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Long Term Evolution 1

LONG TERM EVOLUTION

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Long Term Evolution 2

Table of Contents

List of Acronyms .......................................................................................................................................... 4

1.0 Introduction ............................................................................................................................................. 6

2.0 Overview and Background ..................................................................................................................... 6

2.1 First Generation .................................................................................................................................. 8

2.2 Second Generation ............................................................................................................................ 11

2.2.1 GSM ........................................................................................................................................... 14

2.2.2 IS-95 ........................................................................................................................................... 16

2.3 Third Generation ............................................................................................................................... 17

2.3.1 UMTS: Universal Mobile Telecommunications System ........................................................... 17

2.3.2 CDMA- 2000 ............................................................................................................................. 20

2.4 IMT-Advanced .................................................................................................................................. 21

2.5 3G Evolution to 4G ........................................................................................................................... 22

3.1 Introduction ....................................................................................................................................... 24

3.2 Network Architecture and Interface .................................................................................................. 24

3.2.1 LTE Mobile Device ................................................................................................................... 24

3.2.2 The e-Node-B and eUTRAN ..................................................................................................... 25

3.2.3 Evolved Packet System .............................................................................................................. 26

3.2.4 Home Subscriber Server (HSS) ................................................................................................. 28

3.3 Air Interfaces .................................................................................................................................... 29

3.3.1 OFDMA on Downlink ............................................................................................................... 29

3.3.2 SC- FDMA on the Uplink .......................................................................................................... 31

3.4 The LTE Frame Structure ................................................................................................................. 31

3.4.1 LTE Symbols, Slots, Radio Blocks and Frames ........................................................................ 32

3.5 LTE Channels ................................................................................................................................... 35

3.5.1 Channels on the Downlink Direction ......................................................................................... 35

3.6 MIMO Transmission ......................................................................................................................... 37

3.7 LTE Protocol Layers ......................................................................................................................... 38

3.8 LTE Application ............................................................................................................................... 39

3.9 LTE Benefits ..................................................................................................................................... 40

3.10 LTE Antenna ................................................................................................................................... 40

4.0 Conclusion ............................................................................................................................................ 40

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Reference List ............................................................................................................................................. 41

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List of Acronyms

Advanced Mobile Phone System (AMPS)

Base Transmission Station (BTS)

Broadcast Channel (BCH

Broadcast Control Channel (BCC)

Charging Gateway Function (CGF

Code Division Multiple Access (CDMA

Data Network Gateway (PDN-G)

Dedicated Control Channel (DCCH)

Dedicated Traffic Channel (DTCH)

Enhanced Data for GSM Revolution (EDGE)

Evolved Packet System (EPS)

Evolved UMTS Terrestrial Radio Access Network (EUTRAN).

Gateway MSC (GMSC)

General Packet Radio Service (GPRS)

Global System for Mobile Communications (GSM)

High Speed Downlink Packet Access (HSDPA)

Home Location Register (HLR

Home Subscriber Server (HSS)

Internet Protocol (IP)

LTE-Long Term Evolution User Equipment (UE)

Media Access Control (MAC

Mobile Assisted Hand Off (MAHO)

Mobile Station International Number (ISDN)

Mobile Switching Center (MSC)

Mobility Management Entity (MME

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Multi Media messaging service (MMS

Multicast Traffic Channel (MTCH

Multiple Input Multiple Output (MIMO)

Nippon Telegraph and Telephone (NTT)

Nordic Mobile Telephone (NMT

Orthogonal Frequency Division Multiple Access (OFDMA)

Packet Gate Way (PGW

Radio Link Control (RLC

Radio Network Subsystem (RNS)

Radio Resource Control (RRC

Serving Gateway (s-Gw),

Serving GPRS Support Node and the Gateway GPRS Support Node (SGPRSSN)

Short Text Messages (SMS)

System Architecture Evolution (SAE)

The common control channel (CCCH

The Paging Control Channel (PCCH)

Time Division Multiple Access (CDMA)

Total Access Communication System (TACS)

Universal Mobile Telecommunication System (UMTS)

Universal Subscriber Identity Module (USIM)

Wide –Code Division Multiple Access (WCDMA)

3GPP (third generation Partnership projects)

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Long Term Evolution

1.0 Introduction

The Long Term Evolution (LTE) is a wireless communication platform that is an

advancement of the older generation 2G and 3G, which were based on the GSM and GPRS

technology. LTE brings multiple improvements in wireless communications including enhanced

data transfer speeds, reduced latency, better reliability and accommodation for various operation

conditions as well as improved adaptability to terminal mobility (Lescuyer & Lucidarme, 2008).

The LTE’s main objectives include reduction of user equipment (UE) complications as

well as allow smooth coupling of the new advanced technology with older existing platforms

such as GPRS/EDGE and UMTS/HSPA, thus enabling a seamless communication experience

(Sesia & Toufik 2011). This paper will explore the history of mobile wireless communication

and Long Term Evolution (LTE) as a wireless communications alternative: its strengths,

challenges and its possible future.

2.0 Overview and Background

The mobile communications sector has undergone many changes since its inception

towards the end of the last century. Even though there were no strict demarcations to mark

generation one through four, there were certain universally perceived characteristics of each

generation (Myung 2010). For instance, generation one or 1G featured analogue communication

equipment, while generation two (2G) was mainly digital equipment. Generation three or 3G

involved dynamic changes in bandwidth capabilities, as well as audio to video support. All these

changes happened progressively with no real onset times due to the fact that various regions

experienced different developments. For instance, Europe had the 3GPP standardization while in

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US and parts of Asia, there was the 3GPP2 evolution. The first generation was the analogue

communication solely for voice traffic and was represented by such systems as AMPS

(Advanced Mobile Phone System) in the USA as well as the Total Access Communication

System (TACS) in European countries. Analogue technology had several shortcomings including

signal noise, intrusion and low bandwidth (Lescuyer & Lucidarme, 2008).

