In the wireless world, the game is now one of capacity. Whereas the Advanced Mobile Phone Systems (AMPS) of the 1980s were purely analog and voice-oriented, digital modulation modes and rising data rates (Fig. 1) dominate today’s wireless landscape, driven by new digital standards and the wireless carriers’ desire to maximize revenue. New technologies such as multiple-input multiple-output (MIMO) are among many factors that are raising the complexity stakes in wireless. As a result, handset testing is a growing challenge.
A major shift in the wireless industry has been in efforts to use bandwidth more efficiently, delivering more capacity within the same amount of spectrum. With the move from those early AMPS systems to digital 2G systems in the 1990s, data first joined the mix, augmenting voice-only transmission. By this time, GSM systems using digital modulation modes like quadrature phase-shift keying (QPSK) and orthogonal frequency-division multiple access (OFDMA) took over.
The latest transformation to 3G systems during this decade has led to an explosion in data rates and a profound shift in emphasis. With W-CDMA systems and Long-Term Evolution (LTE) technology, data is now the main driver in the system. Voice transmission has become more or less an application. In some respects, when it comes to voice-over-Internet Protocol (VoIP) technology, the voice transmission is no more than additional data.
DESIGN TRENDS DRIVING TEST
Several technical developments are driving wireless test. For one, new technologies such as LTE and LTE Advanced are delivering increased transmission bandwidths. The LTE specification provides downlink peak rates of at least 100 Mbits/s and an uplink rate of at least 50 Mbits/s. It also affords increased spectrum flexibility, with supported spectrum slices as small as 1.4 MHz and as large as 20 MHz.
“In addition, technologies are building onto existing standards, aggregating channel use,” says Bill Burrows, business development manager at Aeroflex Test Solutions Group. “The net effect of aggregating channels will increase the required analysis bandwidth in our products going forward.”
For Burrows, the push into new slices of spectrum, such as the analog TV bands and the 3.5-GHz band, are not so much of an issue for the test manufacturers. Today’s digital radio test sets typically have more than enough bandwidth to account for the broadening spectrum usage. A prime example is Aeroflex’s 7100 LTE digital radio test set, which sports a 6-GHz frequency range, covering all LTE spectrum allocations.
But what does make for test challenges are the increased bandwidths and data rates. “Each time that happens it drives a new baseband design,” says Burrows. As the protocol stacks grow more complex, it becomes more challenging to stay abreast of them.
Increased bandwidth drives the need for a larger and more powerful baseband processing engine in the test equipment. “Every time you update baseband processing, you have to account for investment in legacy systems,” says Burrows. “People still want plain old GSM in their test set. A complete rewrite of your protocol is required when you go to an updated baseband, which means you have to revisit legacy support.”
A key trend driving wireless test is the rapidly growing complexity of the handsets themselves. For example, any given device now incorporates multiple radio technologies. Today’s smart phones integrate Wi-Fi, Bluetooth, cellular radio, and even an FM-radio receiver. The market success of smart phones, which represent a step in the convergence of telephony and computing, has led to the predominance of Apple’s iPhone as well as up-and-coming Android-based devices such as the Motorola Droid.
New handsets and basestations pose some specific challenges that manufacturers and designers face. “It starts with having to develop models for simulations,” says Frank Palmer, LTE product marketing manager in the Electronic Measurements Group at Agilent Technologies. “When these technologies first appear, there really isn’t any test equipment available and standards are not finished. That in itself is a big challenge: What kinds of tools are available for designers to create with at the very beginning?”
With all the complexity involved, it’s incumbent upon handset designers to be near perfect. “There’s little margin for going back and making changes later in the design cycle,” says Jennifer Stark, wireless connectivity business team leader at Agilent Technologies. “This is often complicated by the fact that emerging standards are not yet set in stone.”
Companies also may have competing versions of what the final standard might look like, with each trying to push its version in some way. Thus, the links between models and simulation and the test environment will be important going forward, as there must be correlation between simulation and measurement.
MIMO MAKES AN IMPACT
As stated earlier, the wireless industry has moved, broadly speaking, from analog modulation to digital modulation. Its next gambit for increasing the amount of data that can be carried in the same amount of spectrum will involve MIMO technologies. The myriad new and emerging wireless standards, including LTE, LTE Advanced, and HSPA+, all rely on orthogonal frequency-division multiplexing (OFDM) and MIMO.
“MIMO is particularly big because it can mean two or four transmitters in the device you’re testing as well as in the test equipment,” says Mike Barrick, Anritsu’s business development manager representing wireless test. Today, 2×2 MIMO configurations (two transmitters/two receivers) are commonly in use. In the near future, 4×2 (four transmitters/two receivers) will be more prevalent.
“In the past, we were always trying to correct for multipath fading in the communication channel,” says Agilent’s Frank Palmer. “Now, MIMO lets us take advantage of that channel interference to increase the performance of the system and achieve higher data rates.”
Going forward, MIMO will be a key trend in the move to achieve the data rates specified in next-generation digital modulation schemes such as LTE Advanced (also known as 4G).
ENERGY EFFICIENCY ISSUES
According to Darren McCarthy, Tektronix’s technical marketing manager for RF/microwave products, two major factors are driving wireless test trends. One is the move to lower frequencies for high-data-rate technologies. The other is power efficiency in amplifier design.
