Understanding LTE

February 11, 2008

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). Both LTE and HSPA+ are being developed in Release 8 of the 3GPP specifications, which also include work on the evolution of EDGE, a technology based on the GSM air interface.

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.

Technology Evolution

Unlike UMTS, which is based on Wideband Code Division Multiple Access (W-CDMA) technology, LTE is based on OFDMA and in this regard, LTE 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 modulation format 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 new 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.

LTE Timeline

Recognizing the need to support an insatiable demand for bandwidth while reducing the cost per bit and improving service provisioning, 3GPP in late 2004 put LTE development on a fast track. The first technical specifications for LTE radio access were approved in late 2007. Final approval of the remaining specifications will occur in the first half of 2008 with the initial conformance test specifications scheduled for September 2008. Operators and equipment vendors have started to announce their timelines for LTE rollout, and they are planning the first equipment shipments for 2009 with the first commercial deployments to begin in 2010.

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. In late 2007 the first multi-user tests in an urban setting achieved mobile rates above 170 Mbps. The success of these tests demonstrates that LTE can indeed deliver high peak data rates as targeted by the specifications.³

Market Impact

Enthusiasm for LTE has been growing 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. Analyst firm Visiongain (www.visiongain.com) expects the combined total of LTE and HSPA subscribers to exceed 250 million by 2015. Although it’s been estimated that more than half of the LTE subscribers will be in Western Europe, in late November 2007 Verizon – the second largest service provider 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 platform⁴. 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 could include the following: HSPA+, which is an evolved version of 3GPP HSPA; 3GPP EDGE Evolution; 3GPP2 Ultra-Mobile Broadband (UMB), which is an evolution of CDMA2000 and 1xEV-DO; and Mobile WiMAX. Of all these technologies, Mobile WiMAX is considered by many observers to be the major competitor. While LTE is gaining momentum and is a natural evolution of the established GSM-UMTS cellular legacy, WiMAX technology 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.

Design and Test Challenges

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 have begun to work on components for LTE base stations and user equipment. Operators have to upgrade their core networks to support LTE, and they undoubtedly want to have user devices available in quantity at the time of commercial launch. Suppliers of design and test equipment have 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 2ms 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.

Solutions

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 we are 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, with the promise of delivering the same breadth and depth of measurement coverage to product developers working on LTE chip sets, modules, and systems.

Design Simulation: The Agilent 3GPP LTE Wireless Library provides signal processing models and preconfigured simulation setups for the Agilent Advanced Design System (ADS) electronic design automation software. Using these tools, product developers can create spectrally correct test waveforms that comply with 3GPP requirements for LTE. The models and simulation test benches can be used as  “golden references” for simulation and verification of baseband algorithms, digital baseband/IF systems, and verification RF circuitry used in LTE designs. The LTE Wireless Library also can be imported into Agilent's RF Design Environment (RFDE), allowing RF integrated circuit designers to access LTE test benches within the Cadence Virtuo Custom IC platform.

Waveforms created with the LTE Wireless Library comply with the latest LTE specifications. They can be used, for example, to measure error vector magnitude (EVM), peak-to-average power ratio (PAPR), and adjacent channel leakage ratio (ACLR) performance of system RF components such as power amplifiers, antennas, and filters.

Design Verification: Unique to Agilent’s offering are the LTE Connected Solutions, which combine instrument functionality with the ADS simulation tools and provide early access to evolving LTE signals. A product developer can test a hardware device within a simulated design by downloading the LTE signals created using the ADS Wireless Library into an Agilent ESG or MXG vector signal generator, which produces the real-world, physical test signals. Output from the device under test can be captured with an Agilent MXA signal analyzer, PSA spectrum analyzer, or logic analyzer and then post-processed using the ADS LTE Wireless Library.

Uplink and Downlink Signal Generation: Agilent Signal Studio is PC-based signal creation software that will cut the time spent on uplink and downlink signal generation. The software provides an Agilent-validated, performance-optimized reference signal to better characterize, evaluate, and fine-tune designs under parametric and functional test conditions. Agilent Signal Studio software for 3GPP LTE configures coded physical layer LTE test signals to verify RF performance in the uplink and downlink by measuring EVM, ACLR, and CCDF. When used with the Agilent MXG signal generator, it provides the industry’s best ACLR performance for the characterization and evaluation of BTS components such as multi-carrier power amplifiers. It can also be used with the Agilent ESG signal generator early receiver test and UE component test that requires lower phase noise, excellent level accuracy, better ACLR, fading capabilities, and digital I/Q inputs and outputs. 

