Chapter 10. Data Flow, Radio Resource Management, and Mobility Management

Building on the physical layer procedures discussed in previous chapters, in this chapter we describe higher-layer protocols and mobility management in LTE. Radio resource management and inter-cell interference mitigation techniques will also be discussed in this chapter. However, before discussing higher-layer protocols, we first introduce the concept of bearer for Quality of Service (QoS) control and the protocol architecture of LTE.

LTE is a packet-switched network from end to end that is designed primarily for highspeed data services. To efficiently support the varying QoS requirements of different IP applications, LTE uses the concept of a bearer as the central element of QoS control. Each EPS (Evolved Packet System) bearer (bearer for short) is defined between the Packet Data Network Gateway (PDN-GW) and the UE, and maps to a specific set of QoS parameters such as data rate, latency, and packet error rate. Applications with very different QoS requirements such as e-mail and voice can be put on separate bearers that will allow the system to simultaneously meet their QoS requirements. The end-to-end connectivity through the network is made via the bearer service, and the bearer service architecture is shown in Figure 10.1. An EPS bearer has to cross multiple interfaces, and across each interface it is mapped to a transport layer bearer. An S5/S8 bearer transports the packets of an EPS bearer between a Serving GW (S-GW) and a PDN-GW, while an S1 bearer transports the packets of an EPS bearer between an eNode-B and an S-GW. Over the radio interface the bearer is referred to as the radio bearer, which transfers data between a UE and the E-UTRAN. Signaling Radio Bearers (SRBs) carry the Radio Resource Control (RRC) signaling messages, while the Data Radio Bearers (DRBs) carry the user plane data. Radio bearers are mapped to logical channels through Layer 2 protocols.