6G Challenges for Radio Testing
What should we expect to see in 6G radios?
Now that 5G deployments are well under way, eyes are turning towards the next steps. There are still plenty of open questions regarding full commercial exploitation of 5G capabilities, and we can expect lots of innovation on this field. On technical side current focus is on 5G Advanced which refers to the evolution of 5G in 3GPP releases 18-20.
However, the next mobile generation – 6G – is also gaining momentum. Counting backwards from the anticipated commercialization in 2030 and 3GPP standardization roadmaps, we can sketch the following timeline:
Various 6G research programs have existed in leading research institutes for some years. Initially the focus has been on somewhat high level 6G drivers, motivation and targets. As the role of hard wireless technology research has increased other related topics such as Machine Learning/Artificial Intelligence and energy harvesting have also gained interest. On the industry front, the evaluation of implementation technologies is starting and will eventually lead to platform technology development.
However, currently there is limited understanding of the concrete nature and content of 6G. For obvious reasons, radio spectrum is among topical items. Discussion and actions center on the so-called Frequency Range 3 (FR3), which resides between 5G bands FR1 and FR2, that is 7-24 GHz. Within this range the lower part, roughly 7-12 GHz, has received most attention. Bands beyond FR2 which are also called FR4 and FR5 and which are above 71 GHz, have raised academic interest but they will most likely have limited usability for mobile applications. Other topics that have been addressed in academic white papers include combined sensing and communication as well as combating the scarcity of available radio spectrum by means of Dynamic Spectrum Sharing (DSS).
Can we make guesses as to what other developments to expect? It appears clear that the exploitation of massive antenna panels (MIMO) will continue and lead to an ever increasing number of antenna elements. It is also possible that the integration of antenna elements with transceivers will become more common, especially at the upper end of the FR3 spectrum. The drive for openness in Radio Access Network technology and business promoted in the Open RAN exercise is not disappearing. Virtualization and Cloudification are likely to enable diverse centralized distributed RAN implementations.
Other technical areas of recent attention include Reconfigurable Intelligent Surfaces (RIS) and full duplex operation. The former aims to improve coverage in cluttered environments, while the latter has the potential to improve spectrum efficiency, i.e. cell and network capacity. It remains to be seen which what role these technologies may have in 6G.
How shall testing solutions evolve?
Evidently new technologies and functionalities will introduce new challenges for test developers. The increase of system complexity is reflected in the amount of required testing. Open system architectures, such as Open RAN, also increase the need to ensure interoperability and system performance. These factors require the application of test automation. At the same time the cost of instrumentation as well as lab facilities rises and the demand for highly competent test personnel increases. These trends promote maximizing the utilization of equipment and supporting remote operation which are enabled through laboratory automation.
Regarding radio testing there are some clear directions. First, over-the-air (OTA) measurement becomes the base line. This is due to two factors: highly integrated radio units without connectors for test instruments and the expected prevalence of MIMO solutions.
Second, the frequency range of test facilities must be stretched to cover the FR3 band. Since the lower part of this band is of primary interest, the solutions will most likely be based on current FR1 test facilities. If at some point the upper part of FR3 is introduced, perhaps present FR2 test solutions will be extended downwards to cover this band. In practice, measurement instrument upgrades and re-design of measurement chambers will be necessary. The necessary changes will be quite drastic if verification of equipment for FR4/5 frequency bands is necessary.
Third, the potentially increasing size of MIMO antenna panels means that narrower antenna beams can be created and the capability for spatial multiplexing and multi-user MIMO is enhanced. Simultaneously the distance required for far-field measurements increases. These factors accentuate the need to re-design measurement chambers. It may also be that instead of far-field measurements the so-called mid-field range is optimal for some test applications. Another direction may be the increased use of Compact Antenna Test Ranges (CATR).
Various questions arise from the speculated new functionalities: combined communication and sensing, DSS and full duplex operation. It is yet unclear how these would be implemented and which supporting features would be needed. Hence it is difficult to assess the subsequent requirements for related testing solutions. Reconfigurable Intelligent Surfaces (RIS) technology may have different degrees of capabilities and dynamics. In the most advanced forms, testing and the necessary facilities could become extremely complicated. But at this point this is only speculation.
Finally, even today the range of mobile devices is quite wide. These include base stations, smart phones and various types of IoT nodes. In the future we can expect the selection to become even more diverse, with perhaps more varied verification needs.
Timeline for new testing needs
In reference to the picture above, certain stages can be identified. Current phase of basic research and technology evaluation involves mostly manual test arrangements with perhaps ad-hoc instrumentation and tuned-up facilities. In the period 2025-2026, as the content of 6G becomes more clear, testing technologies will be developed in a targeted manner and first versions of actual 6G test instruments and environments will be introduced.
R&D phase of 2027 and beyond sets the highest demands on the test coverage and capabilities. At this stage commercial 6G test equipment shall be available. Test volumes will also increase and automation will be necessary.
Finally starting in 2029, test solutions in 6G radio production are quite sensitive to cost, test time and production line throughput. In part, the R&D test arrangement may be reduced or optimized for production, but a great deal of production-specific test development will be required. High level of automation will also be essential.
Much is already known about 6G radio testing or can be extrapolated from recent 5G experiences. Still many topics remain speculation or educated guesses, since standardization will not start for another two years or so. It is already clear that the need for extensive test technology development and competent test developers will not subside, quite the contrary. 6G also offers an opportunity for new players to enter the test development game.
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