Uplink and Downlink Signal Analysis: The complexity of LTE systems requires signal analysis with in-depth modulation analysis as well as RF power measurement. Agilent signal and spectrum analyzers measure complex LTE signals with world-class accuracy and repeatability. They can be used with the high-performance Agilent Vector Signal Analysis (VSA) software, which provides RF and baseband engineers with the industry’s most comprehensive, up-to-date LTE signal analysis based on the 3GPP standard. The software provides downlink and uplink measurement capability in a single option; measures all LTE bandwidths and modulation formats; and, with the PSA high performance spectrum analyzer, delivers industry-leading EVM of < -50 dB (< 0.35%).

Baseband Analysis: On LTE user equipment, communication between the front end and baseband occurs over a digital bus, which may be serial or parallel. Special signal analysis and signal generation tools are needed to properly characterize this digital interface. By combining an Agilent logic analyzer with signal analysis and signal generation tools, designers can comprehensively characterize the behavior of their systems from baseband to antenna. The logic analyzer provides a physical connection into the circuit, while the signal analysis software interprets the data from a wide range of measurements to be analyzed and displayed.

The logic analyzer also can be used with the VSA software, creating the industry’s only Digital VSA (DVSA) package for digital baseband, IF and RF signal analysis. With this software, digital signal processing (DSP) designers can design and debug interfaces that once were analog and now are digital. The VSA software performs functions such as I/Q analysis, EVM, and Fourier spectrum analysis on the digital signal.
The Agilent N4850A digital acquisition probe and N4860A digital stimulus probe operate with Agilent 16800 and 16900 Series logic analyzers, providing digital acquisition and serial stimulus capabilities required for DigRF v3 based IC evaluation and integration. The integration of DigRF v3 logic analysis tools with the Agilent RF portfolio provides cross-domain solutions for rapid deployment of DigRF v3-based designs.

Real-time Digital Decode and Debug: Agilent Infiniium DSO90000A Series High Performance Real-Time Oscilloscope provides superior signal integrity and deep application analysis so that engineers can quickly debug and characterize digital systems. Applying Agilent’s RF design expertise, proprietary packaging technologies, and unique CMOS ADC architecture, the Infiniium scope offers the industry’s lowest noise floor. The InfiniiScan Plus event identification system is based on the world’s fastest hardware trigger system and can identify glitches faster than 250 ps. No other oscilloscope provides this level of trigger accuracy. With more than 29 applications, the Infiniium 90000A verifies application compliance and debugs the most difficult electronic designs in the shortest possible time.

UE Development Solutions Platform: The Agilent E6620A Wireless Communications Test Set provides a scalable, advanced platform for developing LTE user equipment. As the specifications are released, the test set will support the development process from initial protocol development through RF and protocol conformance test, functional test, and interoperability test (IOT). The E6620A will use the same 3GPP-compliant LTE protocol stack across all solutions to provide consistency leading to shorter design cycles and the highest quality designs.

Protocol Development: The complexity of LTE means that the importance of protocol development cannot be over-emphasized. New handset designs must meet the expectations of the consumer and the standards bodies, which means carrying out earlier and more comprehensive development, design verification, and regression testing. Agilent has partnered with Anite to offer versatile but rigorous testing solutions. The Anite SAT LTE solution analyzes UE product designs early in the development process, simulates and tests a broad range of protocol functionality, and assures that even the most advanced products will meet or exceed industry certification and quality requirements.

The Anite SAT LTE Development Toolset (DT) using the Agilent E6620A is a comprehensive suite of tools that supports all phases of UE development, from pre-silicon protocol module development through system integration and verification, to shorten development times and validate confidence in designs.  

UE Protocol Conformance Test: In the wireless industry, Agilent and Anite offer proven conformance test solutions to ensure the performance of the protocol components of a handset. Anite’s conformance toolset solutions, based on the Agilent E6620A, incorporate comprehensive campaign management and analysis tools to assess the quality of handsets under evaluation. They provide extensive automation and a remote-control interface to help maximize test throughput and are used throughout the product lifecycle for integration, conformance and certification testing of handsets. Today these solutions support a wide range of radio technologies including GSM, EDGE, W-CDMA and HSPA. When the LTE conformance specifications are published, Agilent and Anite will be ready with a standards-compliant solution.

RF Conformance Test: The RF conformance test specifications for LTE will be defined by the 3GPP towards the end of 2008. They will cover the following measurement areas:  transmitter requirements, receiver requirements, performance requirements, and radio resource management. When these conformance specifications are published, Agilent will be ready with standards-compliant system solutions.

Network Protocol Signaling Analysis: The Agilent Signaling Analyzer software platform is adding LTE and SAE technology support. Together with a new high density probing solution, the Signaling Analyzer software will enable passive probing and analysis of LTE network interfaces, including S1, X2, S3, S4, and S5. The powerful combination of distributable hardware pre-processing with scalable software architecture meets current and future performance needs to ensure a successful deployment of integrated LTE/SAE network systems.

Conclusion

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.

Footnotes:
1) 3GPP TR 25.912 v7.2.0

2) From 3GPP technical specification, 3GPP TS36.300 V8.2.0

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/.