Cellphones and Digital Networks

Cell phones have been around for nearly 15 years and are now everywhere you look. Over a quarter of Americans and a half of Europeans own cell phones and the numbers have been increasing exponentially. With the continuing increase in technology cell phones have become smaller, cheaper, and thanks to the move from analog to digital the calls are much clearer. They offer a great amount of convenience, and can be very economically for the busy businessman on the go. Advancements in cell phones are always being made, giving a clearer sound and lighter feel, as well as a longer life. The cell phone industry has been one of the fastest growing in the world. The electronics are fairly simple, but they are so small that they are truly and engineering marvel. This paper will discuss in depth the many different components of the average cell phone, and talk about how it converts your voice into something that can be sent through a digital network. The paper will also look at how the inner workings allow for a phone to act as a microcomputer, with Internet access, address books, and even games. Finally, it will review the many exciting ideas for this growing market and look to the future of the industry, and how the industry plans on overcoming various limiting factors. Alexander Graham Bell invented the telephone in 1876, 18 years later Guglielmo Marconi created the first radio. It was only natural that these two great technologies would eventually be combined to create the cellular craze. In the 80's few people used radiophones, these phones were the precursor to cellular, but they had several limiting factors preventing them from every becoming a major part of everyday society. In the radio telephone system, there was one central antenna tower per major city, and no more than 25 channels available on that tower. Each phone needed a powerful transmitter, big enough to transmit 40 or 50 miles. It also meant that not many people could use radiotelephones due to the lack of channels. With the current cellular system any none adjacent cell can use the same frequency, so the amount of phones that can be used are nearly limitless. These cells also mean that each phone does not need a strong transmitter, so the phone can be a lot smaller. With the innovation of digital phones, many great features are now available, such as caller id, Internet access, and several other new features. It also meant that the phone would need a microprocessor to convert from analog to digital, this complicated the circuitry, but left it with new technology available the industry was able to make the phone as small as possible. The only restriction in size became the user-input devices, and the screen size. Usefulness of the Digital Cell Phone The digital cellular phone offers many advantages to today†s society. The conveniences that it offers over simply not having one are obvious and they vary from person to person. But there are many advantages over other types of phones as well. The cellular phone not only allows people to communicate with others while they are on the go, but it also offers many other features to help people. With the services that digital provides, people can access email and find information almost anywhere in the world for a reasonable fee. In the future, as the integration of phones and computers grow, people will be able to access tutorials in the field, and use them to communicate with specialists saving a great amount of time for many researchers. Today digital cell phones, such as the one shown in Appendix C figure 1, can process millions of calculations per second in order to compress and decompress the voice stream. In order to do this each phone is equipped with a circuit board that contains many different chips. The circuit board of a common phone is shown in Appendix C figure 2. Two chips described earlier are the Analog-to-Digital and Digital-to-Analog conversion chips that translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog. There is also a Digital Signal Processor that is highly customized processor designed to perform signal manipulation calculations at high speed. The microprocessor controls the keyboard and display and deals with command and control signaling with the base station, it also coordinates the rest of the functions on the board. This microprocessor is as powerful as the super computer of the 70's that took up whole rooms, but is now the size of a finger. By using its arithmetic/logic unit or ALU it can perform all mathematical operation that run many of today features in phones. It is also responsible for the transfer of data throughout the phone. It will also make decisions and then run a new set of instructions. In Appendix C figure 3 a very simple microprocessor is shown. Cell phones use microprocessors that are much more complex, but the use the same idea. The ROM and flash memory chips provide storage for the phone's operating system and customizable features, such as the directory and various simple games. (Appendix C figure 4) The RF and power section handles power management and recharging, and also deals with the hundreds of FM channels. Finally, the Radio Frequency amplifiers handle signals in and out of the antenna. The Radio Frequency amplifier is the same device as you would find in your car's radio. The display has grown considerably in size as the number of features offered by cell phones has increased. Most phones currently available offer built-in phone directories, calculators and even games. It some new products that will be discussed later, cell phone counter as PDA's offering very large screen and offer all of the benefit you would find in today's hand held computers. The display is a liquid crystal display (LCD). It is made of thousands of tiny crystals with two possible colors. They have recently announced that they will be offering color screens on some new phones that work like the display of a laptop computer. Very small speakers and microphones, about the size of a dime, amplify the analog waves. These devices are just like that of a portable radio and the microphones used on television talk shows. They are both wired to the microprocessor. In order for digital cell phones to take advantage of the added capacity and clearer quality, they must convert your voice into binary information. This means that it must break it down to 1's and 0's. The reason that this is so advantageous is that unlike analog, digital is either on or off, 1 or 0, instead of oscillating between the two. For the conversion, the device must