LTE (telecommunication)

Long Term Evolution (LTE) describes the latest standardization work by 3rd Generation Partnership Project(3GPP) in the mobile network technology tree previously realized the GSM/EDGE and UMTS/HSxPA network technologies that now account for over 85% of all mobile subscribers . In this latest standardization work which started in late 2004, the 3GPP (set out in December, 1998) defines a set of high level requirements (new high-speed Radio Access method) for mobile communications systems to compete with other latest cellular broadband technologies particularly WiMAX.

In preparation for further increasing user demands and tougher competition from new radio access technologies, LTE is enhanced with a new radio access technique called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) . Via this technology LTE is expected to improve end-user throughputs, sector capacity, reduce user plane latency and consequently offers superior user experience with full mobility.

Unlike other latest deployed technologies such as HSPA, LTE is accommodated within a new Packet Core architecture called Enhanced Packet Core (EPC) network architecture. Technically, 3GPP specifies the EPC to support the E-UTRAN. EPC is designed to deploy TCP/IP protocols thus enabling LTE to support all IP-based services including voice, video, rich media and messaging with end-to-end Quality of Service (QoS). The EPC network architecture also enables improved connections and hand-over to other fixed-line and wireless access technologies while giving an operator the ability to deliver a seamless mobility experience .

To achieve all the targets mentioned herein, LTE Physical Layer (PHY) employs advanced technologies that are new to cellular applications. These include Orthogonal Frequency Division Multiple Access (OFDMA) and multiple-input and multiple-output (MIMO) data transmission. Smart Antennas are also deployed to accomplish those targets. Furthermore, the LTE PHY deploys the OFDMA for the Downlink (DU) - that is from the Base Station (BS) to the User Equipment (UE) and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the Uplink (UL). These technologies will further minimize the LTE system and UE complexities while allowing flexible spectrum deployment in existing or new frequency spectrum .

LTE enjoys the support from a collaborative group of international standards organizations and mobile-technology companies that form the 3GPP. However, a strong support comes from Ericsson and Qualcomm who decided not to support WiMAX, the competitor of LTE . Motorola which also supports WiMAX claims to be the leading contributor in LTE standards such as Radio Access Network (RAN) 1 & 2 and a top three contributor to EPC 1 & 2 standards . The standardization work on LTE is continuing, and Motorola claims to introduce LTE in ‘Q4 2009 . However, LTE is assumed to dominate world’s mobile infrastructure markets after 2011 .

Standardization Path

The standardization of LTE started in November 2004, when the RAN Evolution Workshop in Toronto Canada accepted contributions from more than 40 operators, vendors and research institutes that included 3GPP members and nonmembers organizations. Contributions were merely a range of views and proposals on the UTRAN. Following those contributions, 3GPP started a feasibility study, in December 2004, so as to develop a new framework for evolution of the 3GPP Radio Access technology towards:

• Increased data rates
• Reduced cost per bit
• Increased service provisioning - that is more services with better user experience
• Flexibility in usage of both new and existing frequency bands
• High-data-rate
• Low-latency
• Simple architecture, open interface and packet-optimized RAN technology

Put simply, the study maps out specifications for RAN that are capable to support the wireless broadband internet scenario which is already enjoyed in today’s cable networks – adding full mobility to enable exciting new service possibilities .

Currently LTE specifications are described in 3GPP Release 8. The 3GPP Release 8 is the latest set of standards that describes the technical evolution of 3GPP mobile network systems. It is the successor of 3GPP Release 7 that includes a set of specifications for HSPA+, the ‘missing bridge’ between HSPA and LTE. Actually HSPA+ is described in both, the 3GPP Release 7 and 8, allowing the designing of simpler ‘flat’, all-IP based network architecture and bypassing many of the legacy equipments required for UMTS/HSPA.

