An Introduction to LTE

LTE, LTE-Advanced, SAE, VoLTE and 4G Mobile Communications

Christopher Cox

• Publisher: Wiley
• Year: 2014
• ISBN:
  9781118818015
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Mobile & Wireless Communications
• Index: Technology & Engineering

Following on from the successful first edition (March 2012), this book gives a clear explanation of what LTE does and how it works. The content is expressed at a systems level, offering readers the opportunity to grasp the key factors that make LTE the hot topic amongst vendors and operators across the globe. The book assumes no more than a basic knowledge of mobile telecommunication systems, and the reader is not expected to have any previous knowledge of the complex mathematical operations that underpin LTE.

This second edition introduces new material for the current state of the industry, such as the new features of LTE in Releases 11 and 12, notably coordinated multipoint transmission and proximity services; the main short- and long-term solutions for LTE voice calls, namely circuit switched fallback and the IP multimedia subsystem; and the evolution and current state of the LTE market. It also extends some of the material from the first edition, such as inter-operation with other technologies such as GSM, UMTS, wireless local area networks and cdma2000; additional features of LTE Advanced, notably heterogeneous networks and traffic offloading; data transport in the evolved packet core; coverage and capacity estimation for LTE; and a more rigorous treatment of modulation, demodulation and OFDMA. The author breaks down the system into logical blocks, by initially introducing the architecture of LTE, explaining the
techniques used for radio transmission and reception and the overall operation of the system, and concluding with more specialized topics such as LTE voice calls and the later releases of the specifications. This methodical approach enables readers to move on to tackle the specifications and the more advanced texts with confidence.

Chapter 1
Introduction

Our first chapter puts LTE into its historical context, and lays out its requirements and key technical features. We begin by reviewing the architectures of UMTS and GSM, and by introducing some of the terminology that the two systems use. We then summarize the history of mobile telecommunication systems, discuss the issues that have driven the development of LTE and show how UMTS has evolved first into LTE and then into an enhanced version known as LTE-Advanced. The chapter closes by reviewing the standardization process for LTE.

1.1 Architectural Review of UMTS and GSM

1.1.1 High-Level Architecture

LTE was designed by a collaboration of national and regional telecommunications standards bodies known as the Third Generation Partnership Project (3GPP) [1] and is known in full as 3GPP Long-Term Evolution. LTE evolved from an earlier 3GPP system known as the Universal Mobile Telecommunication System (UMTS), which in turn evolved from the Global System for Mobile Communications (GSM). To put LTE into context, we will begin by reviewing the architectures of UMTS and GSM, and by introducing some of the important terminology.

A mobile phone network is officially known as a public land mobile network (PLMN), and is run by a network operator such as Vodafone or Verizon. UMTS and GSM share a common network architecture, which is shown in Figure 1.1. There are three main components, namely the core network, the radio access network and the mobile phone.

Figure 1.1 High-level architecture of UMTS and GSM

The core network contains two domains. The circuit switched (CS) domain transports phone calls across the geographical region that the network operator is covering, in the same way as a traditional fixed-line telecommunication system. It communicates with the public switched telephone network (PSTN) so that users can make calls to land lines and with the circuit switched domains of other network operators. The packet switched (PS) domain transports data streams, such as web pages and emails, between the user and external packet data networks (PDNs) such as the internet.

The two domains transport their information in very different ways. The CS domain uses a technique known as circuit switching, in which it sets aside a dedicated two-way connection for each individual phone call so that it can transport the information with a constant data rate and minimal delay. This technique is effective, but is rather inefficient: the connection has enough capacity to handle the worst-case scenario in which both users are speaking at the same time, but is usually over-dimensioned. Furthermore, it is inappropriate for data transfers, in which the data rate can vary widely.

To deal with the problem, the PS domain uses a different technique, known as packet switching. In this technique, a data stream is divided into packets, each of which is labelled with the address of the required destination device. Within the network, routers read the address labels of the incoming data packets and forward them towards the corresponding destinations. The network's resources are shared amongst all the users, so the technique is more efficient than circuit switching. However, delays can result if too many devices try to transmit at the same time, a situation that is familiar from the operation of the internet.

The radio access network handles the core network's radio communications with the user. In Figure 1.1, there are actually two separate radio access networks, namely the GSM EDGE radio access network (GERAN) and the UMTS terrestrial radio access network (UTRAN). These use the different radio communication techniques of GSM and UMTS, but share a common core network between them.

The user's device is known officially as the user equipment (UE) and colloquially as the mobile. It communicates with the radio access network over the air interface, also known as the radio interface. The direction from network to mobile is known as the downlink (DL) or forward link and the direction from mobile to network is known as the uplink (UL) or reverse link.

A mobile can work outside the coverage area of its network operator by using the resources from two public land mobile networks: the visited network, where the mobile is located and the operator's home network. This situation is known as roaming.

1.1.2 Architecture of the Radio Access Network

Figure 1.2 shows the radio access network of UMTS. The most important component is the base station, which in UMTS is officially known as the Node B. Each base station has one or more sets of antennas, through which it communicates with the mobiles in one or more sectors. As shown in the diagram, a typical base station uses three sets of antennas to control three sectors, each of which spans an a...