Enabling seamless WiMAX fabric

WiMAX stands poised to extend coverage in PC networking and mobile phone communications as semiconductor makers roll out WiMAX chipsets and as test-equipment vendors offer the lab instrumentation and production ATE systems necessary to test the chipsets and the products they populate.

WiMAX has been suggested as a technology for cellular telephony, but Paul Argent of Aeroflex said he expects WiMAX to initially bring broadband wireless access to laptops. Over the next two years, Argent said, WiMAX will primarily provide high-speed data access to PCs in coffee shops as well as in moving vehicles. (See “WiMAX markets and opportunities.”)

The N6430A WiMAX protocol conformance test and development solution provides a suite of test solutions for 802.16-2004/Cor2 D3 Mobile WiMAX protocol conformance test (PCT) and protocol development testing. Courtesy of Agilent Technologies.

As for WiMAX competing with WLANs, she expects them rather to complement each other, with appliances making the most effective connection based on the conditions of the moment. For instance, she said that WiMAX will be the choice if you’re on a train going 50 mph, but if you later find yourself in a stationary situation, WLAN might be the best choice.

To test WiMAX chips in production volumes, ATE makers are adapting their RF-capable systems to handle WiMAX test requirements, while makers of bench and rack-mount test equipment are tailoring their instrumentation to handle a potential onslaught of components, modules, and WiMAX-compatible appliances; these vendors are also addressing the test needs of service providers who will be installing and maintaining WiMAX infrastructure.

The M2690A/M2691A signal analyzers operate from 50 Hz to 6 GHz and can measure the transmit power of mobile WiMAX devices. Courtesy of Anritsu.

Adam Smith, a business development engineer at Verigy, said WiMAX test represents an evolutionary step from WLAN test, with WiMAX imposing stricter requirements as designers try to cram ever more features into a tighter space. “From a test equipment point of view,” he said, “your equipment needs to have very good noise performance—it needs to be very sensitive.” Agilent’s Stark added that WiMAX’s underlying orthogonal frequency division multiplexing (OFDM) modulation scheme results in high peak-to-average power levels, putting a premium on highly accurate power-amplifier measurements.

If WiMAX is going to reach wide acceptance, test costs—including the cost of silicon overhead to support testing—have to be as small as possible. Ultimately, said Smith at Verigy, someone will want to fit WiMAX capability within a $99 mobile device.

When asked whether performance or cost is the most important aspect of a WiMAX test regimen, Schaub at Advantest responded, “Both, unfortunately. The cost has to be similar to today’s WiFi products.” He commented on WiMAX going into a laptop: “We as customers already have an expectation of cost, so it can’t be too far out of alignment. Additionally, a lot of companies intend to use WiMAX to extend their cellular networks. It saves them a ton of money in infrastructure cost.” And, he added, there is an expectation that the cost of a cellphone will be low in many poorer countries. As for performance, he said, “All of this must work while riding in a car, on a train, etc. So, the performance requirements are similar to today’s cellular phones.”

The CMW270 single-instrument production tester makes signaling and nonsignaling measurements of WiMAX mobile stations and customer premises equipment.  Courtesy of Rohde & Schwarz.

Even more troubling, you may find that you don’t have access to signals between functional blocks. Ken Harvey, senior product technologist at Teradyne, pointed out that as WiMAX chips become increasingly integrated, you’ll find you might not have direct access to I/Q signals—“You’ll go straight from RF to bits,” he said.

Lacking standards against which to test the signal between the RF mixer and the ADC, for example, and potentially lacking access to that signal, you’ll have to rely on system-level requirements, such as the error vector magnitude (EVM) of the resolved signal and bit-error rate of the data stream. Stark of Agilent elaborated on this point. She divides the WiMAX market into four segments: chips and components, modules (for which chip-maker reference designs sometimes substitute), appliances, and service providers. She concurred that EVM is a system-level spec that must be measured at the appliance level but cautioned that designers must beware of component and module EVM (of a power amplifier, for example), so they can stay within an overall EVM budget.

Ultimately, Stark said, test will play a key role as vendors try to differentiate their components in terms of features, RF performance, and power consumption. She pointed out that, fortunately, WiMAX appliance vendors won’t be creating brand new devices that we’ve never seen before. The use case for WiMAX, she said, is to add WiMAX to existing devices, such as laptops, PDAs, and cellphones.

