Features And Techniques Of The 3 Gpp Lte System Transmissio Nx

FEATURES AND TECHNIQUES OF THE
SYSTEM 3GPP-LTE TRANSMISSION

Author Ing. Jean-Michee Masso Ntonga
Date 12/30/2011 8:43:00 AM

INTRODUCTION

The way to which are intended for the modern communication technologies mobile, in order to
maintain the necessary competitiveness in a market which always request more in capacity of
deployment of services in regime of ubiquity, but maintaining the same requirements of speed
and quality that would be in its own context home or to work, seems already trace: the support of
radio access OFDMA technique and the convergence toward the networks packet-oriented and
all-IP.

These two requirements, the support of the radio access OFDMA technique and the convergence
toward the networks all-IP, provide a sound basis on which ones can implement the MIMO
techniques , among other words, the realization of multiple transmissions of data flow parallel to a
terminal, an individual device. By analyzing the prospective development of the standard 3G
mobile telecommunications and technology wireless broadband, Figure 1 shows the process of
the convergence of various standards. The WiMAX Forum, successively to the recognition of the
standard 802.16e (Mobile WiMAX) as belonging to the family IMT-2000, has determined that the
so-called "Long Term Evolution" of the WiMAX a door in 2010 as a definition of the standard
802.16m, whose characteristics will make the quite compatible with the defined IMT-Advanced
systems; which systems will form the basis for the development of fourth generation of mobile
telephony(4G).

Indeed, the linear evolution of the current 3G networks is zest quickly to achieve its own limits. To
achieve the "real" high-speed, the mobile providers will need to evolve toward the technology 4G.
Without it, the communication channels will not have enough high capacity and there will be little
economic actually to provide broadband services. Although 3G technology (the most advanced
wireless technology deployed today) agrees quite for the network voice and is great for some data
services, It is insufficient to support the applications more "greedy" in bandwidth, such as the
video, which emerge today.

In order to obtain the performance even better, it must incorporate the OFDM and MIMO
technologies on CDMA and GSM networks. OFDM and MIMO are the milestones of all future
wireless networks(4G). Because the strengths of OFDM transmissions , associated with the
possibilities of antenna of tip of MIMO, allow to condense more users in the spectrum available to
speeds of up to five times higher than those available with 3G technology.

features and techniques of the system 3GPP-LTE system transmission >> 2

In this context, the technology that seems the best candidate for transfer current mobile
operators up to the fourth generation of mobile telephony, is the one defined as LTE or also super
3G. In fact, the initiative LTE is part of the draft partnership of 3° generation(3 GPP) to increment
the performance and the ability of UMTS networks using HSDPA and HSUPA technologies.

With the LTE technology, the operators will have to provision a powerful platform that will provide
services to broadband mobile and ubiquitous, with a good share of the market compared to the
old technologies.

The mobile data traffic ( Figure 2) [ 2] has recently exceeded the voice traffic. Because the data
traffic increased exponentially from a whereas that of the voice decreased radically. Mobile data
present an interesting opportunity for the operators, but the cost of providing the service is significant.
This is the main reason for which the mobile data are still relatively more expensive compared
with the fixed line broadband. The Ethernet interfaces offer a high data capacity at much lower
cost per bit as illustrated in figure 2.

Figure 1 – Convergence of mobile technologies (Source: WiMAX Forum)


Figure 2 - The growth of mobile data traffic

MIGRATION FROM THE 2G/3G TO THE LTE

[3] In a radio access network 2G/ 3G or 2G/ 3G RAN(Radio Access Network), the station database
manages the radio interface with the mobile station and the controller handles more of a base
station to provide the functions of control such as a radio-channel, the set-up, the handovers, etc.
A hub-and-spoke topology activates the communication from the base station to the controller
and the controller at the base station as shown in figure 3.
In a radio access network LTE or LTE RAN, the base station itself, includes the functionality of the
controller and can communicate directly with a base station via an any-to-any topology. A base
station LTE communicates with a entity of mobility management or MS(Mobility Management
Entity) and a serving GateWay(S-gw) via a star topology as shown in Figure 4.