The second generation saw migration to digital radio networks and noise elimination.

Second generation equipment also featured Time Division Multiple Access (CDMA) and Code

Division Multiple Access (CDMA). The TDMA was adapted as Group Special Mobile (later

Global System for Mobile Communications (GSM) mainly in Europe. In the US, CDMA was

adopted and had the advantage of better spectrum use, thus more users. CDMA one had

48.5Kbps and CDMA two had 115bps. A momentary advancement in the 2G was introduction of

packet switching as opposed to circuit switching, a platform on which, the 2G was built

(Dahlman, Parkvall & Per Beming 2008).

Third generation 3G featured an increased bandwidth with the limit minimum at

144Kbps. Most 3G networks far surpassed the minimum at 5-10Mbps. Universal Mobile

Telecommunication System (UMTS) and Wide –Code Division Multiple Access (WCDMA) are

the industry representatives of 3G. Further, development of 3G into 3.5G was through

introduction of High Speed Downlink Packet Access (HSDPA), in order to allow data transfer

rates up to 7.5Mbps (Sesia & Toufik 2011). The long term evolution (LTE) succeeded the 3.5G

technologies through 3GPP (third generation Partnership projects) with actual data rates at

100Mbps in the downlink and 50Mbps in the uplink. The network data rates for 4G networks are

in the range of 1Gbps. This study will explore the technical aspects of the 4G.

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2.1 First Generation

Prior to 1950, mobile communication systems were used exclusively for military and

maritime communication purposes. The equipment used was expensive and restricted. In 1946,

however, a car based analogue system was developed for St Louis for use in police cars, taxis

and other private push-to-talk communication arrangements. This network was supported

through installation of high power (typically 200Watts) transmitters/receiver stations usually

placed on high ground (Lescuyer & Lucidarme 2008).

The first generation of mobile telecommunications marked the onset of modern wireless

telephony. Previous to this, telephone communication was mainly through fixed land line. In

1979, the first 1G cellular communication network was introduced in Japan by Nippon Telegraph

and Telephone (NTT). It covered just a small area of metropolitan Tokyo and was purely

analogue. Nordic Mobile Telephone (NMT), a similar network, was launched in several

European countries in 1981, while other areas such as the UK, Canada and Mexico among

others, received 1G network within the same decade. Total Access Communication Systems

(TACS) was another close competitor to those mentioned above. The major shortcoming with all

the 1G networks is the fact that they lacked inter-operation capabilities between countries.

However, it is should be noted that roaming capabilities were already incorporated (Sesia &

Toufik 2011).

Advanced mobile phone system (AMPS) was launched in the US in 1982 and had a

bandwidth of 40 MHz in the 800-900 MHz frequency range. In 1988, 10 MHz of bandwidth

expansion were allowed, thus increasing the capability of the network. The network was started

in Chicago and covered an area of 2100 square miles. Further, AMPS supported 832 channels,

with the maximum data rate fixed at 10Kbps (Sesia & Toufik 2011). Initially, the IG systems

Long Term Evolution 9

used Omni-directional aerials, limiting their coverage efficiency as well as frequency reuse

factor greatly. Later, bi-directional as well as tri-directional orientations were introduced. Most

networks adopted the current trend of connecting back to back three antennas in one mast at 120

degrees to cover the most area. A frequency re-use factor of 7 was adopted an indication that, a

frequency would only be re-used after traversing a seven cell polygon (Myung 2010).

Cell Structure for Cellular Communication Networks

Although the mobile networks have evolved through the last three decades, the basic

cellular arrangement of communication systems has remained the same. In a mobile phone cell

structure, each mobile phone is located in a cell spanning a certain radius from several hundred

meters in densely populated areas to several tens of kilometers in very remote areas (Myung

2010). The typical radius for a cell is 1-4 Km in digital systems, but was as large as 20Km in the

analogue system. This factor meant that analogue systems could accommodate far fewer users

than the newer digital networks. In every moment, the mobile device transmits a signal to the

base station located at the middle of each cell to update its location as well as get/receive other

important information. It is notable that, every device must be allocated a different

communication channel, thus avoiding channel overlap. Typically, one channel maybe allocated

64 channels. In order to ensure large area coverage, the networks are designed so as to recycle

the available frequencies after every cell. The strength of the signal sent from the device reaching

the base transmission station (BTS) is proportional to the device’s distance from the station

(Lescuyer & Lucidarme, 2008).

Figure1: Cell Structure for Cellular Communication Networks

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(Myung 2010)

In the diagram, there are three frequency re-use regions of seven cells each. Adjacent

cells in different cell jurisdictions ought not to apply similar frequency to avoid interference.

Cells using similar frequencies are designated using similar letters, and are not adjacent

(Dahlman, Parkvall & Per Beming 2008). When a mobile user moves away from one cell into

another, the cellular network initializes a process referred to as hand-off. This is where, the BTS

in the cell from where the device is moving into, informs the two nearest BTS stations of the

migration. The station receiving the greater signal level from the moving device accepts custody

of the device and the process takes about 300 milliseconds, enabling a seamless communication

link (Dahlman, Parkvall & Per Beming 2008). Once the hand-off is successful, the previous host

releases all control over the device and is free to allocate the channel to a new user. The

foundation for communication in all cellular networks is based on this arrangement, but the

advancements have taken place as a result of efficiency in the cellular structures (Myung 2010).

