HSPA+ Evolution to Release 12: Performance and Optimization

15 LTE Interworking

Harri Holma and Hannu Raassina

15.1 Introduction

Long Term Evolution (LTE) networks are being rapidly rolled out but the deployment process will take some time. Therefore, there is a clear need for smooth interworking between 3G and LTE for coverage, capacity, and for service reasons. The interworking requires a number of different functionalities depending on the LTE use cases. We can differentiate the following phases of LTE network deployment:

• Data only LTE network for USB modems but not for smartphones. The interworking for the data connection is required from 3G to LTE when LTE coverage is available. Also the coverage trigger from LTE to 3G is needed to transfer the connection to 3G when running out of LTE coverage area.
• Data only LTE network, also for smartphones. The interworking solution for voice calls is required, Circuit Switched (CS) fallback, to move the UE from LTE to 3G when the voice call is initiated.
• Enhanced interworking for data, including redirection and handovers to minimize the connection break during the data transition. The first phase of data interworking may rely on simple functions based on reselections.
• Voice over LTE (VoLTE) to also carry voice in LTE requires interworking between Voice over IP (VoIP) and CS voice when running out of LTE coverage area. This is called Single Radio Voice Call Continuity (SRVCC).

The selection of the target system and target cell in the idle mode is done by the UE based on the network parameters, ...