Figure 3 - Topology of the 2G/ 3G RAN

Figure 4 -Topology of the LTE RAN

While the techniques WCDMA and HSPA have made significant progress on the mobile data
efficient and on multimedia information exchanges , LTE will provide the performance of the
extended network and reduced costs by byte which will enable it to keep the promises and the
recommendations of the mobile broadband.
When the new network technologies are introduced, the services suppliers expect to what their
existing investment is protected and that the infrastructure deployed can be reused in most of the
possible cases.
The main theme of the migration is thus geared toward the topics which represent a substantial
part of the total cost of ownership of a service provider, among others:

Deployment of LTE on the existing sites and the sharing of common infrastructure(such as the
air conditioning, the energy supply, the antenna masts, the power cables and antennas);
Sharing of connecting(backhaul) facilities between the LTE and other technologies in network
access provided on the same site: this means that the same CSG( Cell Site Gateway) deployed
to connect a 2G BTS(Base Transceiver Station) and a 3G Node B to the switching network to


packets, can be used to connect a LTE eNodeB. The sharing of the CSG to achieve the
aggregate mobile traffic 2G/ 3G/LTE, allows the use of the same fiber if only this fiber is
available on the site node B. The desired goal of the suppliers of these services is to connect
the traffic mobile 2G/ 3G/LTE to the fixed traffic from the DSL, the micro-wave and of nodes of
FTTx access, through a basic converged network IP/MPLS for a efficiency of the costs.

Compatibility of transportation solutions used for the 2G, 3G and LTE technologies.
For services suppliers, it is important that the solutions used in the connection of the transport
network IP layer- IP TNL(Transport Network Layer)- for the 2G, 3G and LTE technologies are
similar in order to use and unify the operational tasks such as the supply, the controls and
procedures OAM(Operations And Management).
In fact, the 2G and 3G networks have already suffered the migration to IP transport; however
with the LTE, this will be required because the 3GPP has specified the transport IP as the single
transport network layer for the so-called technology.Therefore, other than a hybrid approach
with the technical TDM and the transport of packages in parallel, the services suppliers will
need to renovate the connection network (backhaul network) and add the transport capacity
of the various packages, typically using the pseudowires to emulate some interfaces(TDM
pseudowire for the 2G, ATM pseudowire for the 3G).

Common Management of network platforms.

GENERAL CHARACTERISTICS OF THE 3GPP-LTE SYSTEM

The LTE technology represents the latest evolution of the standards dedicated to mobile networks
in broadband. It was wrapped in its final definition in 2009, thanks to the agreement of
collaboration 3GPP(3rd Generation Partnership Project) established in December 1998 between
the main institutions of standardization in telecommunications.
The LTE technology is located in an intermediate position between the standards of third
generation 3G and the standards of fourth generation 4G, yet at the preparation stage. The LTE's
aim is to promote the use of the Broadband in mobility, taking into account the experience and
the investments made for the 3G networks and in anticipation of the time in relation to the
availability of standards 4G, whose goal is to reach the speeds of wireless connection superior to
1Gbps.

In the LTE networks, the IP is the only protocol used to support the connectivity between different
mobile nodes as defined by the 3GPP. The transportation solutions based on the Layer 2 VPN or
the Layer 3 VPN MPLS are convenient solutions that allow for the support of the IP connectivity
between the mobile nodes, in particular through the aggregation and the basic network.

For the moment the improvements provided for compared with 3G technologies are summarized
in:

Data transfer speed in download up to 100 Mbps;
High data rate to a reduced price by bit;
Data transfer Speed in upload up to 50 Mbps;
Spectral efficiency(or number of bits/s transmitted for each Hz deployed) three times higher
than that of the version the most mature of the UMTS, that is to say the HSPA;
Low latencies(less than 10 milliseconds for the passage of the idle state to the activated state,
and less than 5 milliseconds for the small dimension IP packets);
Support for at least 200 users per cell with powers of more than 5 MHz of bandwidth;
Best support of mobility: the mobility is guaranteed up to 15 km/h, with the high performance
which range from 15 to 120 km/h. Of all ways, this mobility is functional with a limited located
to 350 km/h;
Performances insured with coverage obtained through the cells from 5 km, while considering
a minimal degradation for the cells to 30 km.

With a throughput of the order of 100 Mbps and a lower latency to 10 milliseconds, the LTE will
provide a rich user experience comparable to what the users possess today at home with the xDSL
and cable connections, with mobility.

The improvements generated can be effective through the implementation of the specific
technical characteristics, such as:

The use of the OFDM modulation for the downlink and the modulation Single-Carrier FDMA
for the uplink;
The use of a minimum of 1.25 MHz and a maximum of 20 MHz of band for each canal( 1.25 ;
1.6 ; 2.5 ; 5; 10; 15; 20), attributable with ample flexibility either in uplink and downlink;
Applicability in various frequency bands, including those of GSM, UMTS-WCDMA and new
bands at 2.6 GHz, with the possibility to add new bands in the time following the necessities;
The support of transmissions relatively to the MIMO technique;
The support of modulation schemes QPSK, 16 QAM and 64QAM either in uplink and downlink;
The support of FDD and TDD duplexing techniques(Figure 5).


