Why is LTE going to win over WiMAX?

After that Qualcomm, in November 2008, announced it was ending development of the Ultra Mobile Broadband (UMB) technology favoring LTE instead, LTE and WiMAX are the only two technologies that are competing to become the most widely adopted standard for 4G networks. WiMAX refers to the IEEE 802.16 family of standards which include notably IEEE 802.16-2004, the first major WiMAX standard for fixed access. This was superseded by IEEE 802.16e-2005, known as Mobile WiMAX, which provides both fixed and mobile access. In October 2009, the IEEE 802.16 Working Group submitted its proposal for IMT-Advanced based on IEEE 802.16m, which enhances IEEE 802.16e-2005 to meet the IMT-Advanced requirements. Although WiMAX benefits from its earlier development and deployment and today it has about 519 deployments worldwide with more than 10 million subscribers, LTE has quickly gained substantial momentum and it is expected to surpass WiMAX and become the accepted standard for 4G systems. This common thinking is supported by the Wireless Market Research and Analysis’ (Maravedis) forecasts which estimate 75 million WiMAX subscribers by 2014 while over 200 million LTE subscribers by 2015. It is also expected a migration of WiMAX operators towards LTE anytime before 2013. The aim of this post is to explain the reasons why LTE is going to win the “4G-battle”.

- LTE vs WiMAX performance

Table 4 shows the main technical specifications for WiMAX and LTE.

Table 4: LTE and WiMAX technical specifications

The performance of LTE (3GPP Release 8) and IEEE WiMAX 802.16m are similar in both data rates and latency, and mobility support. It is worth mentioning that both LTE and WiMAX use OFDMA in the downlink, but they differ in the uplink. WiMAX continues to use OFDMA, while LTE’s approach (SC-FDMA) is more advanced and allows a reduced peak-to-average ratio (PAPR) for the uplink signal. This makes it easier for the mobile terminal to maintain a highly efficient signal transmission using its power amplifier.

At the moment, 3GPP is working on LTE-Advanced (3GPP Release 2010) which promises further better performance and this is a clear advantage of 3GPP over WiMAX.

- Industry Support

The main difference between LTE and WiMAX is that they are supported by companies specialized in different fields. WiMAX supporters are mainly computer companies such as Intel and Cisco and there is a waning support from telecommunications companies. At the moment Sprint and Clear are the biggest service providers that have shown their commitment to WiMAX, while Samsung is the main smartphone manufacturer. On the other hand, as we saw in the previous paragraph, 3GPP counts more than 350 individual members, including major service providers, vendors, chipset providers, User Equipment (UE) providers, and test equipment vendors. Moreover, a strong confidence of service providers towards LTE arises from the close cooperation of 3GPP with NGMN alliance for defining the requirements of the standard and with LSTI for collaborative technology trials. As result of this cooperation, in July 2008, NGMN approved LTE as it first compliant technology, instead of WiMAX. Hence, since LTE was developed with support of the major mobile operators in the world, we can aspect that the great majority would adopt LTE, and not WiMAX.

- Evolution from previous standards and interoperability

LTE comes form the 3GPP organization. An organization which has acquired broad confidence among users, vendors and service providers with GSM and UMTS. On the contrary, WiMAX comes from an industry association, which is quite new in the cellular networks. As a consequence, service providers and vendors are inclined to evolve their system from UMTS towards LTE instead of adopting a completely new standard. Moreover, LTE provides seamless integration with 3GPP-based 2G/3G networks and with 3GPP2-based 2G/3G networks and provides seamless handoff to CDMA/1xEV-DO. This interoperability capability of LTE with legacy systems is attractive to such providers as this will allow them to roll-out their LTE network in several phases without interrupting their existing services.

- Smartphone manufacturers

Along with the mobile operators, also the smartphone and laptop manufacturers play an important role in the battle of 4G. Indeed, end-users will go for handsets with the coolest factor. The iPhone completely changed the way we look at mobile phones and what we expected from such devices, and it has contributed to increase exponentially the 3G data traffic generated by the mobile users. The continuing commitment of Apple in developing devices that only support 3GPP standards suggests that the successor of the iPhone 4 would probably support LTE standard. As Apple, other popular devices only support 3GPP standards, driving non-3GPP networks based service providers to converge to the 3GPP standards.

- Result of the battle

From this analysis LTE has emerged as the favourite winner of the “4G-battle” and in the following years it will become the most widely adopted standard for 4G mobile communications. On the other hand, WiMAX will not disappear, indeed it has been targeting emerging markets that have little infrastructure, because WiMAX deployment would be faster and more cost-effective than laying a wired infrastructure. But we will see a migration of WiMAX operators towards LTE.

Why is LTE going to be a success?

Success of a technology is expressed in terms of its adoption by customers in existing and new market segments. In the following we present the key factors that in our opinion will lead LTE to be a successful technology.

