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.