1. A survey on LTE Architecture
Abstract
This paper provides a comprehensive overview of the
network architecture of a Long Term Evolution (LTE)
system according to the Release 8 version of the
specifications. It is designed to enable the reader to
become conversant rapidly with the main principles of
the LTE network architecture. Engineers involved in
the design of LTE interfaces and network equipment, as
well as those involved in the first deployments of this
new technology, should find this paper invaluable.
Not only does this paper provide a straightforward
introduction to the definitive but complexspecifications
defined by the Third-Generation Partnership Project
(3GPP), but it also particularly highlights aspects of the
network architecture and interfaces that enable LTE
networks to be deployed in an optimized and efficient
manner. References are provided throughout so that the
interested reader can readily access more detailed
material.
Introduction
GSM
GSM(Global System for Mobile Communications,
originally Groupe Spécial Mobile), is a standard
developed by the European Telecommunications
Standards Institute (ETSI) to describe protocols for
second generation (2G) digital cellular networks used
by mobile phones. It is the de facto global standard for
mobile communications with over 90% market share,
and is available in over 219 countries and territories.[1]
The GSM standard was developed as a replacement for
first generation (1G) analog cellular networks, and
originally described a digital, circuit-switched network
optimized for full duplexvoice telephony. This was
expanded over time to include data communications,
first by circuit-switched transport, then packet data
transport via GPRS (General Packet Radio Services)
and EDGE (Enhanced Data rates for GSM Evolution or
EGPRS).
Subsequently, the 3GPP developed third generation
(3G) UMTS standards followed by fourth generation
(4G) LTE Advanced standards, which are not part of
the ETSI GSM standard.
CDMA
Code division multiple access (CDMA) is a channel
access method used by various radio communication
technologies.
CDMA is an example of multiple access, which is
where several transmitters can send information
simultaneously over a single communication channel.
This allows several users to share a band of frequencies.
To permit this to be achieved without undue
interference between the users, CDMA employs spread-
spectrum technology and a special coding scheme
(where each transmitter is assigned a code).
CDMA is used as the access method in many mobile
phone standards such
ascdmaOne, CDMA2000 (the 3Gevolution of
cdmaOne), and WCDMA (the 3G standard used
by GSM carriers), which are often referred to as
simply CDMA.
2G
2G(or 2-G) is short for second-
generation wireless telephone technology. Second
generation 2G cellular telecom networks were
commercially launched on the GSM standard
in Finland by Radiolinja (now part of Elisa Oyj) in
1991.[2] Three primary benefits of 2G networks over
their predecessors were that phone conversations were
digitally encrypted; 2G systems were significantly more

2. efficient on the spectrum allowing for far greater
mobile phone penetration levels; and 2G introduced
data services for mobile, starting with SMS text
messages. 2G technologies enabled the various mobile
phone networks to provide the services such as text
messages, picture messages and MMS (multimedia
messages). All text messages sent over 2G are digitally
encrypted, allowing for the transfer of data in such a
way that only the intended receiver can receive and
read it.
After 2G was launched, the previous mobile telephone
systems were retrospectively dubbed 1G. While radio
signals on 1G networks are analog, radio signals on 2G
networks are digital. Both systems use digital signaling
to connect the radio towers (which listen to the
handsets) to the rest of the telephone system.
2G has been superseded by newer technologies such
as 2.5G, 2.75G, 3G, and 4G; however, 2G networks are
still used in many parts of the world.
3G
3G, short form of third Generation, is the third
generation of mobile telecommunications
technology.[3]This is based on a set of standards used
for mobile devices and mobile telecommunications use
services and networks that comply with
the International Mobile Telecommunications-2000
(IMT-2000) specifications by the International
Telecommunication Union. 3Gfinds application in
wireless voice telephony, mobile Internet access, fixed
wireless Internet access, video calls and mobile TV.
3G telecommunication networks support services that
provide an information transfer rate of at least
200 kbit/s. Later 3G releases, often
denoted 3.5Gand 3.75G, also provide mobile
broadband access of
several Mbit/s to smartphones andmobile modems in
laptop computers. This ensures it can be applied to
wireless voice telephony, mobile Internet access,fixed
wireless Internet access, video calls and mobile
TV technologies.