- Trend of the mobile data traffic and LTE performance

Over the last year, global mobile data traffic has increased by 160 percent and is growing faster than expected five years ago. The rapid consumer adoption of smart phones, netbooks, e-readers and Web-ready video cameras as well as machine-to-machine applications like eHealth monitoring and asset-tracking systems, is continuing to place unprecedented demands on mobile networks. In spite of the economic downturn, the demand for mobile services has remained high, posing both challenges and opportunities for service providers worldwide. The Cisco Visual Networking Index (VNI) Global Mobile Data Traffic Forecast update of 2010, predicts that mobile data traffic will double every year through 2014, increasing 39 times between 2009 and 2014. Mobile data traffic will reach 3.6 exabytes per month by 2014 when almost 66 percent of the world’s mobile data traffic will be video. Figure 7 shows the Cisco forecast for mobile data traffic.

Figure 7: Cisco VNI forecast for mobile data traffic

According to this forecast, in the next years user will require high throughput to support mobile video, along with low latency to support real-time applications such as VoIP, gaming and video-conference. Pre-commercial trials of LTE has already shown the capacity of this technology to fulfill such high requirements and a wide adoption of this technology by the mobile user will be an easy consequence.

- Advantages for service providers

From a service provider point of view, there are several motivations that would convince it into adopting the LTE technology:

• Lower operational expenditure (OPEX) to operate the network, made possible by the incorporation of self-organizing network capabilities in the LTE standard;
• High spectral efficiency and reduced cost of delivery per bit as compared to legacy wireless technologies;
• Promising performances allowing the user to use several applications and real-time services;
• Co-existence with legacy systems and standards;
• Tendency of the world’s major wireless service providers to show their commitment to LTE;
• Be part of a global ecosystem.

- Provision of interoperability with legacy systems

The interoperability capability of a new technology with legacy systems is a driver for its adoption. LTE will allow smooth and seamless service handover in areas where there is no LTE service (i.e. have HSPA, WCDMA, GSM, CDMA or 1xEV-DO coverage). Furthermore, provisions in LTE to be deployed as overlay network on existing non-3GPP systems (i.e. CDMA or 1xEV-DO) is attractive to such providers as this will allow them to roll-out their LTE network in several phases without interrupting their existing services.

- Collaboration with service providers and network vendors

A technology solving real customer problems has better chances of getting adopted widely. The LTE development saw participation and collaboration by various strong service providers and network vendors. These companies brought their experience and innovation to realize a list of requirements and recommendation (administered through the NGMN alliance) for 4G mobile communication systems and they participated in technology trials under the LSTI initiative. The LTE standard incorporated the service providers’ requirements in the standard and therefore achieved confidence of the service provider community in the technology. Since members of the NGMN alliance represent well over one half of the total mobile subscriber base world-wide, sponsors of the NGMN alliance account for more than 90% of the global footprint of mobile network development, and LTSI membership includes 26 vendors and 13 service providers, the collaboration between 3GPP and these support organizations assures a widely adoption of the LTE technology in the following years.

- Low royalty for IPR licenses

LTE technology IPR is owned by various companies involved in the technology development and their collaboration via the NGMN and LSTI alliances enabled them to agree on low royalty for IPR licenses to each other. This is unlike CDMA technology where IPRs were mostly owned by a single company (i.e Qualcomm). Thanks to the low royalty for IPR licenses, small companies can easily set up to be a service provider, increasing competition in the market with the result of a lower final price of the service for the user. Also the price of the complementary products (e.g. USB adapters) will be more accessible for the final user.

- Pre-commercial trials

TeliaSonera was the first operator in the world with the launch of commercial 4G services to customers in Stockholm, Sweden, and in Oslo, Norway, in December 2009. During 2010, the extensive network roll out continues in 25 cities and recreation areas in Sweden and in 4 in Norway. TeliaSonera has secured deliveries for 4G modems with support for 3G and 2G during the second quarter 2010 and it will launch 4G services to Danish customers during the spring 2011. The whole world had its eyes fixed on Sweden and Norway for the first period of deployment and this contributes to build confidence of customers on the feasibility, and performances of this technology, giving the feeling of a coming wide adoption.

- Success of previous 3GPP standards

LTE standard has been developed by 3GPP which is the progenitor of the widest adopted standards in cellular networks (i.e. GSM, UMTS, and following enhancements). For this reason, LTE is seen as the “natural evolution” of UMTS and this will bring to a wide migrations of GMS/UMTS users to LTE technology.

Classification and development of LTE standard

Standards may arise from different processes and an useful distinction can be made between formal (de jure) standards and de facto standards. Formal standards includes mandatory standards, which are promulgated by government agencies with regulatory authority, and voluntary standards, which are developed by international, regional or national standards organizations, broadly known as Formal Standardization Organizations (FSOs). De facto standards, on the other hand, emerge from market processes and are developed by individual companies or by industry trade associations or consortia.

LTE standard is a voluntary formal standard developed by the 3rd Generation Partnership Project (3GPP). 3GPP is a collaboration agreement that was established and formalized in December 1998 and that brings together a number of regional and national telecommunication standardization bodies of Europe, USA, China, Japan, and Korea. A detail description of the 3GPP Standardization Body including its members, scope and structure is given in a dedicated post along with a description of the 3GPP support organizations, i.e. NGMN (alliance of service providers and activities sponsored by vendors) and LSTI (an initiative of all types of stakeholders to trial the technology).