Figure 5 – Duplexing techniques (Source: Ericsson)

TRANSMISSION TECHNIQUES

[1] The 3GPP LTE technology deploys, for the transmission of voice and data, the following
techniques: OFDM, OFDMA and SC-FDMA.
In addition, although the specific of the LTE admit either the FDD technique than TDD to separate
the traffic uplink(UL) traffic of the downlink(DL) one, it seems that the orientation of the
manufacturers is to develop in principle the systems based on the FDD technique.
Before addressing the technical details relating to the physical layer, it is wise to provide a brief
description of the functional characteristics of the main transmission technologies, mentioned
above, which constitute the basis for systems LTE.

OFDM
In the systems which rely on the OFDM technique, the available bandwidth is divided into several
sub-bands which allow data transmission on the parallel-flows. The transmitted data on each sub-
band, after are modulated through the available digital modulations (QPSK, QAM, 64QAM, etc),
depending on the quality of the received signal.
The deployment of the OFDM technique in the systems of communication has consented to
significantly reduce the problems that arose when, to achieve the higher data rates in
transmission, the only possible solution was the one to increase the symbol rate.
In practice, each OFDM symbol result be a linear combination of signals present on each sub-
carrier. As the data are transmitted in parallel, rather than in series, the OFDM symbols are
generally longer than the transmitted symbols on the single carrier systems to the parity of date
rate.
Two subsequent advantages for transmission systems based on the OFDM technique consist in:
Each transmitted OFDM symbol is preceded by a cyclic prefix(CP or cyclic Prefix), that is to say,
a brief replica of the final part of the signal from the sum of the symbols on each sub-band.


This feature allows to reduce significantly the effort produced by the Intersymbol Interference
(ISI) or the effect determined by the superposition of the signal replicas(or echoes) in receipt;
The sub-band obtained from the division of the entire band are orthogonal to each other, that
is to say, the distance is chosen so that the impulsive response of each signal carrier has a
maximum where the impulsive responses of adjacent channels carriers have, on the contrary,
a value of zero. This determines, ideally, a lack of interference from a adjacent channel(ICI or
InterChannel Interference);
In the OFDM technique, the conversion of the signal symbols to transmit is obtained by
applying the simple technique of Fast Fourrier Transformation(FFT). In effect:
In transmission(TX), the Inverse Fast Fourrier Transformation(IFFT) is used in order to
obtain the signal to be transmitted from the symbols associated with the sub-carriers;
In receipt(RX), the FFT allows to acquire from the received signal, the symbols that are
associated in correspondence with the various sub-carriers.

However, the OFDM technique presents two fundamental limits:
The susceptibility to errors due to the stability of carrier frequencies. These errors are
determined either by the local oscillators that by the shift Doppler (or the echoes which come
from objects in motion) that cause the instability;
The need for a high PAPR , because the instantaneous value of the power in RF(Radio
Frequency) or power of great value in transmission may vary from an abnormal way, also on
the inside of a same symbol. This phenomenon marks a reduction in efficiency of the power
amplifier in transmission.

OFDMA

The main innovation of LTE compared with the UMTS standard is the use of the OFDMA
modulation for the downlink and the SC-FDMA modulation (a technology refined to the OFDM) for
the uplink. These techniques are willing to manage the system with the bandwidths of the canal,
which range from 1.25 MHz to 20 MHz.

The OFDMA is a multiple access technology, based on the OFDM modulation, which allows to
assign to each user, a sub-set of sub-carriers from the division of the available band, for a certain
period time. Indeed, it is for this reason sometimes, that there is a tendency to consider the


OFDMA as a technique, the result of a combination of the OFDM modulation and of the access
technology TDMA.

Figure 6 shows the difference between OFDM and OFDMA.

Figure 6 - Comparison between OFDM and OFDMA

Referring to Figure 6, it is noted that the OFDMA is a "type Adaptive" technique . In other words, it
adapts to the nature of the radio channel. Its main characteristic is to assign each user only a
portion of the available sub-bands. The OFDMA allows to:
Vary the transmission power required to communicate with each of the users according to
their needs;
Adapt the quality of service (QoS) depending on the type of application(voice, video
streaming, Internet access , etc..) that the user intends to use.

features and techniques of the system 3GPP-LTE system transmission >>
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SC-FDMA

In this kind of the user’s access system (Figure 7), as is the case of the OFDMA, a range of
orthogonal sub carriers are deployed which allow to transmit the information contained in the
modulated symbols.

