IRAT Basics: WCDMA to GSM Handover Explained

This page describes Inter Radio Access Technology (IRAT) and IRAT handover basics, with an example of an IRAT handover between WCDMA and GSM Radio Access Technologies (RATs).

What is IRAT?

IRAT stands for Inter Radio Access Technology. RAT can be GSM, CDMA, WiMAX, LTE, TD-SCDMA, or any other wireless technology used to provide an air interface for mobile devices to obtain various services like voice, email, internet, and VOIP.

IRAT Handover from WCDMA to GSM

IRAT is mainly used for handovers between different RATs. While a mobile terminal is operating on one RAT, it continuously performs neighbor cell measurements and sends measurement reports to the network. Based on these measurement reports, the network can initiate a handover from one RAT to another. This is called an IRAT handover.

The following figure depicts a typical message flow between a mobile terminal and the network for a handover from WCDMA to GSM. A handover from GSM to WCDMA is also performed in a similar manner. The key difference is that the measurement report is sent by the GSM mobile terminal to the GSM network (BTS). The information then reaches the core network.

As the core network is interconnected between both RATs (GSM and WCDMA), it plays a vital role in performing handovers between the RATs. Once the handover to the new RAT is completed, the previous RAT channels are released.

The decision for a handover is based on the measurements made by the UE (User Equipment) in WCDMA mode. Handover is possible even without the UE performing measurements, relying instead on inputs from the Core Network (CN) and Base Station Subsystem (BSS).

The procedure is outlined step-wise below:

1. HANDOVER FROM UTRAN command: Initiated at the UTRAN (UMTS Terrestrial Radio Access Network) and sent to the UE. This command includes all the information needed by the mobile to complete the handover to the GSM cell.
2. The RRC (Radio Resource Control) layer sends the message contents to the GSM RR (Radio Resource) layer.
3. At this step, the UMTS radio resources are released.
4. Now, the mobile sends a GSM HO Access message to the BSS of the GSM network, as it is now operating on the GSM RAT.
5. The BSS transmits a PHYSICAL INFORMATION message to the mobile with timing information.
6. The mobile sends the HANDOVER COMPLETE message.
7. At this step, other UMTS resources are also released.

The HANDOVER FROM UTRAN command consists of integrity check information, activation time, RAB (Radio Access Bearer) information, and the GSM message.

The GSM HANDOVER COMMAND consists of cell description, channel description, handover reference, multi-slot configuration, time information, and codec information.

For IRAT handovers, inter-RAT measurements are very important. Even while the mobile (UE) is active in one mode (or RAT), it continuously performs measurements for other RATs to aid in the handover decision. The network also continuously measures the received signal strength from the mobile to command the UE (mobile) to perform a handover.