IRAT basics and WCDMA to GSM IRAT handover
This page describes Inter RAT, IRAT and IRAT handover basics with example of IRAT handover between WCDMA to GSM RAT.
What is IRAT
IRAT stands for Inter Radio Access Technology. RAT can be either GSM,CDMA,WiMAX,LTE,TD-SCDMA or any other wireless technology used to provide air interface to mobile to obtain various services(voice,email,internet,VOIP).
IRAT handover from WCDMA to GSM
IRAT is mainly used for handover between different RAT. Mobile terminal while in one RAT performs neighbour cell measurements and sends measurement report to the network. Based on this measurement report provided by mobile terminal, network can initiate handover from one RAT to the other RAT. This is called IRAT handover.
Following figure depicts typical message flow between mobile terminal and network for handover from WCDMA to GSM. The same way handover from GSM to WCDMA network is also performed. The difference here is that measurement report is sent by GSM mobile terminal to the GSM network(BTS). The information reaches to the core network.
As core network is interconnected between both the RATs(GSM and WCDMA), it plays a vital role to perform handover between RATs. Once the handover with the new RAT is completed previous RAT channels are released.
Based on the measurements made by UE (in WCDMA Mode) decision for handover is taken. Handover is possible even without UE performing measurements. This is based on inputs from CN and BSS. The procedure is outlined stepwise below.
1. HANDOVER FROM UTRAN command is initiated at UTRAN and sent to UE.
The command includes all the information needed by mobile to complete handover with GSM cell.
2. The RRC layer sends the message contents to GSM RR layer.
3. At this step, UMTS radio resources will get released.
4. Now, Mobile sends GSM HO Access message to BSS of GSM as it is now in GSM RAT.
5. BSS transmits PHYSICAL INFORMATION message to Mobile with timing information.
6. Mobile sends the HANDOVER COMPLETE message.
7. At this step other UMTS resources are also released.
HANDOVER FROM UTRAN consists of integrity check information, activation time, RAB information and GSM message. GSM HANDOVER COMMAND consists of cell description, channel description, handover reference, multi-slot configuration, time information and codec information.
For IRAT handover inter RAT measurements is very important as even while mobile (UE) is active in one mode(or RAT), it will continuously does measurements for other RATs for handover decision. Network also keep measuring received signal strength from the mobile for commanding UE(mobile) to perform handover. Refer IRAT MEASUREMENTS for more.