Lte basics

3GPP LTE (Long Term Evolution)

Abstract
The 3GPP Long Term Evolution (LTE) represents a major advance in
cellular technology. LTE is designed to meet carrier needs for high-speed data and
media transport as well as high-capacity voice support well into the next decade. LTE
is well positioned to meet the requirements of next-generation mobile networks. It will
enable operators to offer high performance, mass-market mobile broadband services,
through a combination of high bit-rates and system throughput – in both the uplink
and downlink – with low latency.
LTE infrastructure is designed to be as simple as possible to deploy and
operate, through flexible technology that can be deployed in a wide variety of
frequency bands. LTE offers scalable bandwidths, from less than 5MHz up to 20MHz,
together with support for both FDD paired and TDD unpaired spectrum. The LTE–
SAE architecture reduces the number of nodes, supports flexible network
configurations and provides a high level of service availability. Furthermore, LTE–
SAE will interoperate with GSM, WCDMA/HSPA, TD-SCDMA and CDMA.

Outline
•Introduction
•3GPP Evolution
•Motivation
•LTE performance requirements
•Key Features of LTE
•LTE Network Architecture
•System Architecture Evolution(SAE)
•Evolved Packet Core(EPC)
•E-UTRAN Architecture
•Physical layer
•LTE Frame Structure
•Layer 2
•OFDM
•SC-FDMA
•Multiple Antenna Techniques
•Services
•Conclusions
•LTE vs WiMAX
•References

Introduction
LTE is the latest standard in the mobile network technology tree that previously realized
the GSM/EDGE and UMTS/HSxPA network technologies that now account for over 85%
of all mobile subscribers. LTE will ensure 3GPP’s competitive edge over other cellular
technologies.
Goals include
Significantly increase peak data rates, scaled linearly according to spectrum allocation
improving spectral efficiency
lowering costs
improving services
making use of new spectrum opportunities
Improved quality of service
better integration with other open standards

3GPP Evolution
Release 99 (2000): UMTS/WCDMA
Release 5 (2002) : HSDPA
Release 6 (2005) : HSUPA, MBMS(Multimedia Broadcast/Multicast Services)
Release 7 (2007) : DL MIMO, IMS (IP Multimedia Subsystem), optimized real-time
services (VoIP, gaming, push-to-talk).
Release 8(2009?) :LTE (Long Term Evolution)
Long Term Evolution (LTE)
•3GPP work on the Evolution of the 3G Mobile System started in November 2004.
•Currently, standardization in progress in the form of Rel-8.
•Specifications scheduled to be finalized by the end of mid 2008.
•Target deployment in 2010.

Motivation
Need for higher data rates and greater spectral efficiency
Can be achieved with HSDPA/HSUPA
and/or new air interface defined by 3GPP LTE
Need for Packet Switched optimized system
 Evolve UMTS towards packet only system
Need for high quality of services
Use of licensed frequencies to guarantee quality of services
Always-on experience (reduce control plane latency significantly)
Reduce round trip delay
Need for cheaper infrastructure
Simplify architecture, reduce number of network elements

LTE performance requirements
Data Rate:
•Instantaneous downlink peak data rate of 100Mbit/s in a 20MHz downlink spectrum (i.e.
5 bit/s/Hz)
•Instantaneous uplink peak data rate of 50Mbit/s in a 20MHz uplink spectrum (i.e. 2.5
bit/s/Hz)
Cell range
•5 km - optimal size
•30km sizes with reasonable performance
•up to 100 km cell sizes supported with acceptable performance
Cell capacity
•up to 200 active users per cell(5 MHz) (i.e., 200 active data clients)

LTE performance requirements
Mobility
•Optimized for low mobility(0-15km/h) but supports high speed
Latency
•user plane < 5ms
•control plane < 50 ms
Improved spectrum efficiency
Cost-effective migration from Release 6 Universal Terrestrial Radio Access (UTRA) radio
interface and architecture
Improved broadcasting
IP-optimized
Scalable bandwidth of 20MHz, 15MHz, 10MHz, 5MHz and <5MHz
Co-existence with legacy standards (users can transparently start a call or transfer of data
in an area using an LTE standard, and, when there is no coverage, continue the operation
without any action on their part using GSM/GPRS or W-CDMA-based UMTS)

Key Features of LTE
• Multiple access scheme
 Downlink: OFDMA
 Uplink: Single Carrier FDMA (SC-FDMA)
• Adaptive modulation and coding
 DL modulations: QPSK, 16QAM, and 64QAM
 UL modulations: QPSK and 16QAM
 Rel-6 Turbo code: Coding rate of 1/3, two 8-state constituent encoders, and a contention-
free internal interleaver.
• Bandwidth scalability for efficient operation in differently sized allocated spectrum bands
• Possible support for operating as single frequency network (SFN) to support MBMS

