Chipset vendors, mobile-device manufacturers, and telecommunications companies are all working to deploy high-bandwidth LTE networks while maintaining compatibility with existing 3G and 2G networks. As LTE rolls out, these organizations will need capable test equipment that ensures the quality of the products and services they are developing. An understanding of the challenges involved can help you select test equipment that is up to the task.
Jung-ik Suh, wireless program manager at Agilent Technologies, lists three key challenges that Agilent customers are facing. The first relates to design and simulation for complicated LTE systems. The second stems from LTE’s use of enhanced technologies such as MIMO and carrier aggregation to provide faster and more stable wireless data rates as well as higher spectrum efficiencies. Adding to this challenge, he said, “LTE developers are required to make their LTE designs and products work with existing 2G and 3G wireless technologies to solve LTE coverage issues.”
As for the third challenge, “Our customers have greater pressure for earlier time-to-market to compete against fierce competition in the wireless industry. Reducing manufacturing time and cost is a key challenge to solve.”
8100 Mobile-Device and Chipset Test System
Courtesy of Spirent Communications
Erik Org, senior marketing manager at Azimuth Systems, agreed that MIMO presents a key challenge: “The use of the MIMO technology in 4G/LTE has resulted in one of the most profound changes in the way that mobile devices are tested.” He noted that, because MIMO performance is heavily dependent on the real-world time-varying characteristics of radio propagation channels, devices need to be tested throughout the product design and development cycle using real-world conditions.
Delivering Network Capacity
Per Kangru, worldwide LTE business development manager at JDSU’s Communications Test and Measurement business segment, said, “Mobile operators face diverse challenges in 4G/LTE trials, deployment, and service assurance” as they work to deliver sufficient network capacity and end-user quality of service while ensuring interoperability among multiple vendors. “Network equipment vendors must work fast to develop, test, and deploy their products,” he continued, adding that they need the right tools to be successful under challenging time constraints.
“Growing competitiveness in the network equipment market often leads to fragmented networks,” Kangru added. “As a result, radio-access network vendors face the challenge of working with existing core and Global System for Mobile Communications/Universal Mobile Telecommunications Service vendors to ensure a full end-to-end solution is available.”
Raajit Lall, product manager for RF and wireless test at National Instruments, sees software as a key technology for meeting the test challenges of constantly evolving LTE. “Test engineers struggle to find high-performance analyzers and generators that can keep up with the constantly changing requirements of LTE,” he said. “The NI PXI platform utilizes a software-defined approach where the hardware is abstracted from the cellular/data standard. All the modulation, demodulation, and processing capabilities are implemented in software on a PC, allowing engineers to field-upgrade their test equipment to the latest LTE standard without having to swap hardware or upgrade firmware. This abstraction of hardware from software allows test engineers to quickly adopt changes in cellular standards.”
Chipset Issues
Some specific issues related to LTE rollout emerged in research that the Signals Research Group conducted recently using the Spirent 8100 Mobile Device Test System to run independent data-performance benchmark tests of commercially available LTE chipsets.1 The test involved chipsets from four different vendors deployed in five commercial devices from LG, Sierra Wireless, Novatel Wireless, Samsung, and Fujitsu. Although the SRG report highlighted the capability of some chipsets to realize the promise of LTE’s high data rates, large performance differences were found between commercially available implementations in devices.
Nigel Wright, vice president of wireless marketing at Spirent, said his company has been working with Signals Research Group since the very early days of UMTS and on through the various evolutions of HSPA. The key advantage of LTE—its much higher data rates—leads to some significant differences in performance measurement compared with earlier standards.
Noted Wright, “It was obviously interesting to see how well some of the early commercial devices delivered [with respect to data rate] under at least the most representative set of real-world conditions that the industry has so far agreed on, because the role that devices play in the data performance that subscribers experience is very much underestimated. Everyone thinks, ‘Oh yes, there are bottlenecks in the network and the backhaul,’ but the actual design of the device and, to significant extent, the chipsets also has a pretty substantial bearing on how the end-to-end data performance is perceived by consumers.”
