There are times when even the most “techie” of consumers begins to wonder if there’s any way of making sense out of the barrage of features available in the rapidly evolving world of communications. As soon as you think you understand something, it gets changed or placed. Everything is a jumble of letters and numbers. First there was CDMA, then 4G/LTE, now 5G. And, of course there’s 802.11b/g/n and ac! We’ve talked about these in previous blog posts. In this post I’m going to discuss a feature that goes by the acronym MIMO which stands for Multiple Input/Multiple Output. You may have heard people saying that you “have to get MIMO antennas”; today we’ll talk a bit about what that means!
I’m sure that virtually everyone has, at one time or another, stared at the top of a cell phone tower and wondered why there were so many antennas clustered up there. Why do they have to have so many antennas side by side? We usually think about connecting to an antenna tower as if we drew a line from the top of the tower to our device. But what if we could draw more than one line from a cellular tower to our device? What if we could draw lines from our device to several of the antennas on the tower? Could we get more data to flow between the tower and our device?
It’s easy to understand that if we were connecting water hoses from a water source to our RV we could get more water to flow if we connected several hoses in parallel. Several hoses in parallel would act as if they formed a bigger pipe.
It’s also easy to understand that if we were running electrical current through wires, we could safely pass more current through several wires than we could any single wire.
With digital radio signals the concept is similar, but the process is a lot more complicated. If we had several antennas on the tower and several on our device, there’s no way to ensure that the signal from Antenna X on the tower gets to Antenna A on your device. In fact, what Antenna A is actually going to see is a mixture of the signals from Antennas X, Y, Z, etc. Likewise, Antenna B on our device is going to see a similar mixture of signals coming from each of the antennas that are broadcasting to you.
Even if the exact same signal is transmitted by both antennas, what will be received by A and B is going to be a mix of all of that and that mix will also be supplemented by reflected signals which may even have slight time delays. Quite often what’s done is to broadcast the same signal using two different polarizations as is shown in Fig 1. Even though the both polarizations contain the same information, from a signal processing perspective we can consider them to be two different data streams and use digital signal processing to separate them.
[WARNING—MATH ALERT! This next section uses a little bit of algebra; if you’ve given up math for retirement, you are free to skip to the next section!]
As a simple example, lets assume that the tower has two antennas broadcasting to you and we’ll call them X and Y. We’ll assume that your phone has two antennas and we’ll call them A and B. Mathematically, the signal seen by antenna A on your phone can be represented as:
Signal A(t) = AxX(t) + AyY(t) where Ax and Ay are the signal strengths of antennas X and Y as seen by antenna A all of which are functions of time (t)
Similarly, the signal seen by antenna B on your phone is going to look something like:
Signal B(t) = BxX(t)+ ByY(t)
For some of you these equations are going to bring back faint (painful?) memories from algebra because what we have in this example is nothing more than two equations with two unknowns.
Now the good news is that our little algebra course will end here—we’re not going to have to solve those equations ourselves. But, thanks to modern signal processing techniques, our cellular modems do just that. In fact, by solving these equations the two pairs of antennas on the tower and on your device can act as if they are two separate data transmission “pipes” so the amount of data you can receive in a given period of time is twice as much.
[This is the end of the MATH ALERT!]
A simple MIMO setup with twin antennas as we’ve described is called a 2×2 MIMO (2 transmit antennas and 2 receive antennas) and such simple systems are now common on smartphones, tablets, hotspots, etc. In fact, 2×2 MIMO is now being superseded by 4×4 MIMO on some newer devices and nothing prevents systems from having even more than four.
If all of this wasn’t complicated enough, there’s actually a bit of difference between how MIMO operates in urban environments compared how it works in more rural ones. In a rural environment multiple antennas on a cell tower essentially transmit the same signal but with coding differences (such as different polarizations) so they can be distinguished from each other. A cell phone with multiple antennas can receive these transmissions and “compare” them. By doing this the “accuracy” of the received signal is improved which results in an overall improvement in phone performance. This most basic use of MIMO is called “transmit diversity and it can enable phones to achieve fairly high speeds with relatively weak signals.
However, in a more urban environment, where there are more cell towers within range and more surfaces to cause reflections, different data streams can be transmitted from different antennas so that the data speeds achieved can be significantly higher that would be possiblle with a single data stream. MIMO operating in this manner is said to be using “spatial diversity.” For those of us who grew up in an analog broadcast world, an amazing aspect of spatial diversity MIMO is that it is actually beneficial to have reflections, the very things that used to cause “ghosts” on our old TV pictures. It’s the use of these reflected signals that enables MIMO to differentiate the signals coming from different antennas. Figure 2 illustrates how urban reflections can be used in the MIMO process. The red and purple signals travel on different paths and have different delays as a result. Using signal processing both data streams are recovered and the total speed can be twice or more than the speed of either stream
So how does all affect how a phone performs?
Performance tests have demonstrated that going from 2×2 MIMO to 4×4 MIMO can give you improved wireless signal strength and speed. For example, some tests compared the iPhone XR to the iPhone XS. The iPhone XR and iPhone XS have the same wireless modem, but the XR has 2×2 MIMO whereas the XS has 4×4. When both phones were both connected to a 4×4 MIMO LTE network, the 4×4 iPhone XS topped out at a download speed of just under 400 Mbps. The 2×2 MIMO iPhone XR topped out at right under 200 Mbps at the same signal strength. That’s a pretty amazing performance improvement without any other differences between the two phones. Figure 3 [reference 1] shows the insides of a Samsung Galaxy S8. The cellular antennas are along the top and near the bottom. It’s amazing how much is stuffed into these devices.
So, the next time you upgrade your phone, ask what type of MIMO it uses.
One additional consideration worth noting about MIMO is that using it reduces the benefit of having a simple cellular amplifier. In fact, using an such an amplifier can actually result in a performance decrease because it will prevent the phone or hotspot from taking advantage of the speed increases that derive from MIMO. When a MIMO-equipped cell phone is combined with a single-channel cellular amplifier all the embedded MIMO information is lost. Yes, the signal seen by the phone will be stronger, but all the advantages provided by MIMO will be lost. Essentially, the phone will revert back to a 1×1 MIMO which is how we define a single antenna configuration. The “rule of thumb” these days is that if you can obtain a usable data signal without an amplifier, you’ll probably be better off without it! That’s not to say that an amplifier is never beneficial, but in many cases you may be better off just relying on MIMO to achieve maximum speed.
By now you should have basic understanding of how MIMO can improve your cell phone reception. Next month we’ll talk about how the same concepts can be applied to WiFi communications.
IEEE Spectrum, “Building Smartphone Antennas That Play Nice Together”, Sampson Hu and David Tanner, 10/23/2018