Communication Handling in 1G

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The 1G network’s traffic was split into four channels. These include the control channel,

from BTS to mobile and manage the system, paging channel, base to mobile and alerts mobile of

calls/short messaging service (sms)/pages for them. Others include access channel, two way, and

allows calls and other communication between mobile and BTS and data- bi-directional- which

allowed for faxes and voice traffic (Dahlman, Parkvall & Per Beming 2008). Once a user

switches on a mobile device, the phone broadcasts its 10 digit identification number to its host

BTS through the strongest of the 21 pre-loaded channels. The BTS receives the signal and

informs the local MSC of the new subscriber whose mobile device has been allocated network

resources. The MSC performs a location update for the new user, and continues to do so once

every 15 minutes. When a user places a call, the BTS forwards the request to the MSC which,

after ascertaining the caller’s identity as one of its own, proceeds to locate an idle channel and

sends the channel information to the caller. The mobile station user adjusts its signal to the

allocated channel and the call is then placed (Dahlman, Parkvall & Per Beming 2008). The

calling station continues the call signal till the receiver mobile station picks up or network

terminates the call request as unanswered. This procedure has not changed through the various

generations (Myung 2010).

2.2 Second Generation

Second Generation (2G) networks were started towards the end of 1980s. This

generation’s main difference from the 1G was that it allowed, in addition to voice services, data

capabilities. The initial data capacity was low, major enhancements were made to improve

bandwidth. To enable the transformation, the basic architecture of networks in 2G incorporated

Time Division Multiple Access (TDMA) as well as Code Division Multiple access (CDMA) as

alternatives to the circuit switched networks in 1G. Code division basically means that, multiple

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users can access the same channel at the same time but using different coding for each user’s

stream (Dahlman, Parkvall & Per Beming 2008). The data is encoded before transmission and

then decoded in the receiving end to reveal as many channels as were encoded into the signal.

Time division works much like code division, but slots short bursts of time for each user in the

same channel, the bursts themselves being so short and frequent that voice and other output is

not significantly altered (Lescuyer & Lucidarme, 2008).

In addition to CDMA and TDMA, there were two other networks in the 2G category: D-

AMPS and PDC. The latter was used in Japan only as a compatibility network for its 1G NTT

platform. D-AMPS (Digital Advanced Mobile Phone System) is fully digital. It was structured to

accommodate in, a single cell, the newer digital channels as well as the analogue channels. The

MSC allocates the available bandwidth accordingly. D-AMPS operate in the 1850 to 1910 MHz

for upstream and 1930 to 1990 Mhz range for downstream traffic. In D-AMPS, standard waves

were shorter at 16cm, allowing shorter ¼ wave antenna of 4 cm on mobile phones (Myung

2010). This revolutionized mobile phone sizes to smaller, lighter phones. In the digital realm,

voice picked in the microphone is digitized using complex algorithms inside the phone’s module

before being sent to over the network. The digitization process also compresses the audio band

from the 56Kbps in analogue systems to only 8Kbps in the 2G networks, freeing up bandwidth

by a factor of 7 implying more channel capacity for the 2G networks (Ergen 2009). In D-AMPS,

three users may share a single frequency through time division, with each pair of frequencies

supporting 25 pulses per second. It should be noted that, frame (pulse) has 6 time slots and has a

total of 40milliseconds (Dahlman, Parkvall & Per Beming 2008).

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(Dahlman, Parkvall & Per Beming 2008)

In the first arrangement, three users share a TDM time frame, with each user receiving

324 bit allocation (Ekström et.al. 2006). Out of those, 64 bits are used up for network control and

addressing, 101 bits are used for parity error correction and 159 bits incorporate the actual user

data such as voice. The second arrangement shows six users sharing the same TDM frame. To

achieve this capacity, compression has to be done to 4Kbps, down from 8Kbps (Dahlman,

Parkvall & Per Beming 2008). In D-AMPS, the mobile device actually assist the MSC in hand-

offs by taking the idle time it has during a frame to compare signal strengths. If it discovers a

signal stronger than the one from its local BTS, it seeks to be handed over to the stronger signal

BTS. This scenario is called Mobile Assisted Hand Off (MAHO). D-AMPS is used mainly in

Japan and US, while everywhere else in the world, GSM is used. GSM is similar to D-AMPS in

most aspects. However, in GSM, the mobile receives at a frequency 55MHz higher than it can

transmit, unlike in D-AMPS where mobile receives at 80MHz higher than it transmits. In

addition, GSM channels are wider than D-AMPS by 170KHz, allowing higher data rates per

user. The section below will discuss GSM in detail (Dahlman, Parkvall & Per Beming 2008).

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2.2.1 GSM

GSM is an acronym of Global System for Mobile communication, a system having

channels each of 124 frequency pairs (Ergen 2009). Each channel holds 8 separate connections,

and has a bandwidth of 200Khz. Any active mobile has assigned to it one slot in a one channel

pair. Even though a cell may support as many as 992 channels, only a fraction of them are

utilized to avoid traffic conflicts with neighboring cells. GSM stations cannot transmit and

receive simultaneously; therefore, at times GSM is taken up in switching from one mode to the

other (Ergen 2009). Therefore, for the eight time slots allocated per connection, four are used in

either direction.

Figure 2: GSM transmission architecture

(Ergen 2009)

In the figure, the shaded slots indicate time series allocations for a mobile station. The

device inputs its traffic into each slot successively until all the payload has been sent or received.

Only one slot per frame is dedicated for user traffic, mainly to avoid conflict as mentioned

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above. Every slot contains one 148-bit data frame, which takes up 577 microseconds in channel

time and every frame must begin and end with three 000 bits meant for delineation purposes. In

addition, each frame contains a double 57 bit information field immediately followed by control

bits for specifying the nature of the following frame (whether voice or data). Information fields

are separated by a 26 bit synchronization region during which a receiver is allowed time to synch

up its transmission properties to those of sender.

Figure 3: TDM Frame Structure

(Ekström et.al. 2006)

From the figure, the GSM multi-frame is made up of twenty six 1250-bit TDM frames,

which in turn are made up of seven 148-bit data frames. Each transmitter is shares a frame with

seven others, limiting its maximum data rate to 33.854Kbps, more than double the rate of D-

AMPS 1G technology. In addition, another multi-frame with 51 slots is used with GSM and

these slots are used for other network control services. One such service is the Broadcast Control

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Channel (BCC), which continually broadcasts the base station’s location, signal strength and

other statuses. This information can help the mobile station know when it moves in to a new cell.

