An Introduction to LTE: LTE, LTE-Advanced, SAE, VoLTE and 4G Mobile Communications, 2nd Edition

Chapter 15Inter-operation with UMTS and GSM

In the early stages of rolling out the technology, LTE has been available only in large cities and isolated hotspots. In other areas, network operators have continued to use older technologies such as GSM, UMTS and cdma2000. Similarly, most LTE mobiles are actually multiple mode devices that also support some or all of those other technologies. To handle this situation, LTE has been designed so that it can inter-operate with other mobile communication systems, particularly by handing mobiles over if they move outside the coverage area of LTE.

In this chapter, we discuss the most important issue, namely inter-operation with the earlier 3GPP technologies of UMTS and GSM. There are two possible inter-operation architectures: one requires enhancements to the 2G/3G packet switched domain to make it compatible with LTE, while the other requires extra functions in the evolved packet core that make it backwards compatible with the older systems. The specifications support mobility between LTE and UMTS or GSM in both RRC_IDLE and RRC_CONNECTED, and include the option for optimized handovers that transfer mobiles with no packet loss and with a minimal break in communications. We delay any discussion of non-3GPP technologies until the next chapter and any discussion of the 2G/3G circuit switched domain until Chapter 21.

This chapter uses similar specifications to the previous one. The most important ones cover the procedures that a mobile should ...