The wireless service users are now increasing faster than ever before. According to Ericsson, 1.5 billion people will have broadband by 2011. More than half of these people will have mobile broadband [1]. The telecom industry is often driven by market forces. Smart-phones like BlackBerry and iPhone with their bandwidth-hungry applications; such as web, mobile TV, gaming etc., are spreading out around the world [2]. There is a continuous pressure on wireless mobile networks to accommodate such bandwidth-hungry applications and also deliver reliable, high transmission rate wireless
Evolution in 3GPP technologies from GSM-EDGE to UMTS-HSPA-HSPA+ play important role to provide increased capacity and user experience, and the evolution will continue. 3GPP (3rd generation partnership project) has completed the Release-8 (LTE) specifications in March 2009, release 8 specification provide further enhancements to the previous High Speed Packet Access+ (HSPA+) technology. The LTE standard has been focused on enhancement of UMTS toward the 4G of mobile access technologies, and to all-IP flat architecture. LTE standard is actually 3.9G (PRE 4G), as a starting point of development process of mobile networks towards IMT Advanced [3].
Service Architecture Evolution (SAE) and Long Term Evolution (LTE) are the main items of 3GPP Release 8. LTE is related to radio side improvements. Release 8 defines new OFDMA-based (Orthogonal Frequency Division Multiple Access) technology and this new technology is also referred to as the E-UTRA (Evolution UMTS Terrestrial Radio Access). High Speed OFDM Packet Access (HSOPA) is new interface system in release 8 which aims to simplifying the combined network of Circuit Switched and Packet Switched, to an entirely IP system. SAE addressed network side improvements also known as Evolved Packet Core (EPC). EPC is responsible for the optimization of IP traffic and services and provide deployment of new and existing technologies [4].
3GPP has turned their focus to Release-9 and 10 after the completion of Rel-8. Release-9 adds new functionality to both HSPA and LTE and targeted to be completed by 2010.
This paper is organized as follows: First section gives an overview about evolution of mobile network from previous mobile network technologies such as Universal Mobile Telecommunication System (UMTS), High-speed Packet Access (HSPA), and HSPA+ toward recent pre 4G and 4G LTE mobile network technologies. Next section focuses on the Long Term Evolution (LTE) which is the successor technology of HSPA and HSPA+ and explains in detail its architecture with its functional elements. Final section gives an overview about the future mobile network technology which is LTE-Advanced a real 4G mobile network technology. At the end the conclusion is drawn.
Evolution of Mobile Network
This section is about one possible evolution path from 2G/3G network to LTE and LTE advanced. The main evolution in the telecommunication industry is moving from Circuit (switched) based network towards packet based data networks. In GSM network the first data services brought by the GPRS. Similarly UMTS with HSPA provide the same trend with higher data speeds.
Data rates pushes higher by new generation of technologies deployments.
Figure 1: Data rate evolution of Radio Access Technologies [5]
Figure 1 shows the evolution of the data rates. As shown in the figure 1, EDGE offered 400 to 472 kbps theoretical data rates and WCDMA first deployments offered 384 kbps. Similarly high speed packet access technology (HSPA) offered 7.2-14 Mbps, HSPA+ 42 Mbps and Long Term Evolution (LTE Released 8) 100 to 150 data rates that is 300 times higher data rate over previous 8 years and 300 Mbps expected to achieve in Advanced LTE [5].
Figure 2 shows the time schedule of 3GPP specification and the commercial deployments.
Figure 2: Deployment Timeline of Mobile Network Standards [5]
Release 99
As shown in the figure 2, Release 99 (WCDMA) was the initial standards for UMTS, standardized in mid 1999 and 3GPP published it in March 2002. The main requirement for Release 99 was to mitigate the impacts on the core network when introducing the UMTS Terrestrial Radio Access Network (UTRAN). A Wideband Code Division Multiple Access (WCDMA) radio interface was designed to allow the release-99 a peak rate of 2 Mbit/s [6]. General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) technologies were provided with 3GPP Release-99 UMTS specifications. Rel-99 provided the evolutionary path for technologies like EDGE, GSM, GPRS and enabling better voice and more spectrally efficient data services with the introduction of a 5 MHz UMTS carrier.
