This article is the continuation of my previous article. I request all to go on first article before studying it. In first article I explained the handover that was Intra-LTE handover using X2 interface. Now I am trying to put up the words for Inter RAT LTE handovers.
What do you mean by Inter RAT Mobility : LTE to 3G/GPRS/GSM or vice versa is called inter-RAT
- Done when there is limited LTE coverage.
- To balance load of new system 3G to LTE inter-RAT -Triggered by traffic management policies.
Why Inter-RAT HO is required?
- LTE will be available only in Urban areas during initial phase.
Therefore mobility across Radio Access Technology will be critical.
- Historically, handover is main reason for call drop so it is significant to minimize failures and drops during HO.
- Non-real time data should be considered as important and should not be dropped while executing HO procedure.
- Some instances Applications may have to re-authenticate to already established services e.g. streaming have to be restarted.
Steps to complete inter-RAT handover:
- Setting up measurements
- Evaluating measurement reports
- Handover Preparation
- Handover Execution
- Handover Completion
- Data forwarding
- If inter-RAT is to GSM, NACC(Network assisted Cell Change) is used and no resources are prepared in the target system. UE enter GSM system in RRC_Idle mode and start the RRC Connection Setup Procedure.
Description of above diagram:
eNB configure the UMTS measurements in the UE using the “RRC Connection Reconfiguration” message:
- It sets thresholds (serving Cell and inter-RAT neighbor cell) for activations and set events
- It gives WCDMA center frequency and scrambling code to be used.
- UE measures UMTS cells and sends RRC: Measurement Report
- eNB takes decision to perform HO to UMTS
- In order to prepare target UMTS cell, S-eNB sends S1AP: “Handover Required” Message to MME.
- This message contains target UMTS cell id
- MME then contacts SGSN based on UMTS Cell id and allocates the resources (using FWD Relocation Req. : Out of the scope or this discussion)
- Once MME gets confirmation (receipt of Relocation Complete message from SGSN) MME sends S1AP: “Handover Command” to eNB
eNB then sends “RRC: Mobility from E-UTRA Command” to UE indicating move to UTRAN.