Key Features of LTE(contd.)
 Multiple Antenna (MIMO) technology for enhanced data rate and performance.
 ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer.
 Power control and link adaptation
 Implicit support for interference coordination
 Support for both FDD and TDD
 Channel dependent scheduling & link adaptation for enhanced performance.
 Reduced radio-access-network nodes to reduce cost,protocol-related processing time &
call set-up time

LTE Network Architecture
[Source:Technical Overview of 3GPP Long Term Evolution (LTE) Hyung G. Myung
http://hgmyung.googlepages.com/3gppLTE.pdf

[Source:Technical Overview of 3GPP Long Term Evolution (LTE) Hyung G. Myung]

System Architecture Evolution(SAE)
•System Architecture Evolution (aka SAE) is the core network architecture of 3GPP's future
LTE wireless communication standard.
•SAE is the evolution of the GPRS Core Network, with some differences.
•The main principles and objectives of the LTE-SAE architecture include :
A common anchor point and gateway (GW) node for all access technologies
IP-based protocols on all interfaces;
Simplified network architecture
All IP network
All services are via Packet Switched domain
Support mobility between heterogeneous RATs, including legacy systems as GPRS, but also
non-3GPP systems (say WiMAX)
Support for multiple, heterogeneous RATs, including legacy systems as GPRS, but also non-
3GPP systems (say WiMAX)

SAE
[Source:http://www.3gpp.org/Highlights/LTE/LTE.htm]

Evolved Packet Core(EPC)
MME (Mobility Management Entity):
•-Manages and stores the UE control plane context, generates temporary Id, provides
UE authentication, authorization, mobility management
UPE (User Plane Entity):
•-Manages and stores UE context, ciphering, mobility anchor, packet routing and
forwarding, initiation of paging
3GPP anchor:
•-Mobility anchor between 2G/3G and LTE
SAE anchor:
•-Mobility anchor between 3GPP and non 3GPP (I-WLAN, etc)

E-UTRAN Architecture
[Source: E-UTRAN Architecture(3GPP TR 25.813 ]
7.1.0 (2006-09))]

User-plane Protocol Stack
[Source: E-UTRAN Architecture(3GPP TR 25.813 ]7.1.0 (2006-09))]

Control-plane protocol Stack
[Source: E-UTRAN Architecture(3GPP TR 25.813 ]7.1.0 (2006-09))]

Physical layer
• The physical layer is defined taking bandwidth into consideration, allowing the physical layer to
adapt to various spectrum allocations.
• The modulation schemes supported in the downlink are QPSK, 16QAM and 64QAM, and in
the uplink QPSK, 16QAM.The Broadcast channel uses only QPSK.
• The channel coding scheme for transport blocks in LTE is Turbo Coding with a coding rate of
R=1/3, two 8-state constituent encoders and a contention-free quadratic permutation polynomial
(QPP) turbo code internal interleaver.
• Trellis termination is used for the turbo coding. Before the turbo coding, transport blocks are
segmented into byte aligned segments with a maximum information block size of 6144 bits.
Error detection is supported by the use of 24 bit CRC.

LTE Frame Structure
One element that is shared by the LTE Downlink and Uplink is the generic frame structure. The
LTE specifications define both FDD and TDD modes of operation. This generic frame structure
is used with FDD. Alternative frame structures are defined for use with TDD.
LTE frames are 10 msec in duration. They are divided into 10 subframes, each subframe
being 1.0 msec long. Each subframe is further divided into two slots, each of 0.5 msec
duration. Slots consist of either 6 or 7 ODFM symbols, depending on whether the normal or
extended cyclic prefix is employed
[source: 3GPP TR 25.814]

Generic Frame structure
Available Downlink Bandwidth is Divided into Physical Resource Blocks
[source: 3GPP TR 25.814]
LTE Reference Signals
are Interspersed Among
Resource Elements

OFDM
LTE uses OFDM for the downlink – that is, from the base station to the terminal. OFDM meets
the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide
carriers with high peak rates. OFDM uses a large number of narrow sub-carriers for multi-carrier
transmission.
The basic LTE downlink physical resource can be seen as a time-frequency grid. In the
frequency domain, the spacing between the subcarriers, Δf, is 15kHz. In addition, the OFDM
symbol duration time is 1/Δf + cyclic prefix. The cyclic prefix is used to maintain orthogonality
between the sub-carriers even for a time-dispersive radio channel.
One resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource element
carries six bits.
The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of
180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms Transmission Time
Interval (TTI) consists of two slots (Tslot).
In E-UTRA, downlink modulation schemes QPSK, 16QAM, and 64QAM are available.

