LTE IRAT Introduction

As par 3GPP there will be both Idle and Connected mode connectivity available/provided between LTE and other RATs. (UMTS/3G and GSM/2G).

Idle mode procedures are always triggered by UE and LTE IDLE UE can reselect target RAT based upon

• SIB's (SIB6 and SIB7)
• Without SIB's

 SIB-6 carries UTRAN broadcasted information whereas SIB-7 carried GERAN broadcasted information.

If there are no SIB's and UE wants to enter into IRAT in IDLE mode then it is done (only possibility) from LTE by triggering Out-of-Service.

Connected Mode Procedures are triggered by Network

• Redirection
• Handover

Handover and Redirected Information UE gets from NW initiated RRC Connection Release message. Different flavors of Redirection/Handover is either RRC message based, Blind / Non-Blind, RF quality based.

To Summaries
[A] Idle mode procedures

• LTE to W-CDMA/GSM cell re-selection
• W-CDMA/GSM to LTE cell re-selection

[B] Connected Mode procedures

• LTE to W-CDMA/GSM redirection/Handover
• W-CDMA/GSM to LTE redirection/Handover