A new generation of cellular standards has appeared
approximately every tenth year since 1Gsystems were
introduced in 1981/1982. Each generation is
characterized by new frequency bands,higher data rates
and non–backward-compatible transmission
technology. The first release of the 3GPP Long Term
Evolution (LTE) standard does not completely fulfill
the ITU 4G requirements called IMT-Advanced. First
release LTE is not backward-compatible with 3G, but is
a pre-4G or 3.9G technology, however sometimes
branded 4G by the service providers. Its evolution LTE
Advanced is a 4Gtechnology. WiMAX is another
technology verging on or marketed as 4G.
Overall LTE architectural overview
LTE,an acronym for Long-Term Evolution,
commonly marketed as4G LTE,is a standard
for wireless communication of high-speed data for
mobile phones and data terminals. It is based on
the GSM/EDGEand UMTS/HSPA network
technologies, increasing the capacity and speed using a
different radio interface together with core network
improvements. The standard is developed by
the 3GPP (3rd Generation Partnership Project) and is
specified in its Release 8 document series, with minor
enhancements described in Release 9.

3. LTE is the natural upgrade path for carriers with both
GSM/UMTS networks and CDMA2000 networks. LTE
is, therefore, anticipated to become the first truly global
mobile phone standard, although thedifferent LTE
frequencies and bands used in different countries will
mean that only multi-band phones will be able to use
LTE in all countries where it is supported.
Although marketed as a 4Gwireless service, LTE (as
specified in the 3GPP Release 8 and 9 document series)
does not satisfy the technical requirements the 3GPP
consortiumhas adopted for its new standard generation,
and which were originally set forth by the ITU-
R organization in its IMT-Advanced specification.
However, due to marketing pressures and the
significant advancements that WiMAX, HSPA+ and
LTE bring to the original 3G technologies, ITU later
decided that LTE together with the aforementioned
technologies can be called 4G technologies. The LTE
Advanced standard formally satisfies the ITU-
R requirements to be considered IMT-
Advanced.[8]And to differentiate LTE Advanced
and WiMAX-Advanced from current 4G technologies,
ITU has defined them as "True 4G".
EPS provides the user with IP connectivity to a PDN
for accessing the Internet, as well as for running
services such as Voice over IP (VoIP). An EPS bearer
is typically associated with a QoS. Multiple bearers can
be established for a user in order to provide different
QoS streams or connectivity to different PDNs. For
example, a user might be engaged in a voice (VoIP) call
while at the same time performing web browsing or
FTP download. A VoIP bearer would provide the
necessary QoS for the voice call, while a best-effort
bearer would be suitable for the web browsing or FTP
session.
Figure 1. The EPS network elements
The core network
The core network is responsible for the overall control
of the UE and establishment of the bearers. The main
logical nodes of the EPC are:
• PDN Gateway (P-GW)
• Serving Gateway (S-GW)
eNodeB(eNB)

4. E-UTRAN Node B, also known as Evolved Node B,
(abbreviated as eNodeB or eNB) is the element in E-
UTRA of LTEthat is the evolution of the element Node
B in UTRA of UMTS. It is the hardware that is
connected to the mobile phone network that
communicates directly with mobile handsets (UEs), like
a base transceiver station (BTS) in GSM networks.
Traditionally, a Node B has minimum functionality,
and is controlled by an RNC (Radio Network
Controller). However, with an eNB, there is no separate
controller element. This simplifies the architecture and
allows lower response times.
MME (Mobility Management Entity):
Mobility Management Unit on a LTE network is
centralized control unit for key operations on Access
Network and Core Network.
Some of the roles of MME are listed below :-
Network access control (NAC)-
NAC is involved in authentication and authorization for
the UE (mobile phone). Required for IP connectivity.
Radio Resource Management (RRM) -
RRM is system level control for managing Radio
parameters involved in wireless communication
between US and base-stations/e-NodeB. The key radio
parameters that needs to be managed efficiently are
listed below.
1. Dynamic transmit power control.
2. Modulation scheme management and data rates
handling.
3. Channel allocation and de-allocation.
4. Handover requirement detection and switching.
5. Error correction schemes.
SGW (Serving Gateway):
Some of the key roles of SGW are listed below:-
(1). SGW is responsible for handovers with neighboring
eNodeB’s.
(2). Data transfer in terms of all packets across user
plane.
(3). Mobility interface (or anchors) to other 3GPP
systems (2G and 3G) etc.
