LTE Advanced Architecture and Protocol Stack

This document outlines the architecture and protocol stack of LTE Advanced (LTE-A). It describes the architecture of the E-UTRAN and details the user and control planes for both the Access Stratum (AS) and Non-Access Stratum (NAS) layers within the protocol stack.

LTE Advanced Architecture

The E-UTRAN architecture for LTE-Advanced is illustrated below.

Figure 1: LTE Advanced E-UTRAN Architecture

The LTE-A architecture for E-UTRAN comprises the following key entities: P-GW, S-GW, MME, S1-MME, eNB, HeNB, HeNB-GW, and Relay Node. Let’s delve into the function of each of these components:

• P-GW (PDN Gateway): The P-GW acts as the gateway to packet data networks (PDNs). It connects to the S-GW via the S5 interface and to the operator’s IP services through the SGi interface. It also interacts with the PCRF (Policy and Charging Rules Function) using the Gx interface. The P-GW assigns IP addresses to User Equipments (UEs) and enables a UE to connect to multiple PDNs simultaneously for access to various services. This component also handles packet filtering, policy enforcement, and charging-related functions. Importantly, it provides connectivity between 3GPP technologies (LTE, LTE-A) and non-3GPP technologies (WiMAX, CDMA, etc.).

• S-GW (Serving Gateway): The S-GW interfaces with the MME using the S11 interface and with the SGSN (Serving GPRS Support Node) via the S4 interface. As mentioned earlier, it connects to the PDN-GW through the S5 interface. The EPC (Evolved Packet Core) terminates at the S-GW. It connects to the E-UTRAN via the S1-U interface. In LTE-A, each UE is associated with a unique S-GW. This entity aids in inter-eNB handovers and inter-3GPP mobility. It’s also responsible for inter-operator charging, packet routing, and packet forwarding.

• MME (Mobility Management Entity): The MME is the primary control-plane element in the LTE Advanced architecture. It handles authentication, authorization, and security functions related to NAS signaling. The MME is also responsible for selecting the appropriate S-GW, PDN-GW, or P-GW.

• S1-MME: The S1-MME provides the connection between the EPC and the eNBs.

• eNB (evolved NodeB): The eNB is the fundamental building block of the LTE-A network. It provides the air interface for UEs (LTE-A phones). Its functionality is comparable to a base station in GSM or other cellular systems. Each eNB serves one or more E-UTRAN cells. The interface between two eNBs is known as the X2 interface.

• HeNB (Home eNodeB): The HeNB, also known as a Femtocell, improves coverage in indoor environments like offices or homes. It can connect directly to the EPC or via a Gateway.

• HeNB-GW (Home eNodeB Gateway): The HeNB-GW provides connectivity for HeNBs to the S-GW and MME. It aggregates traffic from multiple Home eNBs to the core network, using the S1 interface to connect with the HeNBs.

• Relay Node: Relay Nodes improve network performance.

LTE Advanced Protocol Stack

The LTE Advanced protocol stack is depicted below.

Figure 2: LTE Advanced Protocol Stack

The protocol stack is divided into two main parts: NAS (Non-Access Stratum) and AS (Access Stratum). It’s further categorized into the control plane and the user plane.

The user plane of the eNB consists of the PHY, MAC, RLC, and PDCP layers. The control plane of the eNB includes these four layers along with the RRC layer. The functions of each of these layers are described below:

• PHY (Physical Layer): This layer handles frame formation according to the TDD or FDD topology, and based on the OFDMA structure determined by bandwidth (BW) and FFT size. It also manages the modulation and coding of various control and traffic channels, including scrambling and codeword-to-layer mapping. The PHY layer incorporates reference signals (DMRS/SRS in the uplink and C-RS/CSI-RS/UE-RS in the downlink), which are used for channel estimation and equalization.

• MAC (Medium Access Control): The MAC layer is responsible for:

  • Multiplexing/demultiplexing RLC Packet Data Units (PDUs).
  • Scheduling information reporting.
  • Error correction through Hybrid ARQ (HARQ).
  • Local channel prioritization.
  • Padding.

• RLC (Radio Link Control): This layer provides:

  • Error correction through Automatic Repeat reQuest (ARQ).
  • Segmentation (based on transport block size) and re-segmentation (for retransmissions).
  • Concatenation of SDUs for the same radio bearer.
  • Protocol error detection and recovery.
  • In-sequence delivery.

• PDCP (Packet Data Convergence Protocol): The PDCP layer handles:

  • Header compression.
  • In-sequence delivery and retransmission of PDCP Session Data Units (SDUs) for acknowledged mode radio bearers during handover.
  • Duplicate detection.
  • Ciphering and integrity protection.

• RRC (Radio Resource Control): The RRC layer is responsible for:

  • Broadcasting system information related to NAS and AS.
  • Establishing, maintaining, and releasing RRC connections.
  • Security functions, including key management.
  • Mobility functions.
  • QoS management functions.
  • UE measurement reporting and control of the reporting.
  • NAS direct message transfer between the UE and NAS.

• NAS (Non-Access Stratum): The NAS layer deals with:

  • Connection/session management between the UE and the core network.
  • Authentication.
  • Registration.
  • Bearer context activation/deactivation.
  • Location registration management.