2.2.2 IS-95

This term is synonym for International Standard 95, a name mostly used to describe the

Code Division Multiple Access (CDMA) network architecture. CDMA ONE is also used to refer

to the CDMA network type. The essential difference between CDMA and GSM is that instead of

a transmitter using a small fraction of the allocated frequency to send data; it can use the entire

spectrum. In the meantime, the architecture uses multiple coding to encrypt each of the

transmitters’ outputs before feeding them into one channel. According to coding theory, multiple

user signals when added together do not collide, but add up linearly in a way that allows

separation in the receiving end. Each bit time in CDMA is split into chips, typically 64 or 128

(Sesia & Toufik 2011).

It is notable that, every device is assigned a chip sequence in which it may transmit in the

form of a binary string. To transmit a 0, the device sends its sequence compliment. For example,

the string maybe 00001101 for a 1 bit data output or 00001100 for a 0 bit output and no other

protocol is allowed (Ekström et.al. 2006). There are, therefore, no channel allocation conflicts in

CDMA, seeing that each transmitter may utilize the full bandwidth during its active slot. The

strength of the signal from the BTS to the mobile device is proportional to the distance between

them, and a general rule of thumb is for the mobile station to transmit at the inverse of the power

level coming from the station. In addition, the station may direct near-situated devices to lower

their signal levels, to avoid saturating the weak output signals from mobile devices located far

off. CDMA band is typically 1.25 MHz, many times higher than 1G networks and GSM

(Lescuyer & Lucidarme, 2008).

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In an early enhancement of 2G networks, which also became to be known as 2.5G, a

communication platform known as Enhanced Data for GSM Revolution (EDGE) was formed.

This network allowed more bits per slot in the uplink as well as downlink. Thus, higher speeds

were possible than in the GSM. The major drawback in EDGE is that with increase in bit-rate,

errors also increased, and a significant amount of resources had to be allocated for error

correction. Another 2.5G network is the General Packet Radio Service (GPRS). This network

type is an overlay of packet transmission over the original voice structures in either D-AMPS or

GSM. The additional packet bits allocated in the channel necessitates an intelligent co-ordination

of voice versus data in system resource allocation. This is normally done by BTS in accordance

to voice versus data requirements at any one time. Usually voice is given priority, and a data

session in a GPRS network will get halted if device user either initiates or hosts a voice call

(Sesia & Toufik 2011).

2.3 Third Generation

Third generation communication platform revolutionized the wireless communication

domain by enabling integration of high bandwidth with newer features support. These features

include e-mail capability, high level gaming, media playback and data streaming among others.

This section will discuss 3G and the specific additional features that differentiate it from

2G.Further, it will also discuss the various networks that are categorized as being in the 3G

bracket including UMTS, CDMA2000, W-CDMA, HSDPA (Dahlman, Parkvall & Per Beming

2008).

2.3.1 UMTS: Universal Mobile Telecommunications System

The UMTS is a third generation mobile communications network launched in 2002 and

based on the GSM. Rather than be called a new system, UMTS has largely been seen as an

upgrade on earlier systems since packet carrying enhancement on GSM had already been done

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through EDGE and GPRS, both considered as 2.5G networks. UMTS was designed and

maintained by the 3GPP group as a component of IMT-2000 (International Telecommunications

Union- 2000) (Myung 2010). It uses wideband CDMA for greater efficiency in bandwidth and

this new technology employs new facilities as will be discussed below.

UMTS Architecture

Figure 4: UMTS Infrastructure

(Myung 2010)

User Equipment (UE)

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User equipment is a broader name used to refer to the mobile phone among other

equipment, due to the fact that the user equipment may handle more functions with the enhanced

network support. For instance, the original device supported only voice and paging functions. In

the 2G networks, mobile stations could support, in addition to the two functions, also a

rudimentary packet transfer system which was embedded with the circuit switched network

element (GSM/GPRS) (Myung 2010). In the third generation networks, mobile devices could

support all the functions listed above and in addition, other functions requiring higher data rates

such as internet gaming chat, and certain forms of video streaming. In the 4G system, the user

equipment supports even more functions including Mobile TV (Lescuyer & Lucidarme, 2008).

Radio Network Subsystem (RNS)

This system replaces the Base Transmission System (BTS) and it provides the network

management functions such as traffic handling, resource allocation and roaming. Further, it

controls the Node B and the Radio Network Controller (RNC) which encrypts and decrypts

information. The Node B is the replacement of the BTS and handles call routing.

Core Network

This is the central network processing unit and it replaces the Network Switching System

(NSS). This architecture contains both the circuit- switched and packet-switched elements. The

circuit switched elements include the Mobile Switching Center (MSC) as well as the Gateway

MSC (GMSC). These elements operate as they did in the GSM network. The packet switched

elements include the Serving GPRS Support Node and the Gateway GPRS Support Node. The

SGPRSSN is responsible for packet mobility management, active session management, and

billing. The Gateway GPRS Support node is the link between the core network and the external

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networks. It is responsible for protocol harmonization, data filtering and authentication among

other functions.

UMTS Features

The UMTS supports a maximum data rate, theoretically, of up to 384Kbps and a peak

data rate of 42Mbps with HSPA+ implemented, which translates to speeds up to 7.2Mbps for

handsets with HSDPA support. Most countries have already upgraded their UMTS networks to

HSDPA to enable faster data transfer rates, usually with the label of 3.5G.

