Test Solutions Soup Up for 4G

Characterization of digital communication systems and related RF hardware relies on complex test systems. These systems play a critical role in design and production performance evaluation. The building blocks of a digital communication system often require simultaneous time and frequency domain analyses, blurring the lines between the dedicated functions associated with traditional stand-alone scopes and analyzers. Designers must also investigate nonlinear behavior and the response to infrequent events. In turn, test and measurement equipment is evolving dramatically. Test equipment manufacturers are providing more capability by relying on the latest hardware and software for feature-rich and easy to use solutions with impressive raw performance. To understand how test equipment is evolving, Microwave Journal editors talked to representatives from Agilent Technologies, Anritsu, Keithley Instruments, Rohde & Schwarz and Tektronix. The following are some of their impressions on the state of RF test solutions in 2009.

Test Challenges

Release 7 of 3GPP has been designed as an upgrade for existing HSPA networks, and is sometimes called “HSPA+”, or “evolved HSPA”. The release introduces MIMO and 64QAM to increase data rate of the air interface. Beyond Release 7, 3GPP is now developing the standards for a new mobile network. Called Long Term Evolution (LTE) and System Architecture Evolution (SAE) for next generation mobile networks, this is the next step in the continuous move to wider bandwidth and higher data rates.

We asked several vendors to comment on how new technologies such as MIMO, LTE or DigRF were impacting RF and microwave testing. All agreed that the rapid pace of technology has required greater collaboration between vendors, chip set manufacturers and technical specification groups. Test solutions are more comprehensive (integrated) than the single metric test equipment of the past. Software also plays a key role in providing functionality, up-to-date standards compliance and ease of use.

Roland Steffen of Rohde & Schwarz remarked that, “Compared with existing technologies such as WCDMA or GSM, standards such as LTE and HSPA+ place more stringent requirements on test and measurement equipment, due to more complex modulation methods (64QAM), multi-antenna systems (MIMO – multiple input, multiple output) and expanded layer-1 configuration capabilities. Processes are becoming more complicated in the protocols as well. Test equipment has to meet these requirements. Furthermore, LTE is a new technology, and the specifications are still evolving. A close working relationship between the test equipment vendor, wireless device manufacturers and chipset designers is required to synchronize feature implementation, reference specifications and timely delivery of the test equipment software.”

One example of this occurred last September when Rohde & Schwarz and Beceem Communications Inc. announced their collaboration to provide a custom test solution for Beceem’s mobile WiMAX chipsets. The goal of this partnership was to provide a fast, easy-to-handle test solution for all WiMAX equipment manufacturers using Beceem chipsets. The R&S CMW270 WiMAX communication tester provided the all-in-one solution with a continuous frequency range for all WiMAX band classes up to 6 GHz, real-time signaling, a VSA for transmitter verification and VSG for expanded receiver testing. This single box unit was demonstrated with the Beceem chipset at this past year’s WiMAX World in Chicago, IL. Test time optimization was given priority in the CMW270 platform architecture. Multi-evaluation measurements and parallel Tx/Rx test algorithms reduced chipset configuration and handling times. Internal switching between RF channels further improved RF calibration of multi-antenna devices (MIMO) by eliminating external adaptation. The unit also demonstrated a non-signaling mode for RF alignment in production and a signaling mode (base station emulation) for testing under simulated network conditions.

For analyzing MIMO and LTE signals, Rohde & Schwarz’s signal analyzers (R&S FSQ, R&S FSG as well as R&S FSV models) enable detailed testing of the physical layer. The signal analyzers can be equipped with options for LTE and MIMO allowing simultaneous processing of signals transmitted by the multiple antennas. They can also be used to determine whether pre-coding for LTE signals works across the different stages. R&D engineers can observe the performance of both the baseband and RF bands. The analysis software also ensures that engineers, developers and technicians can respond quickly to new developments of the standard.