The specifications of the 3GPP Release 8 standard are assumed to complete at the end of 2008. Obviously the finalization of the 3GPP Release 8 will further progress the market interest in commercial deployment of LTE. The 3GPP Release 8 will compile the completion of 3GPP Release 7 HSPA+ features, Voice over HSPA and EPC specification and Common IP Multimedia Subsystem (IMS) .

LTE Key Features

As it has been discussed earlier, technically speaking, a fundamental objective of the 3GPP LTE project is to offer higher data speeds for both DL and UL transmissions. In addition to that, it is obviously LTE to be characterized by reduced packet latency while promising a superior experience in online gaming, Voice over IP (VoIP) videoconferencing and other real-time professional services. Now that based on the feasibility study under 3GPP, the following are the important features of LTE:

OFDMA on the DL and SC-FDMA on the UL

3GPP Release 8 specifies an all-new RAN that combines OFDMA-based modulation and multiple access schemes for the downlink, as well as SC-FDMA for uplink. These new technologies (OFDM schemes) are deliberately deployed to split available spectrum into thousands of extremely narrowband carriers, such that each carrier is capable of carrying a part of signal. This is what is known as multiple carrier transmission .

To enhance the OFDM schemes, LTE also employs other higher order modulation schemes such as 64QAM and sophisticated Forward Error Correction (FEC) schemes such as tail biting, convolutional coding and turbo coding. Furthermore, complementary radio techniques such as MIMO and Beam Forming with up to four antennas per station are also deliberately deployed for further enhancement of innate spectral efficiency of OFDM schemes .

The results of these radio interface features are obvious, enabling LTE to have improved radio performance. As such they yield the spectral efficiency up to 3 to 4 times that of HSDPA Release 6 in the LTE DL and up to 2 to 3 times that of HSUPA Release 6 in UL . Consequently, theoretically, the DL peak data rates extend up to 300Mbit/s per 20MHz of spectrum. Similarly, theoretical UL peak data rates can reach 75Mbit/s per 20MHz of spectrum as well as supporting at least 200 active users per cell in 5MHz..

All-IP Packet Optimized Network Architecture

LTE has a ‘flat’, all-IP based core network with a simplified architecture, open interface and fewer system nodes. Indeed, the all-IP based network architecture together with the new RAN reduces network latency, improved system performance and provide interoperability with existing 3GPP and non-3GPP technologies. Within 3GPP, all-IP based core network architecture is now known as Evolved Packet Core (EPC). EPC is the result of standardization work within 3GPP which targeted to convert the existing System Architecture Evolution (SAE) to an all-IP system.

Advanced Antenna Techniques

LTE is enhanced with MIMO, Spatial-Division Multiple Access (SDMA) and Beam Forming. These are advanced radio antenna techniques which are complementary to each other. These techniques are deployed for better air interface via enhancing the innate spectral efficiency of OFDM schemes. Furthermore, these techniques can be used to trade-off between higher sector capacity, higher user data rates, or higher cell-edge rates, and thus enable mobile operators to have finer control over the end-user experience .

 

System Architecture

LTE System Architecture

Evolved Radio Access Network (RAN)

The evolved RAN consists of the LTE base station (eNode B) that interfaces with the UE. The eNode B contains the PHY, Media Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. Therefore the eNode B performs some tasks such as resource management, admission control, scheduling and enforcement of negotiated UL QoS.

Serving Gateway (SGW)

The SGW guides and forwards user data packets. Furthermore, during inter-eNode B handover SGW acts as the mobility anchor for the user plane. It can also act as an anchor for mobility between LTE technology and other 3GPP technologies. When the UE is in idle state, the SGW terminates the DL data path of the UE and triggers paging when DL data arrives for the UE..

Mobility Management Entity (MME)

MME handles Control Signaling for mobility [1]. When the UE is in idle mode, the MME is responsible for UE tracking and paging procedure that includes retransmissions [4]. MME is also involved in the bearer activation/deactivation process. In addition MME can choose the SGW for a UE at the initial attach and at time of intra-LTE handover involving core Network (CN) node relocation. MME can interact with the Home Subscriber Server (HSS) so as to authenticate the user..

