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MIMO, SU-MIMO, MU-MIMO, finding NEMO? It’s all buzzword bingo to me!

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

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4G. 5G, 5G+!!! Gee, why do I care?

Written by WiFiRanger Ambassador, Joel Weiss “docj”

To the average person, today’s cellular data marketplace is a jumble of technobabble. Carriers continuously boast of the capabilities of their networks while also claiming that even better service is soon to be available. At the same time several companies planning to establish satellite-based internet systems claim that users will be better off with those (when they exist)! If only there was a way to sift through the “Geek speak” to better understand what the situation actually is!

The acronym 4G LTE actually stands for 4th Generation Long Term Evolution and, believe it or not, it is even a registered trademark. It pertains to cellular transmission standards that were first proposed all the way back in 2004. To be called 4G LTE a cellular system has to be capable of providing at least 100 Mbps capability. 4G LTE is in use essentially all over the world and LTE phones can, with some specific exceptions, be used in most countries. 4G LTE replaced the 3G CDMA network used by some US carriers and that network will be shut down in the near future.

Even though people (and advertisers) use the terms 4G and LTE as if they are synonyms, in reality, the term LTE encompasses futures evolution beyond 4G.

So if LTE is what we have today, what comes next? I’ve heard people talk about Advanced LTE; is that the same as 5G?

Advanced 4G LTE is an improvement on “regular” 4G LTE but it doesn’t represent a whole new technology…For properly equipped cell towers and receivers (phones) Advanced 4G, sometimes called LTE+ in ads, can provide increased download speeds, up to ~300Mbps. To enable this, the cellular network essential permits a receiver to make multiple simultaneous connections to the network. It’s as if you phone or hotspot had two or more parallel connections to the same cellular tower. In “Geek speak” this is called carrier aggregation!

For carrier aggregation to work, the modem in your phone (the device that actually talks to the cellular network) has to be of an advanced type and it has to be communicating with a tower that has the proper hardware on it. Suffice it to say that at present, most of your phone and hotspots won’t yet support this capability and it is not uniformly available in the US.

To make matters even more confusing, some marketing flacks at AT&T decided to create a non-existent standard that they called “5Ge” which is nothing more than AT&T’s implementation of 4G LTE+. Irrespective of anything you hear in an ad, 5Ge is NOT 5G

So, if we don’t yet even have LTE+ why are we worrying about 5G? What would be different about 5G?

The 5G cellular system will be a completely new cellular implementation that will enable users to experience download speeds up to the Gbps range. Although, the actual speed obtained by users on any specific tower will probably be less than that, on the average most people will see download speed improvement of factors of at least 10 to 100. In addition, one of the advantages of 5G will be greatly reduced “ping times” (the time it takes for your “click” to reach the computer on the receiving end.) That would mean that a cellular connection would have plenty of bandwidth to support multiple video streams and/or to engage in real-time gaming

5G technology actually will come in three “flavors” and the implementation you encounter will depend significantly on which carrier you subscribe with and where you live. Different carriers have purchased the rights to use different sets of frequencies for their own 5G implementation. Furthermore, 5G implementation will be different in different parts of the country depending on the population density.

The following graphic depicts a portion of the electromagnetic spectrum and how our current and proposed communications networks fit together. The orange oval in the 0.8-2 GHz region is where today’s cellular phones and hotspots operate. The red oval shows the general spectral region called millimeter wave where the highest performance 5G systems will operate.

At the high performance end of the 5G spectrum there will be very high frequency 5G using what some people refer to as “millimeter waves.” The good news is that systems using mm waves will be capable of download speeds in the ~10 Gbps range. These transmissions will use frequencies of around 25 GHz. The bad news is that these wave are easily blocked by the walls of buildings, trees, rain and other obstacles and there will have to be many small “towers” to serve an area compared with the relatively small number of large towers we have today. Most people expect that this high frequency 5G will mostly be limited to urban and/or suburban environments.

At somewhat lower frequencies, in the 1-6 GHz range, there will be other implementations of 5G. Sprint had made investments in this frequency spectrum and other carriers are expected to use it also. Signals at these slightly lower frequencies will penetrate buildings and other obstacles better than do mm waves, but they won’t have quite as much penetration capability as we are used to with cellular signals today. The download speeds provided by 5G systems operating at these frequencies will be somewhat less than those made possible in the mm wave region.

At the lower end of the spectrum, there will be “low frequency” 5G and the carrier most aggressively pursuing this approach is T-Mobile which made a large investment in frequencies around 600 MHz, the so-called Band 71. T-Mobile is already using Band 71 for 4G LTE service, but later in 2020 it is expected to begin 5G operations using the same band . However, existing phones and hotspots that can receive Band 71 will not, in general, be able to receive 5G broadcasts on Band 71.

Furthermore, the physics of low frequency transmissions, however, limits 5G using these low frequencies to download speeds of ~100 Mbps. That might not compare with the Gbps speeds of higher frequency approaches, but it is sure a lot faster than the 1-10 Mbps speeds many of us live with today!

Wow, that’s a lot of information. When will all this happen?

5G is currently being rolled out by all the major carriers and is available in quite a few major metro areas. Here’s an interactive map of where you can already get 5G:

It will probably take a number of years before the mix of technologies being offered by different carriers shakes out completely. Since I live in a relatively rural area, I doubt I’m going to see much of anything any time soon! But, no doubt our grandchildren will grow up in a word in which everything is wireless. “Grandpa, what’s that funny dish-like thing on the roof of your RV?”

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Bars? Decibels? I Just Wanna Make a Phone Call!

Photo of a smartphone with a speedtest of LTE data.

Written by WiFiRanger Ambassador, Joel Weiss “docj”

From time to time most RVers find themselves in situations where making a simple voice call is difficult and using the Internet is PAINFULLY slooooow or impossible. It’s true that many of these issues start and end with signal strength from our device to the cell tower; but, how do we know for sure that’s the problem?

Continue reading Bars? Decibels? I Just Wanna Make a Phone Call!
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Saving Money on Cellular Data Plans

Photo of elderly couple surfing the internet on a laptop while outside enjoying a picnic.

Nothing quite motivates like money, especially for those on a limited budget while traveling. As Cellular data costs continue to burden those with a mobile lifestyle, alternative internet sources are being sought out. Free WiFi has been ubiquitous and readily available to most travelers, but unfortunately the signal strength, security, and speeds have been lacking. Until an adequate solution is discovered, travelers find themselves stuck between the high cost of Cellular data or the poor performance of free WiFi.  

Continue reading Saving Money on Cellular Data Plans