TUTORIAL SESSION

LTE Air Interface: Enabling Technologies and 3GPP Specifications

Assc. Prof. Murat Uysal
Ozyegin University, Turkey, and University of Waterloo, Canada
muysal@ece.uwaterloo.ca

Abstract

The next major development in wireless market will be the introduction of 3GPP Long Term Evolution (LTE) standard. LTE has been designed to meet the increasing demands for highspeed data and multimedia transmission along with high-capacity voice support. With rather a dramatic departure from the existing CDMA-based 3G (third generation) wireless cellular systems, LTE air interface adopts OFDMA (orthogonal frequency division multiple access) for the downlink and SC-FDMA (single-carrier frequency domain multiple access) for the uplink. It employs AMC (adaptive modulation and coding) as well as a number of optional MIMO (multiple-input multiple-output) antenna techniques and interference management methods for potential performance enhancements. This tutorial will cover major aspects of the LTE air interface providing details on the physical and data link layers. It will provide a comprehensive overview of OFDMA, SC-FDMA, AMC, and MIMO techniques as defined in 3GPP specifications and present justifications for the choice of such technologies. The tutorial will further present the principles of "relay-assisted (cooperative) communications" and "cognitive radio" which are envisioned for adoption in LTE Advanced standard.

Objectives

LTE standardization has come to a mature state by the 3GPP Release 8 (http://www.3gpp.org) and only minor updates are expected in Release 9. Commercial LTE deployment is expected as early as 2010 in some countries while major adoptions are expected to take place within the next five years. In October 2009, 3GPP has already announced that LTE Release 10 and beyond will be named as "LTE Advanced". This can be considered as the real fourth generation (4G) that fully supports the 4G (fourth generation) requirements as defined by the ITU (International Telecommunications Union). LTE and LTE Advanced will be the key players of the wireless market within the next decade. It is therefore vital for everyone involved in wireless communication to get familiar with this technology. Targeting this hot topic, this tutorial aims to provide the basic principles, enabling technologies, and architectures of LTE systems in a nutshell. It will present major aspects of the LTE air interface providing details on the physical and data link layers.

