How “Massive” are the Current Massive MIMO Base Stations?

I have written earlier that the Massive MIMO base stations that have been deployed by Sprint, and other operators, are very capable from a hardware perspective. They are equipped with 64 fully digital antennas, have a rather compact form factor, and can handle wide bandwidths in the 2-3 GHz bands. These facts are supported by documentation that can be accessed in the FCC databases.

However, we can only guess what is going on under the hood – what kind of signal processing algorithms have been implemented and how they perform compared to ideal cases described in the academic literature. Erik G. Larsson recently wrote about how Nokia improved its base station equipment via a software upgrade. Are the latest base stations now as “Massive MIMO”-like as they can become?

My guess is that there is still room for substantial improvements. The following joint video from Sprint and Nokia explains how their latest base stations are running 4G and 5G simultaneously on the same 64-antenna base station and are able to multiplex 16 layers.

This is the highest number of multiuser MIMO layers achieved in the US” according to the speaker. But if you listen carefully, they are actually sending 8 layers on 4G and 8 layers 5G. That doesn’t sum up to 16 layers! The things called layers in 3GPP are signals that are transmitted simultaneously in the same band, but with different spatial directivity. In every part of the spectrum, there are only 8 spatially multiplexed layers in the setup considered in the video.

It is indeed impressive that Sprint can simultaneously deliver around 670 Mbit/s per user to 4 users in the cell, according to the video. However, the spectral efficiency per cell is “only” 22.5 bit/s/Hz, which can be compared to the 33 bit/s/Hz that was achieved in real-world trials by Optus and Huawei in 2017.

Both numbers are far from the world record in spectral efficiency of 145.6 bit/s/Hz that was achieved in a lab environment in Bristol, in a collaboration between the universities in Bristol and Lund. Although we cannot expect to reach those numbers in real-world urban deployments, I believe we can reach higher numbers by building 64-antenna arrays with a different form factor: long linear arrays instead of compact square panels. Since most users are separable in terms of having different azimuth angles to the base station, it will be easier to separate them by sending “narrower” beams in the horizontal domain.

15 thoughts on “How “Massive” are the Current Massive MIMO Base Stations?”

  1. Professor Bjornson,

    Thank you very much for interesting blog as always.

    I am wondering if each user gets one “layer” at a time, or if a user can get two layers from two separate frequency bands (5G & LTE) simultaneously.

    I also wonder how they assign antennas for each layer. When supporting 16 layers simultaneously, I guess they use 4 antennas per layer?

    Lastly, I assume every commercial massive MIMO operation is handled in TDD? What will they do with the FDD legacy bands if massive MIMO station takes over all wireless providers?

    1. According to the video, each user get two layers: One per polarization. It doesn’t matter how many bands you have, this doesn’t increase the number of layers. If you have 10000 subcarriers and two layers per subcarrier, then you do MU-MIMO with two layers, not 20000 layers. This is the point that I want to make.

      One does not assign antennas to layers, but transmit each layer over all the antennas but with different power and phase-shifts to form a spatial beam in a desired direction. This is how precoding works.

      Sprint has TDD spectrum in the considered band so I suppose that is what was used. I would guess that FDD bands will be continued to used for the time being, even if the performance is lower than in TDD.

  2. Hello, Dr. Bjornson.
    According to the definition of the massive MIMO in your book, in these systems the number of BS antennas should be very greater than the number of usres they serve.
    So, in a system with 64 antennas at BS, the number of the users they serve should be fewer than 10 or maybe fewer than 5.
    Do not we look at this as a limitation?
    Will we see the implementation of massive MIMO with more antennas in the near future? How many the most number of antennas are?
    What are the main limitations of using more antennas at the BS?

    1. Definition 2.1 in my book only says that we should have M/K>1, so that there are more antennas than users. I have purposely avoided to make claims regarding what the ratio should be since it depends on many different factors, such as array geometry and propagation environment. If you have a look at Figure 7.7, the highest sum spectral efficiency is achieved when M ≈ 2K. So with 64 antennas, I think it is fully possible to serve up to 32 users.

      We will most likely see base stations with more antennas in the future, particularly at mmWave frequencies, where I think there are already 256-antenna base stations in the pipeline.

  3. If layers “are signals that are transmitted simultaneously in the same band, but with different spatial directivity”, then transmitting 2 signals with different polarisations wouldn’t count as different layers.

    1. I suppose you interpret “spatial directivity” in a different way than I had in mind. What I mean is that the signals are precoded with different precoding vectors and therefore will propagate in different ways to the receivers.

  4. How will the much anticipated high density users be served in a 5G network? It’s expected that 5G user density will be about 1mil/km2. What level of MU-MIMO will suffice? I think even 256 antenna arrays won’t give best user experience.

    1. Even if there is 1 million devices in a 1 km x 1 km area, all users won’t be active simultaneously. Even if they run applications simultaneously, the data traffic on the physical later is intermittent. Maybe 0.1% of the users will be active.

      I like to view it from the opposite direction. In current networks, most base stations only have 1 or 2 active users per resource block. If we can increase that to 8 or 16 users per resource block by MU-MIMO, then we have taken a big step forward.

  5. Thank you for this post ! But the used precoding vectors are generally based on fixed codebooks and thus fixed beams. Do you think there is way to improve spectral efficiency by using linear precoders such as ZF or MRT, which should adapt better to the environment ?
    And if so, how comparable would these results be to lab experiments in terms of power restrictions and environments ?

    1. Even if codebooks are used in the training and/or feedback phases, the precoding doesn’t have to be selected from that codebook. If a better precoder can be selected, it will improve performance. The gain from ZF and regularized ZF will be larger the better the SNR is. It is the SNR that might differ from a lab environment and real deployments.

  6. Professor can one say in a massive MIMO scenario, the maximum number of layer is equal to the maximum number of beams that can be generated simultaneously at a specific band ?

    1. Yes, this is the maximum number of layers that one would like to multiplex. (It is theoretically possible to send more but the performance won’t improve.)

  7. Hello, Professor Bjornson.
    I have a question:
    Is it essential in massive MIMO sysrems, that BS transmit with all antennas?
    For example if we transmit with only uncorrelated antennas to a user, is not it more efficient according to diversity gain?

    1. No, from a performance perspective, it is always preferable to use all antennas. It doesn’t matter if you consider the capacity, ergodic capacity, or outage probability. It is true that one can achieve the same diversity or multiplexing gain using fewer antennas, but that doesn’t mean that the performance is the same, just that it scales in the same way.

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