Understanding LTE

1. Technology Evolution
2. LTE Timeline
3. Market Impact
4. Design and Test Challenges
5. Solutions
6. Conclusion
7. Related Information

Long Term Evolution (LTE) is the project name of a new air interface for wireless access being developed by the Third Generation Partnership Project (3GPP). LTE is the evolution of 3GPP's Universal Mobile Telecommunication System (UMTS) towards an all-IP network. The LTE specifications provide a framework for increasing capacity, improving spectrum efficiency, improving cell-edge performance, and reducing latency. Many of the targets for LTE are similar to those for the continuing development of High Speed Packet Access (HSPA) – generally known as HSPA+ – although LTE has some specific additional capabilities such as flexible channel bandwidths and the advantages of Orthogonal Frequency Division Multiple Access (OFDMA). LTE is being developed in Releases 8 and 9 of the 3GPP specifications.

To meet the demand for ever-higher data rates, LTE offers a 100 Mbps download rate and 50 Mbps upload rate for every 20 MHz of spectrum. Support is intended for even higher rates, to 326.4 Mbps in the downlink, using multiple antenna configurations. To allow the use of both new and existing frequency bands, LTE provides scalable bandwidth from 1.4 MHz to 20 MHz in both the downlink and the uplink. LTE is optimized for low speeds (0 - 15 km/h) but will still provide high performance to 120 km/h with support for mobility maintained up to 350 km/h. 3GPP are considering support for even higher speeds up to 500 km/h.

Unlike UMTS, which is based on Wideband Code Division Multiple Access (W-CDMA) technology, LTE is based on OFDMA and in this regard is similar in concept to Mobile WiMAX™, another emerging wireless broadband technology, although the systems operate with different frame structures, sub-carrier spacing, and channel bandwidths. A notable feature of LTE is a new, OFDM-derived transmission scheme for the uplink. This format, called Single Carrier Frequency Division Multiple Access (SC-FDMA), combines the low peak-to-average power ratio (PAPR) of single-carrier systems with the multi-path resistance and flexible sub-carrier frequency allocation offered by OFDMA.

Within the 3GPP specifications for LTE, the evolved radio access network is split into two parts: the Evolved UMTS Terrestrial Radio Access (E-UTRA), which describes the mobile part of LTE, and the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), which describes the base station part containing the Evolved Node B (eNB). Along with the LTE specifications, 3GPP is working on a complementary project called the System Architecture Evolution (SAE), which defines the split between LTE and a new Evolved Packet Core (EPC). This architecture is a flatter, packet-only core network that will help deliver the higher throughput, lower cost, and lower latency that is the goal of LTE. It is also designed to provide seamless interworking with existing 3GPP and non-3GPP access technologies.

Work on LTE began in 2004. The first completed Release 8 specification was published in March 2009, and the specification is now considered sufficiently stable for commercial development. LTE standards development continues with 3GPP Release 9, with completion of the specification expected by December 2009. Trials are underway, and initial deployment of LTE is expected in 2010 and 2011.

Base station and user equipment design and testing are following close on the heels of specification development. To help with the rapid commercialization of LTE a group of operators and vendors formed a consortium in 2006 called the LTE/SAE Trial Initiative (LSTI). The work of the group is in three phases: proof of the LTE concept, interoperability and finally field trials. The first results became available during 2007 and showed sustained high speed test calls using single antenna and MIMO configurations, demonstrating that LTE can indeed deliver high peak data rates as targeted by the specifications.³ In September 2009, the group announced the successful completion of its phase-one Proof of Concept tests for both FD-LTE (LTE frequency division duplex mode for deployments using paired spectrum) and TD-LTE (LTE time division duplex mode for deployments where unpaired spectrum is not available). Completion of the phase one testing clears the way for additional trials including interoperability development testing (IODT), interoperability testing (IOT), and friendly customer trials.

Today LTE is well on its way to becoming the first single global standard for cellular communications. It is being adopted by both GSM and CDMA operators alike, and was selected by theNext Generation Mobile Networks Alliance (NGNM) as the first technology to successfully meet its requirements. LTE has been endorsed by3G Americas, GSMA, UMTS Forum, and other industry organizations.

LTE is expected to fulfill the wireless industry's needs for a decade or more. However, to meet the International Telecommunication Union (ITU)'s IMT-Advanced requirements for a true 4G technology, 3GPP is developing LTE-Advanced, defined in Release 10 and beyond. In October 2009, LTE-Advanced was submitted to the ITU as an IMT-Advanced candidate.

Enthusiasm for LTE has grown rapidly as the 3GPP's aggressive development plan nears fruition. A November 2007 report by Juniper Research (www.juniperresearch.com) predicts that the number of LTE subscribers will be greater than 24 million by 2012, just two years after LTE's commercial launch. An October 2009 release from Infonetics Research (www.infonetics.com) puts the number of LTE commercial launches scheduled for 2010 at 14, with the first major deployments getting underway by NTT DoCoMo in Japan and Verizon Wireless in the U.S. The infrastructure market is expected to top $5 billion and the number of LTE subscribers to exceed 72 million by 2013. Analyst firm Visiongain (www.visiongain.com) expects the combined total of LTE and HSPA subscribers to exceed 250 million by 2015.

