LTE Tutorial Basics v1

8/3/2019 LTE Tutorial Basics v1

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Goodman Networks

NS&T

Network Services and Technology

LTE Basics (3GPP)


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Table of Contents

Section 1 Introduction & Purpose

Section 1.1 3G LTE Beginnings

Section 1.2 3G LTE Development

Section 1.3 3G LTE Technologies

1.3.1 OFM (Orthogonal Frequency Multiplex)

1.3.2 MIMO (Multiple Input Multiple Output)

13.3 SAE (System Architecture Evolution)

Section 1.4 3G LTE Summary

Section 1.5 OFDM Basics

1.5.1 Note on OFDM

Section 1.6 LTE Channel Bandwidths

Section 1.7 LTE OFDM cyclic prefix, CP

Section 1.8 OFDMA in the Downlink

1.8.1 Downlink carriers and resource block

1.8.2 LTE SC-FDMA in the Uplink

Section 2.0 LTE MIMO Basics

2.0.1 Note on MIMO

Section 2.1 LTE FDD Frequency Band Allocations

Section 2.2 Abreviations

2

Proprietary Information

The information contained herein is not for use or disclosure outside GOODMAN NETWORKS, Supplier, their Affiliates

and subsidiary companies, and their third party representatives, except under written agreement.


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1.0 3G LTE Basics Tutorial Introduction &

Purpose

The following will discuss information, overview, and tutorial about the basics of 3G

LTE, the long term evolution plans for the next generation of cellular

telecommunications services.

With services such as WiMAX offering very high data speeds, work on developing the

next generation of cellular technology has started. The UMTS cellular technology

upgrade has been dubbed LTE - Long Term Evolution. The idea is that LTE will enable

much higher speeds to be achieved along with much lower packet latency (a growing

requirement for many services these days), and that LTE will enable cellular

communications services to move forward to meet the needs for cellular technology to

2017 and well beyond. HSPA (High Speed Packet Access), a combination of HSDPA

(High-Speed Downlink Packet Access) and HSUPA (High-Speed Uplink Packet Access),

and HSPA+ are now being deployed, the 3G LTE development is being dubbed 3.99G

as it is not a full 4G standard, although in reality there are many similarities with the

cellular technologies being touted for the use of 4G. However, regardless of the

terminology, it is certain that LTE will offer significant improvements in performance

over the existing 3G standards.

Many operators have not yet upgraded their basic 3G networks, and LTE is seen as the

next logical step for many operators, who will leapfrog straight from basic 3G straight

to LTE as this will avoid providing several stages of upgrade. The use of LTE will also

provide the data capabilities that will be required for many years and until the full

launch of the full 4G standards known as LTE Advanced.

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1.1 3G LTE Beginnings

3GPP, the Third Generation Partnership Project that oversaw the development of the

UMTS 3G system, started the work on the evolution of the 3G cellular technology. A

workshop was held in Toronto Canada in November 2004 alongside this work. The

work on 3G LTE started with a feasibility study started in December 2004, which wasfinalized for inclusion on 3GPP release 7. LTE core specifications were then included in

release 8. The workshop set down a number of high level requirements for 3G LTE:

Reduced cost per bit

Increased service provisioning - more services at lower cost with better user

experience

Flexibility of use of existing and new frequency bands

Simplified architecture, Open interfaces

Allow for reasonable terminal power consumption

In terms of actual figures, targets for LTE included download rates of 100Mbps, and

upload rates of 50Mbps for every 20MHz of spectrum. In addition to this LTE was

required to support at least 200 active users in every 5MHz cell (i.e. 200 active4





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phone calls). Targets have also been set for the latency in IP packet delivery. With

the growing use of services including VoIP, gaming and many other applications

where latency is of concern, figures need to be set for this. As a result a figure of

sub-10ms latency for small IP packets has been set.

1.2 3G LTE Development

Although there are major step changes between LTE and its 3P predecessors, it is

nevertheless looked upon as an evolution of the UMTS / 3GPP 3G standards. Although

it uses a different form of radio interface, using OFDMA / SC-FDMA instead of CDMA,

there are many similarities with the earlier forms of 3G architecture and there is scope

for much re-use. LTE can be seen for provide a further evolution of functionality,

increased speeds and general improved performance.

