Let's now try to understand some of the fundamental factors that impact the data rate or the speed that you get on your phone every day, ie., what factors determine how fast your connection will be. First of which is how many bars does your phone have, which ultimately translates to how good is your connection. This would be the equivalent of the quality of the rules. If you have more bars, it naturally means that your signal is of better quality. And that means that your SNR or signal to noise ratio, a metric that we learned about earlier, that metric is pretty good. Higher the SNR given everything else equal, higher data rate you will be able to achieve. The second metric is how wide is my channel and how crowded it is. That is equivalent of how wide my road is and how crowded the road is, how many other cars there are at the same time. In technical terms, how wide is your channel? It's called channel bandwidth or channel capacity and how crowded it is, it's called network load. So you can understand that in terms of an everyday road. If a road is wide, it is said to have better capacity. Just like that, if a channel is wide, it is said to have higher channel capacity. And on the other hand, if the road is wide, but if it is chock full of cars going bumper to bumper, that road will seem to have a lot of traffic congestion. Just like that, if there are many users trying to access the same wireless channel at the same time, the channel be loaded with users, so to speak, and that is what we call network load. So the width of your channel and the crowd on that channel, meaning network load are also a couple of important factors that impact your data rate. The third factor is how frequently and how efficiently can the phone use this available channel? If the phone can utilize the channel more frequently, your phone will be able to get higher data rates. And if your phone, whenever it gets to utilize the wireless channel can utilize that channel with more efficiency, the phone will be able to get bigger bang for the buck, so to speak. That is, within a given amount of time and using a given amount of frequency spectrum, your phone, if it operates more efficiently, will be able to download or upload more data as compared to another phone that operates with lower efficiency. And spectral efficiency, nothing but is the efficiency with which you can utilize your frequency spectrum. And goes without saying that higher the spectral efficiency, higher the data rate or higher the speed will be because you are using the existing resources with more efficiency. Now, spectral efficiency granted tends to get a little technical explanation and manifestation. But as always, there is a one easier way to understand spectral efficiency. Let's go back to our original example of letter writing. Let's say that you have one page and you have to send a certain letter to your friend. If you write your letter in double spaces, then let's say you will be able to accommodate 100 words on the same page. Whereas if you could write with single space instead of double spacing, then you have instantaneously doubled the number of words that you can accommodate on the same page. That page will now have 200 words instead of 100. So without using additional resources, i.e., without using an extra paper or without having to pay for extra postage because it is just one paper. Without using additional resources or without spending any more money, you have effectively doubled the amount of message that you can convey to your friend simply by writing your message more efficiently, i.e., writing in single spaces rather than in double spacing. So spectral efficiency can be understood in similar terms. If you could operate your system more efficiently, you can utilize the same existing amount of resources more efficiently and improve your performance, which in this context would be your speed or your data rate, technically also known as throughput. And given that these three factors are closely impact throughput and throughput ultimately impacts user experience and customer satisfaction. Wireless network operators spend a good deal of time, money, and other resources in order to optimize all these factors, i.e., maximize the SNR, maximize channel capacity but minimize network load, and maximize spectral efficiency in order to maximize user experience and customer satisfaction. And now that we have learned about some of the general concepts of wireless communications, seeing some of the factors that impact wireless signal, and what kind of considerations determine your data rate. Let's try to translate this theoretical background into practical domain by focusing on a specific wireless communication system, that of a cellular network.
