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The many facets and evolving nature of multi-input, multi-output (MIMO) antenna systems have test companies scrambling to stay ahead of industry requirements. Depending on where the tests are performed, from academia and industry R&D labs to product qualification and manufacturing, or from ICs to basestations to handsets, the requirements vary considerably. The latest test and measurement techniques to verify leading-edge performance in research labs as well as cost-effective production testing for the U.S. and other regions are among the recent changes.
Different Flavors Of MIMO
A lot is happening in the MIMO area in 2012. Part of smart antenna technologies, MIMO offers a higher data rate and better spectral efficiency (bits/second/Hz) without additional bandwidth or increased transmission power. Of course, the use of multiple antennas and added data processing at both transmitting (Tx) and receiving (Rx) ends increases system complexity and the associated test requirements.
Perhaps the first issue in dealing with MIMO testing is understanding the variations that exist. The different flavors of MIMO and confusion with other multiple antenna technologies abound (see the table).
MIMO is part of 4G/Long-Term Evolution (LTE). The major difference between the two different types of MIMO—time division (TD) LTE or LTE TDD (TD duplexing) and frequency division (FD) LTE or LTE FDD (FD duplexing)—is the type of spectrum required to deploy each protocol, says Erik Org, senior marketing manager at Azimuth Systems.
“With frequency division LTE, you need paired spectrum, one band for the uplink and one band for the downlink because the uplink and the downlink happen simultaneously,” says Org. Each radio transmits and receives at the same time, all the time. With TD, transmission occurs in one slot and reception occurs in the next slot. As a result, implementing the TD protocol does not require paired spectrum.
With multiuser or MU MIMO (802.11ac), instead of doubling the data rate for a single user, two users share the double data rate. “Instead of increasing the data rate available, you actually increase coverage,” says Mike McKernan, product marketing manager at Spirent Communication. “There is one more user in that cell that you can support using that single MIMO channel.”
“With MU MIMO you can have multiple receivers so you can have a single transmitter with four antennas and you can have multiple receivers or up to four receivers each with their individual antenna, or even a combination where you have three receivers and one of them has two antennas,” says Raajit Lall, product marketing manager for RF and wireless test at National Instruments (Fig. 1).
Different results from laboratory testing obtained by handset makers that generally use a conducting or wired test method and real-world results in the field conducted by service providers have initiated MIMO over-the-air (OTA) testing. The 3rd Generation Partnership Project (3GPP) standards body is working hard to determine how to reduce the gap between the lab tests and the field test through MIMO OTA testing according to Jung-ik Suh, wireless marketing program manager for Agilent Technologies, Electronic Measurement Group.
Test equipment vendors provide different approaches for OTA. Agilent provides two-stage MIMO OTA test methodology intended to maximize the user’s return on investment (ROI) and OTA test results. “There are around three proposals for MIMO OTA, and the 3GPP are working with test vendors like Agilent to see which way will be the best,” says Suh.
Beamforming is not necessarily MIMO. “Beamforming with multiple antennas improves wireless system performance through directional beam, which enhances data rates and coverage with less interference with the targeted receiver,” says Suh. However, when combined with MIMO, MIMO beamforming is a planned part of TD-LTE and of LTE-Advanced technology.
“The initial LTE-Advanced R&D focuses more on carrier aggregation, which provides wider frequency bandwidth to achieve higher data rates up to 1 Gbit/s. However, up to 8x8 MIMO is also under development by leading R&Ds,” says Suh.
An Issue: Specification Status
Today’s testing concerns evolve around many specifications that are not finalized and approved. As part of its validation process for MIMO LTE-Advanced, CTIA, the International Association for the Wireless Telecommunications Industry is performing reference antenna testing. The CTIA has known good devices and known bad devices that are used for test equipment comparisons.
“Those known good and known bad devices are being passed around so that CTIA can verify that the results that people are getting are as expected and a lot of that is going on this month and next month,” explains Spirent’s McKernan.
TD LTE MIMO improvements are also in the works. Although a wide range of frequency bands was defined for both FDD and TDD use, many have not been used. “Over the last year, a lot more attention has been focused on the TDD bands 38-41,” says Azimuth’s Org. He expects changes to occur in the 2.5-to 2.7-GHz frequency bands. Carriers around the world, especially in China, will consider TD LTE because of the advantages of being able to assign spectrum or assign capacities asymmetrically.
For wireless local-area networking (WLAN), the Wi-Fi alliance basically decides which MIMO versions are mandatory and which versions are optional. For the latest WLAN standard, 802.11ac, testing 3x3 is becoming mandatory for many of the chipset vendors. “The 3x3 is definitely required for the latest standard for WLAN,” says NI’s Lall. “For LTE, 8x8 is not required, but it is being tested in a lot of the R&D labs.”