Open standard or proprietary standard?

There are several definitions of open standards which emphasize different aspects of openness, including the openness of the resulting specification, the openness of the drafting process, and the ownership of rights in the standard. According to the general definition, open standards are those considered to have diffused widely and been adopted by most companies in the market. The companies using them are not required to pay for the right to manufacture products in accordance with the standard, other than nominal fees (if any) imposed by the FSOs. Proprietary standards, on the other hand, are controlled by a single company or by a handful of companies and are protected by Intellectually Property Right (IPR). Other companies are required to pay for the rights to use these standards.

Being a formal standard, the standardization process of LTE is characterized by openness, due process, and consensus, which assume fairness and equity among participants. Anyway, the intellectual property and patents of contributors of the standard impact the openness of the standard. In the following we will analyze the openness of the LTE standard through the requirements that make a standard open defined by Krechmer in 2006:

1. Open Meeting: participation is open in various technical specification groups (TSG) and working groups (WG) to all members of 3GPP;
2. Consensus: 3GPP’s project co-ordination group (PCG) and technical specification groups (TSG) endeavor to reach consensus on all issues;
3. Due Process: 3GPP’s PCG, TSG, and WG have mandate to handle appeal process from individual members;
4. Open IPR: members are required to declare their IPRs that are essential and grant licenses on fair terms, reasonable terms and conditions, and on a non-discriminatory basis;
5. One World: 3GPP standards are applicable worldwide;
6. Open Change: all changes and proposals are discussed in TGS and WG, and become enforced after consensus;
7. Open Documents: 3GPP specifications and reports are available without any charges to all;
8. Open Interface: specifies and sets open interfaces for various components of a wireless system;
9. Open Access: interoperability, accessibility, and safety aspects are handled in the specifications and reports;
10. On-going Support: 3GPP standards are evolving continuously release over release.

Note that 3GPP LTE fulfills all the Krechmer’s requirements except for “Open IPR” which is fulfilled only in part. Indeed, the intellectual property, instead of being available on a royalty-free basis, have to be licensed to all applicants on fair terms and conditions and on a non-discriminatory basis. LTE technology IPR is owned by various companies involved in the technology development and their collaboration via the NGMN and LSTI alliances enabled them to agree on low royalty for IPR licenses to each other. This is unlike CDMA technology where IPRs were mostly owned by a single company (i.e Qualcomm). Moreover, the rules for standards published by the major internationally recognized standards bodies such as the IETF, and ITU-T allow an open standard to contain specifications whose implementation require payment of reasonable and non-discriminatory patent licensing fees. Therefore, it can be concluded that 3GPP LTE is considered to be an open standard.

LTE standard development stages

The 3GPP work on the Evolution of the 3G Mobile System started with the RAN Evolution Work Shop, 2-3 November 2004 in Toronto, Canada. The Work Shop was open to all interested organizations, members and non members of 3GPP. Operators, manufacturers and research institutes presented more than 40 contributions with views and proposals on the evolution of the Universal Terrestrial Radio Access Network (UTRAN). A set of high level requirements was identified in the Work Shop:

• Reduced cost per bit
• Increased service provisioning
• more services at lower cost with better user experience
• Flexibility of use of existing and new frequency bands
• Simplified architecture
• Open interfaces
• Allow for reasonable terminal power consumption

It was also recommended that the Evolved UTRAN should bring significant improvements to justify the standardization effort and it should avoid unnecessary options. With the conclusions of this Work Shop and with broad support from 3GPP members, a feasibility study on the UTRA & UTRAN Long Term Evolution was started in December 2004. The objective was “to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology”.

In september 2005 the feasibility study resulted in the first stage of the HSPA+ (Evolved High Speed Packet Access) Standard. HSPA+ represents the first step towards LTE, indeed, it increases the downlink/uplink data rate and reduces the latency in respect to HSDPA/HSUPA, and introduce MIMO technologies along with higher order modulation (64QAM). As further step towards LTE, it specifies an optional all IP-base architecture. 3GPP synchronizes specification development into releases and HSPA+ corresponds to the 3GPP Release 7.

In December 2008 the first release of the LTE Standard, namely 3GPP Release 8, was completed. It includes more than 1000 specifications divided in series with numbers from 21 to 36. For instance, in the following we report some specifications that we have used as reference for the Introduction of this blog and for other previous works:

• TR 25.913 collects the Requirements of the Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN);
• TS 33.400 series describe the Security Architecture;
• TS 36.200 series describes the Physical layer specifications of E-UTRA: General description (TS 36.201), Physical channel and modulation (TS 36.211), Multiplexing and channel coding (TS 36.212), Physical layer procedures (TS 36.213), Physical layer measurements (TS 36.214);
• TS 36.300 series describe the Layer 2 specifications of E-UTRA: Medium Access Control (MAC) protocol specification (TS 36.321), Radio Link Control (RTL) protocol specification (TS 36.322), Packet Data Convergence Protocol (PDCP) specification (TS 36.323), etc.;
• TS 36.400 series describes the E-UTRAN architecture.