Wright said that with the exception of North American operators, service providers generally have been tolerant of any chipset that passes conformance tests, saying, in effect, “It’s good enough; it won’t bring the network down.” That view is diminishing as LTE rolls out. “LTE is different because operators realize the subscribers are looking for enhanced performance,” Wright explained.
TM500 LTE Test Mobile System
Courtesy of Aeroflex
That is one factor pushing operators to demand more performance-oriented test. Another is the desire for operators themselves to obtain high performance under real-world conditions. From a network-operator point of view, a device that works poorly can waste network resources.
Suppose, explained Wright, a subscriber’s device says, “’I’m in a good signal situation—give me the fat pipe, I’m ready.’ Then if the device performs poorly, you can get really high error rates, so all the network resources that the operators dedicate to that subscriber, or a significant proportion of them, are wasted. If you’ve got millions of poorly performing devices on your network vs. millions of good performing devices, that can really make a big difference to an operator’s costs in terms of the amount of resources it has to build out to support those devices.”
And in fact, said Wright, commenting on Spirent’s testing in conjunction with Signals Research, “One of the biggest issues we found was the chipset misrepresenting the channel reception conditions back to the network,” leading to wasted network resources. He added, “If a chipset isn’t accurately reporting its receiving conditions back to the network, and then the device fails to properly demodulate what the network sends, you get lots of errors. As a result, the actual application-layer data rate that the subscriber experiences goes way down.”
The relative immaturity of LTE deployments is the reason for the big differences that you see between the headline numbers and real-world performance. “In a 10-MHz channel, theoretically you can get over 70 Mb/s as we demonstrated when testing devices on a completely clear channel with no interference,” said Wright. “That’s great, but then you start looking at real-world conditions—you bring in interference, noise, multipath—and in many cases, the throughput can drop dramatically to 20 or 10 or even less, which still is very respectable but dramatically less than the headline rates would have you believe.”
Wright continued, “It was a surprise to us that things behaved as well as they did. We didn’t see huge numbers of problems with drivers—certainly not in the limited test cases that we ran, and we didn’t see many problems with devices dropping calls, for example. So this was significantly better than some of the earlier generation changes with UMTS to HSPA and HSPA+ where some of the early devices exhibited some pretty flakey behavior.” With LTE, he said, “By and large, things were more stable than we expected.”
LTE Test Equipment
The list of test equipment that can help meet the challenges of LTE includes, of course, the Spirent 8100, used in conjunction with the Signals Research testing. The 8100 supports a range of application, location, and radio access test areas, and its flexible option-based design allows customers to add capabilities. Spirent also offers the CS8 emulated Evolved Packet Core (EPC), which supports design verification testing (DVT) in the mobile device design lab. CS8 addresses RF and baseband development, radio protocol testing, and system- or platform-level development testing.
Other LTE equipment includes the Aeroflex TM500 LTE Test Mobile System and the EAST500 LTE Capacity Test System. Nicola Logli, product manager at the Aeroflex Stevenage division, cited a number of recent enhancements to the TM500 and milestones it has reached. The system now offers category 5 support as well as support for 4×4 DL MIMO and 64QAM modulation in the UL, providing 300-Mb/s operation in the DL and 75 Mb/s in the UL.
Several release 9 headline functionalities have been introduced, Logli said, including LTE positioning, release 9 dual-layer beamforming, and eMBM (enhanced multimedia broadcast multicast) support. In addition, Aeroflex has released an advanced receiver for neighboring cell interference cancellation (in the TM500 SUE FDD system) and demonstrated an LTE-Advanced carrier aggregation prototype. He added that the number of UE devices per cell has reached 200.
