Let's now look at a simple example of a wireless system that most of us use every day that of a cell phone. Let's say that you're trying to call your mom, which you should do as often as possible and we'll try to see at least at a high level what happens when we greet our mom and how our words are transmitted to the other end using different blocks of a typical wireless system. So let's say that you have called your mom and the phone rang. She picked up the phone and you greeted her saying, hi, mom. When you say those words at a high level, that sound goes to the modulator inside your phone among other things. And as we know, modulator has the job of impressing that information, which is our greeting here on to a carrier wave. So modulator impresses that information onto a carrier wave. But the phone's internally work on a default frequency, which may be different from the frequency assigned to your phone by the network operator or the cellular provider. So your phone intrinsically converts that carrier wave into the specific frequency assigned to it by other network operator which has got upconversion. And once the signal is at the desired frequency, it undergoes a power amplification, just like before you go for a long hike or long run, you fuel up, the wireless signal before it goes the long distance between two phones, it has to fuel up itself and the power amplification is the process that does exactly that. And once the signal has been converted to the specific carrier wave frequency and it has been amplified in power, it goes through the antenna. And this is the point where the electrical signal which resides here is converted into a wireless or electromagnetic or radio frequency signal. Now, that radio frequency signal travels over the air and when it travels over the air it faces certain impediments. The most important and most significant of which is called attenuation or part loss or transmission loss. Just like you happen to spend some amount of energy walking, hiking or running or just like when you send a letter to your friend, the postal carrier service has to spend a certain amount of fuel in its van trying to carry the letter from one point to the other. The wireless signal also has to spend some energy when going from one point to another. And the manifestation of that loss of energy is quantified in technical terms as part loss or transmission loss. Let's say that after a certain amount of transmission loss, that signal reaches your mother's phone. And the good thing is that most of the procedures that occur on her phone are pretty much the mirror of the procedures that occur on your phone as a transmitter. For example, her phone will also have an antenna, but the antenna will have the opposite responsibility to convert the incoming electromagnetic or wireless signals into electrical signals. And because that electromagnetic signal will have lost a certain amount of energy in transit, that signal needs power boost or amplification at your mother's phone as well, just like how you are near some fueling up after you come back from a long run or a long hike or just how the postal service carrier van might near refueling every now and then. So the incoming signal is also amplified inside your mother's phone. And because the transmitter phone had to upconvert in frequency the receiver's phone, we need to downconvert in frequency to convert that carrier away frequency into the default frequency that your phone can understand. And just like the transmitter's phone had to use the modulator functionality, the receiver's phone will need to use the demoderator functionality in order to extract the information that has been impressed upon the incoming carrier wave. And when your mother's phone is able to demodulate that information, that greeting is then converted into voice and that is the same greeting you sent her, hi, mom. And this is how when you talk to someone over phone they get to hear your voice and you in turn get to hear their voice. Now, a few disclaimers here and a few notes, this is a very high level or a simplified description of a real life wireless communication system. There are many other blocks that are crucial from engineering standpoint that we don't show here simply because we don't have the requisite background in order to understand their functionality and significance. But whatever we have learned so far is enough for us to put in practice and at least put on paper some of the basic building blocks of a simple wireless system. That's the first note I wanted to give you. Second note is that although this figure shows that the wireless signal goes directly from your phone to your mother's phone, that is not exactly how things work in reality. We show it this way just for simplification, but in reality and some of you may know this already, the signal from your phone actually goes over to a cell tower, technically known as a base station. So this cell tower is something that is deployed by your cellular network operator, towers that we see all around us. So the signal from your phone doesn't directly go to your mother's phone, it rather goes to a cell tower deployed by the network operator. The operator might perform some additional processing on that signal and then that signal is transmitted by the network operator cell tower to your mother's phone. So, in reality, this communication exchange is mediated by the cell tower and the signal doesn't directly go from one phone to another. But for simplicity of discussion, it suffice it to say so for the time being. Another thing to note is that not all radio signals are created equal in terms of the amount of energy that they lose in transit. That is the path loss or transmission loss suffered by different electromagnetic signals is different and it roughly depends on their frequency. In general, higher the frequency of the radio signal higher the pathloss that it encounters when going from one point to another. And network operators or any owner of wireless system needs to keep that fundamental fact in mind, that the transmission loss or path loss is a roughly proportional operational frequency. Higher the frequency of operation, higher the corresponding path loss will be. But that isn't the end of impediments suffered by wireless signals when they go from one point to another. In addition to path loss, there are a couple of additional impediments as well and they are called noise and interference. Now, the technical description of noise and interference and their significance is coming up shortly. But is there a high level way to understand what noise and interference are? Yes, there absolutely is. Imagine that you have written a letter to your friends and you have now sent that letter to your friend using a postal carrier. As I already mentioned, the amount of fuel that the postal carrier's van will spend when going from your house to your friend's house, mediated, of course, by the post office, which would be the equivalent of the base station or cell tower here, the fuel spent would be equivalent of path loss or the transmission loss. Noise and interference, however, would be the equivalent of the impediments that happened to the letter itself. For example, in transit, your letter might get crumpled a little, it might catch a little bit of dirt, it might catch a little bit of rain, it might catch a little bit of snow. And all those factors might change the appearance of the letter when your friend actually receives that letter. And just like those factors changed the appearance of the letter, noise and interference also have the potential to change the appearance of the wireless signal when it actually reaches the receiver. And that has a couple of profound implications that will learn about shortly. But before then, we'll learn about some of the technical details of noise and interference.