Providing a detailed list of specifications is far from the goal of this blog, but to a determinate and curious reader, we recommend to visit the official web site of 3GPP, where all specifications can be downloaded for free.

3GPP Release 9 followed in 2009 and specifies some enhancements to the LTE System Architecture Evolution (SAE) and defines methods for interoperability between LTE and other access systems such as UMTS and WiMAX.

At the moment 3GPP is working on the 3GPP Release 10 known as LTE-advanced. LTE-advance is a preliminary standard that was formally submitted at ITU-T in the fall 2009, and expected to be finalized in 2011 with the target of meeting the IMT Advanced requirements for 4G as defined by ITU such as peak data rates up to 1 Gbps.

Table 3 summarizes the events occurred in the LTE standardization process and the actual work in progress. In order to understand correctly the content of the table, note that 3GPP standards are typically released in 3 stages:

• Stage 1 refers to the standard description from a standards’ user point of view;
• Stage 2 is a logical analysis, breaking the problem down into functional elements and the information flows amongst them;
• Stage 3 is the concrete implementation of the protocols between physical elements onto which the functional elements have been mapped.

Table 3: events in LTE standardization process

The presence of many actors (stakeholders) in formal standardization makes it difficult to reach agreement in the process, resulting in longer time for the finalization of the standard than de facto standardization. Note, indeed, that the 3GPP Release 7 took more than two years to be completed. However, the situation is different in the case of the 3GPP Release 8 whose completion was achieved within one year. This can be attributed to the collaboration among various stakeholders and support organizations to test and validate the technology in a real-world setup and feeding the results of such tests back to 3GPP.

LTE Technology convergence phases

If the previous paragraph treated with the standardization stages accomplished by 3GPP in the LTE standardization process, this paragraph focuses on the roles that NGMN and LSTI alliances, and service providers have played in the overall developments towards LTE standardization. Three phases can be identified.

Phase 1 started in September 2006 with the NGMN alliance formation among major service providers to formulate their requirements for a 4G wireless communication system and set the stage for various vendors to standardize the same in LTE specification. The formation of the LSTI alliance and LTE Proof-of-Concept (PoC) trials occurred in this phase as well. Phase 1 ended with the stage 2 freeze of the LTE standard.

Phase 2 has seen LTE gaining more emphasis and importance between vendors at the expense of other competing technologies, and also it has seen major development in LTE technology. This phase started in July 2008 when NGMN approved LTE as it first compliant technology. That triggered Sprint, which was one of the founders of the alliance, to leave NGMN. This is partly due to Sprint’s decision to adopt WiMAX as choice of technology for their next generation network. In November 2008, Qualcomm halted the UMB project and shifted focus fully on to LTE technology. In January 2009, Nokia ended the production of its only WiMAX device. In October 2008, Bell and Telus declared a joint plan to move from CDMA to HSPA in 2010 and then to LTE in 2012. Regarding the alliances, LSTI members continued their focus on performing the technology trials and interoperability trials to refine the technology, while NGMN released its final requirements. Stage 3 freeze of the LTE standard was achieved in this phase.

Phase 3 (1Q 2009) started with commitments to adopt LTE by the biggest service providers which started to award the commercial contracts to deploy LTE to the vendors. In February 2009, Verizon announced their LTE deployment plans and selected various vendors for the network. The global mobile suppliers association announced that 26 major service providers have committed to deploy LTE systems.

3GPP Standardization Body

The 3rd Generation Partnership Project (3GPP) is a collaboration agreement that was established and formalized in December 1998. The collaboration agreement brings together a number of regional and national telecommunication standardization bodies which are known as "Organizational Partners". The current Organizational Partners are:

• ARIB (Japan) - Association of Radio Industries and Businesses
• ATIS (USA) - Alliance for Telecommunications Industry Solutions
• CCSA (China) - China Communications Standards Association
• ETSI (Europe) - European Telecommunications Standards Institute
• TTA (Korea) - Telecommunications Technology Association
• TTC (Japan) - Telecommunication Technology Committee

The 3GPP Organizational Partners determine the general policy and strategy of 3GPP and perform tasks such as approval and maintenance of the 3GPP scope and the Partnership Project Description, taking decisions on the creation or cessation of Technical Specification Groups, and approving their scope and terms of reference, approval of Organizational Partner funding requirements, allocation of human and financial resources provided by the Organizational Partners to the Project Co-ordination Group, and acting as a body of appeal on procedural matters referred to them.

Along with the Organizational Partners, there are 12 Market Representation Partners (MRPs), i.e., IMS Forum, TD-SCMA Forum, GSA, GSM Association, IPv6 Forum, UMTS Forum, 3G Americans, TD-SCMA Industry Alliance, Info Communication Union, Femto Forum, CDMA Development Group, and Cellular Operators Association of India (COAI), which provide for the maintenance of the Partnership Project Agreement and the approval of applications for 3GPP partnership.