R&S also noted that it has added MIMO pre-coding and channel coding to its LTE signal generators. The company has demonstrated a 2x2 MIMO receiver test using its R&S SMU200A RF signal generator as a one-box solution, already including real time fading capability. For baseband testing, the R&S AMU200A baseband generator and fading simulator is available. The combination supports MIMO test for LTE, WiMAX, WLAN and HSPA+.

In addition to its existing RF portfolio, Rohde & Schwarz unveiled its initial solution for protocol tests (layer 1 to layer 3) for 3GPP LTE at last year's Mobile World Congress in Barcelona, Spain. The R&S CMW500 (see Figure 1) for LTE and HSPA+ protocol tests offers multiple interfaces for communicating with the protocol layers to be tested in the wireless device (DUT). Depending on the DUT’s level of integration, a purely software interface, a digital I/Q interface or an RF interface can be used. Tests carried out with the RF interface require a protocol implementation that has been fully integrated into the baseband and the RF. The interface used is transparent for the protocol layers above and the tester’s software modules. The user can therefore switch between interfaces without having to modify test routines and software tools.

Carla Feldman, Wireless Business Unit Marketing Manager at Agilent Technologies, responded to our question by stating that, “There is no doubt that both 3GPP LTE and WiMAX have, and will continue to have, a significant and lasting impact on the communications industry in 2009. R&D spending in LTE, for example, has already begun ramping up from chipset development to components and integrated hardware. The same can be said of the technologies that go into making a LTE system work such as MIMO and DigRF V4—the latest version of the DigRF electronics interface standard for the cellular market.”

Feldman used the complexity of MIMO technology to illustrate Agilent’s solution to one particular test challenge. “(To) ensure this technology’s optimal operation requires the engineer to accurately test the MIMO receiver—a challenging task given the large combination of variables that must be tested in a given MIMO configuration. A critical part of testing a MIMO receiver is replicating real-world conditions and channels and performing real-time fading of MIMO signals. Agilent’s PXB MIMO Receiver Tester provides up to 4 baseband generators, 8 faders, 120 MHz bandwidth, custom MIMO correlation settings, and supports testing and troubleshooting of 2x2, 2x4 and 4x2 MIMO. Signal Studio signal creation software runs in the instrument and provides the engineer with up-to-date standards-compliant signal creation.”

The Agilent PXB (see Figure 2) is a recent advance in MIMO receiver testing marrying the signal source, noise source and fader together in a fully-integrated solution to successfully accomplish these tasks. Rather than offering signal generation only, this measurement approach offers an exceptionally versatile platform for testing LTE and WiMAX receivers. It not only allows the engineer to replicate real-world MIMO conditions and channels, but also to generate realistic fading scenarios including path and channel correlations.

With regard to DigRF, Feldman noted that, “Agilent recently introduced the industry’s first Digital Radio Frequency (DigRF) V4 test solution. It enables comprehensive stimulus and analysis for developers of radio-frequency integrated circuits (RFIC) and baseband ICs as well as integrators of wireless handsets. ‘Cross-domain’ test, such as DigRF V4, offers new insights that reach from individual digital bits all the way through to IQ-modulated RF signals. Agilent’s test solution allows engineers to work in the domain (digital or RF) and abstraction level (physical or protocol layers) of their choice to quickly characterize RFICs and rapidly solve cross-domain integration problems.”

Engineers at Keithley Instruments pointed out that, “LTE and WiMAX both use the same underlying technology—OFDM (orthogonal frequency-division multiplexing) and MIMO. While this has some advantages, in terms of similar radio architectures and the ability to interchange product development engineers easily between the two types of technology, a number of differences must be considered. Base stations offer the highest opportunity for a common platform, since the cost, size and power requirements are far different from the handset, or “user equipment.” A common 20 MHz IF can be employed, combined with the multiple RF systems required for the different transmission bands specified for WiMAX and LTE and the multiple streams required for a MIMO transmission. A handset presents a number of challenges because cost, size and power performance are critical for market acceptance. LTE has been optimized for power amplifier efficiency transmitting SC-TDMA; while a WiMAX UE transmits OFDM, which requires a higher cost and more power-hungry amplifier, it has a smaller signal processing footprint.