Timeline

Chapter I: Introduction (15 minutes)
Tutorial will start with a brief historical summary of cellular standards leading to the LTE. It will then provide an overview of the current mobile broadband market along with some forecasts for the next decade. The main performance requirements of LTE such as data rate, spectral efficiency, latency, call set-up time, mobility, QoS etc. will be also presented. The aim of this part is to understand the market drivers and technological trends in wireless systems and help position LTE with respect to its competitors.
Chapter II: Overview of LTE Network Architecture and Protocol Stack (15 minutes)
The second part of the tutorial will start with an architectural overview of LTE network. LTE core network architecture -called as "System Architecture Evolution (SAE)" in 3GPP specifications- will be described along with architectural components EPC (Evolved Packet Core), MME (Mobility Management Entity), S-GW (Serving Gateway), and P-GW (Packet Date Network Gateway). Building on an all-IP packet-based network, LTE has a simpler network architecture in comparison to its predecessors. Similarities and improvements over 2G and 3G standards will be particularly emphasized to fully appreciate the efficiency and flexibility of LTE architecture. In this part of the tutorial, LTE protocol stack will be also described with reference to the well-known OSI seven-layer model identifying the related 3GPP specifications for each layer.
Chapter III: Enabling Technologies for LTE (45 minutes)
The main objective in physical layer design is to maintain certain performance requirements such as high data rate (i.e., better spectral efficiency) and good link reliability (i.e., better error rate performance) under power consumption constraints. In order to fulfill its ambitious performance requirements, physical layer of LTE has been designed dramatically different from that in its predecessors. Key enabling technologies adopted in LTE is OFDMA (in the downlink), SC-FDMA (in the uplink) and MIMO antenna techniques. In this part of the tutorial, OFDMA and SC-FDMA will be described providing details on transmitter and receiver structures. Advantages/disadvantages of both techniques will be described and the reasons behind their choices for uplink/downlink will be explained. Another underlying technology in LTE is MIMO communications. Variants of MIMO antenna techniques such as transmit diversity, spatial multiplexing, and multi-user MIMO are supported in LTE. Each of these techniques will be described explaining their differences and possible deployment scenarios along with discussions on their practical implementation in LTE.
Chapter IV: Physical Layer of LTE (45 minutes)
This part of the tutorial will discuss the physical layer (Layer 1) of the LTE. It will start with the introduction of two types of LTE radio frame structures defined in 3GPP specifications for FDD (frequency division duplex) and TDD (time division duplex). Information will be provided on channel bandwidth, number of resource blocks, number of carriers, FFT/IFFT size required in OFDMA/SC-FDMA, sample rate, etc. Downlink and uplink structures will be explained including details on physical channels, channel processing (i.e., CRC, segmentation, encoding, rate matching etc), reference/synchronization signals, supported modulation/coding formats, available MIMO options etc. Some basic LTE procedures such as cell search, synchronization, power control, CQI (channel quality indication) reporting, channel sounding will be further explained.
Chapter V: Data Link Layer of LTE (30 minutes)
This part of the tutorial will discuss the data link layer (Layer 2) of the LTE. This layer has three sub-layers known as MAC (Medium Access Control), RLC (Radio Link Control), and PDCP (Packet Data Convergence Protocol). The tasks of each sub-layer along with the interaction among them will be described. LTE transport and logical channels which, respectively, link the MAC-physical layers and PDCP-MAC layers will be defined and categorized according to their functions. Channel mappings will be then described for the uplink and downlink. RRC (Radio resource control) is not strictly part of Layer 2, but for the completeness of presentation, RRC functions and states will be further discussed.
Chapter VI: LTE Advanced (30 minutes)
As earlier mentioned, 3GPP has already initiated studies on evolution of LTE which will be named as LTE Advanced. Besides some proposed modifications on LTE (i.e., enhanced precoding and forward error corrections, hybrid OFDMA/SC-FDMA in uplink etc), initial proposals on LTE Advanced involve the possible deployment of emerging technologies such as relaying and cognitive radio. This part of the tutorial will present a glimpse into the future on how these can be integrated into LTE and potential performance improvements they can bring along with a discussion on the possible implementation challenges.

Target Audience

The tutorial is intended for a broad audience from both academia and industry. Background in wireless communication at an undergraduate level is a prerequisite. Some familiarity with 2G and 3G cellular systems is beneficial, but not required.
Physical layer technologies incorporated in LTE as well as the ones envisioned for LTE Advanced are rather new to cellular systems. This presents challenges for cellular operators, vendors, regulators, and service providers. The tutorial will be useful for engineers and technical management staff in telecommunications industry who need a detailed technical understanding of the LTE with a particular emphasis on radio air interface. It will save them time by avoiding the laborious search for specific information on LTE through standards/white papers and benefit them through interpretation of such documentation by an expert in the field.
Such a tutorial on LTE also appeals to graduate students, researchers, and faculty members working in the area of wireless communications. Rather than generic descriptions that can be found in some textbooks on wireless communications, details of OFDMA, SC-FDMA, AMC, and MIMO techniques will be presented in the context of LTE with discussions on their performance in "real-life" environments and potential challenges in their deployment. Graduate students will further benefit from discussions on LTE Advanced in which hot research topics such as relaying and cognitive radio will be covered. Faculty members who teach undergraduate and graduate courses on digital and wireless communications can use the tutorial material to enrich their course content through real-life deployment examples.

Qualifications of the Instructor(s)

Murat Uysal received Ph.D. degree in electrical engineering from Texas A&M University, College Station, Texas, in 2001. Since 2002, he has been with the Department of Electrical and Computer Engineering, University of Waterloo, Canada, where he is now an Associate Professor. He is currently on sabbatical leave at Ozyegin University, Turkey. His research interests are in the area of communication theory and signal processing with applications to wireless communication systems.