Release 5 (HSDPA)
As shown in the figure 2, Release 5 (HSDPA) of UMTS was completed in March 2002 and commercial deployments followed in 2005. Release 5 defined features like High Speed Downlink Packet Access (HSDPA) channel, IP UTRAN and the IP Multimedia Subsystem (IMS) that provide better performance and functionality advantages over previous release standards [7]. HSDPA improves the theoretical down-link speed up to 14 Mbit/s.
Release 6 (HSUPA)
High Speed Uplink Packet Access (HSUPA) standard was completed in December 2004 and was commercially deployed in 2007. HSUPA create the improvements to previous UMTS networks and this improvement was on the software side that runs in the node Bs and radio network controllers. HSUPA was the first step towards Long Term Evolution (LTE). HSUPA improves theoretical up-link speed up to 5.8 Mbit/s [8].
Release 7 (Evolved-HSPA)
As shown in the figure 2, release 7 was completed in 2007 and commercial deployment was started in 2009. 3rd Generation Partnership Project (3GPP) adds new functionalities into HSPA and gives it the name evolved HSPA (HSPA+). 64 Quadrature Amplitude Modulation (Downlink) / 16QAM (Uplink) modulation and multiple input / multiple output modulation (MIMO) antenna capability were main enhancements in HSPA+. These enhancements are fully backwards compatible with Rel-99/Rel-5/Rel6. HSPA+ provides 14 Mbps to 42 Mbps in the downlink and from 5.8Mbps to 11 Mbps in the uplink. In Release 7 Internet-HSPA and direct tunneling brings up major changes to the previous releases [5].
Release 7 reduces the power usage for packet based services like VoIP and web browsing. Release 7 introduces discontinuous downlink and uplink transmission concepts for saving the end terminal power consumption. In Release 7 transmitter remains shutdown when there is no data channel transmission and this will save transmitter power consumption [9].
Long Term Evolution (LTE)
Background and Overview
In 2005 3rd Generation Partnership Project (3GPP) standardization body decided to start work on a next generation wireless network design and the main idea was that this design only based on packet-switched network. Research related to this design was performed in two different study programs. Research related to the design of a new radio network and air interfaces architecture was done under the LTE program. Afterwards, Service Architecture Evolution (SAE) program started and this program focused on the design of new core network architecture. Later, both of these programs combined into a single work program, the Evolved Packet System (EPS) Program. By that time, in literature the abbreviation LTE was dominant and most documents still refer to LTE rather than EPS [10]. 3GPP had approved Release-8 specifications in January 2008 and in March 2009 completed final standard were published.
The following design goals were set for the new network (LTE) [11]:
To shift fully packet-based network.
Increase throughput, targets were to have throughput 100 Mbps in the downlink this is of three to four-times the release 6 HSDPA levels and 50 Mbps throughput in the uplink which is two to three times the HSUPA levels.
In HSPA network, a mobile device takes a long time to connect to the network and start communication which has a negative impact. So it was decided to reduce time for state changes in HSPA network.
HSPA cellular networks had a transmission delay of around 50 ms which was much higher as compared to fixed line networks. For new network (LTE) design, it was decided that delay should be 5ms equaling to fixed line networks.
To support scalable bandwidth channels of 1.4, 3, 5, 10, 15, and 20 MHz in both directions. If we compare this with HSPA networks that was limited to a bandwidth of 5MHz.
Operation in unpaired spectrum and paired spectrum is possible.
Compatibility and internetworking with previous and existing systems and non-3GPP systems shall be ensured.
LTE Network Architecture
The Service Architecture Evolution (SAE) and Long Term Evolution (LTE) are the two main work items of 3GPP release 8. LTE create radio side improvements while SAE create improvements on the core side. LTE/SAE network allows more better and efficient transfer of data. Architecture of the LTE is a comprehensive mobile architecture that covers a multitude of network elements. New network elements are included in the LTE network like Mobility Management Entity (MME) and Service Architecture Evolution Gateway.
Several principles are used in the development of LTE/SAE architecture [1]:
A common anchor point and gateway node for all access technologies.
An optimized architecture for the user plane with only 2 node type (base stations and gateways).
An IP based system and protocol used on interfaces.
A split in the user/control plane between the gateway and mobility management entity.
