>> The other things that you mentioned earlier like latency, traffic, I think those are pretty straightforward. So I don't understand that. How about spectral efficiency, can you tell us more about that? >> Absolutely, so the easiest way to understand spectral efficiency is to split it into two words. First word is spectral, there is efficiency and once you do that you kind of have a good guess of what it means. Spectral efficiency in one sentence would be the efficiency with which your wireless device, for example, your phone is able to utilize the spectrum that has been given to it by the network. Let's say that a certain device is able to send 100 bits of data in a given second on a given part of the spectrum. And the other device is able to send let's say 200 bits of data within the same portion of the spectrum. Then you would say that the second device has twice as much spectral efficiency as compared to the first device because while utilizing the same amount of underlying fundamental resource, which is your spectrum, the second device is able to send more data. In that it is able to operate more efficiently and because it is able to use the spectrum allocated to it more efficiently, we would say that the second device has higher spectral efficiency as compared to the first device. Spectral efficiency technically depends on a myriad other factors like the sophistication of your device, kind of capabilities that are available in the network. What are the operating conditions in your wireless channel? And that discussion tends to get a little detail oriented. But suffice it to keep in mind that there are multiple factors that impact the spectral efficiency with which your phone operates. There could be an easier way to understand the importance of spectral efficiency because trust me, millions of dollars and thousands of hours are invested every year in order to improve the spectral efficiency by even a fraction of a person. That is how important spectral efficiency is. Because think about this, spectrum costs millions or billions of dollars. If you could utilize the same amount of spectrum and deliver more data IE, If you could get bigger bang for your buck, so to speak, that would positively affect every network operator and service providers bottom line. And that is what makes spectral efficiency one of the most important metrics in modern day wireless communications. That's it, there is an easier way to understand what spectral efficiency means. Consider a popular book, let's say that the city library has only one copy of that book and there is a long queue of people waiting to get that book to borrow that and read it at home. Let's say the book is 100 pages and an average person can read maybe ten pages a day. So a typical person will nearly ten days to finish that book. Hundred divided by ten. So the second person who's waiting for that book will need to wait 10 days before he can get the book for the first time. But imagine if all the readers were more efficient in that they could read 20 pages a day instead of ten pages a day. So a typical reader will now be able to finish the same book in five days instead of ten, that has two implications. Not only the current reader is finishing the book much earlier than expected, but the second the reader who would earlier have to wait ten days to get the book, now we'll get the same book within five days. So not only are individual readers completing the books quickly, subsequent readers in the queue have to wait shorter and shorter amount of time in order to get access to that book. And spectral efficiency in this analogy would be the speed at which users read the book. If one user operates with better spectral efficiency, not only can you improve the data rate or throughput for that specific user, but because more resources are now available for other users, you can similarly improve through ports and user experience or data rate that the other users in the network get as well. So that is another reason why spectral efficiency gets as much attention in the wireless world that you see it getting. >> Wow, that's so fascinating. I think the other interesting point that you brought up is at the edge cells call drops can happen. Is still the case for 5G network? >> Call drops are unfortunately a part and parcel of pretty much every cellular technology over the years. But what we can definitely see is that as cellular generations have progressed we have made more and more progress in minimizing or somewhat eliminating the possibility of those coverage holes as they call them. Now to answer your question, the possibility of certain spots lacking coverage in a geographical area that is still existent. Although the possibility of that happening is going significantly down over the years. Case in point. If you try to recall ten or 15 years ago when cell phones were somewhat new, we wouldn't really be able to make voice calls from deep down in the basement or while we were in an elevator. But if you fast forward to today, I don't think we have any difficulty in making phone calls or browsing high speed internet even when we are in the basement or going up and down in an elevator. So that is one progress that technologies have made over the years at a high level. Specifically to 5G, what kind of steps it will take to mitigate or altogether eliminate the possibility of certain blind spots. So to speak well there are multiple such solutions and we will discuss some of those in the subsequent modules. But one thing that we have already discussed is the relationship between wireless propagation and the operating frequency. We know that lower the operating frequency, wider the wireless coverage because that's how electromagnetic signals travel through the air. So if a certain area is extremely challenging to cover, you can consider covering that area by using a lower frequency because given everything else equal, that lower frequency will have broader network coverage as compared to the higher frequency. So possibility to operate at a lower frequency is one solution that 5G can possibly implement. Another solution and something that we will talk about in detail in one of the later modules is 5G's ability to sharply focus the outward signal in a desired direction. It's like trying to light a room by a light bulb as opposed to trying to light a specific area of the room by using a flashlight. The light bulb will not only use more energy but it won't light up all the areas of the room to the intensity that you want it to. Whereas if you want a certain area of the room to be highlighted brightly, then you can just flash a light on that area and that area will be as bright as you want it to be. So at a high level, 5G has provisions to focus its outward energy in certain desired directions so that the signal strength and by definition the wireless coverage in that area is improved. So that is another solution that 5G has in mind. Another important way 5G can solve this problem is what we engineers call cell densification. What does it mean? Let's say that in a certain geographical area had only ten base stations or ten0cell phone towers, so to speak, it is not necessary that when you upgrade your technology from 4G to 5G. It's not necessary that you have to be limited to those ten base stations, it is always possible for network operators to deploy more than ten cell phone towers. Let's say, the network operator deploys 20 cell phone towers in the same area instead of ten. Logic suggests that everything else being equal, the network with 20 cell phone towers will be able to cover the geographical region better and more thoroughly, more ubiquitously as compared to a network with just ten cell phone towers. So self identification is another way 5G can try to solve this problem of minimizing coverage holes or areas with wireless blind spots