Then, 3GPP currently has three observers which are Standards Development Organizations (SDOs) who have the qualifications to become future Organizational Partners:

• TIA (USA) - Telecommunications Industries Association
• ISACC (Canada) - ICT Standards Advisory Council of Canada
• Communications Alliance - former Australian Communications Industry Forum (ACIF)

Along with the above partners there are more than 350 individual members. All entities registered as members of an Organizational Partner and eligible for participation in the technical work of that Organizational Partner can become Individual Members of 3GPP if they are committed to support 3GPP and to contribute technically or otherwise to one or more of the Technical Specification Groups (TSGs) within the 3GPP scope. An Individual Member has the right to participate in the work of 3GPP by attending meetings of the TSGs and subtending groups.

Scope and objectives of 3GPP

3GPP shall prepare, approve and maintain the necessary set of Technical Specifications and Technical Reports for the Global System for Mobile communication (GSM) including GSM evolved radio access technologies (e.g. General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE)).

Furthermore, 3GPP shall prepare, approve and maintain the necessary set of Technical Specifications and Technical Reports for an evolved 3rd Generation and beyond Mobile System including:

• Evolved UTRAN and beyond (including UTRA in Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes);
• 3GPP Core Network and evolutions (Third Generation networking capabilities originally evolved from GSM. These capabilities include mobility management, global roaming, and utilization of relevant Internet Protocols);
• Terminals for access to the above (including specifications for a UIM);
• An evolved IMS developed in an access independent manner;
• System and service aspects.

Finally, 3GPP shall also consider the long term evolution. The results of the 3GPP work shall form the basis of member contributions to the ITU in accordance with existing procedures. In the framework of agreed relationships, the 3GPP Technical Specifications and Technical Reports will form the basis of standards, or parts of standards, of the Organizational Partners.

Structure of 3GPP

3GPP consists of a Project Co-ordination Group (PCG) and Technical Specification Groups (TSGs). The PCG is the highest decision making body in 3GPP, it meets formally every six months to carry out the final adoption of 3GPP TSG work items, to ratify election results and the resources committed to 3GPP. Each TSG has the responsibility to prepare, approve and maintain the specifications within its terms of reference, may organize their work in Working Groups (WGs) and liaise with other groups as appropriate. The TSGs report to the PCG. Figure 7 depicts the structure of 3GPP.

Figure 6: structure of 3GPP

Support Organizations

3GPP enjoys the help from certain support organizations that help to define the standards. Next generation mobile networks (NGMN) and LTE/SAE trial initiative (LTSI) are such organizations. Various vendors and operators have come together to facilitate the LTE standard setting by providing recommendations and providing feedback by knowledge gathered during trials.

- Next Generation Mobile Network (NGMN) alliance

NGMN is an alliance of major service providers who are also early adopters of mobile communication technology. Several operators (Sprint Nextel, China Mobile, Vodafone, Orange, T-Mobile International, KPN Mobile, and NTT DoCoMo) formed the alliance in September 2006. Subsequently, NGMN defined the high-level requirements for all next generation broadband wireless networks – not just LTE. This type of initiative is one of the key differences between LTE and its predecessors, which were primarily vendor driven technologies. The NGMN alliance’s mandate is to complement and support the work within standardization bodies by providing a coherent view of what the operator community is going to require in the decade beyond 2010. NGMN has provided 3GPP with recommendations on optimized networks, self organized networks and higher performance networks. These recommendations, which LTE has been developed around, are incorporated into the standards. Another interest of NGMN is to further the mobile ecosystem.

One of the benefits of NGMN is that service providers have buy-in throughout the standardization process. As a result, they will be more comfortable with the standards when they are completed and LTE will be optimized for operators.

In addition to a service providers’ role as NGMN members, various types of vendors play the role of sponsors of NGMN alliance’s activities. Universities and non-industrial research institutes are also contributing to NGMN’s activities in their role of advisors to the alliance.

- LTE/SAE Trial Initiative (LSTI)

LSTI is a global, collaborative technology trial initiative focused on accelerating the availability of commercial and interoperable LTE mobile broadband systems. Major vendors (Nortel, Alcatel-Lucent, Ericsson, Nokia and Nokia Siemens Networks) and operators (Orange, T-Mobile and Vodafone) founded the initiative and have added more stakeholders (chipset vendors, operators and equipment vendors) since its inception. Vendors and operators began testing LTE early in the development process. The test results are shared with operators and 3GPP in an effort to improve the standards as the technology is being defined. Moreover, LSTI collaboration ensures that operators can rely on published results since they participate in the process. LSTI testing helps remove the hype from LTE and make the results more realistic. The efforts of NGMN and LSTI in conjunction with 3GPP are driving LTE to be a comprehensive technology with early interoperability testing and operator confidence. Objectives of LSTI include:

• driving the industrialization of 3GPP LTE and SAE;
• demonstrating the 3GPP LTE capabilities;
• promoting 3GPP LTE to operators, vendors, analysts and regulators;
• simplifying the technology with a full packet-based network and developing newer business models for service providers, vendors and operators;
• evolving the 3GPP LTE standard with findings from the proof-of-concept and interoperability trials.