According to Lall at NI, National Instruments announced support for LTE-FDD hardware and software in 2010 and recently introduced the NI PXIe-5665 Vector Signal Analyzer, which provides ACLR and EVM measurement capabilities for LTE. In addition to FDD, NI also provides software support for LTE-TDD, allowing customers to utilize NI generators and analyzers for complete LTE testing.
Lall said NI primarily focuses on UE testing for mobile devices, power amplifier testing for chip vendors, and transceiver testing for both characterization and production environments. He added that NI’s approach enables engineers to use the same hardware for both lab and manufacturing test, making correlation and analysis of test data from both environments easy. “The flexibility of the PXI platform and NI software also allows the same equipment to be used for a variety of different tests, including base-station testing, custom or unusual tests such as frequency domain triggering, and long-duration tests with record and playback capabilities,” he explained.
Agilent offers a variety of products for LTE test. Agilent’s Jung-ik Suh said the company introduced LTE-Advanced design software plus signal analysis and signal generation solutions in early 2011. The company has added various LTE design and test solutions, including the new EXT wireless test set, which reduces customers’ manufacturing test time and cost. Also new is the 89600 WLA (Wireless Link Analysis) software, which, said Suh, “gives system-integration engineers and verification engineers greater visibility into the increasingly complex and dynamic interaction between MAC and physical layers in LTE designs.”
89600 WLA Software
Courtesy of Agilent Technologies
Agilent’s MIPI M-PHY Transmitter Compliance Test Software supports a range of the MIPI Alliance’s M-PHY specifications and conformance test suites. The company also offers a new wideband DPD (digital pre-distortion) modeling platform for LTE-Advanced. In addition, said Suh, Agilent provides various upgrades and enhancements for existing products, enabling, for example, Inter-RAT handover test with the PXT and 8960 Wireless Test Sets to help customers make LTE products compatible with 2G and 3G wireless technologies.
Agilent supports VoLTE test with its PXT Wireless Communications Test Set and offers its N6700 Modular Power System to help solve critical battery-life issues. It also has wider bandwidth on its X-series Signal Analyzers to get up to 160 MHz on the PXA to provide bandwidth analysis capabilities beyond LTE-Advanced’s 140-MHz bandwidth and to cover 802.11ac technologies.
Anritsu’s MD8475A Signaling Tester, according to Paul Innis, senior manager for wireless solutions, enables testing of wireless applications for the newest 4G/LTE networks and builds on existing 2G/3G capability offered by the company’s MD8470A Applications Tester. “The MD8475A contains a flexible hardware architecture for multi-base-station capability, allowing a user to conduct a wide range of application/mobile-user stress-test scenarios under a wide variety of real-world wireless network conditions, including 3/4G handovers, high-speed data transfers, web browsing, multimedia streaming, IMS, and other data-intensive applications,” said Innis. “The MD8475A is well suited as a one-box, multilayer application network simulator for R&D, integration, and UE performance applications.”
Noted Innis, “The MD8475A’s SmartStudio user interface enables efficient setup of wireless network simulations by eliminating the need to edit complex scenarios, resulting in extensive timesavings in test-sequence completion.” In addition, Anritsu offers the ME7834A/L, a scalable system for protocol conformance and carrier-acceptance testing of 2G, 3G, and 4G/LTE wireless technologies. Wireless device OEMs, carriers, test houses, and other vendors of and participants in the device and carrier ecosystems are able to use this system to quickly and easily determine a device’s conformance to 3GPP and carrier-specific standards, Innis said.
For LTE deployment, installation, and maintenance, Anritsu introduced the MW8209A PIM Master, a passive intermodulation analyzer that covers the 900-MHz band to address the growing need to measure PIM in E-GSM networks including UMTS Band VIII and LTE Band 8. Designed for use with Anritsu handheld analyzers, the MW8209A comes with a proprietary distance-to-PIM function that helps field engineers, technicians, and contractors pinpoint PIM faults, eliminating the unknown of whether the PIM source is from the antenna system or surrounding environment.