With respect to test, two approaches could be used. It’s possible to measure the products to the documented specifications. The software-defined radio architecture in instruments such as the Keithley Model 2820 RF Vector Signal Analyzer and Model 2920 Vector Signal Generator fully support standards-based test for both LTE and WiMAX, using a single hardware platform (see Figure 3). Or, one can take advantage of the power of this type of test equipment architecture and characterize the RF performance in terms of magnitude and phase quality. This would provide a generic way to derive specifications such as WiMAX or LTE modulation quality. When MIMO comes into play, one will need a generic indication of the isolation between each RF module or unit as well. The test equipment needs to cover the required frequency range and transmission bandwidth, such as a frequency range of 400 MHz to 6 GHz with a bandwidth of 20 MHz. If it’s necessary to add WLAN (802.11n) into the architecture, 40 MHz of bandwidth would be required. This approach may provide very fast test times and a higher utilization of test equipment, but it requires an investment in time and resources to be able to verify that the product meets both its regulatory and interoperability requirements.”

Anritsu Co. supports HSPA+, LTE, MIMO and DigRF through flexible cost-effective solutions that take advantage of customers’ existing investments, an important aspect in today’s market, according to Wade Hulon, Vice President and General Manager of Anritsu’s Americas Sales and Marketing.

“In our present economic environment, companies must walk the tightrope of continuing to develop 4G products while controlling costs. Our test platforms allow customers to easily integrate hardware and software so they can address emerging technologies such as LTE, MIMO and DigRF,” said Hulon.

The MS269xA Series is an example of that philosophy. The company recently introduced three software packages for the signal analyzers to address LTE testing: the MX269020A LTE downlink measurement software, the MX269021A uplink measurement software and the MX269908A LTE IQ producer. Last October, Anritsu introduced a test platform consisting of the MS269xA equipped with a vector signal generator, a DigRF interface and measurement software. As a single-instrument alternative to existing DigRF test systems, the platform reduced bench space, cost, and time to accurately evaluate transmit and receive performance of DigRF RFICs.

LTE support is also provided by Anritsu’s MD8430A, the first LTE base station simulator. The MD8430A allows manufacturers of LTE chipsets and mobile devices to quickly, accurately and cost-effectively evaluate their products and improve time to market. The MD8430A can be used with the MF6900A, a fading simulator, to create an LTE physical layer and protocol test solution.

Additionally, the MD8430A (see Figure 4) can be combined with Anritsu’s MD8480C W-CDMA Signalling Tester and used for the simulation of both W-CDMA/HSPA/HSPA Evolution and GSM/GPRS/EGPRS base stations. This combination supports LTE-UTRAN/GERAN Inter-RAT handover tests and maximizes customers’ investments in existing hardware. The MD8480C, along with the MT8820B Radio Communications Analyzer, MS269xA and MG3700A Vector Signal Generator, all support the evolution of HSPA Release 7, including 64QAM and MIMO.

Advances in Test Equipment

We asked vendors what they felt was among their most significant new products and technologies and how it was impacting test and design. Several of the manufacturers pointed to advances in raw performance, speed, upper-frequency range and display technology as key enablers to more capable testing. Among these advances, recent improvements in fast triggering, digitizer performance and deep memory are also making oscilloscopes a powerful tool for studying how components respond to wideband signals.

Digital Oscilloscopes

Agilent’s Carla Feldman remarked that, “RF engineers are increasingly using oscilloscopes in addition to signal analyzers for analyzing both the growing digital content in their radio designs along with direct radio signal analysis especially in the emerging ultra-wideband (UWB) measurements. Technologies such as wireless USB, pulsed radar, WiMedia, ‘60 GHz’ UWB and other UWB technologies can couple the native wideband capabilities of an oscilloscope with vector signal analyzer software for powerful modulation domain measurements. We address this market opportunity by coupling our low noise (high dynamic range) oscilloscopes, the industry’s deepest memory and our powerful 89601A VSA Software.”

