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Simplifying and Improving Advanced-Technology MIMO Receiver Testing

Spirent VR5 HD Product Summary

August 15, 2011
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Spirent Communications’ new VR5 HD Spatial Channel Emulator addresses the problems associated with testing RF receivers used in modern high-antenna-count wireless technologies. The VR5 is designed to bring efficiency and ease of use to testing cutting-edge mobile devices and base-stations.

Advanced wireless technologies like LTE (including LTE-Advanced and TD-LTE) rely on multiple-antenna techniques (MIMO and beamforming) to meet the public’s perpetual demand for faster data rates. This poses a number of problems for those charged with RF testing. The first and most obvious problem relates to the sheer number of channels which must be emulated and controlled.

Figure 1 - Spirent's VR5 HD Spatial Channel Emulator

VR5 Hardware Platform

Emulating a MIMO channel requires m × n separate emulated radio channels, where m is the number of antenna elements at the transmitter and n is the number of elements at the receiver. If bi-directional or handover testing is required, the number of required links immediately doubles. In the near future there will be deployments using 4x2 MIMO, followed by 4x4, 8x2 and 8x4 schemes. The number of channels required for testing now ranges in the dozens rather than in single digits. The VR5 design addresses these test cases and many more with a single 6U hardware unit.

The VR5 also integrates RF components that are often required for testing. Passive components such as splitters, combiners and duplexers are integrated within the unit, while both the system’s output power and dynamic range are designed to eliminate the need for outboard amplifiers.

The VR5 renews some of the hardware features that made its sibling, Spirent’s SR5500 Wireless Channel Emulator, an industry favorite over the years. Internal power meters at each input and output help ensure accuracy and repeatability, and also feed real-time updates on the front-panel display. Controlled additive interference (AWGN) generated by the unit is also measured in real time and those measurements are displayed to the user.

New test equipment targeted to the wireless market should always be “future-proofed” by implementing an order-of-magnitude increase in the quality of RF specifications. The VR5 does not disappoint, bringing with it completely new RF handling capabilities, including cutting-edge output power range, noise floor, and overall channel quality. With this performance headroom, the VR5 can easily handle any relevant test requirements that may come over the next several years.

Control and Feedback

The next problem to address is that of managing these massive and complex emulated channels. While the technology gets ever more intricate, those tasked with testing it find themselves having to do more with fewer resources, usually in step with very aggressive product schedules.

The VR5 design team took a step away from traditional channel-emulator control and built the interface on a new usage philosophy specifically designed to simplify control over a complex MIMO environment. The result, implemented in a front-panel touchscreen, offers substantial control with just a few swipes of the finger, all while minimizing the opportunities for user error.

During setup and configuration, the front panel presents a step-by-step process offering combinations of test cases, environment scenarios and operator parameters. Graphical configuration information is presented at each step to help the operator quickly recognize and correct setup errors. While most test cases can be configured strictly through this high-level control, the user still has the option to set lower-level parameters for customized testing. Most test cases, even complex high-antenna-count scenarios, can be set up in less than a minute.

Figure 2 - Thanks to new concepts in test equipment control, a customized 4x2 bi-directional MIMO test is set up in seconds.

Graphical feedback continues during test execution. The Channel Player view, familiar to SR5500 users, shows real-time updates of the power and delay associated with individual fading paths. The VR5 adds the Temporal Player view, which provides real-time updates of selectable measured parameters such as C/N, input power, or output power.

The Temporal Player is most handy when used in conjunction with the Dynamic Environment Emulation (DEE) feature offered with both the SR5500 and VR5. To use DEE, the user maps out per-path fading parameters in a spreadsheet, varying them over time via the spreadsheet. The emulator then reads the spreadsheet file and physically creates the defined RF channel.

Figure 3 - Front-panel feedback includes continual RF measurements and temporal views

DEE can also be used to minimize the cost and time spent drive-testing mobile devices. Drive-route data can be captured and stored using a commercial cellular scanner. Optional Spirent software converts the captured data into a DEE file for playback in the lab. Aside from the convenience and cost savings of re-using data captured from a single drive route, this “virtual drive test” method adds a level of repeatability that is not possible by repeated physical drive-testing.

For special cases where statistically anomalous fast fading is required, a second fading engine is built into the unit. The fine-time engine, called Fading Lab, lets advanced users create RF environments based on sample-by-sample RF data parameters. For example, RF researchers can generate sample data by using commercial mathematical software (e.g. MATLAB®), ray-tracing software or custom software. Channel-sounder data is another potential source of this fine-time information. The Fading Lab engine processes this data and creates the corresponding physical RF environment.

MIMO and MIMO-Over-The-Air (MIMO-OTA) Testing

Large-scale commercial MIMO deployment brings with it new challenges in the test lab. For example, in a traditional single-in, single-out (SISO) wireless connection, the orientation of a receiver antenna had little effect on performance. In MIMO, however, the effectiveness of the channel is a direct function of the spatial relationships between receiver antenna elements and transmitter antenna elements.

The VR5 offers full control of MIMO channel correlation in order to address all these test cases. Spirent also adds a unique correlation-related feature called Dynamic Correlation, which emulates the real-time changes in correlation caused by motion. For more information relating measured channel correlation and the effects of dynamically changing correlation of a MIMO channel, please see "Dynamic Correlation in Virtual Drive Testing" (Gurumurthy, Madhu Microwave Journal August 2010: 76 - 82).

Another wrinkle that results from MIMO deployment is the effect of the physical design and orientation of the antenna and antenna elements. In most testing using a channel emulator, the emulator is connected to the receiver antenna(s) via a cable. Since the physical antenna’s receiving capabilities are now critical, MIMO Over-the-Air (MIMO-OTA) testing becomes part of the testing equation. Often confused with field testing, “MIMO-OTA testing” refers to controlled radiated testing in a shielded enclosure.

MIMO-OTA uses either an anechoic chamber or a reverb chamber to isolate the DUT from outside interference. In the former, the chamber’s internal walls absorb and dissipate RF energy so that the emulator -chamber combination explicitly controls all of the RF impinging on the DUT. In the reverb chamber, reflections from the chamber walls are used to purposely create a rich over-the-air multipath environment.

In both OTA methods described here, testing requires a channel emulator that can generate controlled RF conditions which take into account the characteristics of the chamber. Thanks to Spirent’s involvement in the industry bodies most engaged in MIMO-OTA, the VR5 is uniquely positioned to deliver feasible MIMO-OTA testing. A software option maps desired channel characteristics to chamber parameters and creates the channel to be generated by the VR5.

Figure 4 - Spirent’s Spatial Channel Mapping Toolbox creates models for MIMO-OTA testing.

Summary

The VR5 is a timely response to the problem of testing highly complex RF technologies given streamlined resource pools. Step-by-step setup guidance and graphical feedback make it possible for even inexperienced team members to run very sophisticated testing without introducing operator errors. Continual graphical feedback during execution is further insurance against getting misleading results.

The bench-top unit can replace racks of hardware in those labs where real estate is an issue, and the design can save weeks of time that might otherwise be spent locating, ordering, and waiting for critical outboard components. Finally, the system was designed to ensure that the VR5 will prove itself valuable for years to come, no matter how challenging the requirements.

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