Introduction to LTE

The recent increase of mobile data usage and emergence of new applications such as online gaming, mobile TV, Web 2.0, streaming contents have motivated the 3rd Generation Partnership Project (3GPP) to work on the Long-Term Evolution (LTE). LTE is the latest standard in the mobile network technology tree that previously realized the GSM/EDGE and UMTS/HSxPA network technologies that now account for over 85% of all mobile subscribers. LTE, whose radio access is called Evolved UMTS Terrestrial Radio Access (E-UTRA), is expected to substantially improve end-user throughputs, sector capacity, spectrum efficiency, and reduce user plane latency, bringing significantly improved user experience with full mobility. In order to achieve these targets, LTE makes use of technical principles that are innovative for cellular networks, such as, Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) antenna schemes. Furthermore the 3GPP defined a new core network architecture, called System Architecture Evolution (SAE), which reduces the number of network elements, introduces a full IP-based protocol for both data and voice traffic, and provides support for, and mobility between, legacy systems like GSM and UMTS, and also non-3GPP system like WiMAX.

This post is structured as follows. The first paragraph presents the target requirements of LTE specified in the standard. The second paragraph deals with the Frame Structure used in LTE. The third and the fourth paragraphs present the LTE downlink physical layer and the LTE uplink physical layer, respectively. Finally the fifth paragraph introduces the core network of LTE (SAE).

1) LTE Target Requirements

Main requirements for the design on an LTE system are specified in the 3GPP TR 25.913 Feasibility Study of Evolved UTRA and UTRAN. They can be summarized as follows:

• Peak Data Rate: instantaneous peak data rates of 100 Mbps (downlink) and 50 Mbps (uplink) for a 20 MHz spectrum allocation, assuming 2 receive antennas and 1 transmit antenna at UE.
• Latency: the one-way transit time between a packet being available at the IP layer in either the UE or radio access network and the availability of this packet at IP layer in the radio access network/UE shall be less than 5 ms. Also C-plane latency shall be reduced, e.g. to allow fast transition times of less than 100 ms from camped state to active state.
• Average user throughput per MHz: downlink target is 3-4 times better than Release 6 HSDPA. Uplink target is 2-3 times better than Release 6 HSDPA.
• Spectrum Efficiency (bit/sec/Hz/site): downlink target is 3-4 times better than Release 6 HSDPA. Uplink target is 2-3 times better than Release 6 HSDPA.
• Mobility: the system should be optimized for low mobile speed from 0 to 15 km/h. Higher mobile speed between 15 to 120 km/h should be supported with high performance. Mobility across the cellular network shall be maintained at speeds from 120 km/h to 350 km/h.
• Coverage: up to 5 km: full performance targets should be met, up to 30 km: slight degradations in the achieved performance, up to 100 km: should not be precluded by the specifications.
• Capacity: at least 200 users per cell should be supported for spectrum allocation of 5 MHz, and at least 400 users for higher spectrum allocation.
• Spectrum flexibility: support for spectrum allocations of different sizes: 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, and 20 MHz in both uplink and downlink.
• Spectrum allocation: operation in paired (Frequency Division Duplex / FDD) and unpaired spectrum (Time Division Duplex / TDD) shall be supported.
• Interworking: Interworking with existing UTRAN/GERAN systems and non-3GPP systems shall be ensured. Multimode terminals shall support handover to and from UTRAN and GERAN as well as inter-RAT measurements. Interruption time for handover between E-UTRAN and UTRAN/GERAN shall be less than 300 ms for real time services and less than 500 ms for non real time services.
• Costs: Reduced CAPEX and OPEX including backhaul shall be achieved. Cost effective migration from Release 6 UTRA radio interface and architecture shall be possible. Reasonable system and terminal complexity, cost and power consumption shall be ensured. All the interfaces specified shall be open for multi-vendor equipment interoperability.
• Co-existence: Co-existence in the same geographical area and co-location with GERAN/UTRAN shall be ensured. Also, co-existence between operators in adjacent bands as well as cross-border coexistence is a requirement.
• Quality of Service: End-to-end Quality of Service (QoS) shall be supported. VoIP should be supported with at least as good radio and backhaul efficiency and latency as voice traffic over the UMTS circuit switched networks.

2) LTE Frame Structure

To support transmission in paired and unpaired spectrum, two duplex modes are supported: Frequency Division Duplex (FDD), supporting full duplex and half duplex operation, and Time Division Duplex (TDD). Therefore two frame structures are defined: frame structure type 1 for FDD mode, and frame structure type 2 for TDD mode.

The 10 ms FDD frame is divided into 10 subframes of 1 ms each. Each subframe is further divided into two slots of 0.5 ms duration, as shown in Figure 1.