Packet Data Network Gateway (PDN GW)

PDN GW is a point of exit and entry of traffic for the UE. PDN GW performs packet filtering and acts as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2.

An "All IP Network" (AIPN)

A characteristic of next generation networks are that they are based upon the core internet protocol Transmission Control Protocol/Internet Protocol (TCP/IP). This provides users with richer communications experience including enhanced voice, video, and messaging services and advanced multimedia solutions.

In 2004, 3GPP proposed TCP/IP as the future for next generation networks and began feasibility studies into the so-called All IP Networks (AIPN.) Proposals developed included recommendations in 2005 for 3GPP Release 7, which are the foundation of higher level protocols such as LTE. These recommendations are part of the 3GPP System Architecture Evolution (SAE). Some aspects of All-IP networks, however, were already defined as early as release 4).

From an architectural point-of-view the SAE/EPC architecture is defining an access independent, IP-based, flat network architecture with optimized interworking between legacy 3GPP and 3GPP2 networks. This means that both CDMA service providers and GSM/WCDMA service providers will be able to evolve their networks to LTE–SAE. A LTE–SAE network is designed for efficient support of mass-market usage of any IP-based service. The architecture is based on an evolution of the existing GSM/WCDMA core network, with simplified operations for a smooth and cost-efficient deployment.

The UMTS back-end becomes accessible through a variety of means, such as GSM's/UMTS's own radio network (GERAN, UTRAN, and E-UTRAN), WiFi, Ultra Mobile Broadband (UMB) and Worldwide Interoperability for Microwave Access (WiMAX). Users of non-UMTS radio networks would be provided with an entry-point into the IP network, with different levels of security depending on the trustworthiness of the network being used to make the connection. Users of GSM/UMTS networks would use an integrated system where all authentication at every level of the system is covered by a single system, while users accessing the UMTS network via WiMAX and other similar technologies would handle the WiMAX connection one way (for example, authenticating themselves via a MAC or ESN address) and the UMTS link-up another way.

E-UTRA Air Interface

Release 8's air interface, E-UTRA (Evolved UTRA, the E- prefix being common to the evolved equivalents of older UMTS components) would be used by UMTS operators deploying their own wireless networks. It's important to note that Release 8 is intended for use over any IP network, including WiMAX and WiFi, and even wired networks.

The proposed E-UTRA system uses Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink (tower to handset) and Single Carrier FDMA (SC-FDMA) for the uplink and employs multiple-input and multiple-output (MIMO) with up to four antennas per station. The channel coding scheme for transport blocks is turbo coding and a contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver.

The use of Orthogonal Frequency Division Multiplexing (OFDM), a system where the available spectrum is divided into thousands of very thin carriers, each on a different frequency, each carrying a part of the signal, enables E-UTRA to be much more flexible in its use of spectrum than the older CDMA based systems that dominated 3G. CDMA networks require large blocks of spectrum to be allocated to each carrier, to maintain high chip rates, and thus maximize efficiency. Building radios capable of coping with different chip rates (and spectrum bandwidths) is more complex than creating radios that only send and receive one size of carrier, so generally CDMA based systems standardize both. Standardizing on a fixed spectrum slice has consequences for the operators deploying the system: too narrow a spectrum slice would mean the efficiency and maximum bandwidth per handset suffers; too wide a spectrum slice, and there are deployment issues for operators short on spectrum. This became a major issue with the US roll-out of UMTS over WCDMA, where WCDMA's 5 MHz requirement often left no room in some markets for operators to co-deploy it with existing GSM standards.

LTE supports both FDD and TDD mode. Each mode has its own frame structure within LTE and these are aligned with each other meaning that similar hardware can be used in the base stations and terminals to allow for economy of scale. The TDD mode in LTE is aligned with TD-SCDMA as well allowing for coexistence. Ericsson demonstrated at the Mobile World Congress 2008 in Barcelona for the first time in the world both LTE FDD and TDD mode on the same base station platform.