Software provides the functionality and versatility for today’s test equipment to maintain compliance with evolving standards and to improve engineering productivity. The Infiniium supports the installation of third-party software packages on the Windows XP Pro operating system such as Excel, LabVIEW, Agilent VEE, MATLAB and anti-virus software. Test engineers can perform customized processing and automation. In addition, Agilent recently announced the U7239A MB-OFDM PHY test software, which automatically configures the Infiniium 90000 oscilloscope for each test and generates an informative HTML report at the end of the test. It compares the results with the specification test limit and indicates how closely the device passes or fails each test. The complex analysis runs seamlessly within the scope, which saves users time and effort compared to making and analyzing measurements manually.

Agilent’s DSO/DSA 90000A family of oscilloscopes introduced in March 2008 offers industry-leading noise performance along with the first ever 1 GB deep memory to a family of high performance oscilloscopes having bandwidths from 2.5 to 13 GHz. According to Agilent, these scopes offer a low noise floor (147 µVrms at 5 mV per division - 2.5 GHz model), deep memory, fast hardware trigger system, fast off-load speed (remote), large front-panel display and fast measurement speed. All these features are designed to provide engineers with better insight through superior signal integrity and debugging capability.

By providing ever larger amounts of acquisition memory, engineers are able to capture an ultra-long record of signals, such as 25 ms of signal data at 40 Giga-Samples per second (GS/s). Using deep memory with peak detect allows engineers to capture infrequent glitches. MegaZoom III is the third generation of the fast and deep memory architecture introduced by Agilent in 1996. It combines fast, responsive deep memory with a high-resolution display system to make it easier for engineers to find elusive signal anomalies. The new InfiniiScan Plus event identification system is based on the fastest hardware trigger system. This new trigger system can identify glitches faster than 250 ps. Infiniium 90000A will offer the only three-level trigger system, combining multiple hardware triggers with the InfiniiScan software, providing virtually infinite trigger combinations for any debug situation. Applications include DDR, PCIe, DisplayPort, HDMI, Serial ATA, Serial Attached SCSI, Ethernet families, USB, wireless USB, jitter analysis, RF signal analysis, eye pattern analysis and protocol decoding analysis.

Darren McCarthy, Technical Marketing Manager of Tektronix, was in agreement, commenting that “Sampling technology now extends to 50 GS/s with real-time bandwidths to 20 GHz. The oscilloscope technology has enabled faster high-speed serial data designs and the discovery of the true high-frequency behavior of fast rise-time phenomena. These advances allow scopes to address the wireless design market and ultra-wideband technologies from RFID to satellite. Synthetic aperture radar (SAR) is also a developing market that can directly take advantage of current oscilloscope technology with direct measurements thru to the Ku-band.”

With the recent introduction of the DPO/DPO70000B series digital oscilloscopes in January 2009, Tektronix offers some impressive performance with regard to vertical resolution (ENOB) and wide bandwidth real-time oscilloscopes. The integration of the SignalVu™ software turns the real-time oscilloscope into a high performance wideband spectrum analyzer. The user interface is designed to allow simple control of frequency, span, resolution bandwidth and acquisition time. The software is integrated directly to the acquisition engine of the oscilloscope, which enables the analysis engine to utilize the entire memory for measurements. McCarthy claims, “This gives the user the best frequency, time resolution and capture depth available for wideband signal analysis.”

Spectrum Analyzers

McCarthy feels that the patented real-time technology from Tektronix, which includes the display, trigger and streaming technology would “radically change how spectrum analyzers are used in development and field operations. The DPX™ Live RF spectrum technology available across the company’s entire portfolio from field portable to high-performance benchtop spectrum analyzers is 100s to 1000s of times faster than previously available spectrum trace technology and enables engineers to see signals-within-signals and find spectrum anomalies that have never been seen (see Figure 5). It is not just about finding what might be wrong with a transmitter, but also providing the confidence that a design is done properly when the behavior shows no anomalies.”

Another real-time technology pointed out by McCarthy “is the unique Frequency Mask Triggering, which enables the efficient isolation of transient spectrum events when traditional level or event triggers won’t work. With efficient event isolation, engineers have reduced troubleshooting time and have often found the frequency selective triggering as the only solution to reliably validate designs.

