The network structure in LTE is quite simple in principle (and actually, simplified with respect to the GSM and WCDMA structure): there is only a single type of access point, namely, the eNodeB (or BS, in our notation).1 Each BS can supply one or more cells, providing the following functionalities:

• Air interface communications and PHYsical layer (PHY) functions;
• Radio resource allocation/scheduling;
• Re-transmission control.

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 by the S1 interface to the 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

$\quad$ (i) a Mobility Management Entity (MME)
$\quad$(ii) the serving gateway (connecting the network to the RAN)
$\quad$(iii) the packet data network gateway, which connects the network to the Internet.

In addition, the Home Subscriber Server is defined as a separate entity. The structure is sketched in Figure.The core network fulfils, inter alia, the following functions: - Mobility management - Subscriber management and charging; - Quality of service provisioning, and policy control of user data flows - Connection to external networks.