first record an analog wave, such as the one in Appendix B figure 1. To create the highest fidelity possible, it records number to represent the wave, instead of the wave itself as represented in Appendix B figure 2. The cell phones analog-to-digital converter, a device that is also found in a CD player, does this process. On the other end a separate digital-to-analog converter is used for playback. The quality of transfer depends on the sampling rate, that controls how many samples are taken per second, and the sampling precision. The precision controls how many different levels are possible in the sample. The better these two are the clearer the sound, but it takes a higher speed processor and requires a greater amount of data transfer. In Appendix B the benefits are shown in figure 3. Most common digital cellular systems use Frequency Shift Keying to send data back and forth. This system uses one frequency for 1's and another for 0's and rapidly switching between the two. This requires optimal modulation and encoding schemes for recording, compressing, sending, and then decoding without loss of quality. Because of this digital phones contain an amazing amount of processing power. The cellular network is web of towers covering areas, generally thought of as hexagonal cells as shown in APPENDIX A Figure 1. The genius of the cellular system is because cell phones and base stations use low-power transmitters, so the same frequencies can be reused in non-adjacent cells. Each cell is about 10 square miles and has a base station that consists of a tower and a small building containing the radio equipment. As more people join the cellular world, companies are quickly adding more towers to accommodate them. Every digital carrier is assigned different frequencies, an average carrier may get about 2400 frequencies per city, and this number is about three times the amount as analog. The reason that more channels are available is because digital data can be compressed and manipulated much easier than analog. Each tower uses one seventh of the available frequencies, so none of the surrounding 6 towers interfere. The cell phone uses two frequencies per call, called a duplex channel. The duplex channel allows one channel to be used for listening and the other for talking, so unlike a CB or walkie-talkie, both people can talk at the same time. This system currently allows for about 168 people to talk in each cell, for each system. The cellular approach requires a large number of base stations in a city of any size, but because so many people are using cell phones, costs remain low per user. Every cell phone has a special code associated with it, called an electronic serial number (ESN). It is a unique 32-bit number programmed into the phone when it is manufactured. When the phone is activated another five digit code called a system identification code (SID), a unique 5 digit number that is assigned to each carrier by the FCC, is imprinted in the phones memory. When you first power up a cell phone, it checks a control channel to find the SID. If the phone cannot find any control channels to listen to, it knows it is out of range, and displays a no service message. After finding the SID, the phones check to see if it matches the SID programmed in the phone, and if it does not match it knows that the phone is roaming. The central location that the cell phone is registered to keeps track of the cell that your phone is in, so that it can find you when someone calls the phone. When the phone is turned on it sends its ESN to the control channel. If the phone goes out of range, it will take a short while to locate your phone when it enters back into service. This can cause loss of calls, even though the phone is in service, but this problem is very temporary. When someone does call your phone it is sent to the central tower called the Mobile Telephone Switching Office (MTSO). This office is continually communicating with the cell phone. It sends and receives the calls, as well as telling it what frequencies to use. This is all done through the control channel, so it does not impair any calls. As you move toward the edge of your cell, the cell's tower will see that your signal strength is diminishing. At the same time, the base station in the cell you are moving toward, which is listening and measuring signal strength on all frequencies, will be able to see your phone's signal strength increasing. The two base stations coordinate themselves through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. There are three common technologies used by cell phone providers. These are Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). In FDMA every call is done on a separate frequency. FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. This is very similar to the way that radio stations operate. Each station is assigned a signal at a different frequency within the available band. FDMA is used mainly for analog transmission, so it is slowly being phased out. It is capable of carrying digital information, but it is not considered an efficient method for digital transmission. Time Division Multiple Access gives each call a certain amount of time on a frequency. The Electronics Industry Alliance and the Telecommunications Industry Association use TDMA. In TDMA, a narrow bandwidth that is 30 kHz wide and 6. 7 milliseconds long is split time-wise into three time slots. (Appendix D, figure 1) Each conversation gets the radio frequency for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800 MHz or 1900 MHz frequency bands. Some phones have the ability to switch between bands. This function is called simply Dual-Band, and is important when traveling between different band frequencies. TDMA is also the access technology for Global System for Mobile communications. The Global system uses different frequencies in different areas of the world and is not compatible with other TDMA systems. GSM operates in the 900 MHz and 1800 MHz bands in Europe and Asia and in the 1900 MHz band in the United States. GSM systems use encryption to make phone calls more secure. GSM is the international standard in Europe, Australia and much of Asia and Africa. In covered areas, cell-phone-users can buy one phone that will work anywhere else the standard is supported. To connect to the specific service providers in these different countries, GSM-users simply switch SIM cards. SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider. Unfortunately, the 1900 MHz GSM phones used in the United States are not compatible with the international system.