Figure. 7 –Blocks Schema of SC-FDMA system.

However, with the SC-FDMA technique, the sub-carriers are transmitted in sequence and not in
parallel(Figure 8). This type of solution has the following advantages:

Consent of considerably reducing the fluctuations of the transmitted signal in determining of
the PAPR value much lower as compared to that observed on the signals based on the
OFDMA;
Allows to avoid the deployment, at the level of the user’s device, of power amplifiers with the
high linearities and a low efficiency(measured by the ratio of the transmitted power and the
one needed for power to the amplifier).

However, the use of the SC-FDMA technique in the cellular systems(inherently subject to the
spread characterized by multiple paths) includes that the signal received in correspondence of the
radio base station, either likely to the phenomenon of the ISI. To alleviate this problem, it is
therefore necessary to deploy, at the level of the radio base station, the adaptive equalizer
systems in the area of the frequency; which systems, of course, determine a more increased load

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of elaboration work also well the high costs of implementation which will lie totally to the
telecommunication operator.

Figure 8 – Comparison between OFDMA and SC-FDMA

From the point of view of the sub-carriers allocation for the user access , we can cite two different
approaches (Figure 9):

Localized SC-FDMA (LFDMA)
In the LFDMA approach, each device uses, to forward the information, a set of adjacent sub-
carriers. In substance, it is as if to each user is allocated a specific portion of frequency for the
communication with the radio base station ;

Distributed SC-FDMA

A possible realization of the distributed SC-FDMA, is known as interleaved FDMA (IFDMA). In
this case, the individual sub-carriers assigned to each user are located remotely
predetermined and with periods compared with those reported by other users.

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This solution is particularly immune to transmission errors, because the information to be
transmitted is distributed on the entire available band.

Figure 9 – The LFDMA and IFDMA solutions

MIMO - MRC

At the physical level, the systems LTE may, in option, deploy multiple transceivers relatively to the
radio base station as to the user’s device in order to improve the robustness of the connection and
to increase the capacity of the transmitted data. For this, the techniques used are the following:

The MRC technique is used to improve the reliability of the connection in the critical
propagation conditions when the amplitude of the signal is low and when one is in the
presence of multiple paths.
In the case of the MRC, the signal is received through two or more antennas/transceivers
systems, separated between them in space, and therefore characterized by various impulsive
responses. The processor performs the operation to equalize the channel in separated mode
on the received signals in order to combine them after in a single composite signal(Figure 10).
It follows two advantages:
In operating in this way, while the received signals are combined in a coherent manner,
the thermal-noise introduced by each transceiver is resulting independent and
determining, in the case of a two channels receiver MRC , a total increase of the
signal/noise ratio(SNR) of 3db;

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The separation in the space of two receivers allows the drastic reduction of the effects
related to the selective fading due to the multiple paths, improving the overall quality of
the received signal.

Figure 10 – Example of application of the MRC technique

The MIMO technique is necessary to obtain the best performances in terms of
transmission speed.
The use of the MIMO technique consents to increment the total data rate of the
transmission system, obtained through the deployment of multiple antennas in
transmission and reception. To be able to acquire this result, the receiver must calculate
the impulsive response channel that characterizes each antenna in transmission.
In practice, considering a MIMO 2x2 system, four impulsive responses channel are
computed (Figure 11). Once that the impulsive responses are known , the data can be
transmitted simultaneously on the two antennas. As what, the linear combination of two
data streams to the two receiver antennas generates a set of two equations with two
unknowns which can be solved in order to obtain the two data streams at the origin.

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Figure 11 - Example of application of the MIMO technique

CHARACTERISTIC PARAMETERS OF THE PHYSICAL LAYER

This paragraph allows, according to the specificities knowledge of 3GPP LTE systems, to express
some characteristic parameters of the physical level in downlink and uplink.

DOWNLINK

Communications between the radio base station and the user’s device deploy the OFDM
modulation and the OFDMA access technique. The admitted bandwidths vary from 1.25 MHz to 20
MHz. The distance between the sub-carriers is evaluated at 15 KHz, with the possibility to opt also
to the 7.5 KHz frequency for some specific applications.
In general, the LTE frame structure is represented by Figure 12.