2.3.2 CDMA- 2000

CDMA-2000 is mainly the American 3G equivalent and it evolved from IS-95 which was

represented by CDMA. According to Sesia & Toufik (2011), CDMA-2000 can also be referred

to as IMT-Multi Carrier (IMT-MC). Cdma- 2000 is an evolutionary name for a series of

advancements performed on the CDMA architecture upon which it is based. The evolution from

CDMA to cdma2000 took the following stages: CDMA2000 1xRTT, CDMA2000 1 xEV-DO (

with 3 releases 0,A and B), the release C also called Ultra Mobile Broadband(UMB) and the

CDMA2000 1xEVDV

CDMA2000 x1

The cdma2000 1x was also called IS-2000 is the core wireless interface standard for

CDMA and has the same Radio Frequency Bandwidth as the IS-95 standard (thus the 1x). The

1xRTT, however, was developed to increase the uplink bandwidth by addition of 64 more

channels orthogonal with the existing 64, effectively doubling the network’s capacity. The 1X

enhancement supports network data rates up to 154Kbps and actual data rates between 80- 100

Kbps (Dahlman, Parkvall & Per Beming 2008).

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1X- EVDO

This revolution uses both the CDMA as well as TDMA for high speed internet access

(Myung 2010). It is in use in many mobile networks especially where high speed internet is

required. Its third revision (Release C) is called the Ultra Mobile Broadband and preceded the 4G

layout. This has been used extensively for high speed mobile browsing with such features as

gamming support and video streaming by end users with compatible mobile stations (Sesia &

Toufik 2011).

2.4 IMT-Advanced

IMT-2000 is a wireless communication standard approved in 2008 that basically laid

down requirements for a network to be termed as 3G. IMT-Advanced, however, proceeded to

extend requirements on 3G networks and devices to be able to offer mobile broadband on smart

phones, wireless laptop modems, and other machines. It is also expected that such a network

should support gaming, video streaming and chat, Multi Media messaging service (MMS),

mobile TV and HDTV (Furht 2009). The properties of the IMT Advanced standard include the

following:

 It is a packet switched network based on the Internet Protocol (IP) concept

 Offers high speed rates of 1Gbps when user and Base station are stationary and up to

1Mbps when user is moving at speeds, which are high in regard to the base station

 A spectral efficiency of 15bits/Hz in downlink and 6Bs/Hz in the uplink

 Seamless link between networks and smooth handovers

 High quality multimedia support (Lescuyer & Lucidarme, 2008).

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2.5 3G Evolution to 4G

The increasingly demanding IT platform forced researchers to advanced yet further on

equipment and network abilities in order to come up with systems that would support even

higher data rates, handle user mobility and handovers better, support more services and programs

and guarantee a longer period of reliability (Sesia & Toufik, 2011). These conditions were

driving the industry towards a new wireless communication experience- 4G. This concept meant

performing certain changes to the existing 3G facilities and standards, or even coming up with

entirely new machines. As expected, the major changes would be along the dimension of data

rates (both uplink and downlink), variety of services or programs that could be operate on the

network, advanced multimedia support among other factors (Dahlman, Parkvall & Per Beming

2008).

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Figure 5: Wireless Networks Evolution

(Sesia & Toufik 2011)

The first change required that network support of channels with radio frequencies up to

40Mbps with maximum spectral efficiency. AMT-Advanced had already envisioned these

changes. In addition, spectral efficiency was expected to hit a peak 2.2bps/HZ/cell and support

mobility at speeds up to 350Km/h. As earlier stated, the task force behind the migration of 3G

into 4G was the Third Generation Partnership Project (3GPP) which ensured the roll-over into

4G by the ear 2010 (Guerra & Ramlogan 2008). While some aspects of the platform change were

new and unprecedented, it is worthwhile noting that, numerous changes were really just upgrades

Long Term Evolution 24

of existing mechanisms. In addition, some of the envisioned attributes of the 4G networks are yet

to be realized. The section below will discuss in length the fourth generation wireless networks

including the properties, variations, current usability and the future of 4G (Guerra & Ramlogan

2008).

3 Long Term Evolution (LTE)

3.1 Introduction

LTE was formulated with a view to enhance the performance of the existing 3G and 4G

platforms. In addition, a number of facilities (either hardware of just the hardware name) in the

network are changed (Guerra & Ramlogan 2008). The LTE’s main objectives include reduction

of user equipment (UE) complications as well as allow smooth coupling of the new advanced

technology with older existing platforms such as GPRS/EDGE and UMTS/HSPA to enable a

seamless communication experience (Sesia & Toufik 2011).

3.2 Network Architecture and Interface

The section below will discuss the architectural components of the LTE. Long Term

Evolution is based on the Evolved Packet System (EPS) infrastructure. Unlike in the older

platforms where circuit switching was used, the EPS uses packet switching. The Evolved Packet

System employs a new structure of equipment including the User Equipment (EU), eNodeB-B

and eUTRAN, the Mobile Management Entity (MME), Serving Gateway (s-Gw), the PDN

Gateway, the Home Subscriber Server (HSS) and the Air Interfaces.

3.2.1 LTE Mobile Device

The LTE mobile device, also caller User Equipment, refers to the end user device like a

mobile phone, a high speed data modem, smart phone, digital TV or any other equipment

designed to use the features availed by 4G (Sesia & Toufik 2011). The name user Equipment

Long Term Evolution 25

(UE) was a change from the previous platforms use of the name mobile station/device, because

the new device could perform a range of different functions. The user equipment maybe

conceptualized using its various components including the Radio Frequency circuitry, baseband

processing, battery and the Universal Subscriber Identity Module (USIM) (Myung 2010). The

RF circuitry contains the transmitter as well as the receiver in addition to an amplifier. This

circuit houses the frequency synthesizer that aids in frequency hopping between channels if need

arises. Further, the baseband processing aids in signal processing. The Universal Subscriber

Identity Module (USIM) is similar to, but more advanced than the GSM sim card. It contains the

International Mobile Subscriber Identity Number (IMSI) and the Mobile Station International

Number (ISDN). Most USIM modules may contain a limited storage for user data such as Short

Text Messages (SMS) and Phonebook entries (Myung 2010).

3.2.2 The e-Node-B and eUTRAN

The eNODE

This part of the system replaces the Base Trans-receiver Station (BTS) used in lower

networks. It transmits as well as receives signals from both the end user and the core network.

