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 complex specifications 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 duplex voice 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 3G evolution 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

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. 3G finds 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 3.5G and 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 1G systems 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.

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 4G wireless service, LTE (as

specified in the 3GPP Release 8 and 9 document series)

does not satisfy the technical requirements the 3GPP

consortium has 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)

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)- SGWSGW 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 from and to

the mobile stations within its geographical service area.

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 other sources

(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

LTE Architecture interfaceS1-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 Functionality1. SAE bearer user plane tunneling2. Supports Inter eNodeB path switching during handover.S3: Interface between SGSN and MMEFunctionality1. Enables user and bearer information exchange for inter 3GPP access.2. Network mobility in idle and/or active state.S4: Interface between SGSN and Serving SAE Gateway

Functionality1. It provides control and mobility support between GPRS Core & 3GPP.2. Anchor function of Serving GW. 3. Alternate plane tunnel in case of no Direct Tunnel.S5 - Interface between Serving GW and PDN GW.Functionality1. 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 MMEFunctionality1. 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 betweenServing GW in the VPLMN and the PDN GW in the HPLMN. FunctionalityInter-PLMN reference point providing user and 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

FunctionalityQoS policy transfer and charging control information. It supports local breakout function.S10 - Interface between Multiple MMEs

FunctionalityMME relocation and information transfer.S11 - Interface between MME and Serving GW

FunctionalityReference point.S12 - Interface between UTRAN and Serving GWFunctionalityGTP-U protocol is used to define this reference point for user plane tunnelling with Direct Tunnel.. Usage of S12 is an operator configurationS13- Interface between MME and EIR.

FunctionalityEnables UE identity check procedure.Sgi - Interface between PDN GW and the packet data network.

FunctionalityPacket data network interface for 3GPP accesses.1. Operator external public2. Private packet data network3. Intra operator packet data network, e.g. provision of IMS services.Rx- Interface between AF and the PCRFFunctionalityRx reference point.Sbc- Interface between 

CBC and MMEFunctionalityWarning message delivery and control functions.

ConclusionIn 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.

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