An Introduction to 5G

17Mobility Management in RRC_IDLE

This chapter covers the second of 5G's mobility management states, RRC_IDLE. In this state, the mobile is on standby, and is not expected to be transmitting or receiving. The mobile moves from one cell to another by a procedure known as cell reselection, and keeps the core network informed about its location by means of registration updates.

As in Chapter 16, we will assume that the mobile is controlled by the next‐generation radio access network (NG‐RAN) and the 5G core. When discussing the procedures for cell reselection in Section 17.2, we will also assume that the mobile supports standalone operation of the 5G New Radio and is camping on a 5G cell that is controlled by a gNB. We will discuss reselection between 5G and 4G cells within the NG‐RAN as part of that section, but we will leave reselection between the NG‐RAN and the evolved UMTS terrestrial radio access network (E‐UTRAN) until Chapter 19. The main specifications cover the signalling procedures in the NG‐RAN and the 5G core, and the mobile's procedures in idle mode [1-3].

17.1 Introduction to RRC_IDLE

17.1.1 Principles

In the state of RRC_IDLE, the mobile is registered with the 5G core network, and may have PDU sessions established with one or more external data networks. However, the mobile is on standby, and is not expected to be transmitting or receiving. The objectives are to maximize the mobile's battery life and minimize any signalling communications with the network.