Integration of non-3GPP access technologies using client and network based mobile IP.
Figure 3 gives an overview of the Evolved System Architecture. LTE has the flat IP architecture to allow for better and efficient packet transfer between Packet Data Network and User Equipment (UE) and also make it possible to remain compatible with legacy wireless networks. The network elements of LTE shown in the figure 3 will now be briefly described in the next sections.
Figure 3: High level architecture for 3GPP LTE [12]
Functional Elements
As shown in the figure 4, LTE architecture is divided into four main domains or segments. The architectural development is limited to Core Networks and Radio Access while User Equipment and Services domains remain unchanged.
User Equipment (UE)
LTE Evolved UMTS Terrestrial Radio
Access Networks (EUTRAN)
SAE Evolved Packet Core (EPC)
Services domain
Figure 4: System architecture for LTE
network [5]
User Equipment
User Equipment is end user device that uses for communication. UE is typically a device like smart phone or a data card those used in 2G, 3G and it could be embedded device to a laptop. UE handles the tasks like mobility management, session management, call control, and identity management. Universal Subscriber Identity Module (USIM) also known as Terminal Equipment is a module separate from the UE. Universal Subscriber Identity Module is used for security purpose like to identify and authenticate the end user and also provide protection to the radio interface transmission [5].
Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)
E-UTRAN is the radio access component of LTE and is responsible for radio resource management in LTE. LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) in downlink and Single Carrier FDMA (SC-FDMA) in uplink. These multiple access techniques will reduce the interference between users and improve the network capacity. The evolved RAN consists of the eNodeB (eNB) that interfaces with the User Equipment (UE).
E-UTRAN Node B (eNodeB)
The base station in the LTE/SAE network is known as eNodeB. 3GPP Release 8 reduce the number of network elements to minimize the end-to-end latency and therefore header compression, all radio protocols, mobility management and all packet retransmissions are located in the eNodeB. Release 8 removes the Radio Network Controller (RNC) element and put all those algorithms into eNodeB that are located in RNC. When UE comes up then eNB is responsible to perform radio resource management functionality, i.e. it shall perform radio admission control, allocation of downlink and uplink to user equipment and shall do radio bearer. eNodeB work as a layer2 bridge between the Evolved Packet Core (EPC) and User Equipment (UE). The eNB hosts the layers like Medium Access Control (MAC), Packet Data Control Protocol (PDPC), Physical (PHY), and Radio Link Control (RLC) that perform the encryption and Radio Resource Control (RRC) functionality that corresponding to the control plane [12]. When packets arrive to eNB, eNB shall perform compression on the IP header and encrypt the data stream. Another responsibility for eNB is choosing a MME by using MME selection function.
Figure 5: The E-UTRAN architecture [11]
eNB is also responsible for taking care of overall Quality of Service because it is the only entity on the radio side. As shown in figure 5, eNBs are interconnected with each other by using the X2 interface and connected to the EPC by means of the S1 interface.
To sum up eNodeB performs the following functions [8]:
Compression of IP header and encrypting of user data stream.
Radio resource management.
Measurement and measurement reporting for scheduling and mobility.
Ciphering and deciphering of control plane and user data.
Cell information broadcast.
SAE Evolved Packet Core (EPC)
Evolved Packet Core also known as SAE core is the main component of the SAE architecture. EPC is an IP based multi-access core network for 3GPP radio access, non-3GPP radio access, and fixed access. EPC supports only packet-switched traffic so there is no need of switching center type MSC (Mobile service Switching Center) or MSS (Mobile Soft Switch). This flat architecture design model will save total cost of ownership, optimize network performance, and facilitate the IP-based services [12].
Figure 6: Hierarchical to a flat LTE core network [13]
This flat architecture reduces the total number of nodes and only two nodes exits in the SAE user plane: the base station and the gateway, as shown in Figure 6. By incorporation of radio network controller (RNC) functionality inside eNodeB, handovers will be managed directly between eNodeBs. Interfaces in EPC are based on IP protocols. All services including voice will be delivered through packet connections and by using a single packet network for all services will save operators overall expenditures [13].
Figure 7: LTE Evolved Packet Core [13]
As shown in figure 7, Evolved Packet Core (EPC) consists of serving Gateway (SGW), Packet Data Network Gateway (PGW), Mobility Management Entity (MME), and Policy and Charging Rules Function (PCRF).
