Charles Schroeder, Business and Technology Fellow, National Instruments
As cellular communication continues to evolve, test engineers are working 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 re-evaluate optimised testing techniques. Viable commercialisation of 5G technology depends on the fulfilment of this need for new testing methods such as over-the-air (OTA) testing and efforts must be geared towards meeting this goal.
Boosting bandwidth
Increasing data capacity to keep pace with the growing user demand is one of the key goals of the 5G standard, with the focus being on achieving the target peak bandwidth of 10 Gbps per user. Many new technologies such as multi-user MIMO (MU-MIMO), which allows the simultaneous use of the same frequency band by multiple users, are being introduced to achieve this target. 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 centimetre and millimetre 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 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. Although important for increasing signal power, these antenna arrays and beamforming techniques are also crucial for 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 antennaes 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) challenge test engineers to maintain quality standards while limiting increases in both capital costs (the cost of test equipment) and operating costs (the 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 characterisation, optimal code-book calculation, and antenna parameter characterisation. 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 look at balancing product quality and the impact on time-to-market, the capital cost, the operating cost, and the floor space. In response to these challenges, the test and measurement industry will, over the next few years, 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 that 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 packaged antenna arrays as a system rather than individually, which could lead to the greater efficiency promises of the system-level test.
In the past, test equipment suppliers and test engineers have risen to the challenge of testing increasing performance and complexity while minimising the time-to-market and the cost of testing. Assuredly, this will also be the case for 5G. Although 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.
