6G Midband Spectrum Discussions from B6GS 2024

It seems like discussions about 6G spectrum never end! Despite my efforts to limit presentations focusing solely on spectrum, they remain a hot topic at events and conferences.

At this year’s Brooklyn 6G Summit, one session stood out: '6G Midband Technology and Spectrum'. It featured two insightful talks, one by Professor Sundeep Rangan from NYU WIRELESS and another by Yoji Kishi, Executive Director at KDDI Research.

Professor Rangan introduced the concept of the upper mid-band frequency, also known as FR3. Then, Kishi-san shared KDDI’s simulation results for FR3 coverage in downtown Tokyo. Compared to 3.5 GHz, FR3 experiences a 6 dB loss in free-space pathloss for Line of Sight (LoS) and a 3 dB diffraction loss for Non-Line of Sight (NLoS). However, these losses are partially offset by a 6 dB antenna performance gain, thanks to the higher frequency allowing for more antenna elements.

The session was followed by a panel discussion, moderated by Michael Thelander, President and Founder of Signals Research Group (SRG). The panel featured the two speakers, Professor Sundeep Rangan and Yoji Kishi, along with other industry leaders: Ari Kynäslahti (VP of Strategy & Technology, CTO at Nokia), Mikael Hook (Research Area Director at Ericsson Research), Mike Millhaem (5G Technical Architect at Keysight Technologies), and Emad Farag (Principal Engineer at Samsung Research America).

The panel session is embedded below, thanks to IEEE Tv:

Mike Thelander also shared a Signals Flash report summarising the presentations and discussions from the Brooklyn 6G Summit. You can access the full report here. Below is an excerpt from the report about this session:

We hosted a panel to end the festivities on Friday afternoon. We had academia (NYU), infrastructure suppliers (Ericsson, Nokia and Samsung), operators (KDDI), and T&M (Keysight) on stage with us. As the moderator, we got to throw out a few questions before tossing the responsibility to the audience so they could do our job for us. Since we didn’t have the luxury of being able to take notes, we are going to refrain from quoting individual’s verbatim, instead relying on paraphrased statements that hopefully captured the essence of what was being said. We also include personalized views, when appropriate.

The general view of the panelists was that they were cautiously optimistic regarding the coverage profile of FR3 spectrum (6-8 GHz) versus today’s midband spectrum (3.5 GHz). Specifically, they felt FR3 coverage would be sufficiently good enough that it could be overlayed on top of an existing midband cell grid without generating gaps in coverage. In its presentation which preceded the panel, KDDI showed results from various simulations it had done involving FR3 coverage in downtown Tokyo. Comparing FR3 with 3.5 GHz, the operator noted there is a 6 dB free-space pathloss with Line of Site (Los) and a 3 dB diffraction loss with NLoS. These shortfalls are subsequently partially offset by a 6 dB gain in antenna performance since the higher frequency allows for more antenna elements.

KDDI modeling results also showed that while the downlink SINR at 7-8 GHz wasn’t as good as it was at 3.5 GHz, it wasn’t meaningfully different either. During lunch, one of the panelists also indicated to us that having some coverage gaps wouldn’t necessarily be entirely bad, because it would help minimize inter-cell interference, thereby helping to increase cell capacity. Edge of cell coverage would presumably come from the 3.5 GHz bands.

KDDI modeling results also showed that while the downlink SINR at 7-8 GHz wasn’t as good as it was at 3.5 GHz, it wasn’t meaningfully different either. During lunch, one of the panelists also indicated to us that having some coverage gaps wouldn’t necessarily be entirely bad, because it would help minimize inter-cell interference, thereby helping to increase cell capacity. Edge of cell coverage would presumably come from the 3.5 GHz bands.

Our view is that we’ll believe it when we see it. If nothing else, the one-for-one overlay also assumes a solid 3.5 GHz cell grid, and we’re not sure such a grid always exists outside of dense urban areas based on our tests. We’re also concerned about the uplink since it takes both directions to sustain good bidirectional throughput. As we commented during the panel discussions, with 5G mmWave, the downlink coverage is actually quite impressive – something we initially proved with a Rohde & Schwarz scanner back in 2018 when we tested Verizon’s 5GTF network in Houston. The problem, as we noted in subsequent tests, was the uplink direction – an issue that got partially addressed with high-power CPEs and the FWA use case. Panelists felt that using FR3 for the uplink was best, but they wouldn’t rule out the potential use of FDD-TDD CA whereby a lower FDD band could be used for the uplink data and control traffic. FDD-TDD CA is used today by operators with Band n77/n78 and Band n41.

Our question on spectral efficiency resulted in a surprising response. We asked panelists for their views on 6G spectral efficiency in FR3 compared with today’s midband 5G networks. The response we got back was that there could be a 10x increase in spectral efficiency relative to today’s networks when taking into consideration both the new air interface and the new frequency band. We assume the panelists were addressing the question from a bps/Hz perspective on not a total capacity perspective since that is how we phrased the question. A 10x spectral efficiency gain would be very impressive – if it was 2x (bps/Hz) we would be impressed. Worth noting, in a recent 5G Americas whitepaper, the stated view from the member companies was that initial 6G capacity gains will come primarily from access to wider channels (400 MHz), followed by spectral efficiency gains of up to 4x due to advanced beamforming techniques.

One big issue with FR3 is spectrum availability and global harmonization. In the US, the 7-8 GHz frequencies will likely end up being allocated as unlicensed bands due to incumbents. As we noted during the panel, the spectrum sharing approach used with CBRS spectrum has been less than successful. In our view, it is overly conservative in terms of when/where the spectrum can be used for cellular transmissions, plus there are transmit power limitations that make the frequencies unsuitable for providing macro-level coverage. The only way the higher frequencies can be shared while meeting basic coverage goals is to invoke a much better means of spectrum sharing. Some form of AI-based sharing is possible, but in the words of one of the panelists, 3GPP should get the task to develop a much better spectrum sharing technology versus leaving it to outside forces. We agree, as long as there is participation and buy-in from the incumbents.

We asked the panelists to cite a potential unforeseen challenge with the new FR3 frequencies – the challenge could be technical, logistical, or economical. One panelist noted that harmonics from radiation in lower frequencies, like Wi-Fi, can have a huge impact on performance in FR3. They observed as much in some of their lab testing. From our perspective, we think 6G coverage in lower bands will play an important role in driving the adoption of 6G. No one will want to spend a few extra hundred dollars on a 6G phone if 6G coverage is very isolated. Consumers are more likely to purchase a 6G smartphone if 6G is available “everywhere,” even if its performance in a lower FDD band is modestly incremental to 5G.

Interestingly, 5G and 6G sharing of the same radio channel will likely happen, despite the poor performance of DSS (dynamic spectrum sharing), involving 4G and 5G. We’ve tested DSS on a couple of occasions, and we showed with excruciating detail how the LTE channel overhead severely impacted the performance of both technologies. DSS worked in that it allowed two generational technologies to share the same radio channel, but it just didn’t deliver good spectral efficiency. With MRSS (Multi-RAT Spectrum Sharing), the inefficiencies of DSS are removed because the 5G radio channel doesn’t have the same overhead as a 4G radio channel. The downside, we believe, is that the use of MRSS inherently implies the characteristics of 6G waveform will be incremental to 5G. So much for innovation!

The report has other interesting bits worth reading too. It's available here.

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