But just how a particular system, no matter how familiar, reaches adequate system-level performance standards will vary depending on the baseband software, the system design, and the intended operating environment of that system. There is no straightforward translation between WiMAX’s system performance specifications and testable behaviors on WiMAX silicon.

That said, the testing problem is easier in some functional blocks than in others. From a testing point of view, the digital baseband is just another fast signal processor. In a written response, an Intel engineering spokesperson said, “WiMAX silicon is not very different from any other SOC [system-on-chip] testing we perform at Intel. The part goes through Intel’s strict product reliability and qualification guidelines that include wafer testing, ESD [electrostatic discharge] stressing, burn-in, and analog/mixed signal testing across a broad range of temperature, environmental, and power supply variability conditions. WiMAX silicon can use the same DFT [design-for-test] techniques and hardware structures that are common in SOC design, such as at-speed scan, ATPG [automatic test-program generation], and logic and memory BIST [built-in self-test]. The process also includes package qualification and silicon performance testing on multiple skew lots, as well as normal silicon lots.”

Basically, the baseband silicon is a specialized, but still programmable, signal processor. It is working correctly or it isn’t. It’s the software that needs to be adjusted to the specifics of a particular application, and that’s not a testing problem.

If you talk to a vendor of RF silicon, though, you get a rather different view. In the digital world, chip variations don’t alter the functionality of the device until they get so severe that they actually break the circuit. In the RF and analog domains, variations in the chip are variations in the functionality. As one old chestnut has it, you test digital circuits, but you characterize analog ones.

“WiMAX is all over the place right now,” said Tom Gratzek, business director for the WiMAX silicon program at Analog Devices (ADI). “There are different frequency bands, different bandwidth requirements, different baseband filtering schemes—everyone has an approach.”

ADI offers WiMAX front-end silicon that includes the RF stages, mixers, ADC/DAC, and some digital filtering. Gratzek said that the digital portions of the chips get tested the way any other digital circuitry would be: with scan-based BIST. But from there, things get more complex.

“We have to examine the analog signal chain for defects,” Gratzek said. “That by itself requires hundreds of milliseconds of test time. After that, the only approach we have found to predict how the chip will work in the customer’s system is to stimulate the silicon at-speed.” But it is not, Gratzek explained, a full characterization. Rather, the test program is an artful compromise, based on the full characterization of skew lots done in the engineering lab, on the ability of test engineers to elegantly check many degrees of freedom with a few tests, and on continuous feedback from ADI’s applications engineers, working on customer designs.

The solution ADI has found is to drive the receiver with multiple gigahertz-band test signals and to drive the transmitter with corresponding digital vectors. “We sweep three frequencies in each band,” Gratzek said. “Unfortunately, that forces us onto mainframe RF testers, and it adds seconds of test time.”

This is not a unique approach. Infineon engineers report that they generally stimulate their WiMAX silicon at-speed as well. They use a standard 64 QAM 2/3 modulation of a 3.5-MHz-bandwidth signal, sampled at 4 MHz, as a starting point. But Infineon is seeing increasing pressure for customers with video-over-broadband applications to expand the bandwidth to 10 or even 20 MHz, causing changes from the silicon on up through the testing program.

Even moving to mainframe RF ATE isn’t the whole solution, though. Gratzek said that ADI has augmented its testers’ already formidable hardware with some custom spectral-analysis gear. There are also some proprietary design features built into the silicon and the device-under-test card to increase coverage and reduce test time. This allows the test team to sweep frequencies for an end-to-end test: On the receive side, for instance, they can drive the antenna inputs to the low-noise amplifier and analyze the output stream from the ADC for EVM and noise figures.

These top-line numbers give ADI a go/no-go indication on each die, and they let the engineers infer the signal-to-noise ratio and linearity of the ADC from the end-to-end test. But the test team can extract even more detailed information as well, due to the high degree of digital configurability of the RF design.

“We can manually control the automatic gain control loop, and we do so during test,” Gratzek illustrated. “We can also disembed the digital filters on the output of the chain to examine the raw digital data. And we can force the analog filters to specific characteristics, step through the gain settings on the LNA [low-noise amplifier], and so forth.” This allows the test team to move, if necessary, from the end-to-end test to an almost diagnostic level of examination, while still on the production test head.