The push to lower frequencies is driven by efforts such as the White Spaces Coalition, a loose federation of eight major companies (Microsoft, Google, Dell, HP, Intel, Philips, Earthlink, and Samsung Electro-Mechanics) seeking to employ unused frequencies in the U.S. VHF television bands (54 to 698 MHz) to deliver high-speed broadband Internet access.
The move to lower frequencies affects issues such as network power-control groupings. “As you go lower in frequency, you need fewer groupings,” says McCarthy. “That’s smart, as it allows for a truly distributed IP-type of network that doesn’t require the physical layer to be adapted so often.” Not only are there benefits in terms of propagation, but the implementation of the physical layer also is less demanding on those central resources required to control power for a given user.
These developments translate into challenges for the test engineer in terms of energy efficiency. Power amplifiers naturally are at their best when operating in full saturation, as when employing higher-order modulation techniques that try to operate a transistor like a diode. “To operate that way you must really understand how your transistor behaves in the non-linear region,” says McCarthy.
Measurement of non-linear behavior is more complex than when an amplifier is operating in its linear region. A relatively new technique that comes into play in characterization of non-linear devices is open-loop active load pull (Fig. 2). This technique uses a separate signal source to stimulate either the source or load side of the device under test (DUT), removing any uncontrolled interaction between the DUT and load-pull system.
The open-loop system absorbs the signal that is generated by the DUT and injects back into the device a signal that is generated by an independent source. The amplifier bandwidth is large enough to cover all harmonic frequencies that require impedance control.
“Other test companies look at this problem in the frequency domain,” says McCarthy. “We look at it in the time domain. Further, most approaches use passive technology to physically load a circuit. We look at it as an active technology.”
Tektronix’s approach fully exercises the entire Smith chart to understand non-linear amplifier behavior. The technology, which was developed at Cardiff University and commercialized by Mesuro LLC, allows designers to go beyond simply characterizing a device. Rather, it results in convergence on the device’s maximum efficiency point. “Rather than extrapolate behavior, we are interpolating it,” says McCarthy. The technique allows users to determine device efficiency at all impedances and find the operating point that’s most efficient for the amplifier.
The researchers say that the approach is frequency- and technology-agnostic. This is a result of its being a time-domain approach as opposed to a frequency-domain approach. It’s also due to the use of active loading and not passive loading.
All of this technology is embodied in the Mesuro MB series (Fig. 3) active load pull solutions, which comprise Tektronix’s AWG7000 series arbitrary waveform generator and DSA8200 series sampling oscilloscope.
MANUFACTURING TEST CHANGING
A large portion of the wireless-test market is manufacturing test. Today’s handsets have multiple radios and are dense with technologies, each of which requires a full testing regimen in its own right. “If you tested phones using the traditional serial method, your test times are too long,” says Aeroflex’s Bill Burrows. “What used to be commonly called call testing, where you actually place a call on the phone, has been replaced by test modes built into the phone. This is a demanding scenario.”
As a result, manufacturers are moving away from classical methods of test and embracing test methodologies that yield greater test throughput with the same resources. One option is the use of communications testers that lend more parallelism and intelligence to the test process.
An example is Rohde & Schwarz’s CMW500 wideband radio communication tester (Fig. 4). According to A.K. Emarievbe, product manager for the company’s mobile radio testers, the platform multitasks by performing multiple evaluations simultaneously.
“With one capture, we can evaluate power measurements, spectral analysis, and modulation,” says Emarievbe. “This cuts testing time dramatically.” Further, the resulting data is correlated, so if there are issues in the power measurement that affect the spectrum measurement, it will be seen at once.
The CMW500 also takes advantage of test routines built into wireless chipsets. By working in correlation with the chips’ manufacturers, Rohde & Schwarz builds intelligence about these test routines into the system. This enables test engineers to write test routines ahead of time, saving even more time in the test process.
Moreover, the tester permits parallel operation with two independent test systems in a single chassis. This saves on both space and cost by enabling higher test throughput.
THE FUTURE IS PARALLELISM
With test taking a growing percentage of the overall cost of manufacturing handsets, parallelism is sure to be a big part of the solution to amortizing the cost of test. One recent entry into the market, LitePoint, has made a splash with its IQxstream 3G/4G manufacturing test system (Fig. 5).
“In developing our system, we looked at how test time is proportioned,” says Spiros Bouas, LitePoint’s chief operating officer. “The biggest slice out of the pie chart is DUT control. Of all the ways we can cut testing time, the biggest opportunity is in DUT control. We have patented innovations related to reducing the number of transactions between tester and DUT.”
LitePoint’s approach is a dual parallelism, in which it tests up to four handsets at one time and also tests multiple standards simultaneously. “We test numerous standards, including Wi-Fi, GPS, Bluetooth, and GSM, among others, all in parallel in about 15 seconds,” says Bouas. “The same tests done in serial fashion take about 60 seconds.”
The tester features 100-MHz orthogonal frequency-division multiplexing (OFDM) capture bandwidth. With LTE having a 20-MHz channel bandwidth, this enables one-shot adjacent channel leakage measurements. Also, with support for up to four LTE handsets at one time, the tester addresses the technical needs of both LTE and HSPA+ by handling MIMO test as well.