Downlink Physical Layer Procedures
Downlink Physical Layer Procedures
For E-UTRA, the following downlink physical layer procedures are especially
important:
Cell search and synchronization:
Scheduling:
Link Adaptation:
Hybrid ARQ (Automatic Repeat Request)

SC-FDMA
The LTE uplink transmission scheme for FDD and TDD mode is based on SC-FDMA (Single
Carrier Frequency Division Multiple Access).
This is to compensate for a drawback with normal OFDM, which has a very high Peak to
Average Power Ratio (PAPR). High PAPR requires expensive and inefficient power amplifiers
with high requirements on linearity, which increases the cost of the terminal and also drains
the battery faster.
SC-FDMA solves this problem by grouping together the resource blocks in such a way that
reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR
also improves coverage and the cell-edge performance.
Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so
parameterization of downlink and uplink can be harmonized.

Uplink Physical Layer Procedures
Uplink Physical Layer Procedures
For E-UTRA, the following uplink physical layer procedures are especially important:
Random access
Uplink scheduling
Uplink link adaptation
Uplink timing control
Hybrid ARQ

Layer 2
The three sublayers are
Medium access Control(MAC)
Radio Link Control(RLC)
Packet Data Convergence Protocol(PDCP)
[Source: E-UTRAN Architecture(3GPP TR 25.012 ]

Layer 2
MAC (media access control) protocol
handles uplink and downlink scheduling and HARQ signaling.
Performs mapping between logical and transport channels.
RLC (radio link control) protocol
focuses on lossless transmission of data.
In-sequence delivery of data.
Provides 3 different reliability modes for data transport. They are
Acknowledged Mode (AM)-appropriate for non-RT (NRT) services such as file downloads.
Unacknowledged Mode (UM)-suitable for transport of Real Time (RT) services because
such services are delay sensitive and cannot wait for retransmissions
Transparent Mode (TM)-used when the PDU sizes are known a priori such as for
broadcasting system information.

Layer 2
PDCP (packet data convergence protocol)
handles the header compression and security functions of the radio interface
•RRC (radio resource control) protocol
 handles radio bearer setup
 active mode mobility management
Broadcasts of system information, while the NAS protocols deal with idle mode
mobility management and service setup

Channels
Transport channels
In order to reduce complexity of the LTE protocol architecture, the number of transport
channels has been reduced. This is mainly due to the focus on shared channel operation,
i.e. no dedicated channels are used any more.
Downlink transport channels are
Broadcast Channel (BCH)
Downlink Shared Channel (DL-SCH)
Paging Channel (PCH)
Multicast Channel (MCH)
Uplink transport channels are:
Uplink Shared Channel (UL-SCH)
Random Access Channel (RACH)

Channels
Logical channels
Logical channels can be classified in
control and traffic channels.
Control channels are:
Broadcast Control Channel (BCCH)
Paging Control Channel (PCCH)
Common Control Channel (CCCH)
Multicast Control Channel (MCCH)
Dedicated Control Channel (DCCH)
Traffic channels are:
Dedicated Traffic Channel (DTCH)
Multicast Traffic Channel (MTCH)
Mapping between downlink logical and transport channels
Mapping between uplink logical and transport channels

LTE MBMS Concept
• MBMS (Multimedia Broadcast Multicast Services) is an essential requirement for LTE. The
so-called E-MBMS will therefore be an integral part of LTE.
• In LTE, MBMS transmissions may be performed as single-cell transmission or as multi-cell
transmission. In case of multi-cell transmission the cells and content are synchronized to
enable for the terminal to soft-combine the energy from multiple transmissions.
• The superimposed signal looks like multipath to the terminal. This concept is also known
as Single Frequency Network (SFN).
• The E-UTRAN can configure which cells are part of an SFN for transmission of an MBMS
service. The MBMS traffic can share the same carrier with the unicast traffic or be sent on
a separate carrier.
• For MBMS traffic, an extended cyclic prefix is provided. In case of subframes carrying
MBMS SFN data, specific reference signals are used. MBMS data is carried on the MBMS
traffic channel (MTCH) as logical channel.

Multiple Antenna Techniques
MIMO employs multiple transmit and receive antennas to substantially enhance the air
interface.
It uses spacetime coding of the same data stream mapped onto multiple transmit antennas,
which is an improvement over traditional reception diversity schemes where only a single
transmit antenna is deployed to extend the coverage of the cell.
MIMO processing also exploits spatial multiplexing, allowing different data streams to be
transmitted simultaneously from the different transmit antennas, to increase the end-user data
rate and cell capacity.
In addition, when knowledge of the radio channel is available at the transmitter (e.g. via
feedback information from the receiver), MIMO can also implement beam-forming to further
increase available data rates and spectrum efficiency

Advanced Antenna Techniques
Single data stream / user
Beam-forming
 Coverage, longer battery life
Spatial Division Multiple Access (SDMA)
 Multiple users in same radio resource
Multiple data stream / user Diversity
 Link robustness
Spatial multiplexing
 Spectral efficiency, high data rate support