Serving (S) System Architecture Evolution Gateway
(GW)- SGW
SGW also monitors and maintains context information
related to the UE during idle state and generates paging
requests when data for the UE arrives from the network
PGW (PDN Gateway):
The PDN Gateway provides connectivity from the
UE to external packet data networks by being the
point of exit and entry of traffic for the UE. A UE
may have simultaneous connectivity with more
than one PGW for accessing multiple PDNs. The
PGW performs policy enforcement, packet
filtering for each user, charging support, lawful
interception and packet screening. Another key
role of the PGW is to act as the anchor for mobility
between 3GPP and non-3GPP technologies such as
WiMAX and 3GPP2 (CDMA 1X and EvDO).
HSS (Home Subscriber Server):
The HSS is a central database that contains user-
related and subscription-related information. The
functions of the HSS include functionalities such
as mobility management, call and session
establishment support, user authentication and
access authorization. The HSS is based on pre-Rel-
4 Home Location Register (HLR)
and Authentication Center (AuC).
Serving GPRS support node (SGSN)
A serving GPRS support node (SGSN) is
responsible for the delivery of data packets fromand to
the mobile stations within its geographical service area.

5. Its tasks include packet routing and transfer, mobility
management (attach/detach and location management),
logical link management, and authentication and
charging functions. The location register of the SGSN
stores location information (e.g., current cell,
current VLR) and user profiles (e.g., IMSI, address(es)
used in the packet data network) of all GPRS users
registered with it.
ePDG (Evolved Packet Data Gateway): The main
function of the ePDG is to secure the data transmission
with a UE connected to the EPC over an untrusted non-
3GPP access. For this purpose, the ePDG acts as a
termination node ofIPsec tunnels established with the
UE.
Policy and Charging Rules Function (PCRF)
PCRF is the software node designated in real-time to
determine policy rules in amultimedia network. As a
policy tool, the PCRF plays a central role in next-
generation networks.[11] Unlike earlier policy engines
that were added onto an existing network to enforce
policy, the PCRF is a software component that operates
at the network core and accesses
subscriber databases and other specialized functions,
such as a charging system, in a centralized manner.
Because it operates in real time, the PCRF has an
increased strategic significance and broader potential
role than traditional policy engines. This has led to a
proliferation of PCRF products since 2008.
The PCRF is the part of the network architecture that
aggregates information to and from the
network, operational support systems,and othersources
(such as portals) in real time, supporting the creation of
rules and then automatically making policy decisions
for each subscriber active on the network. Such a
network might offer multiple services, quality of
service(QoS) levels, and charging rules. PCRF can
provide a network agnostic solution (wire line and
wireless) and can also enable multi-dimensional
approach which helps in creating a lucrative and
innovative platform for operators. PCRF can also be
integrated with different platforms like billing, rating,
charging, and subscriber database or can also be
deployed as a standalone entity.
Roaming architecture
A network run by one operator in one country is known
as a “public land mobile network (PLMN).” Roaming,
where users are allowed to connect to PLMNs other
than those to which they are directly subscribed, is a
powerful feature for mobile networks, and LTE/SAE is
no exception. A roaming user is connected to the E-
UTRAN, MME and S-GW of the visited LTE network.
However, LTE/SAE allows the P-GW of either the
visited or the home network to be used, as shown in
Figure 4. Using the home network’s P-GW allows the
user to access the home operator’s services even while
in a visited network. A P-GW in the visited network
allows a “local breakout” to the Internet in the visited
network.
Figure 2. Roaming architecture for 3GPP accesses with
P-GW in home network

6. LTE Architecture interface
S1-MME: Interface between S1 (for E-UTRAN)
and MME
Functionality
Supports control between S1 and MME.
S1-U: Interface between S1 = E-UTRAN and U
= SGW
Functionality
1. SAE bearer user plane tunneling
2. Supports Inter eNodeB path switching during
handover.
S3: Interface between SGSN and MME
Functionality
1. Enables userand bearer information exchange for
inter 3GPP access.
2. Network mobility in idle and/oractive state.
S4: Interface between SGSN and Serving SAE
Gateway
Functionality
1. It provides control and mobility support between
GPRS Core & 3GPP.
2. Anchor function of Serving GW.
3. Alternate plane tunnelin case of no Direct Tunnel.
S5 - Interface between Serving GW and PDN GW.