Tektronix’s real-time digital outputs are also unique on spectrum analyzers. When combined with commercially available data recording systems, this has enabled real-time spectrum analyzers to be used for critical field monitoring applications when all signals need to be collected in the spectrum. Using commercial off-the-shelf spectrum analyzers for mission critical applications, such as real-time data collection, reduces operational expense and improves uptime while providing the flexibility to analyze, trigger, and mark signals of interest as they are recorded.”

When asked about the importance of speed in test today, several vendors offered examples of where speed was critical and what set the standard today. Steffen of R&S commented on the importance of speed to help address intense price competition among wireless equipment manufacturers by allowing them to reduce production and test costs. “For these users, faster measurement speed is necessary to attain shorter test cycles and the best price for a given performance level. The R&S FSV signal and spectrum analyzer and the SMBV vector signal generator, for example, are designed specifically for speed and design considerations.” The R&S FSV is capable of up to 500 sweep repetitions in manual operation and up to 1000 in remote operation. “This high sweep rate is important for more than just production applications. Whenever accurate and repeatable power measurements require averaging, or when a certain number of averages is required by a standard specification, this sweep rate will allow development engineers to achieve their results in a much shorter time.”

Communication Tester

Many wireless devices now combine multiple radio technologies such as GSM, WCDMA, Bluetooth, WLAN and GPS while also operating at the high data rates required for mobile Internet. Many devices must also operate in multiple frequency bands and support multimode operation to support mobile radio service on any continent; as a result testing has become time-consuming and costly.

Rohde & Schwarz has developed a special approach to saving time and costs in production. Utilizing the R&S Smart Alignment test concept, the R&S CMW500 wideband radio communication tester makes alignment times up to ten times faster than with conventional methods. Plus, the tester is optionally equipped with two channels, which allows parallel measurement of two DUTs using different standards. Since a maximum first pass yield is necessary in order to achieve minimum production costs, high standards were placed on absolute accuracy, repeatability and linearity during the development of the tester.

Keithley felt that its SignalMeister™ RF Communications Test Toolkit was among their most significant new products of the past year. This next-generation software tool allows engineers to create and analyze the complex signals used in the most advanced wireless transmission protocols. It generates and analyzes both single-input, single output (SISO) signals and MIMO signals used in the latest versions of the WLAN and WiMAX protocol standards. In addition to creating high quality signals, the SignalMeister RF Communications Test Toolkit can create impairments to model non-ideal transmitter conditions and real channel conditions such as fading and noise.

The SignalMeister software has the unique ability to analyze the transmitted signals, acquiring and demodulating the signals, then computing and displaying a wide range of parametric data. In addition, the SignalMeister toolkit can perform simulation studies without the need to use the actual hardware, which allows researchers and designers to study the impact of transmitter impairments and channel effects on signal transmissions easily. This powerful software platform integrates the signal creation libraries of multiple wireless communication standards into a single package. The built-in toolset can be used to modify all signal types, including non-encrypted waveform files produced by other software packages, to support extensive product testing.

Network Analyzers

Agilent wanted to discuss its ongoing development of the Nonlinear Vector Network Analyzer (NVNA) and X-parameters. New software for use with the PNA-X microwave network analyzer provides linear measurements, but can easily switch into the NVNA mode for nonlinear measurements. Agilent recently introduced new Nonlinear Vector Network Analysis capability featuring a breakthrough in X-parameters (new, nonlinear scattering parameters) that now allows engineers to quickly and accurately design and develop linear components and subsystems by removing the trial and error loops. The 802.16 standard on which Mobile WiMAX is based specifies a tight Error Vector Magnitude (EVM) requirement (−31 dB, based on a 1 percent packet error rate). Meeting this target requires that all system blocks be more linear and phase noise be considerably better than in an 802.11 design. Power amplifiers must also be more linear and boast higher efficiency. The key to developing linear active devices for LTE or Mobile WiMAX-based systems is to first characterize the nonlinear behaviors—those that do not have a linear input/output relation and are a major contributor to information interference and the reduction in effective bandwidth.