Together with the Radio Network Controller (RNC), the eNodeB-B form the UMTS Terrestrial

Radio Access Network (UTRAN)

Radio Network Controller

The RNC replaces the Mobile Switching Centre (MSC) in 1G, 2G,and 3G. It receives

communication from the eNodeB , which is then relayed to the core network (Ekström et.al.

2006). It also performs the work of radio resource management in addition to limited mobility

Long Term Evolution 26

facilitation. Data encryption and decryption also happens in the RNC. Communication between

groups of cells is made possible by the RNCs (Sesia & Toufik 2011).

Core Network

This part includes the circuit switched elements and packet switched elements. The

circuit switched parts of the core network are based on the GSM and handle elements in the

network that are circuit switched such as call processing. The packet switched elements include

parts that bear packet data. This split into circuit and packet switched elements enable more

optimal utilization of the network since bandwidth can be switched between these two elements

to cater for varying demands. The core network incorporates the Serving Gateway (S-GW), the

Packet Data Network Gateway (PDN-G) and a number of Mobile Switching Centers (MSC). The

4G network is based on a packet system, much unlike the circuit switching done in 1G and 2G

networks. The next session will explore the packet switching network system.

3.2.3 Evolved Packet System

The 3G/UMTS evolution introduced by the 3GPP committee to feature packet switching

is called the Evolved Packet System (EPS) (Sesia & Toufik 2011). This evolution essentially

replaces the older GPRS network. It incorporates the radio, core and services systems and is a

major component of System Architecture Evolution (SAE) proposed as a necessary process in

4G features realization (Guerra & Ramlogan 2008). The major focus of the SAE include

simplified circuitry, an all IP network and bandwidth widening and optimization through use of

Orthogonal Frequency Division Multiple Access (OFDMA) as well as the Multiple Input

Multiple Output (MIMO) antenna configuration, incorporating the Mobility Management Entity

(MME) among other equipment (Lescuyer & Lucidarme, 2008).

Long Term Evolution 27

3.2.3.1 The Mobile Management Entity

This unit performs different functions but mostly is responsible for control of the user

equipment/ mobile equipment when the device moves within or across a Evolved UMTS

Terrestrial Radio Access Network (EUTRAN) as well as providing security for the network in

NAS protocols. The MME is responsible for several types of procedures including MME

common procedures, MME specific procedures and the MME connection procedures. Common

procedures include user equipment identification, authentication, relocation, and security control.

The specific procedures are procedures that only pertain to the UE and are unique to the UE

devices (Ahson 2009). They include detach and attach, as well as multiple detach and multiple

attach. These procedures usually are initiated when devices go off the network or establish

connection and it also involves location update per equipment in the area. MME connection

management procedures include usually network to mobile functions such as service requests,

paging requests and transport of messages (Ahson 2009).

3.2.3.2 The serving Gateway (S-GW)

The serving gateway is responsible for higher level network management as well as

providing smooth links with external networks. It oversees the internal communication protocols

as well as manages the information flow outside the network. It supports the EU interface with

the eNODE B infrastructure (S1-U to eNODE), the S8/S5 interface with the Packet Gate Way

(PGW) (Furht 2009). The SGW also undertakes session management for data transfers. In this

function, it facilitates the processes of availing channels over which data packets are passed. In

MME control, the SGW generates mobile management information specific to the mobile device

location. Lastly, the serving gateway is responsible for data billing for user sessions. The SGW

generates billing data for each EU device judging on the user’s data flow summary. This

Long Term Evolution 28

information is then sent to the Charging Gateway Function (CGF) where billing procedures are

used to bill the user (Furht 2009).

3.2.3.3 The PDN Gateway

This gateway is called the Packet Data Network Gateway and is responsible for data

management in the core network (Furht 2009). This element is responsible for harmonizing

packet transfers between the 4G network’s protocol and other external protocols. This

harmonization is every essential because the 4G network may externally be required to

communicate with different network platforms such as 2G and PSDN. The public internet, to

which much of the data packets flowing through the PDN gateway are routed, is an example of a

system that may contain data delivered in different protocols (Ahson 2009). The PDN has

several protocol interfaces including the S5/S8 data planes and control stacks interface with the

Serving gate Way (SGW). In addition, the Packet Data Gateway may perform duties of filtering

packets and policy enforcement. Charging of services may also be incorporated in this system.

PDN gateway serves as the link between 3GPP network and the non 3GPP networks for the EU.

The PDN gateway is often considered as the most sensitive part of the core network (Furht

2009).

3.2.4 Home Subscriber Server (HSS)

The home subscriber server (HSS) is housed within the Evolved Packet Core (EPC) of

the LTE network. It acts as a link between the Home Location Register (HLR) and the

Authentication Centre (AuC) (Furht 2009). This part is not unique to 4G networks but was

common to 2G and 3G networks as well. The server contains information regarding all

registered user equipment in the network. In particular, it contains the IMSI and MSIDSN

numbers for each device registered under its network. It also keeps information about user

Long Term Evolution 29

statuses and settings for individual network facilities usage such as allowed bandwidth category

for each user as well as subscribed services (Sesia & Toufik 2011).

The authentication part of HSS handles security enhancement procedures for user

generated request including integrity protection and user data ciphering in the uplink and

downlink cycles. These procedures help protect both the end user and the enod-B from

eavesdropping and identity theft (Myung 2010). The securitization procedures generated in the

AuC are passed on to the HLR and other network points relevant to security enhancement. The

common procedures include mutual identification and authentication of terminals (both mobile

equipment and the base station or ENOD-B), and joint radio path and data ciphering during data

exchange. The paths are secured using keys known only to the de-ciphering terminals.

3.3 Air Interfaces

The air interface indicates the arrangement of the network in terms of layers in which

certain functions are performed (Furht 2009). In 4G networks, air interface means the radio

communication link between the user equipment and the base equipment. The air interface is

arranged in three layers namely; the network layer, the data link layer and the physical layer.