Functionality
1. User plane tunneling and tunnel management.
2. It is used for Serving GW relocation due to UE
mobility or connections to a non-collocated PDN GW.
S6a - Interface between HSS and MME
Functionality
1. Enables transfer of subscription and authentication
data. Required for evolved system-AAA interface.
Gx- Interface between VPCRF and PCEF in PDN GW.
Functionality: Transfer of (QoS) policy and charging
rules.
S8 - Interface between
Serving GW in the VPLMN and the PDN GW in the
HPLMN.
Functionality
Inter-PLMN reference point providing userand control
plane between the Serving GW in the VPLMN and the
PDN GW in the HPLMN.
S9 - Interface between
Home PCRF and the Visited PCRF
Functionality
QoS policy transfer and charging control information. It
supports local breakout function.
S10 - Interface between
Multiple MMEs
Functionality
MME relocation and information transfer.
S11 - Interface between
MME and Serving GW
Functionality
Reference point.
S12 -
Interface between UTRAN and Serving GW
Functionality
GTP-U protocolis used to define this reference point
for userplane tunnelling with Direct Tunnel.. Usage of
S12 is an operator configuration
S13- Interface between MME and EIR.
Functionality
Enables UE identity check procedure.
Sgi - Interface between
PDN GW and the packet data network.
Functionality
Packet data network interface for 3GPP accesses.
1. Operator external public
2. Private packet data network
3. Intra operator packet data network, e.g. provision of
IMS services.
Rx- Interface between
AF and the PCRF
Functionality
Rx reference point.

7. Sbc- Interface between
CBC and MME
Functionality
Warning message delivery and control functions.
Conclusion
In this paper the mechanisms by which the Evolved
Packet System provides user equipment with IP
connectivity to the packet data network have been
outlined.
It has been shown how the EPS supports multiple data
flows with different quality of service per UE for
applications that need guaranteed delay and bit rate
such as VoIP as well as best effort applications such as
web browsing.
Further, an overview of the EPS network architecture
has been presented,including the functions provided by
the E-UTRAN access network and the evolved packet
CN. It can be seen that the concept of EPS bearers,
together with their associated QoS attributes, provide a
powerful tool for the provision of a variety of
simultaneous services to the end user.
From the perspective of the network operator,
the LTE system breaks new ground in terms of its
degree of support for self-optimization and self-
configuration of the network through the X2, S1 and Uu
interfaces, to facilitate deployment.
References
1. "GSM Global System for Mobile
Communications". 4G Americas. Retrieved
2014-03-22.
2. "Radiolinja's History". April 20, 2004.
Retrieved December 23, 2009.
3. ITU (4 July 2002). "IMT-2000 Project - ITU".
Retrieved 8 April 2013.
4. Clint Smith, Daniel Collins. "3G Wireless
Networks", page 136. 2000.
5. "An Introduction to LTE". 3GPP LTE
Encyclopedia. Retrieved December 3, 2010.
6. "Long Term Evolution (LTE): A Technical
Overview". Motorola. Retrieved July 3, 2010.
7. "Newsroom • Press Release". Itu.int.
Retrieved 2012-10-28.
8. "ITU-R Confers IMT-Advanced (4G) Status
to 3GPP LTE" (Press release). 3GPP. 20
October 2010. Retrieved 18 May 2012.
9. pressinfo (2009-10-21). "Press Release: IMT-
Advanced (4G) Mobile wireless broadband on
the anvil". Itu.int. Retrieved 2012-10-28.
10. Nair, Vikram. "Evolution of QoS and
Charging Framework in WiMAX,"
WiMAX.com.
11. Campbell, Ian and Chase Wolfinger. "PCRF
and the Evolution of Networking."
12. Campbell, Susan J. "Video Interview:
BroadHop helps to overcome congestion,"
TMCnet April 2009.
13. Finnie, Graham. "Mobile Broadband & the
Rise of Policy: Technology Review &
Forecast," Heavy Reading August 2011.
14. McDaid, Cathal, "Overview and Comparison
of QoS Control in Next Generation Networks,"
palowireless 3G/UMTS Resource Center.
15. http://www.elitecore.com/telecompractices/m
ulti-dimensional-policy.html
16. http://www.fullchipdesign.com/tyh/lte_interfac
es_s1_mme_u_utran.htm