Figure 1: frame structure for FDD mode

Slots consist of either 6 or 7 OFDM symbols, depending on whether the normal or extended cyclic prefix is employed.

The 10 ms TDD frame consists of two half-frames of length 5 ms each. Each half-frame is divided into five subframes of each 1 ms, as shown in Figure 2.

Figure 2: frame structure for TDD mode

All subframes which are not special subframes are defined as two slots of length 0.5 ms. The special subframes consist of three fields DwPTS (Downlink Pilot Timeslot), GP (Guard Period), and UpPTS (Uplink Pilot Timeslot). DwPTS, GP and UpPTS have configurable individual lengths and a total length of 1 ms. Seven uplink-downlink configurations with either 5 ms or 10 ms downlink-to-uplink switch-point periodicity are supported. In case of 5 ms switch-point periodicity, the special subframe exists in both half-frames. In case of 10 ms switch-point periodicity the special subframe exists in the first half frame only. Subframes 0 and 5 and DwPTS are always reserved for downlink transmission. UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission. Table 1 shows the supported uplink-downlink configurations, where “D” denotes a subframe reserved for downlink transmission, “U” denotes a subframe reserved for uplink transmission, and “S” denotes the special subframe.

Table 1: Uplink-downlink configuration for TDD frame structure

3) LTE Downlink Physical Layer

The downlink transmission scheme for LTE in both FDD and TDD mode is based on conventional OFDM. In an OFDM system, the available spectrum is divided into many narrower subcarriers, each of these is independently modulated by a low rate data stream using varying levels of QAM modulation. The basic subcarriers spacing in LTE is 15 KHz, with a reduced subcarrier spacing of 7.5 KHz available for some MB-SFN scenarios. One downlink slot consists of 6 or 7 OFDM symbols, depending on whether extended or normal cyclic prefix is configured. The extended cyclic prefix is able to cover larger cell sizes with higher delay spread of the radio channel. The cyclic prefix lengths are summarized in Table 2.

Table 2: cyclic prefix lengths

Note that the CP duration is described in absolute term (e.g. 16.67 μs for extended CP) and in term of standard units, Ts. Ts is used throughout the LTE specification documents. It is defines as Ts = 1 / ( 15000 x 2048 ) = 1 / 30720000 seconds which corresponds to the 30.72 MHz sample clock for the 2048 point FFT used with the 20 MHz system bandwidth.

OFDMA is employed as the multiplexing scheme in the LTE downlink. Although it involves added complexity in terms of resource scheduling, it is vastly superior to other multiplexing methods that use OFDM as the underlying modulation in terms of efficiency and latency. In OFDMA, users are allocated a specific number of subcarriers for a predetermined amount of time. The transmitted downlink signals can be represented by a resource grid as depicted in Figure 3.

Figure 3: downlink resource grid

Each box within the grid represents a single subcarrier for one symbol period and is referred to as a resource element. Note that in MIMO configurations, there is a resource grid for each transmitting antenna. The smallest element of resource allocation is called physical resource block (PRB) and is defined as consisting of 12 consecutive subcarriers for one slot in duration. The PRBs allocated for each users do not have to be adjacent to each other and the scheduling decision can be modified every transmission time interval of 1 ms. Allocation of PRBs is handled by a scheduling function at the 3GPP base station and the scheduling algorithm has to take into account the radio link quality situation of different users, the overall interference situation, QoS requirements, service priorities, etc. The total number of available PRBs depends on the overall transmission bandwidth of the system.

Physical Downlink Channels

A downlink physical channel corresponds to a set of resource elements carrying information originating from higher layers. The following downlink physical channels are defined:

• Physical Downlink Shared Channel (PDSCH): carriers user data (QPSK, 16QAM, 64QAM);
• Physical Broadcast Channel (PBCH): carriers Master Information Block (QPSK);
• Physical Multicast Channel (PMCH): carriers user data to one or more devices (QPSK, 16QAM, 64QAM);
• Physical Control Format Indicator Channel (PCFICH): indicates format of PDCCH (QPSK);
• Physical Downlink Control Channel (PDCCH): carriers downlink control information (DCI), e.g. downlink or uplink scheduling assignments (QPSK);
• Physical Hybrid ARQ Indicator Channel (PHICH): carriers ACK/NACK for uplink data packets (BPSK).

Physical Downlink Signals

A downlink physical signal corresponds to a set of resource elements used by the physical layer but does not carry information originating from higher layers. The following downlink physical signals are defined:

- Reference Signals

The downlink reference signal structure is important for channel estimation. Specific pre-defined resource elements in the resource grid are carrying the cell-specific reference signal sequence. UE must get an accurate Channel Impulse Response (CIR) from each transmitting antenna. Therefore, when a reference signal is transmitted from one antenna port, the other antenna ports in the cell are idle. CIR estimates for subcarriers that do not bear reference signals are computed via interpolation.

- Synchronization signals

The synchronization signals are classified as primary and secondary synchronization signals, depending how they are used by the UE during the cell search procedure. Both primary and secondary synchronization signals are transmitted on 62 subcarriers within 72 reserved subcarriers around DC subcarrier during 0th and 10th slots of a frame.