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In addition to this, LTE is an all IP based network, supporting both IPv4 and IPv6.

There is also no basic provision for voice, although this can be carried as VoIP.

1.3 3G LTE Technologies

LTE has introduced a number of new technologies when compared to the previous

cellular systems. They enable LTE to be able to operate more efficiently with respect

to the use of spectrum, and also to provide the much higher data rates that are being

required

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1.3.1 OFDM (Orthogonal Frequency Division Multiplex):OFDM technology has been incorporated into LTE because it enables high data

bandwidths to be transmitted efficiently while still providing a high degree of resilience

to reflections and interference. The access schemes differ between the uplink and

downlink: OFDMA (Orthogonal Frequency Division Multiple Access) is used in the

downlink; while SC-FDMA (Single Carrier - Frequency Division Multiple Access) is usedin the uplink. SC-FDMA is used in view of the fact that its pear to average power ratio

is small and the more constant power enables high RF power amplifier efficiency in the

mobile handsets - an important factor for battery power equipment.

1.3.2 MIMO (Multiple Input Multiple Output): One of the mainproblems that previous telecommunications systems has encountered is that of

multiple signals arising from the many reflections that are encountered. By using

MIMO, these additional signal paths can be used to advantage and are able to be used

to increase the throughput. When using MIMO, it is necessary to use multiple antennas

to enable the different paths to be distinguished. Accordingly schemes using 2 x 2, 4 x2, or 4 x 4 antenna matrices can be used. While it is relatively easy to add further

antennas to a base station, the same is not true of mobile handsets, where the

dimensions of the user equipment limit the number of antennas which should be place

at least a half wavelength apart.

1.3.3 SAE (System Architecture Evolution): With the very highdata rate and low latency requirements for 3G LTE, it is necessary to evolve the

system architecture to enable the improved performance to be achieved. One change

is that a number of the functions previously handled by the core network have been

transferred out to the periphery. Essentially this provides a much "flatter" form of

network architecture. In this way latency times can be reduced and data can be routed

more directly to its destination.

EPS

Evolved Packet System

IP STACKPROTOCOL

EUTRAN EPC

These technologies are addressed in much greater detail in the following pages of this

tutorial. 3G LTE specification overview It is worth summarizing the key parameters of

the 3G LTE specification. In view of the fact that there are a number of differences

between the operation of the uplink and downlink, these naturally differ in the

performance they can offer.

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These LTE highlight specifications give an overall view of the performance that LTE

will offer. It meets the requirements of industry for high data download speeds as well

as reduced latency - a factor important for many applications from VoIP to gaming and

interactive use of data. It also provides significant improvements in the use of the

available spectrum.

1.4 3G LTE Summary

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The basic work on LTE has now been completed, although the initial drafts were

released in September 2007 and the parallel work on the infrastructure technology

known as LTE System Architecture Evolution (SAE) followed shortly afterwards. In

terms of the deployments of real systems some anticipate that the first deployments

may be seen in 2010 although one of the main problems will be the user equipment.

Initially these are likely to consist of broadband "dongles" for use with laptops withother mobiles appearing later.

One of the key elements of LTE is the use of OFDM (Orthogonal Frequency Division

Multiplex) as the signal bearer and the associated access schemes, OFDMA

(Orthogonal Frequency Division Multiplex) and SC-FDMA (Single Frequency Division

Multiple Access). OFDM is used in a number of other of systems from WLAN (Wireless

Local Area Network), WiMAX (Worldwide Interoperability for Microwave Access)tobroadcast technologies including DVB (Digital Video Broadcasting) and DAB (Digital

Audio Output). OFDM has many advantages including its robustness to multipath

fading and interference. In addition to this, even though, it may appear to be a

particularly complicated form of modulation, it lends itself to digital signal processing

techniques. In view of its advantages, the use of ODFM and the associated access

technologies, OFDMA and SC-FDMA are natural choices for the new LTE cellular

standard.

1.5 OFDM Basics

The use of OFDM is a natural choice for LTE. While the basic concepts of OFDM are

used, it has naturally been tailored to meet the exact requirements for LTE. However

its use of multiple carrier each carrying a low data rate remains the same.