Although it was initially estimated that more than half of the LTE subscribers would be in Western Europe, in late November 2007 Verizon - one of the largest service providers in the U.S. - announced its intention to develop and deploy LTE as its fourth generation (4G) broadband network. The company sees LTE as the technology that can at last bring together divergent wireless formats on a common-access platform with global scale. Verizon also intends to bridge the wireless/wireline gap by integrating LTE with its FiOS fiber-optic-based platform.⁴ If such goals are achieved and other providers follow the lead, LTE may emerge as a major technology for broadband convergence.

As a so-called 3.9G or 4G technology, LTE will be looking for market share in a field that will likely also include HSPA+, which is an evolved version of 3GPP HSPA; 3GPP EDGE Evolution; and Mobile WiMAX. Of these technologies, Mobile WiMAX has most often been labeled the major rival of LTE. However, LTE continues to gain momentum, even though WiMAX has the advantage of a head start in development, testing, and deployment. Regardless of which format ultimately dominates the market, LTE is expected to be a major force.

The compressed timeline for LTE standards development is reflected in the schedules for LTE product development. As specifications for the LTE radio interface stabilize, equipment manufacturers are working on components for LTE base stations and user equipment. Operators are upgrading their core networks to support LTE, and the first LTE devices have been successfully tested on trial LTE installations. Suppliers of design and test equipment are being challenged to keep pace to provide the tools necessary for these tasks. Because of the newness and complexity of LTE technology, there are a range of engineering design and test challenges to address in the different parts of the LTE infrastructure:

• RF: The variable channel bandwidths specified for LTE increase the system's flexibility and capability but also add to its complexity. The use of multiple antenna configurations and OFDMA to support high data rates adds further complication, although it's expected that by the time LTE products reach the RF testing stage, test engineers will be able to apply lessons learned from implementing MIMO and OFDMA in WiMAX. However, the use of SC-FDMA in the uplink will result in some challenges unique to LTE. With performance targets for LTE set exceptionally high, engineers have to make careful design trade-offs to cover each critical part of the transmit and receive chain.

• Layer 1/Baseband: To support the high data rates that are the goal of LTE, exceptionally large amounts of processing power are needed, particularly in the baseband, where all the error handling and signal processing occurs. Baseband designs will be modeled using PC simulation on both the UE and network sides, and reduced-speed emulation of hardware prototypes is also happening.

• Layers 2/3: LTE Layer 2 is split into three sub-layers, including the Medium Access Control (MAC) and Packet Data Convergence Protocol (PDCP). Design challenges at this layer will be the handling of significant amounts of data in the PDCP and implementation of the 2 ms MAC turnaround time. Layer 3 handles the main service connection protocols. Detailed specifications for both of these layers are still under discussion. Although early product development can be accomplished with simulation of these layers, the integrity of a device design cannot be determined until they are properly integrated with the baseband and RF sections at full operating speed.

Along with LTE-specific challenges are those associated generally with wireless design. Overall system performance depends on the performance of both the baseband and RF sections, and each is associated with particular impairments--for example, nonlinearities and noise figure in an RF up-converter or down-converter, phase and amplitude distortion from a power amplifier, channel impairments such as multi-path and fading, and impairments associated with the fixed bit-width of baseband hardware.

Not the least of all these challenges is the fact that LTE is an evolving technology, and as such is open to change and interpretation. The early availability of conformance tests will help alleviate interoperability issues and provide basic testing. However, from day one of commercial launch LTE must deliver an outstanding user experience in terms of voice quality, quality of data services, and battery life. For that reason comprehensive functional testing and real-world verification of LTE products is essential.

As a world leader in test and measurement solutions, Agilent Technologies is at the forefront of emerging wireless and broadband markets. Agilent has up-to-date, reliable LTE design automation and test solutions that are available today, and Agilent is committed to providing the most complete measurement coverage - from RF to digital - throughout the entire product development cycle.

Agilent design and test products are based on the company's long-established expertise in test and measurement and the knowledge gained through active participation in the relevant 3GPP working groups. Agilent's engineers understand the intricacies involved in designing and testing complex devices for evolving wireless technologies, and they have already produced one of the industry's first and most extensive lines of WiMAX solutions. Now this insight is also being applied to LTE, delivering the same breadth and depth of measurement coverage to product developers working on LTE chip sets, modules, and systems.

In addition to a large portfolio of products for LTE baseband, RF, protocol, and network test and development--including TD-LTE, MIMO, and femtocells--Agilent has an extensive online library of white papers, articles, and application notes, as well as a popular hard-bound technical book authored by LTE experts, to help LTE adopters get their products to market most efficiently and cost-effectively.

While LTE has the potential to enhance current deployments of 3GPP networks and enable significant new service opportunities, its commercial success requires the availability of measurement solutions that parallel the standard’s development.

In the measurement domain, Agilent is leading the way with design automation tools and flexible instrumentation for early R&D in components, base-station equipment, and mobile devices. Agilent, along with its partners, plans to provide a broad, comprehensive portfolio of solutions that address the entire product development life cycle – from early design through production test and deployment. LTE may have many challenges, but with early and powerful test equipment solutions, the LTE challenge can be met.

1) 3GPP Technical Report 25.912 v9.0.0 (2009-09)

2) 3GPP Technical Specification 3GPP TS36.300 V9.1.0, Figure 4-1

3) See Agilent Measurement Journal, Issue 3 [www.agilent.com/go/journal], for a detailed discussion of the “real-world” issues surrounding high data rates in cellular systems.

4) Fitchard, Kevin, “LTE--It’s Not Just VZW’s Network, It’s Verizon’s,” Telephony Online, December 4, 2007. Available at http://telephonyonline.com/wireless/news/lte_verizon_vzw_120307/.