Last year, the 128-antenna Massive MIMO testbed at University of Bristol was used to set world records in per-cell spectral efficiency. Those measurements were conducted in a controlled indoor environment, but demonstrated that the theoretical gains of the technology are also practically achievable—at least in simple propagation scenarios.

The Bristol team has now worked with British Telecom and conducted trials at their site in Adastral Park, Suffolk, in more demanding user scenarios. In the indoor exhibition hall trial,  24 user streams were multiplexed over a 20 MHz bandwidth, resulting in a sum rate of 2 Gbit/s or a spectral efficiency of 100 bit/s/Hz/cell.

Several outdoor experiments were also conducted, which included user mobility. We are looking forward to see more details on these experiments, but in the meantime one can have a look at the following video:

Update: We have corrected the bandwidth number in this post.

Frequency-division duplex (FDD) operation of Massive MIMO in LTE is the topic of two press releases from January 2017. The first press release describes a joint field test carried out by ZTE and China Telecom. It claims three-fold improvements in per-cell spectral efficiency using standard LTE devices, but no further details are given. The second press release describes a field verification carried out by Huawei and China Unicom. The average data rate was 87 Mbit/s per user over a 20 MHz channel and was achieved using commercial LTE devices. This corresponds to a spectral efficiency of 4.36 bit/s/Hz per user. A sum rate of 697 Mbit/s is also mentioned, from which one could guess that eight users were multiplexed (87•8=696).

There are no specific details of the experimental setup or implementation in any of these press releases, so we cannot tell how well the systems perform compared to a baseline TDD Massive MIMO setup. Maybe this is just a rebranding of the FDD multiuser MIMO functionality in LTE, evolved with a few extra antenna ports. It is nonetheless exciting to see that several major telecom companies want to associate themselves with the Massive MIMO technology and hopefully it will result in something revolutionary in the years to come.

Efficient FDD implementation of multiuser MIMO is a longstanding challenge. The reason is the difficulty in estimating channels and feeding back accurate channel state information (CSI) in a resource-efficient manner. Many researchers have proposed methods to exploit channel parameterizations, such as angles and spatial correlation, to simplify the CSI acquisition. This might be sufficient to achieve an array gain, but the ability to also mitigate interuser interference is less certain and remains to be demonstrated experimentally. Since 85% of the LTE networks use FDD, we have previously claimed that making Massive MIMO work well in FDD is critical for the practical success and adoption of the technology.

We hope to see more field trials of Massive MIMO in FDD, along with details of the measurement setups and evaluations of which channel acquisition schemes that are suitable in practice. Will FDD Massive MIMO be exclusive for static users, whose channels are easily estimated, or can anyone benefit from it in 5G?

Update: Blue Danube Systems has released a press release that is also describing trials of FDD Massive MIMO as well. Many companies apparently want to be “first” with this technology for LTE.

The IEEE GLOBECOM conference, held in Washington D.C. this week, featured many good presentations and exhibitions. One well-attended event was the industry panel “Millimeter Wave vs. Below 5 GHz Massive MIMO: Which Technology Can Give Greater Value?“, organized by Thomas Marzetta and Robert Heath. They invited one team of Millimeter Wave proponents (Theodore Rappaport, Kei Sakaguchi, Charlie Zhang) and one team of Massive MIMO proponents (Chih-Lin I, Erik G. Larsson, Liesbet Van der Perre) to debate the pros and cons of the two 5G technologies.

For millimeter wave, the huge bandwidth was identified as the key benefit. Rappaport predicted that 30 GHz of bandwidth would be available in 5 years time, while other panelists made a more conservative prediction of 15-20 GHz in 10 years time. With such a huge bandwidth, a spectral efficiency of 1 bit/s/Hz is sufficient for an access point to deliver tens of Gbit/s to a single user. The panelists agreed that much work remains on millimeter wave channel modeling and the design of circuits for that can deliver the theoretical performance without huge losses. The lack of robustness towards blockage and similar propagation phenomena is also a major challenge.

For Massive MIMO, the straightforward support of user mobility, multiplexing of many users, and wide-area coverage were mentioned as key benefits. A 10x-20x gain in per-cell spectral efficiency, with performance guarantees for every user, was another major factor. Since these gains come from spatial multiplexing of users, rather than increasing the spectral efficiency per user, a large number of users are required to achieve these gains in practice. With a small number of users, the Massive MIMO gains are modest, so it might not be a technology to deploy everywhere. Another drawback is the limited amount of spectrum in the range below 5 GHz, which limits the peak data rates that can be achieved per user. The technology can deliver tens of Mbit/s, but maybe not any Gbit/s per user.

Although the purpose of the panel was to debate the two 5G candidate technologies, I believe that the panelists agree that these technologies have complementary benefits. Today, you connect to WiFi when it is available and switch to cellular when the WiFi network cannot support you. Similarly, I imagine a future where you will enjoy the great data rates offered by millimeter wave, when you are covered by such an access point. Your device will then switch seamlessly to a Massive MIMO network, operating below 5 GHz, to guarantee ubiquitous connectivity when you are in motion or not covered by any millimeter wave access points.