3.3.1 OFDMA on Downlink

Orthogonal Frequency Division Multiple Access (OFDMA) is an air interface method

which allows for more data transport and better channel utilization than the CDMA or FDMA

procedures (Ahson 2009). In OFDMA, a frequency channel gets split up into many sub-channels.

Existence of sub channels mean that interference between them is possible, and this would lead

to distorted output. To guard against this, there is the introduction of guard bands in between the

sub-channels. The frequency band is divided up into sub-channels, which are in turn split up into

Long Term Evolution 30

orthogonal parts which are called sub-carriers (Ahson 2009). A sub carrier contains three pieces

of information packets; data carriers, a DC and the pilot carriers.

The data carrier is used to carry the user data while pilot carriers are used for channel

control purposes. A typical sub carrier is modulated using any of different methods including the

conventional Quadra band Amplitude Modulation. In this modulation, each channel user is given

a number (always an integer) of sub-channels for their use in the frequency spectrum, amounting

to a higher number of sub-carriers (Sesia & Toufik 2011). The data the user inputs is carried in a

parallel mode using different sub-carriers but at a low rate. However, spectral efficiency of this

type of network means very many parallel sub channels in the network, whose combined output

leads to the very high data transfer rate. The fact that this technology carries data at very low

rates means that, it has more symbol time and is able to better resist effects of multi-path

attenuation. This kind of network is therefore more preferable to the older CDMA or FDMA

technologies. The overlapping orthogonal channels also increase its efficiency. OFDMA has a

spread spectrum in which energy output in the band is spread evenly, thereby reducing effects of

interference (Furht 2009).

The reason why OFDMA is preferred in the downlink is because in the conventional

wireless network in which data volumes transcend voice volumes, there will always be more

downloads than uploads, requiring a higher data rate. In most 4G networks, network speeds of up

to 1Gbps are achieved, meaning end user rates of up to 100 Mbps. This requirement of a high

rate of data transfer requires a protocol that is not only capable of high data rates, but one which

is least susceptible to interference, hence the choice of OFDMA in the downlink

Long Term Evolution 31

3.3.2 SC- FDMA on the Uplink

Single Carrier – Frequency Division Multiple Access (SC-FDMA) is a wireless transfer

protocol in which the frequency band is split in sub-channels each bearing a single carrier. This

is unlike in OFDMA where a sub-channel is split into multiple sub-carriers (Ahson 2009).

Uplink means the stage where data is sent from the mobile station to the e-NODE-B (Sesia &

Toufik 2011). The data transfer rate typically expected in the uplink is lower than in the

downlink. The use of SC-FDMA in uplink saves spectrum which means more users can be

accommodated by the frequency bandwidth. This structure allows for optimization of resources

in the wireless network.

3.4 The LTE Frame Structure

Figure 6: the LTE frame

(LTE World Website 2012)

The LTE frame is similar to the TDM structure but with certain changes. The band is

divided into slots of 0.5ms duration each called sub-frame and one frame is made up of 20 slots

each of 0.5 ms. These slots carry different information and the sub-frames maybe standard or

Long Term Evolution 32

special. Special sub frames are located in the second, third and fourth positions and carry

information of Downlink Pilot Time Slot (DPTS) in second position, Guard Period (GP) in third

position and the Uplink Pilot Time Slot (UPTS) in the fourth position respectively. Though the

individual lengths of each may vary, the total length of the three must be 1ms. There are

variations in the arrangement of the sub-frames depending on the particular LTE architecture, but

the underlying principle is the same.

3.4.1 LTE Symbols, Slots, Radio Blocks and Frames

Figure 7: LTE generic frame form.

The diagram shows the frame consisting of 20 sub-frames. A slot is a time division in the

LTE protocol that last 0.5 milliseconds. There are 7 OFDM symbols in one slot, also called the

short cyclic prefix. A sub-frame typically consists of 2 or more slots, with a time span of 1.0

msec. the entire frame spans 10 msec.

(LTE World Website 2012)

Figure 8: structure of OFDM networks showing PHY header and its subdivision. It feature 4 rate

bits, a reserved bit, 12 length bits, a parity bit and 6 tail bits.

Long Term Evolution 33

(LTE World Website 2012)

The figure below illustrates the various channels as well as layers of the radio network.

The abbreviations are described in the sections below.

Figure 9: the E-UTRAN Layer

Long Term Evolution 34

(LTE World Website 2012)

In the above diagram, LTE network is highlighted showing the user equipment related

channels and direction of data flow. It also shows the various layers as well as parts of the

eNodeB network segment (Dahlman, Parkvall & Per Beming 2008). The entire system is IP

packet switched. The physical layer shows the antenna in both the eNodeB and the Mobile

Terminal, the type shown is the Multiple Input Multiple Output (MIMO) antenna system. Lastly,

it shows the network core and its major regulatory functions.

Long Term Evolution 35

3.5 LTE Channels

LTE frequency channels are modeled to suit its specific transmission protocol with

maximum spectral efficiency and minimum interference (Myung 2010). The channels are

categorized into downlink and uplink. Downlink involves transmission from the eNODE-B to

the mobile station, and Uplink is from mobile to E-NODE-B. The properties of these two

channels are different (Lescuyer & Lucidarme, 2008).

3.5.1 Channels on the Downlink Direction

The diagram below represents the channels used in the down link in the three LTE

protocol layers (Myung 2010). The Multicast Traffic Channel (MTCH) in the logical channel is

used to transmit data down to the UE from the network but by specific devices only (Sesia &

Toufik 2011). Dedicated Traffic Channel (DTCH) passes user data to one mobile station at a

time, this channel exists also for the uplink. Broadcast Channel (BCH) transmits system control

data to the UE. The Paging Control Channel (PCCH) is used to pass notifications left when users

were offline. The common control channel (CCCH) is a two way channel for transfer between

network and user equipment. The Dedicated Control Channel (DCCH) is used to transfer

dedicated information between network and the mobile station if the later has RRC connection.

