The network structure in LTE is quite simple in principle with respect to the GSM and WCDMA structure. It is illustrated in Figure 1. The main components are as follows:
i) eNode B: There is only a single type of access point, namely, the eNodeB (or BS, in our notation). Each eNode B is connected to one or more cells, providing the following functionalities:
- Air interface communications and PHYsical layer (PHY) functions;
- Scheduling, radio resource allocation dynamically / to UEs in both up link and down link;
- Controlling mobility of UEs in connected mode;
- State transition from IDLE to Connected mode and vice versa;
- Admission control, congestion control and retransmission control;
- Buffering of the data during handover.
ii) Core network: For LTE, a new core network, called System Architecture Evolution (SAE) or Enhanced Packet Core (EPC) was developed. It is based on packet-switched transmission. It consists of, Mobility Management Entity (MME), Serving gateway (S-GW) and Packet data network gateway(PDN-GW).
Mobility Management Entity (MME): MME provides following functionalities:
a) Handles mobility management in idle mode,
b) Maintains UE context during IDLE mode of the UE,
c) Responsible for Non Access Stratum (NAS) signaling and security: The NAS is a functional layer between the User Equipment (UE) and the Mobility Management Entity (MME). It is used to manage the establishment of communication sessions and for maintaining continuous communications with the user equipment as it moves.
d) Does bearer management for the UE,
e) Subscriber management and charging,
f) Quality of service provisioning and policy control of user data flows,
g) Connection to external networks.
Serving GateWay (S-GW):
a) It connects the network to the RAN.
b) It acts as mobility anchor for the data bearers.
c) It buffers the downlink data when UE is in IDLE mode.
Packet Data Network GateWay(PDN-GW):
a) It connects the network to the Internet.
b) It allocates IP addresses to the mobile.
c) Filters downlink user IP packets into different QoS based bearers.
iii) Home Subscriber Server (HSS): It is defined as a separate entity as illustrated in Figure 1.
iv) Interfaces: Two interfaces are defined in LTE, X2 and S1. The X2 interface is the interface between different BSs. Information that is important for the coordination of transmissions in adjacent cells (e.g., for intercell interference reduction) can be exchanged on this interface. Each BS is connected to the core network by the S1 interface. Figure 2 illustrates the interfaces between various components of LTE network.