Let's now learn a little bit about massive MIMO and as I said earlier we have an entire module dedicated to learning massive MIMO. But let's at least take a high level look at what massive MIMO is and why exactly it is beneficial. Before we look at massive MIMO however, let's take a step back and understand what MIMO is to begin with because I understand some of you, may not be familiar with this terminology. MIMO stands for multiple input, multiple output and what are these inputs and outputs were talking about? Well, to understand that let's consider a very simple wireless system reminiscent of what we had considered during module 1. We had mentioned that let's say this is the transmitter and this is the receiver. We have mentioned that the transmitter will have an antenna and the receiver will have an antenna. The wireless signal travels between the transmitter and the receiver. Now this is okay, it will work. However, nothing says that you have to be limited on either transmitter or receiver with just one antenna. What if both the transmitter and the receiver had one more antenna that was working in parallel. So let's say this would be your transmit antenna one, and this would be your transmit antenna two. Its signal might look something different and it will reach receiver antenna two. So instead of sending just one wireless signal between two devices, you are now trying to send to different wireless signals between the same pair of devices. And if you do it carefully, it stands to reason that because those two wireless signals will carry a different data, you will have essentially doubled your data rate or output or your speed because some of the data will be carried on this wireless signal. Whereas at the same time, because you have an additional pair of transmit and receive antennas, you will be able to send a parallel a wireless signal carrying a different set of data. And that is what will significantly improve your data rate or speed or to put. And that is the fundamental premise of MIMO or multiple input, multiple output. Input in this context is the number of antennas on the transmitters and output in this context is the number of antennas on the receiver. So if both the transmitter and the receiver, multiple antennas, the multiple input multiple output, it is said to be a MIMO system. And the fundamental benefit of a MIMO system is that if you implement and design it carefully, it will help you significantly improve your data rate. Because instead of being limited to just one wireless signals between a transmitter and a receiver pair, you'll be able to send many more wireless signals at the same time. How many more? Well, that is the premise of massive MIMO, massive MIMO as you may have guessed at this point is just MIMO that is implemented on a more massive scale. What says that you have to be limited to two antennas, what if you had 10 antennas, 50 Antennas, 100, 200 or 500 antennas on your transmitter and receiver. You could potentially have 100,200 or even 500 data streams at least in theory to simplify things. So MIMO when it is implemented on a massive scale that is called massive MIMO. Now additional or higher throughput is definitely one benefit of my MIMO but there is another and in fact I would argue even more important benefit of my MIMO. And this discussion tends to get a little technical, so stay with me here. Even if you don't understand the technicalities, at least try to grasp the essence of this discussion and you will be good I assure you. We have seen that there are two different wireless signals. But one might ask, hey, what about the frequency channels on which these wireless signals are sent? Are these two wireless signals going to require two different frequency channels and hence more spectrum? Well, surprisingly the answer to that question is no. And that is what requires a bit of technical understanding, but as always, let's try to grasp it at a high level. The headline here is that these two wireless signals are sent on the exact same frequency spectrum using the exact same frequency channels. So you don't need extra spectrum or an additional channel on top of the same channel you have, you can multiplex so to speak these two wireless signals. Hey, didn't you tell us earlier that if you send two different signals on the same channel at the same time, you will have interference. Well technically that's still is correct, but this is where the beauty of MIMO comes into picture. MIMO is not just about having more antennas, it is also about executing some smart signal processing algorithms in the back end over here before you transmit those signals. At a high level the reason these two signals won't interfere with each other is because they are encoded by the transmitter differently. And there is as always an easier way to understand this. Imagine that there are four people in the room, there is person A, person B, Person C and let's say you okay? Now person A is trying to talk to person B and person C is trying to talk to you because you are in the same room,, sitting close to each other, let's say all four of you speak the same language. Then whenever A tries to talk to B you will get some form of crosstalk because you understand that language and your brain naturally will try to interpret the other conversation as well. And in that terms that other conversation will interfere so to speak with the conversation you are trying to have with your partner. On the other hand imagine that A and B suddenly starts speaking in a language that you don't understand even though they are in the same room because they are speaking in a language that you don't understand. Naturally your brain will try to tune that conversation out as a part of background noise and you will be able to better focus on the conversation you're having with your partner. So even though all four people are sitting in the same room trying to talk to each other in pairs. If they use the same language, they will interfere with each other so to speak. But if they use different languages, you will be able to certainly mitigate a large portion of that interference or eliminate it altogether. And although I'm making certain simplifications here, that is pretty much the essence of why MIMO allows two different signals to be sent on the same channel at the same time without them interfering with each other because those signals are encoded differently, so to speak. And they speak different language in simple terms. And if you got at least a high level just of this discussion, you will be able to probably connect the dots. That because you are sending more data using the same frequency channel, you have suddenly improved the spectral efficiency of your system, a metric that we also learned about earlier. So not just high throughput, but higher spectral efficiency is the second. And I would argue perhaps the more important benefit of MIMO or by extension massive MIMO. So this is the fundamental concept of what MIMO is and with that in mind. Let's try to see how massive MIMO would relate to an everyday example in transportation terms. To do that, imagine a double lane road that you have built underground that will have a certain capacity in terms of number of cars it can handle. That would be equivalent to your first pair of antennas over here. But what if you build another deck of the road on top of your existing road in order to build that new deck? Keep in mind, you won't need an extra land because you will be building that exactly on top of your existing road. So the amount of new land required is virtually zero. So if you could build another deck on top of your existing road and have that deck carry an equivalent amount of traffic. You have suddenly doubled the amount of cars that you can handle without using any extra land. Imagine you could Build a third deck on top fourth, fifth and continue doing it until your resources allow you to do so. You will have gradually multiplied your traffic handling capacity without utilizing any additional physical resources i.e without utilizing additional land. And that is the equivalent of massive MIMO everyday terminologies. And if you understand that, it becomes easier to understand the second point about interference. So, this is what shows you the first point of massive MIMO that is instead of just a one story road so to speak, you can build a multi decker road on top of each other. And without using any additional land, they will be able to handle significantly more traffic than your single storey road here. And that will essentially improve your performance, so to speak. And coming back to the point of why different schemes of MIMO don't interfere with each other. I have given you some explanation earlier, but another parallel explanation can be found here. Imagine any two levels of the road, let's say this bottom one and the second one. The roads are pretty much headed in the same direction. However, the car's on one deck won't interfere or collide with cars on a different deck because even though they are going in the same direction at the same time, they are utilizing different dimensions of space, so to speak. This is utilizing the bottom level, whereas this is utilizing the second level of the multi deck rule. And because those cars won't interfere with each other because they are going on parallel but different routes, that can be understood as another intuitive reasons so to speak. As to why different data streams are different wireless signals in the MIMO system won't interfere with each other as long as you're careful about including those signals effectively and the transmitter. So at the end of this long winded explanation, I hope you're able to appreciate the true power of my MIMO and massive MIMO in that by transmitting multiple wireless signals at the same time using the same frequency channel. MIMO significantly increases network coverage and capacity and soldiers massive MIMO. And in terms of everyday analogies, it is equivalent to a multi deck road where in different levels are stacked on top of each other. So without using any additional lands, you are able to send more traffic in the same directions.