Downlink

LTE uses OFDM for the downlink – that is, from the base station to the terminal. OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide carriers with high peak rates. It is a well-established technology, for example in standards such as IEEE 802.11a/b/g, 802.16, HIPERLAN-2, DVB and DAB.

In the time domain you have a radio frame that is 10 ms long and consists of 10 sub frames of 1 ms each. Every sub frame consists of 2 slots where each slot is 0.5 ms. The subcarrier spacing in the frequency domain is 15 kHz. Twelve of these subcarriers together (per slot) is called a resource block so one resource block is 180 kHz. 6 Resource blocks fit in a carrier of 1.4 MHz and 100 resource blocks fit in a carrier of 20 MHz.

Supported modulation formats on the downlink data channels are QPSK, 16QAM and 64QAM.

For MIMO operation, a distinction is made between single user MIMO, for enhancing one users data throughput, and multi user MIMO for enhancing the cell throughput.

Uplink

In the uplink, LTE uses a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA). This is to compensate for a drawback with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR). High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and drains the battery faster. SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.

Supported modulation formats on the uplink data channels are QPSK, 16QAM and 64QAM.

If virtual MIMO / Spatial division multiple access (SDMA) is introduced the data rate in the uplink direction can be increased depending on the number of antennas at the base station. With this technology more than one mobile can reuse the same resources.

Technology Demos

• In February 2007, Ericsson demonstrated for the first time in the world LTE with bit rates up to 144 Mbit/s

• In September 2006, Siemens Networks (today Nokia Siemens Networks) showed in collaboration with Nomor Research the first live emulation of a LTE network to the media and investors. As live applications two users streaming an HD-TV video in the downlink and playing an interactive game in the uplink have been demonstrated.

• The first presentation of an LTE demonstrator with HDTV streaming (>30 Mbit/s), video supervision and Mobile IP-based handover between the LTE radio demonstrator and the commercially available HSDPA radio system was shown during the ITU trade fair in Hong Kong in December 2006 by Siemens Communication Department.

• In September 2007, NTT DoCoMo demonstrated LTE data rates of 200 Mbit/s with power consumption below 100 mW during the test.

• At the February 2008 Mobile World Congress:

Motorola demonstrated how LTE can accelerate the delivery of personal media experience with HD video demo streaming, HD video blogging, Online gaming and VoIP over LTE running a RAN standard compliant LTE network and LTE chipset. [21]

Ericsson demonstrated the world’s first end-to-end LTE call on handheld and LTE FDD and TDD mode on the same base station platform.

Freescale Semiconductor demonstrated streaming HD video with peak data rates of 96 Mbit/s downlink and 86 Mbit/s uplink .

NXP Semiconductors demonstrated a multi-mode LTE modem as the basis for a software-defined radio system for use in cellphones.

picoChip and mimoOn demonstrated an LTE base station reference design. This runs on a common hardware platform (multi-mode / software defined radio) together with their WiMAX architecture.

• In March 2008, NTT DoCoMo demonstrated LTE data rates of 250 Mbit/s in an outdoor test.

• In April 2008, Motorola demonstrated the first EV-DO to LTE hand-off - handing over a streaming video from LTE to a commercial EV-DO network and back to LTE. [22]

• In April 2008, LG and Nortel demonstrated LTE data rates of 50 Mbit/s while travelling at 110 km/h.

• Researchers at Nokia Siemens Networks and Heinrich Hertz Institut have demonstrated LTE with 100 Mbit/s Uplink transfer speeds.

• On September 18, 2008, Mobile operator T-Mobile and Nortel Networks achieved data rates of up to 170 Mbit/s for downloads and up to 50 Mbit/s for uploads. T-Mobile, the wireless business of Deutsche Telekom achieved these speeds in a car in range of three cell sites on a highway in Bonn, Germany at an average speed of 67 km/h.

