All wireless or radio signals are quite similar in nature to sunlight or visible light, at such all wireless signals travel at the speed of light, which is approximately 300,000 kilometers per second or about 186,000 miles per second. Pretty much every wireless, so radio signal or any electromagnetic signal of any practical interest, all those signals happen to be periodic in nature. In that, they show a certain pattern and that pattern repeats as a function of time. That fundamental periodic nature leads us to two basic attributes of radio frequency signals, namely frequency and wavelength. What are those? The description is given over here. Frequency is the number of repetitions that the periodic electromagnetic signal undergoes in a given second, and the frequency is measured in the unit of hertz. Wavelength, on the other hand, is the distance that periodic wireless signal travels during one cycle or one periodicity, and because it is a measure of distance, it is mostly measured in meters or millimeters equivalently. One simpler way to understand frequency and wavelength is in terms of a couple of metrics that marathon runners use. If you are a marathon runner yourself or a sprinter of any kind, you will know about those metrics already. Those metrics are the number of strides or the number of steps that the runner takes per minute and the average stride length or step length that the runner has. Frequency of radio signals would be equivalent to the number of strides the runner can take per minute. The wavelength would be equivalent to the length of each stride or the distance that the runner can cover in one step or one stride. Just like if you multiply the stride rate of a runner by stride length, you will get approximately the average speed of the runner. If you multiply frequency and wavelength of a wireless radio signal, you get the speed of that radio signal, which as we know now is the universal constant, that is, the speed of light. Frequency and wavelength of a wireless signal are inversely related. The relational constant being the universal constant, the speed of light. The radio signals that are used for wireless communications happen to range from somewhere between three kilohertz and 300 gigahertz. This would probably be the lowest frequency that may be used for wireless communication, and this may be somewhat the highest frequency. Traditionally, this particular frequency range has been called the radio frequency signal range. That is the fundamental reason why wireless signals are interchangeably also called radio signals or radio frequency or RF signals because the frequency of those wireless signals happens to lie between the range specified by the traditional radio frequency range. Now that we know the relationship between frequency and wavelength, it should be easy for us to estimate. Given these frequency endpoints, what would be the corresponding wavelength of the radio signals at the highest? Keep in mind that the lowest frequency would correspond to highest wavelength because they are inversely related. At the highest end of the wavelength point, you will have signals whose wavelength is as large as 100 kilometers, whereas, at the other end, you would have wireless signals whose wavelength is as short as one millimeter. Just to keep everything in context, the wavelength of visible light tends to lie somewhere between 450 nanometers to about 750 nanometers, give or take. If you do the math, you will observe that even the shortest radio signal, which is one millimeter in wavelength, it has a wavelength that is approximately 1,500 times higher than the wavelength of the typical visible light. Going back to our analogy of marathon runners, imagine if visible light and the shortest radio signal were to be two marathon runners and if the stride length of a visible light were to be, let's say, one meter, for example, the stride length of the shortest radio signal would be more than 1,500 meters, that is approximately a mile. That is visible light can cover, let's say one meter in one step. If that were the case, then the shortest radio signal would be able to cover one mile in one step. That is the humongous difference between visible light and the shortest radio signal. Now that we have a fundamental idea about some of the basic properties of the radio signals or wireless signals, let's try to ask and answer one fundamental question. How exactly would this periodic electromagnetic signal as they're also called because a wireless or radio signal has two components, an electrical component, and a magnetic component, hence the name electromagnetic signals. One basic question would be, how exactly do those radio signals or electromagnetic signals convey the precise information that the transmitter wants to send to the other end? This is how the information that the transmitter wants to send is carried, so to speak by these radio-frequency signals. The information can be of any nature. It can be analog, like my voice or it can be digital in nature. For example, your email, your web pages, pictures, videos, etc. That doesn't matter as long as you follow certain specific rules in order to impress your information onto the outgoing radio wave. Because RF signals are used as the so-called vehicle for information transfer because they carry your information from the transmitter to the receiver. These periodic RF signals or wireless signals are also known as carrier waves because they carry the information from one end to the other. Just like the postal service that carries your letter from one point to the other, is called a carrier service, so to speak. These waves, because they carry information from one point to another, are also called carrier waves. In technical terms, the exact procedure of impressing your intended information onto the outgoing carrier wave, that procedure is called modulation. Modulation is a process by which we impress or impose information onto the carrier wave for transmission. Modulation at the high level is about carefully changing some of the specific attributes of the radio-frequency signal. Two of those attributes, we have already learned about frequency and wavelength. There are a few other attributes as well but that discussion tends to be very technical. But suffice it to say at a high level, that modulation is about carefully changing some of the specific attributes of the outgoing radio-frequency signal. That way we can impress our intended information onto that carrier wave. Now, depending upon what type of information you originally wanted to send, either analog or digital, the corresponding modulation will be called analog modulation or digital modulation. For example, if you want to send your voice across, you can impress your voice onto an outgoing carrier wave. It might look just as an example, something like this. Keep in mind that it is still periodic in nature but we have changed certain unmentioned attributes of this outgoing carrier wave. On the other hand, if the information happens to be digital in nature, it will be a stream of bits and bytes, ones and zeros. That will also change certain attributes of the outgoing carrier wave. Just as an example, once again, that carrier wave may look something like this. So as you can tell, just as an example, the frequency of this wave is higher than the frequency of the wave at the top. But this is just an example. Carrier waves modulated by different pieces of data, look entirely different. Now, what we have seen so far is what the transmitter does. It impresses the information onto the outgoing carrier wave in the process of modulation. At the receiver, however, the other end of wireless communication, we need to extract that information from the incoming carrier wave. That process of extraction is called demodulation. Demodulation is the process of recovering the data that has been impressed upon the carrier wave by the transmitter. Although we won't be getting into the technical operational details of modulation or demodulation, there is a fun fact I would like to state here. All of you have heard the term modem. We all have a modem in our home for Internet service. Some of you may also know that we have a modem in our phones and pretty much any wireless device that we currently own. What is the word modem? Well, it is an amalgamation of two words, modulation, and demodulation. That's why the word modem. That is because when the wireless device acts as a transmitter, it uses the modulation part of its functionality. Whereas when it acts as a receiver, it uses the demodulation aspects of its functionality. Because it can perform both the functionalities, that particular device is called modem. Now that we have understood some of the basic building blocks of wireless communication, let's now try to translate our fundamental knowledge into a very simple real-life example of a wireless communication system, that of a phone conversation.