4) LTE Uplink Physical Layer

LTE uplink requirements differ from downlink requirements in several ways. Not surprisingly, power consumption is a key consideration for UE terminal. The high peak-to-average ratio (PAPR), and related loss of efficiency associated with OFDM signaling are major concerns. As a result, an alternative to OFDM was sought for use in the LTE uplink.

Thus, the LTE uplink transmission scheme for both FDD and TDD mode is based on Single Carrier Frequency Domain Multiple Access (SC-FDMA) with cyclic prefix. SC-FDMA is a misleading term, since SC-FDMA is essentially a multi-carrier scheme that re-use many of the functional blocks included in the UE OFDM receiver signal chain. The principal advantage of SC-FDMA over conventional OFDM is a lower PAPR (by approximately 2 dB) that is important for cost-effective design of UE power amplifiers.

The uplink uses the same generic frame structure as the downlink. It also uses the same subcarrier spacing of 15 KHz and PRB width. Uplink modulation parameters (including normal and extended CP length) are identical to the downlink parameters.

Subcarrier modulation is, however, much different. In the uplink, data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality. However, rather than using the QPSK/QAM symbols to directly modulate subcarriers (as is the case in OFDM), uplink symbols are sequentially fed into a serial/parallel converter and then into an FFT block. The result at the output of the FFT block is a discrete frequency domain representation of the QPSK/QAM symbol sequence. The discrete Fourier terms at the output of the FFT block are then mapped to subcarriers before being converted back into the time domain (IFFT). The final step prior to transmission is appending a CP. Figure 4 clarifies the conceptual differences between OFDMA and SC-FDMA.

Figure 4: OFDMA vs SC-FDMA

Scheduling of uplink resources is done by the base station which assigns certain time/frequency resources to the UEs and informs UEs about transmission format to use. The scheduling decisions is based on QoS parameters, UE buffer status, uplink channel quality measurements, UE capabilities, UE measurements gaps, etc. The resource grid used in uplink transmission is similar to the one used in downlink transmission.

Physical Uplink Channels

An uplink physical channel corresponds to a set of resource elements carrying information originating from higher layers. The following uplink physical channels are defined:

• Physical Uplink Shared Channel (PUSCH): carriers user data (QPSK, 16QAM, 64QAM);
• Physical Uplink Control Channel (PUCCH): carriers uplink control information (UCI), i.e. ACK/NACK information related to data packets received in the downlink, channel quality indication (CQI) reports, precoding matrix information (PMI) and rank indication (RI) for MIMO, and scheduling request (SR) (BPSK, QPSK);
• Physical Random Access Channel: requests initial access, as part of handover, or re-establishes uplink synchronization (BPSK, QPSK).

Physical Uplink Signals

An uplink physical signal is used by the physical layer but does not carry information originating from higher layers. LTE defines one category of uplink physical signals: Reference Signals. There are two types of uplink reference signals: the demodulation reference signal is used for channel estimation in the eNodeB receiver in order to demodulate control and data channels, while the sounding reference signal provides uplink channel quality information as a basis for scheduling decisions in the base station.

5) LTE/SAE System Architecture Evolution

Along with LTE, that applies more on the radio access technology of the cellular telecommunications system, there is also an evolution of the core network. Known as System Architecture Evolution (SAE). The main principles of the LTE-SAE architecture include:

• a common gateway (GW) node for all access technologies;
• an optimized architecture for the user plane – from four to only two node types (base stations and gateway);
• IP-based protocols on all interfaces;
• a RAN-CN functional split similar to that of WCDMA/HSPA;
• a split in the control/user plane between the mobility management entity (MME) and the gateway;
• integration of non-3GPP access technologies using client- as well as network-based mobile IP.

Figure 5 shows a simplified view of the overall LTE-SAE architecture, where continuous and dotted lines represent traffic data interfaces and signaling interfaces respectively.

Figure 5: simplified LTE/SAE architecture

The LTE base stations (eNodeB) connect to the core network via the RAN-CN interface. The MME handles control signaling (control plane) while user data is forwarded between base stations and gateway (user plane). To support high-speed handover of terminals in active mode, each LTE base station is logically connected to all its neighboring base stations.

The gateway includes both packet data network (PDN) and serving gateway functionality. The PDN gateway serves as a common anchor point for all access technologies, providing a stable IP point-of-presence for all users regardless of mobility within or between access technologies. The serving gateway is the anchor point for intra-3GPP mobility.

The MME functionality is kept separate from the gateways to facilitate network deployment, independent technology evolution, and fully flexible scaling of capacity.

GSM and WCDMA/HSPA systems are integrated into the evolved system through standardized interfaces between the SGSN (serving GPRS support node) and the evolved core network. This includes interfaces to the MME for transferring context and establishing bearers when moving between accesses, and to the gateway for establishing IP connectivity with user equipment (UE). The gateway node thus functions as a GGSN (gateway GPRS support node) for GSM and WCDMA/HSPA terminals.