This flexibility comes in handy. “We are delivering WiMAX chips to all sorts of customers’ evaluation boards, and they use the silicon in many ways,” Gratzek said. “Our applications engineers feed use data back to the test team, and we try to adjust the tests to anticipate the sensitivities of a particular customer application. For instance, many customers change the filter settings to get the EVM they want on a particular board and antenna configuration. We try to adapt to that.” This requires the applications support team to reserve time on the test floor for development purposes.

Of course, commercial test companies are working to streamline WiMAX test. ATE vendors including Advantest, Teradyne, and Verigy are tailoring their systems to test WiMAX devices in multisite configurations.

Adam Smith of Verigy said there is nothing magical about WiMAX. He said that while ultrawideband (UWB) is dealing with new spectrum, WiMAX aims to make more efficient use of spectrum that’s already been allocated. WiMAX test, he said, is well within the capabilities of his firm’s Port Scale RF instrument for the V93000 system. Similarly, Advantest’s 12GWSGA RF module, introduced last fall for the company’s T2000 test system, and Teradyne’s UltraWave, introduced in March, will handle WiMAX chip test.

Semiconductor ATE systems have typically focused on high throughput without necessarily providing the performance of bench and rack-mount instrumentation, but the advent of WiMAX is changing that, said Harvey of Teradyne. Measurement requirements are becoming so stringent, he said, that ATE instruments must approach bench and rack versions in measurement capability. He cited an additional advantage of high-performance ATE: It lets you characterize silicon on the ATE itself, smoothing the transition to high-volume production test.

Kyle Klatka, product manager for Teradyne’s Wireless Business Unit, added that UltraWave “can deliver up to 16 universal RF ports. By 'universal,’ we mean that every RF port on the instrument has the same capability.” In previous instruments, he said, some ports might include features such as an onboard noise source while others might not, complicating the test engineer’s task. “We decided to take those variables off the table to make life easy on test engineers when they are designing their DIB [device interface board].”

Companies that provide test equipment for WiMAX modules and appliances as well as components include Anritsu, Agilent, Aeroflex, Tektronix, and Rohde & Schwarz, all of which make general-purpose test and measurement equipment that can perform tests on WiMAX systems as well as on dedicated WiMAX boxes and software.

In addition, Keithley Instruments signaled its interest in WiMAX by joining the WiMAX Forum in January. Mark Elo, Keithley marketing director for RF products, said the company’s Model 2920 vector signal generator and the Model 2820 vector signal analyzer support the OFDM and MIMO technologies necessary for WiMAX test, and he added that Keithley has been working with WiMAX chipset and appliance vendors.

For its part, Tektronix offers the K1297-G35 WiMAX protocol analyzer, which provides for protocol simulation, emulation, and monitoring. In addition, Tektronix offers for its real-time spectrum analyzers the RSA-IQWIMAX software, which can help detect, diagnose, and resolve WiMAX design errors.

Rohde & Schwarz offers the CMW270 single-instrument production tester as well as the TS8970 WiMAX radio conformance test system. Anritsu offers signal-generation and signal-analysis bench instruments, such as the MS2690A signal analyzer and MG3700A vector signal generator, as well as the handheld MS2724B spectrum analyzer, which can make fixed and mobile WiMAX measurements in the field.

Argent of Aeroflex said his company offers WiMAX test equipment in PXI and traditional rack-and-stack formats for testing WiMAX base stations and mobile devices “from birth to death.” He noted that before vendors submit their WiMAX devices to WiMAX Forum certification labs, they would benefit from doing their own precertification tests to help ensure their devices pass the first time. When WiMAX Forum labs are charging around $500 per hour, he said, customers will want to have maximum confidence that their products will pass quickly.

Stark of Agilent noted that her firm’s offerings extend from the EEsof division’s Advanced Design System (ADS) design and simulation software to WiMAX drive-test systems. Along the way, the company offers a complement of signal-generation and signal-analysis equipment, which can link to ADS via Agilent’s Connected Solutions technology, as well as WiMAX protocol analyzers and logic analyzers for baseband development and troubleshooting.

At the end of the day, test cost will be paramount—for chips, modules, appliances, and infrastructure installation and maintenance. Gratzek of ADI summarized the cost issue from a chip maker’s perspective: “On our GSM product line, we were able to substantially reduce the test cost as the market matured,” he said. “We aren’t at that stage yet with WiMAX, but we have planned for it. We designed our ATE strategy from the beginning with an end cost-point in mind and a path to get there. We may well reduce the test cost by a factor of three as the technology matures.”