What is a Transmit Antenna?

This is supposedly a simple question to answer; an antenna is a device that emits radio waves. However, it is easy to get confused when comparing wireless communication systems with different number of transmit antennas, because these systems might use antennas with different physical sizes and properties. In fact, you can seldom find fair comparisons between contemporary single-antenna systems and Massive MIMO in the research literature.

Each antenna type has a predefined radiation pattern, which describes its inherent directivity; that is, how the gain of the emitted signal differs in different angular directions. An ideal isotropic antenna has no directivity, but a practical antenna always has a certain directivity, measured in dBi. For example, a half-wavelength dipole antenna has 2.15 dBi, which means that there is one angular direction in which the emitted signal is 2.15 dB stronger than it would be with a corresponding isotropic antenna. On the other hand, there are other angular directions in which the emitted signal is weaker. This is not a problem as long as there will not be any receivers in those directions.

In cellular communications, we are used to deploying large vertical antenna panels that cover a 120 degree horizontal sector and have a strong directivity of 15 dBi or more. Such a panel is made up of many small radiating elements, each having a directivity of a few dBi. By feeding them with the same input signal, a higher dBi is achieved for the panel. For example, if the panel consists of 8 patch antenna elements, each having 7 dBi, then you get a 7+10·log10(8) = 16 dBi antenna.

Figure 1: Photo of an LTE site with three 8TX-sectors.

The picture above shows a real LTE site that I found in Nanjing, China, a couple of years ago. Looking at it from above, the site is structured as illustrated to the right. The site consists of three sectors, each containing a base station with four vertical panels. If you would look inside one of the panels, you will (probably) find 8 cross-polarized vertically stacked radiating elements, as illustrated in Figure 1. There are two RF input signals per panel, one per polarization, thus each panel acts as two antennas. This is how LTE with 8TX-sectors is deployed: 4 panels with dual polarization per base station.

At the exemplified LTE site, there is a total of 8·8·3 =192 radiating elements, but only 8·3 = 24 antennas. This disparity can lead to a lot of confusion. The Massive MIMO version of the exemplified LTE site may have the same form factor, but instead of 24 antennas with 16 dBi, you would have 192 antennas with 7 dBi. More precisely, you would connect each of the existing radiating elements to a separate RF input signal to create a larger number of antennas. Therefore, I suggest to use the following antenna definition from the book Massive MIMO Networks:

Definition: An antenna consists of one or more radiating elements (e.g., dipoles) which are fed by the same RF signal. An antenna array is composed of multiple antennas with individual RF chains.

Note that, with this definition, an array that uses analog beamforming (e.g., a phased array) only constitutes one antenna. It is usually called an adaptive antenna since the radiation pattern can be changed over time, but it is nevertheless a single antenna. Massive MIMO for sub-6 GHz frequencies is all about adding RF chains (also known as antenna ports), while not necessarily adding more radiating elements than in a contemporary system.

What is the purpose of having more RF chains?

With more RF chains, you have more degrees of freedom to modify the radiation pattern of the transmitted signal based on where the receiver is located. When transmitting a precoded signal to a single user, you adjust the phases of the RF input signals to make them all combine constructively at the intended receiver.

The maximum antenna/array gain is the same when using one 16 dBi antenna and when using 8 antennas with 7 dBi. In the first case, the radiation pattern is usually static and thus only a line-of-sight user located in the center of the cell sector will obtain this gain. However, if the antenna is adaptive (i.e., supports analog beamforming), the main lobe of the radiation pattern can be also steered towards line-of-sight users located in other angular directions. This feature might be sufficient for supporting the intended single-user use-cases of mm-wave technology (see Figure 4 in this paper).

In contrast, in the second case, we can adjust the radiation pattern by 8-antenna precoding to deliver the maximum gain to any user in the sector. This feature is particularly important for non-line-of-sight users (e.g., indoor use-cases), for which the signals from the different radiating elements will likely be received with “random” phase shifts and therefore add non-constructively, unless we compensate for the phases by digital precoding.

Note that most papers on Massive MIMO keep the antenna gain constant when comparing systems with different number of antennas. There is nothing wrong with doing that, but one cannot interpret the single-antenna case in such a study as a contemporary system.

Another, perhaps more important, feature of having multiple RF chains is that we can spatially multiplex several users when having multiple antennas. For this you need at least as many RF inputs as there are users. Each of them can get the full array gain and the digital precoding can be also used to avoid inter-user interference.

4 thoughts on “What is a Transmit Antenna?”

  1. Dear Emil,

    I have some questions, but mostly are not related with this article.

    1. Let’s imagine we have a Massive MIMO antenna (128×128), do we have only one array or we can configure it to multiple ?

    2. During beamforming, is the UE beamformed all the time or in some radio condition ?

    3. As mentioned above, we have 128 antennas and one array. All 128 antennas are transmitting to the UE. What will happen during beamforming ? Only some antennas will be directed to the UE and what about others ?

    4. In most of the articles, we see TDD is used for Massive MIMO. Why not FDD ?

    Maybe my questions are funny :), thanks in advance

    1. Hi!

      1. You can deploy the antennas in any way you like. In one or multiple arrays.

      2. When transmitting from an array, the signal will always be beamformed in one way or another. Maybe not in a particular angular direction, but there will always be some locations where the signal components add constructively and some locations where they combine destructively. It is always beneficial to control the beamforming so that the UE is at a location where constructive signal addition appears.

      3. All antennas transmit to the user, but with different phase-shifts to make sure that constructive signal addition appears at the UE. See also: http://ma-mimo.ellintech.se/2017/10/03/what-is-the-difference-between-beamforming-and-precoding/

      4. See the “grand question” in the paper: https://arxiv.org/pdf/1503.06854.pdf

  2. Dear Emil,
    I have some question, about a similar concept:
    1. Is it possible to use cell sectorization techniques in 5G massive MIMO system?
    2. If it is possible, the cell site may be a hotspot area in a heterogeneous network?

    Thank you!

    1. Hi!

      1. Yes! For example, when using 120-degreee sectorization, each site consists of three base stations that have antenna arrays that point in different directions and form three different cells. (Each sector is a cell by itself.) All the massive MIMO theory and methods can be readily applied to that scenario.

      2. I don’t see a direct connection between sectorization and heterogeneous network. Each cell/sector can either cover a hotspot or large area.

      As a side note, I don’t think sectorization is needed in massive MIMO deployment. The three panels that point in different directions can be jointly used to serve all the users in the coverage area. It is better to discriminate between the users using dynamic beamforming/precoding than with fixed sectorization.

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