3.2.1.3.1 Serving Gateway (SGW)
As shown in the figure 4, SAE GW is a combination of the two gateways, Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW) and such implementation represents one deployment scenario, but standards also define the interface between them. In practice, these gateways implemented as one network element, depending on vendor support.
Serving gateway is responsible for forwarding data packets between Internet and mobile devices, while also acting as the anchor for mobility between LTE and other 3GPP technologies [12]. “The GPRS Tunneling Protocol (GTP) is used as a tunnel between the core network and radio access network to provide flexibility in the route changing of IP packets when the user is handed over from one to another cell. In case of interception serving gateway also performs replication of the user dataâ€.
3.2.1.3.2 Packet Data Network Gateway (PDN GW)
PDN GW is responsible for providing connectivity to the UE to external networks and work as a point of entry and exit of traffic for the UE. PDN GW also perform several IP functions, such as IPv4 or IPv6 address allocation to the mobile device, policy enforcement, packet routing, packet filtering for each user, lawful interception, packet screening and classification. Packet Data Network Gateway (PDN GW) act as the anchor for mobility between non-3GPP and 3GPP technologies such as WIMAX and 3GPP2 (EvDO and CDMA 1X) [12].
3.2.1.3.3 Mobility Management Entity (MME)
The Mobility Management Entity (MME) is a signaling entity and its main function is to handle the UE’s Mobility. It provides the mobility between LTE and 2G/3G access networks by using the S3 interface. MME also perform the idle-mode UE tracking, authentication, authorization, and security negotiations [11]. It authenticates the user by interacting with the HSS.
The MME support the following functions [11]:
Terminal-to-network session handling
Security procedures
3.2.1.3.4 Policy and Charging Rules Function
(PCRF)
Policy and Changing Rules Function is a network entity defined by 3GPP that controls network resources, applications and subscriber interaction in real time. PCRF enforces charging policy, detects the service flow. Another most important task of the PCRF is to handles QoS management.
Services Domain
Services domain consists of various sub-systems and that sub-systems in turn may consists of various logical nodes. Types of services that will be made available are given below.
IMS based operator services
Non-IMS based operator services
Also shown in the figure 3, is the Serving GPRS Support Node (SGSN), which control for earlier 3GPP networks and handles user connectivity.
LTE-Advanced
The Real broadband wireless network should be the LTE-Advanced that aims to provide equal or greater data rates than those of wired networks. LTE-Advanced is now in study phase and will be standardized in the 3GPP specification Release 10. Providing better quality of service, reduced network cost per bit and compatibility with previous 3GPP systems are the main requirements for LTE-Advanced. Targets for LTE Advanced are 50% higher than the performance of LTE Rel’8. LTE-Advanced will support bandwidth of up to 100 MHz and offers 1Gbps downlink data rates for low mobility and 100 Mbps for the high mobility [14].
Comparison between LTE and LTE-Advanced:
Technology
LTE
LTE-A
Peak Downlink data rate
100 Mbps
1Gbps
Peak Up-link data rate
50Mbps
500Mbps
Transmission bandwidth Downlink
20MHz
100Mhz
Transmission bandwidth Uplink
20MHz
40 MHz
Mobility
Optimized for low speeds(<15Km/hr)
High performance at speed up to 120Km/h
Same as that in LTE
Coverage
Full performance up to 5 Km
Same as LTE requirement
Scalable Bandwidths
1.3, 3, 5,10 and 20 MHz
Up to 20-100 MHz
Conclusion
In this paper, I have described the main architecture of the next generation access-network technology LTE being developed by 3GPP. LTE standard includes improvements not only at the access network but as well at the core network architecture. LTE will provide higher data rates with lower latency than the current HSPA networks can provide. LTE provide significant savings on the OPEX and CAPEX because of its flat architecture. LTE hardware elements are smaller in size and need less electricity and cooling. These are the advantages that bring saving for operators.
There are great challenges for the telecommunication companies to make the new technology as reliable as the existing one. Despite the great challenges that the LTE is facing I would predict that during the next few years LTE networks will be deployed around the world and end user will get benefits from this technology as soon as possible.