Carrier adoption

• Most carriers supporting GSM or HSPA networks can be expected to upgrade their networks to LTE at some stage:
  • AT&T Mobility has stated that they intend on upgrading to LTE as their 4G technology, but will introduce HSUPA and HSPA+ as bridge standards.
  • T-Mobile, Vodafone, France Télécom, Telia Sonera and Telecom Italia Mobile have also announced or talked publicly about their commitment to LTE.
• However, several networks that don't use these standards are also upgrading to LTE:
  • Alltel, Verizon Wireless, the newly formed China Telecom/Unicom and Japan's KDDI have announced they have chosen LTE as their 4G network technology. This is significant, because these are CDMA carriers and are switching networking technologies to match what will likely be the 4G standard worldwide. They have chosen to take the natural GSM evolution path as opposed to the 3GPP2 CDMA2000 evolution path Ultra Mobile Broadband (UMB). Verizon Wireless plans to begin LTE trials in 2008.
  • Telus Mobility and Bell Mobility have announced that they will adopt LTE as their 4G wireless standard.

Conformance testing

It has been suggested that TTCN-3 test specification language will be used for the purposes of LTE conformance testing. As of March 2008, TTCN-3 test suite development has been underway at ETSI.

See also

• E-UTRA
• System Architecture Evolution
• HSPA

References

• H. Ekström, A. Furuskär, J. Karlsson, M. Meyer, S. Parkvall, J. Torsner, and M. Wahlqvist, "Technical Solutions for the 3G Long-Term Evolution," IEEE Commun. Mag., vol. 44, no. 3, March 2006, pp. 38–45
• 3rd Generation Partnership Project (3GPP); Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)
• 3rd Generation Partnership Project (3GPP); Technical Specification Group Radio Access Network; Physical Layer Aspects for Evolved UTRA
• E. Dahlman, H. Ekström, A. Furuskär, Y. Jading, J. Karlsson, M. Lundevall, and S. Parkvall, "The 3G Long-Term Evolution - Radio Interface Concepts and Performance Evaluation," IEEE Vehicular Technology Conference (VTC) 2006 Spring, Melbourne, Australia, May 2006
• C. Mathas, "LTE: From Zero to 32 Million in Three Years, says ABI Research," Mobile Handset Design Line, June 12, 2008.[23]
• K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems: From OFDM and MC-CDMA to LTE and WiMAX, 2nd Edition, John Wiley & Sons, 2008, ISBN 978-0-470-99821-2

External links for more information

• 3GPP LTE page
• 3GPP TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description
• 3GPP AIPN Workitem
• Nomor Research: White Paper on LTE Advance
• UMTS Long Term Evolution (LTE) Technology Introduction by Rohde & Schwarz
• "The Long Term Evolution of 3G" on Ericsson Review, no. 2, 2005
• "3G Long-Term Evolution" by Dr. Erik Dahlman at Ericsson Research
• "Long-Term 3G Evolution - Radio Access" by Dr. Stefan Parkvall at Ericsson Research
• "3GPP Evolution: LTE and SAE" by Francois Courau at Alcatel (Chairman of 3GPP TSG RAN)
• "3GPP Long-Term Evolution / System Architecture Evolution: Overview" by Ulrich Barth at Alcatel
• Dahlman, Parkvall, Skold and Beming, 3G Evolution: HSPA and LTE for Mobile Broadband, Academic Press, Oxford, UK, 2007
• "3GPP LTE & 3GPP2 LTE Standardization" by Dr. Lee, HyeonWoo at Samsung Electronics
• "Mobile Broadband: The Global Evolution of UMTS/HSPA - 3GPP Release 7 and Beyond" by 3G Americas
• "Trends in Mobile Network Architectures" by Dr. Michael Schopp at Siemens Networks
• Fraunhofer Heinrich-Hertz-Institut
• "Overview of the 3GPP LTE Physical Layer" by James Zyren and Dr. Wes McCoy, Freescale Semiconductor
• Long Term Evolution (LTE): An Introduction – Ericsson White Paper