1.5.1 Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a

large number of close spaced carriers that are modulated with low rate data. Normally

these signals would be expected to interfere with each other. By making the signals

orthogonal there is no mutual interference. This is achieved by having the carrier

spacing equal to the reciprocal of the symbol period. This means that when the signals

are demodulated they will have a whole number of cycles in the symbol period and

their contribution will sum to zero - in other words there is no interference

contribution. The data to be transmitted is split across all the carriers and this means

that by using error correction techniques, if some of the carriers are lost due to multi-

path effects, then the data can be reconstructed. Additionally having data carried at a

low rate across all the carriers means that the effects of reflections and inter-symbol

interference can be overcome. It also means that single frequency networks, where all

transmitters can transmit on the same channel can be implemented.

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The actual implementation of the technology will be different between the downlink

(i.e. from base station to mobile) and the uplink (i.e. mobile to the base station) as a

result of the different requirements between the two directions and the equipment at

either end. However OFDM was chosen as the signal bearer format because it is very

resilient to interference. Also in recent years a considerable level of experience has

been gained in its use from the various forms of broadcasting that use it along withWi-Fi and WiMAX. OFDM is also a modulation format that is very suitable for carrying

high data rates - one of the key requirements for LTE. In addition to this, OFDM can be

used in both FDD and TDD formats. This becomes an additional advantage.

1.6LTE Channel Bandwidths

One of the key parameters associated with the use of OFDM within LTE is the choice

of bandwidth. The available bandwidth influences a variety of decisions including the

number of carriers that can be accommodated in the OFDM signal and in turn

influences elements including the symbol length and so forth. LTE defines a number of

channel bandwidths. The size of the bandwidth is proportional to the size of the

channel capacity.

The channel bandwidths that have been chosen for LTE

are:

1.4 MHz

3 MHz

5 MHz

10 MHz

15 MHz

20 MHz

In addition, the subcarriers that are spaced 15 kHz apart from each other to maintain

orthogonality, giving a symbol rate of 1 / 15 kHz = of 66.7 s. Each subcarrier is able

to carry data at a maximum rate of 15 ksps (kilosymbols per second). This gives a 20

MHz bandwidth system a raw symbol rate of 18 Msps. In turn this is able to provide a

raw data rate of 108 Mbps as each symbol using 64QAM is able to represent six bits. It

may appear that these rates do not align with the headline figures given in the LTE

specifications. This is because actual peak data rates are derived by first subtracting

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the coding and control overheads. Then there are gains arising from elements such as

the spatial multiplexing, etc.

1.7 LTE OFDM Cyclic Prefix (CP)

One of the primary reasons for using OFDM as a modulation format within LTE (and

many other wireless systems for that matter) is its resilience to multipath delays and

spread. However it is still necessary to implement methods of adding resilience to the

system. This helps overcome the inter-symbol interference (ISI) that results from this.

In areas where inter-symbol interference is expected, it can be avoided by inserting a

guard period into the timing at the beginning of each data symbol. It is then possible

to copy a section from the end of the symbol to the beginning. This is known as the

cyclic prefix, CP. The receiver can then sample the waveform at the optimum time and

avoid any inter-symbol interference caused by reflections that are delayed by times up

to the length of the CP.

The length of the cyclic prefix, CP is important. If it is not long enough then it will not

counteract the multipath reflection delay spread. If it is too long, then it will reduce the

data throughput capacity. For LTE, the standard length of the CP has been chosen to

be 4.69 s. This enables the system to accommodate path variations of up to 1.4 km.

The symbol length in LTE is set to 66.7 s. This is defined by the fact that for OFDM

systems the length is equal to the reciprocal of the carrier spacing, so that

orthogonality is achieved. A carrier spacing of 15 kHz gives the symbol length of 66.7

s.

1.8 LTE OFDMA in the Downlink

The OFDM signal used in LTE comprises a maximum of 2048 different sub-carriers

having a spacing of 15 kHz. Although it is mandatory for the mobiles to have capability

to be able to receive all 2048 sub-carriers, not all need to be transmitted by the base

station which only needs to be able to support the transmission of 72 sub-carriers. In

this way all mobiles will be able to talk to any base station. Within the OFDM signal it is

possible to choose between three types of modulation:

QPSK (= 4QAM) 2 bits per symbol

16QAM 4 bits per symbol

64QAM 6 bits per symbol

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The exact format is chosen depending upon the prevailing conditions. The lower forms

of modulation, QPSK (Quadrature Phase Shift Keying) do not require such a large

signal to noise ratio but are not able to send the data as fast. The higher order

modulation format can be used only when there is a sufficient signal to noise ratio.