Beamforming & SDMA
Enhances signal reception through directional array
gain, while individual antenna has omni-directional gain
• Extends cell coverage
• Suppresses interference in space domain
• Enhances system capacity
• Prolongs battery life
• Provides angular information for user tracking
Source: Key Features and Technologies in 3G Evolution,
http://www.eusea2006.org/workshops/workshopsession.2
006-01-1 1.3206361376/sessionspeaker.2006-04-
10.9519467221/file/atdownload

Services
Source: Analysys Research/UMTS Forum 2007]

Conclusions
LTE is a highly optimized, spectrally efficient, mobile OFDMA solution built from the ground up
for mobility, and it allows operators to offer advanced services and higher performance for new
and wider bandwidths.
LTE is based on a flattened IP-based network architecture that improves network latency, and
is designed to interoperate on and ensure service continuity with existing 3GPP networks. LTE
leverages the benefits of existing 3G technologies and enhances them further with additional
antenna techniques such as higher-order MIMO.

LTE vs WiMAX
First, both are 4G technologies designed to move data rather than voice and both are IP
networks based on OFDM technology.
WiMax is based on a IEEE standard (802.16), and like that other popular IEEE effort, Wi-Fi,
it’s an open standard that was debated by a large community of engineers before getting
ratified. In fact, we’re still waiting on the 802.16m standard for faster mobile WiMax to be
ratified. The level of openness means WiMax equipment is standard and therefore cheaper to
buy — sometimes half the cost and sometimes even less. Depending on the spectrum alloted
for WiMax deployments and how the network is configured, this can mean a WiMax network is
cheaper to build.
As for speeds, LTE will be faster than the current generation of WiMax, but 802.16m that
should be ratified in 2009 is fairly similar in speeds.
However, LTE will take time to roll out, with deployments reaching mass adoption by 2012 .
WiMax is out now, and more networks should be available later this year.
The crucial difference is that, unlike WiMAX, which requires a new network to be built, LTE
runs on an evolution of the existing UMTS infrastructure already used by over 80 per cent of
mobile subscribers globally. This means that even though development and deployment of the
LTE standard may lag Mobile WiMAX, it has a crucial incumbent advantage.

References
• http://www.3gpp.org/
• 3GPP TR 25.913. Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN).
• Towards 4G IP-based Wireless Systems,Tony Ottosson Anders Ahl´en2 Anna Brunstrom, Mikael Sternad and Arne
Svensson, http://db.s2.chalmers.se/download/publications/ottosson_1007.pdf
• H. Ekström et al., “Technical Solutions for the 3G Long-Term Evolution,” IEEE Communication. Mag., vol. 44, no. 3, March
2006, pp. 38–45
• The 3G Long-Term Evolution – Radio Interface Concepts and Performance Evaluation
Erik Dahlman, Hannes Ekström, Anders Furuskär, Ylva Jading, Jonas Karlsson, Magnus Lundevall, Stefan Parkvall
http://www.ericsson.com/technology/research_papers/wireless_access/doc/the_3g_long_term_evolution_radio_interface.pdf
• Mobile Network Evolution :From 3G Onwards
http://www1.alcatel-lucent.com/doctypes/articlepaperlibrary/pdf/ATR2003Q4/T0312-Mobile-Evolution-EN.pdf
• White Paper by NORTEL -Long-Term Evolution (LTE): The vision beyond 3G
http://www.nortel.com/solutions/wireless/collateral/nn114882.pdf
• [Long Term Evolution (LTE): an introduction, October 2007 Ericsson White Paper]
http://www.ericsson.com/technology/whitepapers/lte_overview.pdf

References
• Long Term Evolution (LTE) :A Technical Overview - Motorola technical white paper
http://www.motorola.com/staticfiles/Business/Solutions/Industry%20Solutions/Service%20Providers/Wireless
%20Operators/LTE/_Document/Static%20Files/6834_MotDoc.pdf
• Key Features and Technologies in 3G Evolution, Francois China Institute for Infocomm Research
http://www.eusea2006.org/workshops/workshopsession.2006-01-11.3206361376/sessionspeaker.2006-04-
10.9519467221/file/at_download
• Overview of the 3GPP Long Term Evolution Physical Layer,Jim Zyren,Dr. Wes McCoy
http://www.freescale.com/files/wireless_comm/doc/white_paper/3GPPEVOLUTIONWP.pdf
• Technical Overview of 3GPP Long Term Evolution (LTE) Hyung G. Myung
http://hgmyung.googlepages.com/3gppLTE.pdf
• http://wireless.agilent.com/wireless/helpfiles/n7624b/3gpp_(lte_uplink).htm

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