3D Beamforming, is that Massive MIMO?

No, these are two different but somewhat related concepts, as I will explain in detail below.

Contemporary multiantenna base stations for cellular communications are equipped with 2-8 antennas, which are deployed along a horizontal line. One example is a uniform linear array (ULA), as illustrated in Figure 1 below, where the antenna spacing is uniform. All the antennas in the ULA have the same physical down-tilt, with respect to the ground, and a fixed radiation pattern and directivity.

Figure 1: Azimuth 2D beamforming from a horizontal ULA.

By sending the same signal from all antennas, but with different phase-shifts, we can steer beams in different angular directions and thereby make the directivity of the radiated signal different from the directivity of the individual antennas. Since the antennas are deployed on a one-dimensional horizontal line in this example, the ULA can only steer beams in the two-dimensional (2D) azimuth plane as illustrated in Figure 1. The elevation angle is the same for all beams, which is why this is called 2D beamforming. The beamwidth in the azimuth domain shrinks the more antennas are deployed. If the array is used for multiuser MIMO, then multiple beams with different azimuth angles are created simultaneously, as illustrated by the colored beams in Figure 1.

Figure 2: Elevation 2D beamforming from a vertical ULA.

If we would rotate the ULA so that the antennas are instead deployed at different heights above the ground, then the array can instead steer beams in different elevation angles. This is illustrated in Figure 2. Note that this is still a form of 2D beamforming since every beam will have the same directivity with respect to the azimuth plane. This antenna array can be used to steer beams towards users at different floors of a building. It is also useful to serve flying objects, such as UAVs, jointly with ground users. The beamwidth in the elevation domain shrinks the more antennas are deployed.

Figure 3: 3D beamforming from a planar array.

If we instead deploy multiple ULAs on top of each other, it is possible to control both the azimuth and elevation angle of a beam. This is called 3D beamforming and is illustrated in Figure 3 using a planar array with a “massive” number of antennas. This gives the flexibility to not only steer beams towards different buildings but also towards different floors of these buildings, to provide a beamforming gain wherever the user is in the coverage area. It is not necessary to have many antennas to perform 3D beamforming – it is basically enough to have three antennas deployed in a triangle. However, as more antennas are added, the beams become narrower and easier to jointly steer in specific azimuth-elevation directions. This increases the array gain and reduces the interference between beams directed to different users, as illustrated by the colors in Figure 3.

The detailed answer to the question “3D Beamforming, is that Massive MIMO?” is as follows. Massive MIMO and 3D beamforming are two different concepts. 3D beamforming can be performed with few antennas and Massive MIMO can be deployed to only perform 2D beamforming. However, Massive MIMO and 3D beamforming is a great combination in many applications; for example, to spatially multiplex many users in a city with high-rise buildings. One should also bear in mind that, in general, only a fraction of the users are located in line-of-sight so the formation of angular beams (as shown above) might be of limited importance. The ability to control the array’s radiation pattern in 3D is nonetheless helpful to control the multipath environment such that the many signal components add constructively at the location of the intended receiver.

6 thoughts on “3D Beamforming, is that Massive MIMO?”

  1. Dear Björnson,

    As I understand here, if we have a few antennas then 3D Beamforming is possible to serve different located users. So multiple beams can be generated at same time. Right ?

    — Let’s imagine, we have 64 antennas in one array antenna. And two cases:

    1. All these antennas are serving a UE with a single beam. 64 antennas to one UE.

    2. All these antennas are serving multiple UE at same time, with different beams. 64 antennas to different locations and different users.

    What is difference between radiated signal powers between two cases?

    1. Yes, you can do 3D beamforming with few antennas, but each beam will be wide so the users will have to be very well separated in space to avoid having large inter-user interference.

      Regarding the two cases, the total radiated power of a base station is typically limited. Hence, the radiated power will be the same in both cases.

      1. Thanks for feedback.

        But regarding to the cases, I think my question was wrong. It should be “What is difference between received signal powers between two cases?”

        Because, if the reason is to improve the signal in high frequency with Massive MIMO & Bemaforming. In this case the UE (one UE) should has stronger received signal strength with 64 antennas, but in the second case the signal should be less when the 64 antennas start to serve multiple UE at the same time.

        1. Yes, the UE will have a stronger received signal strength when it is the only one being served.

          But suppose there are 10 UEs to serve. How do you serve them?

          1. Serve one UE at a time. Each UE is active 10% of the time, but will get a high received signal strength when it is active.

          2. Serve all UEs at the same time. Each UE is active 100% of the time, but will have a lower received signal strength and also be subject to interference from the other UEs.

          The second option leads to a higher data rates for every UE, as long as you have sufficiently many antennas to control the interference between the UEs.

  2. Hi Emil, as always interesting post on the matter. Anyway, I’d like to argue about the sentence: “Massive MIMO can be deployed to only perform 2D beamforming”.
    Can you comment on that? Why a massive number of antenna elements is more appropriated for a 2D array while instead, it isn’t for a UPA? I mean, Massive MIMO can be deployed also to perform 3D beamforming if it is required an extremely narrow beam in a specific azimuth-elevation direction. Am I right?

    1. I didn’t mean that it is preferred/appropriate to deploy Massive MIMO to perform 2D beamforming, just that it is physically possible – to make the point that neither “Massive MIMO -> 3D beamforming” nor “3D beamforming -> Massive MIMO” are true implications.

      My argument are not made up. You can have a look at Fig. 1 the following measurement paper: https://arxiv.org/pdf/1403.3376.pdf
      The UCA will perform 3D beamforming, while the ULA will perform 2D beamforming.

      If you are interested, you can read more about array deployment in Section 7.4 of my book “Massive MIMO Networks”. For example, we explain why it is preferred to have more antennas in the azimuth domain than in the elevation domain.

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