1.8.1 Downlink Carriers and Resource Blocks

In the downlink, the subcarriers are split into resource blocks. This enables the system

to be able to compartmentalize the data across standard numbers of subcarriers.

Resource blocks comprise 12 subcarriers, regardless of the overall LTE signal

bandwidth. They also cover one slot in the time frame. This means that different LTE

signal bandwidths will have different numbers of resource blocks.

1.8.2 LTE SC-FDMA in the Uplink

For the LTE uplink, a different concept is used for the access technique. Although still

using a form of OFDMA technology, the implementation is called Single CarrierFrequency Division Multiple Access (SC-FDMA).One of the key parameters that affects

all mobiles is that of battery life. Even though battery performance is improving all the

time, it is still necessary to ensure that the mobiles use as little battery power as

possible. With the RF power amplifier that transmits the radio frequency signal via the

antenna to the base station being the highest power item within the mobile, it is

necessary that it operates in as efficient mode as possible. This can be significantly

affected by the form of radio frequency modulation and signal format. Signals that

have a high peak to average ratio and require linear amplification do not lend

themselves to the use of efficient RF power amplifiers. As a result it is necessary to

employ a mode of transmission that has as near a constant power level whenoperating. Unfortunately, OFDM has a high peak to average ratio. While this is not a

problem for the base station where power is not a particular issue, it is unacceptable

for the mobile. As a result, LTE uses a modulation scheme known as SC-FDMA (Single

Carrier Frequency Division Multiplex) which is a hybrid format. This combines the low

peak to average ratio offered by single-carrier systems with the multipath interference

resilience and flexible subcarrier frequency allocation that OFDM provides.

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MIMO (Multiple Input Multiple Output) is another of the LTE major technology

innovations used to improve the performance of the system. This technology provides

LTE with the ability to further improve its data throughput and spectral efficiency

above that obtained by the use of OFDM. Although MIMO adds complexity to thesystem in terms of processing and the number of antennas required, it enables far

high data rates to be achieved along with much improved spectral efficiency. As a

result, MIMO has been included as an integral part of LTE.

2.0 LTE MIMO Basics

The basic concept of MIMO utilizes the multipath signal propagation that is present in

all terrestrial communications. These paths can be used to an advantage, rather than

providing interference.

2.0.1 Note on MIMO:

Two major limitations in communications channels can be multipath interference, and

the data throughput limitations as a result of Shannon's Law. MIMO provides a way of

utilizing the multiple signal paths that exist between a transmitter and receiver to

significantly improve the data throughput available on a given channel with its defined

bandwidth. By using multiple antennas at the transmitter and receiver along with

some complex digital signal processing, MIMO technology enables the system to setup multiple data streams on the same channel, thereby increasing the data capacity of

a channel.

MIMO is being used increasingly in many high data rate technologies including Wi-Fi

and other wireless and cellular technologies to provide improved levels of efficiency.

Essentially MIMO employs multiple antennas on the receiver and transmitter to utilize

the multi-path effects that always exist to transmit additional data, rather than

causing interference.

The schemes employed in LTE again vary slightly between the uplink and downlink.

The reason for this is to keep the terminal cost low as there are far more terminals

than base stations and as a result terminal works cost price is far more sensitive. For

the downlink, a configuration of two transmit antennas at the base station and two

receive antennas on the mobile terminal is used as baseline, although configurations

with four antennas are also being considered.

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For the uplink from the mobile terminal to the base station, a scheme called MU-MIMO

(Multi-User MIMO) is to be employed. Using this, even though the base station requires

multiple antennas, the mobiles only have one transmit antenna and this considerably

reduces the cost of the mobile. In operation, multiple mobile terminals may transmit

simultaneously on the same channel or channels, but they do not cause interference

to each other because mutually orthogonal pilot patterns are used. This techniques isalso referred to as spatial domain multiple access (SDMA).

