字幕表 動画を再生する 英語字幕をプリント - [Presenter] For most of us, a mobile phone is a part of our lives. But I'm sure you're curious minds have always been struck by such questions as how a mobile phone makes a call, and why there are different generations of mobile communications? (upbeat music) Let's explore the technology behind mobile communications. When you speak on your phone, your voice is picked up by your phone's microphone. The microphone turns your voice into a digital signal with the help of MEMS sensor and IC. The digital signal contains your voice in the form of zeros and ones. An antenna inside the phone receives these zeros and ones and transmits them in the form of electromagnetic waves. Electromagnetic waves transmit the zeros and ones by altering the wave characteristics, such as the amplitude, frequency, phase, or combinations of these. For example, in the case of frequency, zero and one are transmitted by using low and high frequencies respectively. So, if you could find a way to transmit these electromagnetic waves to your friend's phone, you would be able to establish a call. However, electromagnetic waves are incapable of traveling long distances. They lose their strength due to the presence of physical objects, electrical equipment, and some environmental factors. In fact, if there were no such issues, even then, electromagnetic waves would not carry on forever, due to the Earth's curved structure. To overcome these issues, cell towers were introduced, using the concept of cellular technology. In cellular technology, a geographic area is divided into hexagonal cells with each cell having its own tower and frequency slot. Generally, these cell towers are connected through wires, or more specifically, optical fiber cables. These optical fiber cables are laid under the ground or the ocean, to provide national or international connectivity. The electromagnetic waves produced by your phone are picked up by the tower in your cell and convert them into high frequency light pulses. These light pulses are carried to the base transceiver box, located at the base of the tower for further signal processing, After processing, your voice signal is routed towards the destination tower. Upon receiving the pulses, the destination tower radiates it outwards in the form of electromagnetic waves, and your friend's phone then receives the signal. This signal undergoes a reverse process, and your friend hears your voice. So, it's true that mobile communications are not entirely wireless, they do use a wired medium too. This is how mobile communications are carried out. However, there was a big issue that we intentionally left unanswered. Mobile communication is only successful when your tower transfers the signal to your friends tower. But how does your tower know in which cell tower area your friend is located? Well, for this process, the cell tower gets help from something called a mobile switching center. The MSC is the central point of a group of cell towers. Before moving further, let's explain more information about the MSC. When you purchase a SIM card, all the subscription information is registered in a specified MSC. This MSC will be your home MSC. The home MSC stores information such as service plans, your current location, and your activity status. If you move outside the range of your home MSC, the new MSC, which serves you instead, is known as a foreign MSC. As you enter a foreign MSC region, it communicates with your home MSC. In short, your home MSC always knows which MSC area you are in. To understand in which cell location the subscriber is within the MSE area, the MSC uses a few techniques. One way is to update the subscriber location after a certain period. When the phone crosses a predefined number of towers, the location update is again done. The last one of these is when the phone is turned on. Let's try to understand all of these procedures with an example. Suppose, Emma wants to call John. When Emma dials John's number, the call request arrives at Emma's home MSC. Upon receiving John's number, the request will be forwarded to John's home MSC. Now, John's MSC checks for his current MSC. If John is in his home MSC, the call requests will be immediately sent to his current cell location, and it checks whether John is engaged on another call, or if his mobile is switched off. If everything is positive, John's phone rings, and the call will be connected. However, if John is not in his home MSC, John's home MSC simply forwards the call request to the foreign MSC. The foreign MSC will follow the previously explained procedure to locate John's phone, and will then establish the call. Now, let's discuss why the frequency spectrum is quite important in mobile phone communications. To transfer zeros and ones in digital communication, each subscriber is allocated a frequency range. However, the frequency spectrum available for cellular communications is quite limited, and there are billions of subscribers. This issue is solved with the help of two technologies, one frequency slot distribution, and two, multiple access technique. In the first technique, different frequency slots are carefully allocated to different cell towers. In the multiple access technique, this frequency slot is efficiently distributed amongst all the active users in the cell area. Now, the big question. Why are there different generations of mobile phone technologies? 1G originally allowed users, for the first time, to carry a phone without a cable attached to it. But 1G suffered from two major problems. The first problem was that the wireless transmission was in an analog format. Analog signals that are easily altered by external sources. So, it provided poor voice quality and poor security. The second problem was that it used the frequency division multiple access technique, which used the available spectrum in an inefficient way. These factors paved the way for the second generation of mobile communications, 2G used digital multiple access technologies, namely TDMA, or CDMA technology. The second generation also introduced a revolutionary data service, SMS, and internet browsing. 3G technology was focused on giving a higher data transfer speed. It used a WCD multiple access technique, along with an increase in bandwidth. To achieve this, the 3G speed of two Mbps allowed the transfer of data for uses such as GPS, videos, voice calls, et cetera. 3G was a huge step in the transformation of the basic phone to a smartphone. Next came 4G, which achieved speeds of 20 to 100 Mbps. This was suitable for high resolution movies and television. This higher speed was made possible due to the OFD multiple access technology, and MIMO technology. MIMO uses multiple transmitter receiver antennas inside both the mobile phone and the towers. The next generation of mobile communication, 5G, to be rolled out soon, will use enhanced MIMO technology and millimeter waves. It will provide seamless connectivity to support the internet of things, such as driverless cars and smart homes. Would you like to learn how a touchscreen works? Please check out this video. Please don't forget to subscribe to Learn Engineering, and your support at patreon.com is invaluable. Thank you.