Per Kangru at JDSU cited a variety of products and initiatives at the company to support LTE test. Customers, he said, can detect and diagnose service-impacting issues up to 10 times faster with JDSU accessLTE, which enables simplified real-time monitoring of LTE network and service performance. Telstra subsidiary CSL has selected accessLTE to support its LTE/DC-HSPA+ network deployment in Hong Kong, Kangru said.
Other offerings include JDSU SART 7.1, which provides real-time capture and analysis at multiple gigabit data rates. The company also has announced a TDD-LTE drive-test system with support for Altair chipsets. In addition, JDSU released an LTE smartphone app2 for Android phones that allows engineers to perform real-life test scenarios as LTE networks are being rolled out. And finally, Kangru said, JDSU is participating in Voice over LTE trials.
Courtesy of Anritsu
MIMO OTA Test
For its part, in 2011 Azimuth Systems announced its fully automated, end-to-end Real World Performance Measurement (RPM) test solution, which supports MIMO over-the-air testing and features a modeling framework for enhanced realism, according to Org at Azimuth. The RPM solution combines a compact mobile isotropic chamber with the ACE MX MIMO Channel Emulator to create an environment where devices can be tested in the same mode as they are used in real-world conditions.
The company also introduced enhancements to the ACE MX MIMO Channel Emulator to enable users to test 3GPP carrier aggregation (R10 and R11) scenarios. The ACE MX offers support for higher-order MIMO topologies, beamforming, and complex transmission schemes. In September 2011, Azimuth released its latest version of the ACE MX Channel Emulator with upgraded features for its Director II Management Software.
Azimuth announced in November that Anite is delivering a complete turnkey LTE performance test system that combines the Anite SAS Network Simulator for device interoperability testing, the Anite 9000 Mobile Test Accelerator Platform, and Azimuth’s ACE MX Channel Emulator. Customers will benefit from a new reseller agreement between Anite and Azimuth, enabling them to purchase the complete offering from one source and have a single point of contact for the sales and the installation process.
And Rohde & Schwarz has a variety of products tailored for LTE: The R&SCMW500 Wideband Radio Communication Tester with an LTE signaling function provides network emulation for call processing (L3 signaling); the R&S CMW500 HSPA+ and LTE Protocol Tester offers accurate and comparable results throughout early R&D, interoperability testing, conformance testing, and production; and the fully automated R&S TS8980 LTE RF Test Systems cover RF test needs from R&D up to conformance tests of LTE mobile devices. In addition, software updates can add LTE test functionality to the company’s platforms for signal generation and spectrum and signal analysis.
Rohde & Schwarz also offers up to 4×2 or 2×4 MIMO testing with real-time fading. Finally, the R&S TSMW Universal Radio Network Analyzer and the R&S ROMES Drive Test Software Platform support network planning, installation, optimization, and monitoring.
Work to be Done
Initiated in 2004, the LTE standardization effort progressed to the first approved technical specifications in 2007, and the first commercial LTE network was launched in Sweden and Norway in December 2009. By the end of October 2011, 35 commercial networks had launched,3 and rapid progress is expected to continue.
Concluded Wright at Spirent, “The work to be done over the next year or two is narrowing the gap between headline rates and what you experience in the real world.” That work will involve meeting the big optimization challenges and involve device and chipset vendors as well as the operators and infrastructure manufacturers as they fine-tune the algorithms that they use to assign all the network resources. Moreover, test-equipment vendors will continue to enhance their offerings to support operators as well as chip, device, and infrastructure manufacturers.
References
1. “Chips and Salsa XIV: The World’s first LTE Chipset Performance Benchmark Results,” Signals Ahead, Signals Research Group. Dec. 15, 2011.
2. Nelson, R., “Mobile Apps Support Communications Test, Data Acquisition,” EE-Evaluation Engineering, January 2012, p. 20.
3. Kottkamp, M., et al., “LTE Release 9 Technology Introduction,” White Paper, Rohde & Schwarz, December 2011.