Long Term Evolution 36

(“Artiza Networks” 2012)

3.5.2 Channels on the Uplink Direction

Figure 10: Uplink Channel Mapping

(“Artiza Networks” 2012)

Long Term Evolution 37

Some channels used in the uplink connection were covered also in the downlink

connection. Several channels, however, are unique to the uplink. The Physical Uplink Shared

Channel (PUSC) is basically used for the purpose of data and multimedia transfer. The Random

Access Channel (RACH) is used to transfer data that is not controlled. The Uplink Shared

Channel (UL-SCH) supports sending data between user equipment and the transport layer of the

network (Lescuyer & Lucidarme, 2008).

3.6 MIMO Transmission

Multiple Input Multiple Output (MIMO) technology allows user devices to send or

receive signals via two or more antennas (Dahlman, Parkvall & Per Beming 2008). This scenario

is very helpful in cases where signals are weak and easily affected by multiple path attenuation.

In a typical single antenna transceiver, a user device sends and receives signals via one antenna.

In a dual antenna single channel operator, a user device may hop between the two antennas but

may only use one at a time. In a true maximal ratio combining MIMO (or MRC MIMO) device,

the transceiver utilizes both antenna simultaneously and combines their output to get a better

quality signal.

Figure 11: MIMO transceiver diagram

Long Term Evolution 38

(LTE World Website 2012)

While MRC increases the reliability of the channel, MIMO configuration helps increase

data rates. This combination makes LTE networks a step better than 3G networks (Sesia &

Toufik 2011).

3.7 LTE Protocol Layers

As mention in section 3.3, LTE is divided into layers. These layers describe the protocols

used by various parts of the LTE network to communicate (Sesia & Toufik 2011). The figure

below illustrates the air interface.

Figure 12: Interface Abbreviations

(LTE World Website 2012)

Physical layer PHY is the layer used for communication between the user equipment and

the eNODE-B. The Media Access Control (MAC) encrypts/ decrypts information in the link

between the physical layer and the Radio Link Control (RLC) whose main work is to organize

Long Term Evolution 39

packet layers in sizes transmittable in the radio interface. The Packet Data Coverage Protocol

(PDCP) links between the user plane and control plane. Ciphering, compression and integrity

issues are covered in the PDCP. Radio Resource Control (RRC) is responsible for sending

information both NAS and dedicated information.

The diagram below illustrates how the various layers interrelate in the communication

model.

Figure 13: the LTE Air Interface Layers

(LTE World Website 2012)

3.8 LTE Application

LTE has found extensive use in the modern day IT arena, but most notably in mobile

broadband communications (“Agilent Technologies” 2009). Faster data transfer rates are finding

widespread applications in various sectors such as video conferencing, video calling, and super-

speed files transfer among other uses. The internet gaming, video streaming, chat and other

mobile phone applications are boosted by LTE (“Agilent Technologies” 2009).

Long Term Evolution 40

3.9 LTE Benefits

LTE has several benefits. Firstly, it is regularly revolutionized wireless communication

system that has achieved more releases and updates than any other specific platform. As a result

of this, it has progressively laid the standards for top of the range wireless mobile

communications. Secondly, it has so far attained some of the highest data transfer rates of any

type of network, with high mobility support as a competitive edge. It also boasts of complex

signal processing and enhancement procedures such as the MRC-MIMO, greatly improving its

reliability. In addition, it has received global approval as a standard, which makes it a guarantee

that equipment using LTE will get enhanced continuity (Lescuyer & Lucidarme 2008).

3.10 LTE Antenna

After development of the MRC-MIMO configurations, 4G wireless networks are able to

use multiple antennas. The single Channel transceiver uses two antennas but may use only one at

a time. The multichannel transceiver is able to use two to four antennas simultaneously for it

processes two signals and combines their output. In the LTE- Advanced platform, between four

and eight antenna will be supported (“Agilent Technologies” 2009).

4.0 Conclusion

LTE is perhaps one of the greatest achievements in the mobile communications era and it

has enabled information transfer at very high rates. This technology has revolutionized multiple

segments of the global community in business, entertainment, information processing, analytical

computing, data archiving, among others. As a result of the constant upgrading performed on the

platform, LTE has continued to meet consumer demands in an environment where information

transfer is becoming the centerpiece of our world.

Long Term Evolution 41

Reference List

“Agilent Technologies” 2009, LTE and the Evolution to 4G Wireless: Design and Measurement

Challenges. John Wiley & Sons

Ahson, S 2009, Long Term Evolution. CRC Press/Taylor & Francis: Boca Raton, FL

“Artiza Networks” 2012. LTE Resources. Accessed on October 23, 2012 from

http://www.artizanetworks.com/lte_tut_lay_2_log.html

Dahlman, E., Parkvall,S. & Per Beming, S. 2008, 3G Evolution – HSPA and LTE for Mobile

Broadband. Academic Press

Ekström, H et.al. 2006. Technical Solutions for the 3G Long-Term Evolution, IEEE Commun.

Mag., vol. 44, no. 3, March 2006, pp. 38–45

Ergen,M 2009, Mobile Broadband – Including WiMAX and LTE. Springer: New York

Furht, B 2009, Long Term Evolution 3GPP LTE radio and cellular technology. CRC

Press/Taylor & Francis: Boca Raton, FL

Guerra, R., & Ramlogan, R 2008, Images and evolution: long term change in the photographic

industry. University of Manchester: Manchester

Lescuyer, P., & Lucidarme, T 2008, Evolved packet system (EPS): the LTE and SAE evolution

of 3G UMTS. West Sussex, England: Chichester, J. Wiley & Sons

LTE World Website 2012. Accessed on October 23, 2012 from www.lteworld.org

Myung, H. G 2010, 3GPP long term evolution: a technical overview. Wiley-Blackwell: Oxford

Sesia, S., & Toufik, I 2011, LTE - the UMTS long term evolution from theory to practice.

Wiley: Chichester

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