Speaking of the fundamental resources that any wireless transmission needs, the radio spectrum is the foremost of all. Radio spectrum essentially is just the range of radio frequencies over which a wireless communication takes place, for a specific purpose. Just like how your car needs certain space on the road to be driven, any wireless or electromagnetic signal needs a certain space, so to speak, in the frequency domain for it to go from one point to another. That place in the frequency domain is nothing but radio spectrum. Now, frequencies or the radio spectrum that can be allocated for different purposes as we saw, can range from three kilohertz at the lowest to 300 gigahertz and the highest, which is our range of radio frequencies, as we established earlier. Specifically for mobile phones, it is still a broad range that those phones can use depending upon their generation. For example, some of the earliest mobile phones are operated in frequencies as low as 450 megahertz., whereas some of the intermediate generations, such as for 4G LTE, operate in frequencies that are as high as 2.6 gigahertz, somewhere over here. The upcoming technology 5G can operate in frequencies that are as high as 39 gigahertz. As we will see later in this class, this frequency of 39 gigahertz has a very special significance. It is near and dear to the hearts of all the wireless engineers these days. In technical terms, that frequency is called millimeter wave, a buzzword that some of you may have heard about already. But even if you haven't, don't worry, we're going to talk a lot about 5G in millimeter wave later in this class. But apart from cellular phones, as you can see, there are many other purposes that the radio spectrum can be used for. For example, your citizen's band radio would use frequencies around 10 - 20 kilohertz. Your AM radio would use frequencies around, let's say between one and two megahertz. Your FM radio would be somewhere around 100 megahertz, and your satellite TV or satellite radio would be somewhere around 25 gigahertz. Cell phones or mobile phones aren't the only wireless entities that utilize the radio spectrum, they are just one subset of the entire wireless ecosystem. There are many other services and devices that need to access radio spectrum, and at such those services are spread out across the entire radio spectrum. But wait a minute, who makes these decisions? Who decided that 5G will operate over here, and FM radio will operate over here. Why not the other way around? Or why not a scheme where any service can pretty much use any frequency spectrum that it desires? Well, answer to the latter question is because as we have seen shortly before if two different services try to use the same spectrum at the same time, they will interfere with each other and that is not desirable. There has to be some regulatory entity whose job it is to carefully assign dedicated slices of radio spectrum to different services. In case of United States, that government regulatory entity is the FCC or Federal Communications Commission. Nearly any country has their own equivalent of FCC. The FCC equivalent has the primary responsibility to allocate and apportion frequency spectrum to different services depending upon the service needs and the needs of the propagation environment. For example, it was the FCC who decided, for example, that AM radio would operate here, and FM radio would operate here, and mobile communications would get these type of frequency ranges. It is the local regulatory authority that determines what spectrum will be used by what service, for what purpose.