Some experts think that 802.11ac may not be completely finished before the end of 2013. Jan Whitacre, LTE market program manager for the Electronic Measurement Group at Agilent Technologies, thinks finalization is at least a year away. None of the suppliers suggest waiting until the specs are finalized before purchasing test equipment.
Strategy For Test
While many of the MIMO changes that are occurring are in flux, these changes have been underway for a while. The right time to implement test changes could be now. MIMO’s anticipated changes have been known for several years. In some cases, they’re finally or imminently being approved. In addition, when test companies have developed their test equipment to address the known or planned changes, the ease of adoption (for setup and use) of the equipment or platform is simplified considerably.
In some cases, a software change is simply required. For example, National Instruments has accounted for the need to address 10x10 and higher systems for future implementations in its platform design. As a result, a customer buying this kind of equipment today can be in a good position to meet changing test requirements for many years.
“That was a conscious decision that we made and that went into the first thoughts behind the PXI platform as well,” says National Instruments’ Lall. “There is no technical limitation to expanding it to even 20x20, and 10x10 is something we have definitely tested.”
Still, companies that are hesitating to make a test equipment purchase today because all the pieces may not be in place have a tough decision. If a company waits too long to purchase equipment to test a finalized specification, it may be way behind its competition. The risk of purchasing a piece of test equipment prematurely can be minimized if the equipment supplier is an intimate part of the specification development and has accounted for that in its platform and product development.
The test supplier’s approach for adaptability can either have the built-in capability or an easy upgrade to minimize the cost and disruptive impact to the customer. Of course, with a wrong decision, the user may have to start all over to achieve an effective test setup, so this is not a decision to be taken lightly. Also, discussing 8x8 or even 4x4 when the current testing involves 2x2 or 4x2 may seem premature to some test customers. However, test equipment suppliers have experienced an increase in user interest and orders in 2012.
Even with the ongoing standards discussions, customers waiting for the specs to get closer to finalization may have reached their limit. “I think in the last year or so that they just got desperate, like we can’t wait,” says Nigel Wright, vice president of wireless marketing at Spirent Communications. “The specs are definitely starting to converge on an approach, so it’s a lower risk strategy.”
The timing of the purchase is an ongoing issue in the test equipment business. “When do people actually make the investment? Do they wait until the standards are all done and take the risk that their competitors have already gone with a solution and are putting out better performing devices?” asks Wright.
Other suppliers confirm the increased customer interest. “8x8 MIMO is in the R&D labs at the present time. This year we’ve been getting a lot of activity around 802.11ac,” says NI’s Lall.
The issue is how users avoid getting something now that limits their flexibility and ability to implement the next phases of the standards.
“Today, my existing solutions support anything that will be deployed commercially in the next few years,” says Azimuth’s Org. It takes a few years for changes and new topology options to propagate through the ecosystem so commercially available devices are ready to support them. “Today, the highest order MIMO that LTE requires is 8x2 (eight basestation antennas/two UE (user equipment) antennas), which can support up to two spatial layers,” says Org.
Azimuth announced a test strategy in 2012 that targets purchasers’ concerns. The company’s planned future products, platforms, and solutions will address real-world test coverage and automation capabilities. In addition to addressing users’ requirements for power, noise, size, and weight in their test equipment, Azimuth’s platforms will support the test challenges of upcoming protocols and network deployments with up to 200 MHz of channel bandwidth and 16x16 bidirectional MIMO topologies.
NI’s Lall encourages customers considering test equipment to narrow the selection process. “They should definitely be looking at flexibility or the upgradability aspect,” he says. If the current need is a 2x2 system, users need to evaluate potential equipment correctly to make sure that it can support a 4x4 or an 8x8 system in the future. While this may seem obvious, Lall has a technique to determine if a potential selection is flexible enough.
“If you’re investing in some kind of test equipment, and your generator or your analyzer is not able to export the ADC (analog-to-digital converter) clock or the reference clock, that itself is a red flag,” says Lall. Simply exporting the sample clock is not enough to implement a full MIMO system. A reference clock must be exported as well. “That’s kind of the first step for a customer to realize that the equipment that I’m investing in might not be upgradable in the future.”
Agilent’s Suh identifies another problem that can occur if a test equipment decision is delayed: lack of data correlation. For example, chipset vendors and RF customers sometimes play the blame game when test results do not agree. Suh emphasizes that a consistent and broad platform from the oldest stage to current R&D for MIMO can help solve this problem.
