The wireless technologies that occupy industry headlines are designed to meet the public’s perpetual demand for faster data rates. To make sure these needs are met, Long-Term Evolution (LTE), including LTE-Advanced and Time-Division LTE (TD-LTE), is being widely adopted. These standards all depend on increasingly complex multi-antenna radio techniques like multiple-input multiple-output (MIMO) and beamforming.
In the face of this increased complexity, mobile operators and device manufacturers are challenged by continual cost pressures, resulting in reduced staffing headcounts, less available real estate in the test lab, and diminished budgets for peripheral equipment and maintenance.
These factors pose a number of serious problems for those charged with RF testing. The solution is a set of new philosophies in the design of equipment used to emulate wireless channels in the lab.
How Have Channel Emulation Requirements Changed?
The first and most obvious problem relates to the sheer number of channels that must be emulated and controlled. 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 discussion of channel density, or the number of emulated RF antenna elements or paths per rack space, has therefore become an important part of every emulator purchasing decision. The physical size of the equipment being used is a key part of the total cost-of-ownership (TCO) calculations now required of many engineers and lab managers.
Another part of the TCO calculation involves the impacts of all required associated components. Many engineers have experienced the frustration of watching schedules slip as they wait for a $30 passive device that has a two-week lead time.
With this in mind, equipment buyers demand that peripheral devices such as splitters, combiners, and duplexers required in channel emulation are integrated within the channel emulation unit. They further demand that the system’s output power and dynamic range eliminate the need for outboard amplifiers.
The equipment procurement decision now also requires closer looks at how the design affects the time impacts of device testing—both calendar time (time-to-market) and engineering hours. While this is always a consideration, the massive increase in the complexity required of equipment must not imply a massive increase in time spent testing or preparing to test.
Test cases themselves have changed drastically. While conducted-mode RF receiver testing (where an emulator is physically cabled to the device under test’s antenna) is more than sufficient for most single-input single-output (SISO) testing, MIMO requires over-the-air (OTA) testing as well. In MIMO, the design of the physical antenna has a substantial impact on performance.
Today’s channel emulator not only must enable OTA testing, it also must include an interface that abstracts the extremely complex calculations involved. OTA testing must be an accessible process instead of an exercise in advanced mathematics.
Of course, not every antenna-related test case can be run in an OTA chamber. Engineers also require a way to emulate the dynamic changes in received RF that come with motion. This is especially important in testing a mobile device. A device may work well in a static condition, but what happens in deployment when users turn their heads? What happens when a data card is used in the backseat of a moving vehicle?
Finally, as with any capital expenditure, a channel emulator being considered today must have “legs.” The purchaser has to be confident that the emulator platform will be sustainable for a number of years and across a generation of possible technology advancements.
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. A new emulator platform must implement cutting-edge output power ranges, lower noise floors, and better overall channel quality.
Addressing Today’s Channel Emulation Requirements
All of these factors lead to a clear conclusion. Today’s channel emulators cannot be an evolution of the emulators that were delivered even a year or two ago. Those emulators do have “legs.” They will be put to service for the next several years, but they will not address the concerns of most purchasers of new equipment.
To address the first point raised earlier, today’s channel emulators must include a considerable increase in the number of RF channels delivered by a single unit. For example, Spirent’s VR5 HD Wireless Environment Emulator delivers the connections required for unidirectional 8x2 MIMO testing in a single 6U enclosure (Fig. 1).
The unit also integrates the “RF plumbing” such as circulators and splitters. This not only addresses “prep time” concerns, it also increases system accuracy. Rather than requiring the user to account for the effects of external passives on signals, measurements by integrated power meters (where available) include the measured effects of passive components.
OTA is a more complex problem. On the one hand, the industry has not yet standardized on a single method of OTA testing. Standards bodies continue to investigate several approaches involving different types of chambers (anechoic versus reverb chambers) and methods. On the other hand, many base-station and mobile device developers realize that they need to know how their MIMO-based products will perform in the real world.
With this in mind, lab-based channel emulation must be able to drive the multiple antenna probes used in OTA testing. Moreover, the user must be shielded from the laborious, time-consuming, and error-prone calculations involved.
Software add-ons like Spirent’s MIMO-OTA Environment Builder present real-time GUIs that hide the complexities of OTA behind intuitive interfaces. Since the sheer volume of calculations required in OTA can be staggering, an effective OTA implementation must be optimized to allow nearly instantaneous changes in channel profiles.
For those cases where the user cannot justify OTA testing but needs more realism than a static test provides, modern channel emulators come equipped to create dynamic channel models (to simulate motion). They can even add “virtual drive testing,” a method where RF environments captured during live-field drive testing can be replicated in the lab.
Abstracting the user from the complexities of testing is not just an OTA-related issue. Even relatively simple MIMO test cases deal with an enormous number of correlated RF paths. As an example, emulating a bidirectional 8x2 beamforming system requires 32 separate RF signal paths through the emulation system. While efficient channel density addresses the inherent physical problems of working at this scale, an equally relevant problem is that of managing all this complexity.
On The Bench
Toward that end, today’s channel emulator interfaces use graphics to present as much information as possible. This includes control and real-time updates in dynamic testing, graphical depictions of noise generated within the emulator, and much more. Setups for most test cases can be performed with a few mouse clicks while internal power meters automatically set levels and correlation values for all related RF paths.
Spirent built its VR5 GUI on a completely new design philosophy specifically meant to enable efficient MIMO testing (Fig. 2). 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.
Some testers provide graphical feedback during test execution. For example, a Channel Player view provides real-time updates of the power and delay associated with individual fading paths. A Temporal Player view delivers real-time updates of selectable measured parameters such as C/N, input power, or output power, providing immediate feedback and ensuring effective implementation of dynamic environment emulation scenarios.
However, an efficient interface must be a convenience and not a limitation. Researchers, for example, may need especially detailed control of the emulated RF environment. With this in mind, effective modern channel emulators offer direct control of MIMO correlation matrices and even the ability to “play back” RF parameter files synthesized in commercial mathematical software such as Matlab, ray-tracing software, or custom software.
The RF channels used in modern wireless communications technologies are far more complex than the channels we used a few short years ago. Requirements for MIMO and beamforming techniques impose significant demands on those charged with testing the RF receivers built into basestations and mobile devices.
While this complexity continues to increase, design and test teams are compelled to streamline the costs of resources involved. This means that the wireless channel emulators being delivered today must shoulder much of the burden. Effective channel emulators like Spirent’s VR5 offer:
- Efficient use of space: Every time a channel emulator is purchased, the phrase “channel density” becomes part of the discussion.
- Integrated RF components: Today’s channel emulators are designed to eliminate the need for external RF “plumbing” such as circulators, combiners, and splitters. The power and dynamic ranges of the emulator should mitigate requirements for outboard amplifiers. This both enhances the accuracy of the test stand and optimizes the use of time in the lab.
- Capabilities that address MIMO-specific testing requirements: MIMO imposes new test requirements beyond those of legacy technologies. Dynamic scenarios, virtual drive testing, and OTA testing must be among the capabilities of the emulator.
- Highly efficient interfaces to the equipment: Modern radio channels are much more complex than in legacy technologies. The ability to control and monitor test equipment requires a very new paradigm. Incremental changes to existing GUI techniques are insufficient. Today’s channel emulator must be extremely efficient during test setup, test execution, and analysis.