RAT measurement. The RNC sends the RL RECONFIG PREPARE message to NodeB to prepare for starting compression mode. The message contains the sampling sequence of compression mode and related parameters of sampling sequence of compression mode, including TGSN, TGL, TGD, TGPL, compression mode method, downlink compression frame type, and power control parameters in compression mode.
After the NodeB prepares resources, it sends the RL RECONFIG READY
message to the RNC.
The RNC sends PHYSICAL CHANNEL RECONFIG message to UE and
prepare for starting compression mode. This includes the activation time, the sampling sequence of compression mode and related parameters of sampling sequence of compression mode. The parameters include TGCFN, TGMP, TGSN, TGL, TGD, TGPL, RPP, ITP, compression mode method, downlink compression frame type, and power control parameters in compression mode.
After the RNC confirmed that the UE has received the PHYSICAL
CHANNEL RECONFIG message, it sends NodeB the RL RECONFIG COMMIT message, indicating the time for NodeB to start compression mode.
After the UE completes related configuration according to new configuration
data, it sends RNC the PHYSICAL CHANNEL RECONFIG COMPLETE message. Now the compression mode is available.
The RNC immediately sends the measurement control message, which
commands UE to perform inter-RAT measurement. The message includes
measurement parameters like the list of GSM cells, the information about frequency of cells, measurement filter coefficient.
The UE sends a measurement report, indicating the RSSI measurement
value of GSM cells.
The UE sends a measurement report, indicating the BSCI confirmation of
GSM cells.
After the handover conditions are met according to judgment, the RNC
sends a SRNS relocation request to CN. The request includes SRNS relocation type (the UE must participate in inter-RAT handover), reason for SRNS relocation
(usually relocation desirable for radio reasons), source PLMN, source SAI, and target CGI (including PLMN and LAC).
After the GSM side allocates related resources, the CN sends RNC the
RELOCATION COMMAND, which includes the IE layer 3 information. The IE contains the related resources allocated by GSM network.
The RNC sends UE the HANDOVER FROM UTRAN COMMAND message.
The message includes the RAB ID, activation time, GSM frequency, and GSM messages in forms of BIT string.
The UE powers off the transmitter according to GSM configuration, so no
signals are in uplink. Consequently the NodeB sends the SIR ERROR report. This message is optional in the flow.
After the UE accesses the GSM network, the CN sends the IU RELEASE
COMMAND message to inform RNC of releasing resources used by UE in the WCDMA network.
The message 16 and message 17 are to release the radio resources of NodeB. What is different from normal releasing flow is that the air interface does not send the RRC connection release message, because the UE is using WCDMA network. Therefore the NodeB releases radio resources without informing UE of the release.