Figure 12 – LTE frame structure

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The LTE frames have a duration of 10 msec(milliseconds). Each frame is divided into 10 sub-
frames. Each sub-frame has a duration of 1 msec and is divided in 2 slot of equal time period of 0.5
msec. The slots consist in 6 or 7 OFDM symbols according that either used the normal cyclic
prefix solution or the extended cyclic prefix solution. The minimum number of adjacent sub-
carriers (Physical Resource Block - PRB) that the scheduler is able to assign to the inside of each
slot is equal to 12.

Table 1 summarizes the characteristic parameters of OFDM modulation provided by LTE
technology.

Table 1

The digital modulations used consist of the QPSK, 16QAM and 64QAM modulations.

UPLINK

For the data transmission from the user’s device to the radio base station, the LTE technology
adopts SC-FDMA as an access system (Figure 13) [ 4].
If ones opts for the duplexing FDD technique, the uplink uses the same frame structure considered
for the downlink and the same parameters of modulation illustrated by Table 1.
In uplink, the data mapping on the QPSK, 16QAM and 64QAM constellations is performed
depending on the quality of the channel. However, in contrast to the OFDM, instead of using the
symbols which have undergone a mapping to modulate directly the sub-carriers, in this case, the
symbols are first processed in a series flow to a parallel flow. Then these symbols in parallel flow,
are transferred in the frequency domain through a FFT block , by obtaining as a result, a discrete

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sequence of symbols. Successively such symbols are associated with the respective sub-carriers
and then are reconverted in the time domain through a IFFT.

Figure 13 – Detail of the SC-FDMA modulation

Among the antagonists of LTE, it is easy to identify the standard WiMAX, which however, even
being in chronological advantage of at least two years in development terms, suffers from the fact
of not being compatible with UMTS as there is LTE. In effect, an operator that has intended to
deploy WiMAX technology, needs to achieve a new independent network, and also in the case of
the dual-mode appliances use (able to operate with two different technologies), the transition
from the coverage area of one technology to an another, would cause an interruption in the
connection, unacceptable especially for voice services.
In addition, all two LTE and WiMAX technologies implement the OFDMA in downlink, but while the
WiMAX continues to deploy the OFDMA for the uplink, LTE technology uses the SC-FDMA. This
latter feature implies a lowest peak-to-average power ratio to the parity of power supplied by the
transmitter(about 5 dB of LTE against the 10 dB of WiMAX, that is to say, the double), and
therefore with a gain of time of batteries use .

CONCLUSION
The introduction of the LTE standard, in the panorama of the Broadband mobile technologies, may
allow for a radical improvement in the capacity of operators to provide value added services.

For consumers, the LTE development is reflected in the availability of new mobile services
broadband on the IP protocol, including the VoIP, offered on the SIP networks. The LTE technique

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will allow the birth of the new market models around new services such as online games, the high
definition video streaming, video blogging and files exchange through the peer-to-peer
architecture networks.

ACRONYMS

CDMA Code Division Multiple Access

DSL Digital Subscriber Line

FDD Frequency Division Duplexing

FFT Fast Fourier Transform

GPP Generation Partnership Project

GSM Global System for Mobile

HSDPA High Speed Downlink Packet Access

HSPA High Speed Packet Access

HSUPA High Speed Uplink Packet Access

IFFT Inverse Fast Fourier Transform

IMT Internet Mobile Telecommunication

LTE Long Term Evolution

MIMO Multiple Input Multiple Output

MPLS MultiProtocol Label Switching

MRC Maximal Ratio Combining

OFDM Orthogonal Frequency Division Multiplexing

OFDMA Orthogonal Frequency Division Multiplexing Access

PAPR Peak-to-Average Power Ratio

RNC Radio Network Controller

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QAM Quadrature Amplitude Modulation

QPSK Quadrature Phase Shift Keeying

SC-FDMA Single Carrier – Frequency Division Multiple Access

SNR Signal Noise Ratio

TDD Time Division Duplexing

UMTS Universal Mobile Telecommunications System

VPN Virtual Private Network

WCDMA Wideband Code Division Multiple Access

WDM Wave Division Multiplexing

REFERENCES

[1] Sistema 3GPP-LTE –Fondazione Ugo Bordoni-2009

[2] Source: “Mobile Data Traffic Analysis”, ABI Research, 3Q 2009

[3]http://www.broadband-forum.org/

[4] M. Rumney, “SC-FDMA – The new LTE uplink explained”, EuMW 2007 Agilent Workshop, Marzo
2008;

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