Anritsu Co. recently introduced the VectorStar™ premium series of microwave VNAs, offered in three standard frequency ranges that go to 20, 40 and 70 GHz. The MS4640A delivers frequency coverage of 70 kHz to 70 GHz, dynamic range of 103 dB at 67 GHz and measurement speed of 20 µs/point.

For true broadband applications such as device modeling, the 70 kHz low-end provides seven octaves of additional information below the traditional 10 MHz cut-off of conventional microwave VNAs. This improves the error-prone task of DC-approximation, providing better device models and circuit simulation. The low-end frequency also improves stability as it eliminates coupler roll-off below 1 GHz.

VectorStar’s 100 dB dynamic range at 70 GHz creates accuracy never before seen in a microwave VNA, according to Anritsu. Supplementing the wide dynamic range is the MS4640A’s excellent receiver compression level. The VNA’s receiver has a +10 dBm 0.1 dB compression level at 70 GHz. With the new Precision AutoCal for 70 kHz to 40 GHz or 70 GHz calibrations, residual directivity of 42 dB can be achieved at 70 GHz, and up to 50 dB at 20 GHz.

Looking Ahead

We ended our conversation with these test equipment vendors by asking, “How is technology improving the capabilities of test equipment?”

Rohde & Schwarz remarked that, “Wireless devices evolve more and more into multi-standard platforms. Therefore, wireless device manufacturers need T&M equipment that supports all important cellular and non-cellular standards. In general, T&M equipment manufacturers are expected to be able to implement new trends in their products very quickly in order to provide the flexibility customers demand. That’s why a key aspect today is the capability to upgrade T&M products by means of software—when new standards require higher data rates, for instance. Rohde & Schwarz develops test instruments that easily accommodate such changes. There’s no need to buy or develop a new box —just perform a software update.

Since the 1950s, the consumer electronics industry, from radios to mobile phones, has experienced a tenfold increase in the upper frequency limit around every 20 years. Modern digital standards such as WiMAX, LTE and WLAN 802.11n or vehicle distance radar are advancing this development. It won’t be long before the 100 GHz boundary for user applications is exceeded. In anticipation of this trend, test equipment must be prepared to serve customers as they step up to very high frequencies.”

Keithley concurred: “The rapid development of new wireless communication standards requires an almost constant re-evaluation of the sourcing and measurement capabilities needed for wireless device research and production testing. One way in which test equipment vendors have risen to address this customer need is to design instruments that are more flexible. New technologies like software-defined radio (SDR) architectures let vendors design instruments that are flexible and adaptable to the changing needs of the industry.

The essence of an SDR implementation is that the modulation and demodulation functions performed on RF signals are done by digitizing the signals and using software and processing techniques, rather than dedicated hardware. This approach allows transmitting or receiving a wide variety of signals more economically than with dedicated, modulation-specific hardware.

The basic principle of software-defined radio is to replace analog circuitry with digital circuitry that can be programmed via software. Functions that were traditionally done in analog hardware, such as frequency generation and conversion, modulation and demodulation, and filtering, are performed with digital hardware. SDR designs also include unique digital functions that can improve the performance of the radio. These functions include decimation and interpolation, which can extend the dynamic range of the radio, and waveform pre-distortion, which can improve the modulation accuracy. In the case of waveform pre-distortion, the modulating signals are modified from the ideal signal to counteract known analog distortion characteristics.”

Agilent noted that, “Many of the same technologies that drive the need for new measurement solutions for our customers are also used by Agilent in the design of our latest test and measurement products. For example, the trend from parallel to serial buses in order to achieve higher system throughput is embodied in our latest oscilloscope products, which use multiple serial data lanes to transmit data from the A/D converters to the memory management system. The continued developments in ASIC technologies are routinely capitalized upon in creating our products. And advancements in simulation technologies are used by our engineers in designing our equipment. Besides allowing us to create our solutions, the fact that we are using the new technologies ourselves enables us to directly identify with the key challenges facing our customers.”