Handover from UTRAN to GSM can happen with or without prior measurements on target cell. The only criteria is UE should be in CELL_DCH state to do a IRAT handover from UTAN to GSM. Otherwise in other RRC states the UE does reselection to move to other radio access technology.
IRAT handover from UTRAN to GSM can be possible without any RABs, CS RAB. CS and PS RABs. But after handover only one CS RAB become active in GSM. That mens if the UE is doing a muliRAB call (CS + PS) before handover, UE will only have the CS RAB active after IRAT handover to GSM.
IRAT Handover from UTRAN to GSM is controlled by network and UE act when it gets command from network. Network sends a request to UE to move to GSM taking all the UE measurement results. It is also possible that network sends handover request to UE without measurements made by UE (bind handover).
Here is the basic call flow of IRAT handover from UTRAN to GSM in successful case
Inter-RAT Handover from UTRAN to GSM Callflow
The basic steps for a successful IRAT handover to GSM are:
- UE receives HANDOVER FROM UTRAN command from network with GSM HANDOVER COMMAND message included. HANDOVER COMMAND have all the information UE requires to continue normal services in GSM system.
- RRC in UE sends the HANDOVER COMMAND message content to RR layer.
- UE releases all UMTS radio resources. Now the UE will work like a GSM MS.
- Now MS send GSM HANDOVER ACCESS message to BSS. This message is sent several times to make sure receives this.
- BSS responds with PHYSICL INFORMATION message. This message contains timing advance information. After receiving PHYSICAL INFORMATION message UE stops sending HANDOVER ACCESS message.
- In response MS sends HANDOVER COMPLETE message to network.
The HANOVER FROM UTRAN message contains the RAB identity which will be active after handover, domain information (CS, PS) and target GSM frequency band (1900, 1800).