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Wireless testing takes the OTA route

December 07, 2018
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By Charles Schroeder, National Instruments Business and Technology Fellow

As cellular communication continues to evolve, test engineers have worked towards adapting an acceptable set of measurements and techniques that enables high volume wireless testing of components such as RF semiconductors, base stations and mobile handsets. The entry of 5G into this space, however, has led to an increase in wireless device complexity, with a concurrent need to reevaluate optimised testing techniques. Viable commercialisation of 5G technology depends on the fulfillment of this need for new testing methods such as over-the-air (OTA) testing and efforts today must be geared towards this goal.

Boosting bandwidth

Increasing data capacity to keep pace with increasing user demand is one of the key goals of the 5G standard, with the focus being on the achievement of the target peak bandwidth of 10 Gb/s per user. Many new technologies are currently being introduced to achieve this aim, such as Multiuser MIMO (MU-MIMO) technology that allows for simultaneous use of the same frequency band by multiple users. Beamforming technology forms the core element of this offering, creating unique, focused wireless connections for each user. The 5G standard also expands the wireless spectrum, specifically into the centimeter and millimeter wave (mmWave) frequencies.

One of the major challenges in the implementation of both these technologies is the requirement for a larger number of antenna elements. As mmWave frequencies tend to attenuate faster than current cellular frequencies, path loss tends to occur, thereby reducing the range of the frequencies. To overcome this path loss, 5G transmitters and receivers will utilise antenna arrays working simultaneously and using beamforming technology to boost the signal power instead of the single antenna per band in current devices. Though important for increasing the signal power, these same antenna arrays and beamforming techniques are also crucial to implementing MU-MIMO techniques.

An added advantage of the antennas at mmWave frequencies is that they will be much smaller than the cellular antennas used for current standards, solving the problem of fitting a number of antennae into mobile devices. New packaging technologies, such as antenna in package (AiP), will ease the integration of these antennas into the small space constraints of the modern smartphone, but the arrays of antennas may be completely enclosed without any directly contactable test points.

Using OTA to address new challenges

Each of these new innovations (increased frequencies, new package technologies, and greater antenna counts) also challenge test engineers today to maintain quality standards while limiting increases in both capital costs (cost of test equipment) and operating costs (time to test each device). While new OTA techniques can help with these issues, they present their own set of unique challenges.

Accuracy of measurement forms the first challenge to be surmounted as unlike cabled tests, test engineers must deal with measurement uncertainty in antenna calibration and accuracy, fixturing tolerance, and signal reflections when making OTA measurements. Secondly, current device test plans must be upgraded with the new measurements that OTA will bring for anechoic chamber integration, beam characterization, optimal code-book calculation, and antenna parameter characterization. Thirdly, with the increase in RF bandwidths processing needs will concurrently rise for calibrating and making measurements adding to test time concerns. Finally, test managers must now look to balance product quality and the impact to time to market, capital cost, operating cost, and floor space. In response to these challenges, the test and measurement industry over the next few years will look to put in place protocols that allow test groups to adopt highly flexible, software-defined test strategies and platforms. This will allow them to ensure their capital expenses can keep pace with this rapid cycle of innovation.

Despite its drawbacks, OTA presents certain distinct advantages. As the only option for AiP technologies, OTA will help test packagedantenna arraysas a system rather than individually, which could lead to the greater efficiency promises of system-level test.

In the past, test equipment suppliers and test engineers have risen to the challenge of testing increasing performance and complexity while minimizing time to market and cost of test. Assuredly, this will also be the case for 5G. Though the challenges of testing 5G look complex today, engineers around the world are already developing the new test instruments and methods, like OTA, that are necessary to make 5G a commercial success.

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