Description of Handover form WCDMA to GRPS
The inter-RAT handover from WCDMA to GRPS caters for the handover from WCDMA PS domain service to GPRS system. The RNC initiatively commands UE to reselect an inter-RAT cell with signaling, which triggers inter-RAT handover. If the traffic flow for slow-speed PS services, the UE might be in CELL PCH or URA PCH state, the UE can perform inter-RAT handover by initiatively originating cell reselection according to system information.
Flows of Handover form WCDMA to GRPS
The inter-RAT handover flow initiatively originated by RNC proceeds as below: 7.10 and 7.10 shows the flow for handover from WCDMA to GPRS.
Figure 1.1 Flow of handover from WCDMA to GPRS (1)
SourceRNC
BSC
SGSN(3G)
Cell change order from UTRAN
UE
BTS
NodeB
I mm Ass
Chl Req
Chl Rqd
SGSN(2G)
RA update Req
SGSN Cntxt
Req
SGSN Cntxt Rsp
SGSN Cntxt Req
SGSN Cntxt Rsp
HLR
Send Authentication Info
Send Authentication ACK
Authentication and Ciphering Request
Authenti cati on and Ci pheri ng Response
SGSN Cntxt
ACK
forward packets
forward
packecks
GGSN
Update PDP cntxt Req
Update PDP cntxt Rsp
Update GPRS Location
MSC/VLR
(2G)
MSC/VLR(3G)
SRNS Data Forward
Command
Figure 1.2 Flow of handover from WCDMA to GPRS (2)
SourceRNC
BSC
SGSN(3G)
SGSN(2G)
NodeB
BTS
UE
HLR
Cancel Loc
GGSN
Iu Rel Cmd
Iu Rel Complete
RL Rel Req
RL Rel Rsp
Rel Req
Rel Conf
Cancel Loc
ACK
Insert Subscriber Data
Insert Subscriber Data ACK
Update GPRS Location ACK
MSC/VLR(2
G)
MSC/VLR(3
G)
Update Location ACK
Update Location
Cancel Location
Cancel Location ACK
Ins Subscriber Data ACK
Insert Subscriber Data
Update Location Accept
RA Update Accept
RA Update Accept Complete
TMSI Reallocation Complete
Location Update Req
7.10 shows the tracing signaling of handover from WCDMA to GPRS.
Figure 1.3 Tracing signaling of handover from WCDMA to GPRS
Signaling Flow at UTRAN Side
The signaling flow at UERAN side proceeds as blow:
The UE sends the measured 2D report, indicating the quality of the serving
cell is worse.
The RNC sends NodeB the RL RECONFIG PREPARE message, indicating
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 RNC the RL RECONFIG
READY message.
The RNC sends UE the PHYSICAL CHANNEL RECONFIG message,
indicating UE to prepare for starting compression mode. The message contains 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 confirms that the UE has received the PHYSICAL CHANNEL
RECONFIG message, it sends NodeB the RL RECONFIG COMMIT message, indicating the time for start compression mode.
After the UE completes related configuration according to the new
configuration data, it sends RNC the PHYSICAL CHANNEL RECONFIG COMPLETE message. This indicates that the compression mode is ready.
The RNC immediately sends the measurement control and commands UE
to perform inter-RAT measurement. The message contains 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.
GSM cells.
After the conditions are met according to judgment, the RNC originates the
SRNS relocation flow and sends UE the CELL CHANGE ORDER FROM UTRAN message. The message indicates UE to handover to the GPRS network by originating cell reselection. The message contains the IEs of target cell like BSIC and BAND IND (900 or 1800), BCCH ARFCN, and NC mode.
Because the UE need to reselect a GRPS cell, it powers off the transmitter
to WCDMA network. The NodeB sends the SIR ERROR report, which is optional in the flow.
Because the UE need to reselect a GRPS cell, it powers off the transmitter
to WCDMA network. The NodeB sends the RL FAILURE report, which is optional in the flow.
After the UE accesses the inter-RAT cell,
− If restoring the PDP context is not required, the RNC directly receives the IU
RELEASE COMMAND at the IU interface.
− If restoring the PDP context is required, the UE obtains the SRNS CONTEXT
information from the source RNC. The source RNC will receive the SRNS CONTEXT REQUEST message with mainly an RAB ID.
The RNC sends CN the SRNC CONTEXT RESPONSE message, indicating
the GTP of each RAB ID and the uplink and downlink sequence number of PDCP.
The CN sends RNC the SRNS DATA FORWARD COMMAND message,
indicating user plane to transmit data. By the message, the CN informs RNC of target transport layer address and tunnel ID of each RAB data forward.
After data is transmitted, the CN sends RNC the IU RELEASE COMMAND
message, indicating RNC to release the sources of the UE.
The RNC sends CN the IU RELEASE COMPLETE message. The message
18 and message 19 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.