2.1 LTE FDD Frequency Band AllocationsThere are a large number of allocations or radio spectrums that have been reserved

for FDD, frequency division duplex, LTE use. The FDD frequency bands are paired to

allow simultaneous transmission on two frequencies. The bands also have a sufficient

separation to enable the transmitted signals not to unduly impair the receiver

performance. If the signals are too close then the receiver may be "blocked" and the

sensitivity impaired. The separation must be sufficient to enable the roll-off of the

antenna filtering to give sufficient attenuation of the transmitted signal within the

receive band.

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LTE TDD frequency band allocations

With the interest in TDD LTE, there are several unpaired frequency allocations that are

being prepared for LTR TDD use. The TDD LTE allocations are unpaired because the

uplink and downlink share the same frequency, being time multiplexed.

2.2 Abbreviations

3GPP Third Generation Partnership

Project

ADM Add Drop Multiplexer

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ADSL Asynchronous Digital Subscriber

Line

ATM Asynchronous Transfer Mode

BSC Base Site Controller

BSS Business Support System

BTS Base Station Transceiver

CDMA Code Division Multiple Access

DAB Digital Audio Broadcasting

DAS Distributed Antenna System

DSL Digital Subscriber Line

DSLAM Digital Subscriber Line AccessMultiplexer

DVB Digital Video Broadcasting

DWDM Dense Wave Digital Multiplexing

E911 Enhanced 911

E911 SR E911 Selective Routing

EDGE Enhanced Data Rates for GSM

Evolution

ECM EPS Connection Management

EMM EPS Mobility Management

EPC Evolved Packet Core

ePDG Evolved Packet Data Gateway

EPS Evolved Packet System

E-RAB E-UTRAN Radio Access Bearer

ETSI European TelecommunicationsStandards Institute

E-UTRAN Evolved Universal Terrestrial

Radio Access Network

EVDO Evolution Data Optimized

GGSN Gateway GPRS Support Node

GigE Gigabit Ethernet

GPRS General Packet Radio Service

GSM Global System for Mobile

Communications

GW Gateway

HLR Home Location Register

HSS Home Subscriber Server

IMEI International Mobile Station

Equipment Identity

IMS IP Multimedia Subsystem

IMSI International Mobile Subscriber

Identity

IP Internet Protocol

IPTV Internet Protocol Television

ISDN Integrated Services Digital

Network

LTE Long Term Evolution

MDU Multi Dwelling Unit

MGW Media Gateway

MIMO Multiple Input Multiple Output

MIPv4 Mobile IP version 4

MIPv6 Mobile IP version 6

MME Mobility Management Entity

MIMO Multiple Input Multiple Output

M-TMSI M-Temporary Mobile Subscriber

Identity

MPLS Multiprotocol Label Switching

MSC Mobile Switching Center

MSP Multiservice Platform

MTSO Mobile Telephony Switching Office

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OFDMA Orthogonal Frequency Division

Multiplexing Access

OLT Optical Line Terminal

OMC-ID Operation and Maintenance enter

Identity

ONT Optical Network Terminal

OSS Operational Support System

PDN Packet Data Network

P-GW PDN Gateway

PDCP Packet Data Convergence Protocol

PDSN Packet Data Switching Node

PMIP/PMIPv6 Proxy Mobile IP version 6

PON Passive Optical Network

PSAP Public Safety Answering Point

PSTN Public Switched Telephone

Network

QAM Quadrature Amplitude

Modulation

QPSK Quadrature Phase Shift Keying

RAN Radio Access Network

RG Residential Gateway

SC-FDMA Single Carrier Frequency Division

Multiple Access

SCP Service Control Point

SDH Synchronous Digital Hierarchy

S-GW Serving Gateway

SHE Super Head End

SGSN Serving GPRS Support Node

SIAD Site Access Device

SONET Synchronous Optical Network

SS7 Signaling System 7

STP Signal Transfer Point

TCP/IP Transmission Control

Protocol/Internet Protocol

TDD Time-Division Duplex

TDM Time Division Multiplexer

UMTS Universal Mobile

Telecommunications System

VHO Video Hub Office

VSO Video Serving Office

VoIP Voice over IP

WAC WiMAX Access Device

WBS WiMAX Base Station

WCDMA Wideband CDMA

WDM Wavelength Division Multiplexing

WMG Wireless Media Gateway

WiMAX Worldwide Interoperability for

Microwave Access

WLAN Wireless Local Area Network

WSS Wireless Soft Switch

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Documents

LTE Tutorial Basics v1