The last point is about higher Pathloss leading to better spatial reuse. That actually brings me to mention a slightly unrelated point in that sometimes the ingenuity of engineering design and implementation lies in utilizing some of the challenges and flipping them on their head to convert those challenges into advantages. If some of you practice martial arts, you will know that a fundamental tenet of multiple martial arts techniques is to use your opponent's momentum against themselves so as to gain an advantage in your fieldwork. Just like that, ingenious engineering design in millimeter-wave allows us to use some of the factors that would otherwise work against us to work in favor of us instead. Using higher Pathloss for better spatial reuse is actually a prime example, and that requires a little bit of more explanation. Let's try to understand these elements in detail. We already know that millimeter wave is prone to higher Pathloss, but what is spatial reuse? Let's try to explain that with the help of a figure here. Let's say that you have access to two frequencies: frequency f_1 and frequency f_2. Let's say that f_2 is substantially higher than frequency f_1. If you were to build a cellular base station at frequency f_1, let's say this is the base station, then at frequency f_1, maybe this would be the coverage of a cell at frequency f_1. Let's say that at frequency f_1 you have access to channel bandwidth of W megahertz and that leads to a network capacity of C bits per second. Let's assume that for now. Now, imagine that at frequency f_2, you have access to the same channel bandwidth, W. Let's not toy around with those variables just yet. If you play your cards right, it should be possible for you to get capacity C while operating at frequency f_2 as shown. There are certain assumptions in wireless and digital communications involved here that I have prepared upon but stay with me here for a minute. Let's assume that at frequency f_1 channel bandwidth of W gives you network capacity of C. This is your cell radius at frequency f_1. Now because frequency f_2 is significantly higher than f_1, its Pathloss will be significantly higher than that for f_1 as well. As such, the signal that you might send from this base station at frequency f_2 will attenuate much quicker as compared to the signal at frequency f_1. That would mean that the coverage radius of a hypothetical cell deployed from the same location at frequency f_2 will be substantially lower than that at frequency f_1. Let's say that this is the cell coverage at frequency f_2. I hope you are with me up to this point. What it means is that if you were to deploy a cellular network on frequency f_2, you would get coverage in the smaller dotted circle but outside that dotted circle, you wouldn't get coverage at f_2. But the situation doesn't have to be that way. For example, what stops you from deploying another base station over here, separated a sufficient distance away? That base station can also operate on the same frequency f_2 and because it does so, it will give an equivalent cell coverage like this. You don't have to stop there either, you can deploy another base station here at f_2 and it'll give you coverage like this and you can continue doing so N times in a given geographical area. Now, once again, one might ask a basic question, hey, if two things operate at the same time on the same frequency, you said earlier that they'll interfere with each other. Well, that premise still holds true but keep in mind that the wireless coverage of this cell, let's say cell 1, ends over here. In this region there is no coverage by cell 1, that is, the wireless signal from cell 1 ends so to speak ''At about this point beyond which you can start building a new cell.'' That is the reason why because coverage is naturally limited at frequency f_2. These cells: cell 1 and cell 2, will not interfere with each other in the coverage area of the other cell. That is how interference will be mitigated or altogether eliminated. Just by taking advantage of the fact that naturally the wireless coverage of cell 1 dies over here and coverage of cell 2 will die over here. Wherever the coverage of one cell attenuates, that is when you can begin a new cell. Given that, a given cell at f_2 is still capable of giving us a capacity of c bits per second. If you have N cells at f_2, the area two-port that you'll be able to get at frequency f_2 will be N times C bits per second, whereas the area throughput at frequency f_1 is just C bits per second because you are able to accommodate only one cell at frequency f_1 in that area. Notice something remarkable that has happened. Without utilizing additional bandwidth just by taking advantage of the natural fact that at higher frequencies coverage is limited, you are able to deploy multiple cells at higher frequency within the same geographical area , and as such, you are able to boost the network capacity offered in that geographical area by N fold equivalent to the number of cells you're able to deploy. Such magnification of network throughput or capacity that is achieved by deploying more cells in the same area or packing the cells more densely is called cell densification. That is the third principle that gives us a significant advantage in terms of millimeter-wave. That is where higher Pathloss leads to better spatial reuse, meaning you could reuse the same space for multiple cells, i.e, fit many more cells, pack them densely, that is, network densification. Spatial reuse and network densification share this relationship. It is higher Pathloss and brilliantly taking advantage of that fact is what allows us and millimeter-wave deployments to densify the network and ultimately significantly improve not just the network capacity, but data speeds that are achievable by individual users. These are some of the fundamental advantages of millimeter-wave, namely a large bandwidth, more antennas leading to higher transmission gains. Some of those sophisticated algorithms which just FYI are called beamforming, but we'll learn about those later. Those beamforming algorithms allowing us high directivity, better focus, and hence higher spectral efficiency, and ultimately a higher Pathloss being used as an advantage rather than being discarded as a disadvantage in order to offer us better spatial reuse and ultimately, network densification which helps magnify user speeds and network capacity.