The alternative of delaying the solution because of test issues means delaying technology evolution and product introductions. So, suppliers and customers need to zero in on where the problem really exists and solve it quickly. Test equipment is an essential part of that process.
Available And Evolving Products
To support all the MIMO alternatives, product choices for implementing MIMO at any phase or for any end application continue to proliferate. For example, Agilent addresses MIMO testing with products that target the LTE-A, LTE-FDD, and LTE-TDD as well as the R&D lifecycle from design, baseband, RF, integration, protocol, and verification/pre-conformance testing.
Some systems cover a maximum of eight channels. Others have a maximum of two or four channels. The X-Series SGs receiver has a maximum of 16 channels. Some products have full capability and others have general capability, especially in the transmitting area.
Agilent’s solution for TD LTE MIMO as well as beamforming is the N7109A multi-channel signal analyzer and the latest 89600 vector signal analyzer software (Fig. 2). The N7109A has established capability for WiMAX and LTE MIMO measurements. Building on those capabilities, the new enhancements address up to eight-channel TD LTE beamforming plus MIMO signal analysis.
In addition, for MIMO 802.11ac, Agilent’s MIMO PXI vector signal analyzer delivers up to 800-MHz signal analysis. This capability, as well as the unit’s accuracy and speed, enables R&D and test engineers to validate their MIMO 802.11ac designs.
Azimuth Systems introduced the ACE MX2 wireless channel emulator as the first product in support of its new test strategy (Fig. 3). Designed to simplify test lab complexity, the channel emulator also addresses power, size, weight, and noise constraints. For example, size and weight are 60% less than previous solutions. Addressing both TDD and FDD with bi-directional operation in a single unit, the ACE MX2 supports up to 8×4 MIMO systems.
In 2012, China Telecommunications Technology Labs (CTTL), an organization that performs wireless test and certification under the China Academy of Telecommunications Research (CATR), selected the Spirent VR5 HD Spatial Channel Emulator for TD LTE device testing, including implementations of advanced MIMO beamforming. The VR5 provides a foundation for other Spirent test equipment as well.
“The abilities to create realistic MIMO beamforming channels, automate the associated stringent phase-calibration process, and simplify testing were among the key design criteria for our recently released Spirent VR5 solution,” says Spirent’s Wright. Built around the VR5, the MB5 beamforming test system supports the testing of 8x2 and 8x4 MIMO beamforming systems and other applications that require advanced phase calibration as specified in 802.11ac (Fig. 4).
National Instruments has demonstrated an 8x8 test solution for LTE based on the PXI platform. The results for synchronization are quite impressive (Fig. 5). “We’ve tested up to a 10x10 system daisy chained across multiple chassis and we get within 0.1° of phase offset between every channel, which is really good considering all of the different wireless standards. That’s way better than what you need,” assures Lall.
In addition to testing 802.11a/b/g/n devices, NI’s 802.11ac WLAN test solution provides flexibility in testing 802.11ac devices. The solution addresses signal bandwidths including 20, 40, 80, and 160 (80+80) MHz for both Tx and Rx for up to 4x4 MIMO configurations. NI says it is working with several early access partners, including silicon suppliers, OEMs, and electronic manufacturing services (EMS) providers, to test the latest 802.11ac devices.
To Test Or Not To Test: Timing Is The Issue
MIMO has certainly created a buzz in the test world and 2012 appears to be a turning point for implementation. “The biggest change that we’re seeing for MIMO has definitely been on the wireless LAN side of things, where people are really scrambling to implement the 3x3 multiuser MIMO,” says NI’s Lall. But with the number of specifications in progress, timing and technology issues remain.
“In the long-term view, MIMO in the UE will be one of the major test challenges in the future,” says Agilent’s Suh. He does not expect this to occur in the very short term because the industry is focusing on carrier aggregation first. Once that is resolved, organizations will move to MIMO in the UE side.
Even though the finalization of 802.11ac is at least a year out, R&D companies and academia need to make the measurements now. “There is a lot of disagreement and a lot of discussions happening because it’s not a simple problem,” says Agilent’s Whitacre. Today’s testing will be essential to the resolution.
Spirent’s Wright expresses concern for handset issues. “You’ve got the physical spacing issue with the antennas because you need to separate them for decoupling, so you need some level of physical space related to wavelength,” he says. Operation from 700 MHz up to 3.5 GHz poses significant RF implementation challenges.
However, there are indications that the industry may have solutions from innovative antenna manufacturers. “Maybe it has, but it seems to us like there is still a significant challenge there to address,” says Wright. Even if the suppliers are convinced they have a solution, they still need to perform the testing to validate it.