Digital Cellular
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Stanley Cheung Telecommunication, Dr. Schwebel Research Paper May 17, 2003 Digital Cellular Cellular technology has been and continues to be one of the fastest growing areas of technology. The need to be constantly connected to each other and to the network and be mobile (wireless) is the driving force for this growth. The purpose of this paper is to develop an understanding of how this technology works from when it first began to where it will be going. We will begin by looking at the first generation systems which are based on analog voice communication using frequency voice modulation. The most popular of which is the Advanced Mobile Phone Service (AMPS). Then we will move on to the second generation systems, Global System for Mobile (GSM), which uses FDMA, TDMA, and CDMA. Finally, conclude with a brief discussion of 3rd generation systems, better known as 3G. First Generation Systems In the 1980s, the Advanced Mobile Phone Service was developed by AT&T. The AMPS system is the most popular of the first generation systems and is used in North America, South America, Australia, and China. This system consists of three basic devices which allow it to function: the mobile, the base transceiver, and the mobile switching center (MSC). In the diagram below you can see that the mobile device has to communicate with the base transceiver, and the base transceiver must communicate with the mobile device as well as the MSC. The function of the MSC is to coordinate the communication between the base transceivers and connect all calls to the public wire telephone network. 
One of the leading problems with wireless communication is the limitation of how often one can use the same frequency for different communications, because a signal can get in the way of another signal, even though they are geographically separated. In North America we have two 25-MHz bands allocated to AMPS, one for transmission between the mobile device and the base station, the other vice-versa. The communication between the mobile device and the base station operates on the 869-894 MHz frequencies, while the communication between the base station and the mobile device operates on the 824-849 MHz frequencies. Each of these 25-MHz channels is divided into two for competition purposes. In doing so, we now can accommodate two operators. Each operator is given 12.5 MHz in either direction for its system. The channels are spaced 30 kHz apart, which allows a total of 416 channels per operator. Twenty-one channels are allocated for control, leaving 395 to carry calls. The control channels, are essentially data channels which operate at 10 kbps. From this we can gather that the amount of channels we have just is not enough for most major markets. Two possibilities for solving this problem are finding ways to amount of bandwidth necessary for each conversation and the reuse of frequencies. AMPS exploited the latter of the two. The method by which extensive frequency reuse was made possible is the idea of cells. In a typical analog cell phone system in the United States, the cell phone carrier is given 800 frequencies to use across the city. In any given city, the cell phone carrier divides up the city into cells. Each cell is typically sized at approximately 10 square miles. The best way to view a cell is to think of them as hexagons on a hexagonal grid, like the following: 
“The objective is to use the same frequency in another nearby cell, thereby allowing the frequency to be used for multiple simultaneous conversations.” (Stallings 342) Because mobile devices and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells. In the diagram above, the two purple cells can reuse the same frequencies. A single cell in an analog system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies which ultimately means no collisions or interference between signals. Every cell contains a base station, which consists of a cell-tower. The cellular approach requires a large number of base stations in a city of any given size. A typical large city can have hundreds of towers. How it works. A cell phone carrier typically gets 832 radio frequencies to use in a city. Each cell phone uses two frequencies per call—a duplex channel—so typically there are 395 voice channels per carrier. Therefore on a 7 cell hexagonal grid, each cell has about 56 voice channels. In other words, in any cell, 56 people can talk simultaneously on their cell phones. With digital transmission methods, the number of available channels increases. For example, a TDMA based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available. Second Generation Systems Global Systems for Mobile Communications (GSM) was developed in Europe to provide a standard for mobile technology. The GSM system’s structure is quite similar to that of the AMPS system, only there are four main elements to this system: the subscriber, the base transceiver, the base station controllers, and the mobile services switching center (MSSC). The diagram below shows the interaction between these elements. 
On a GSM system, both the base transmission and the mobile device are allocated 25 MHz: 935-960 MHz for base transmission and 890-915 MHz for mobile transmission. Users will have to access the network using a combination of frequency-division multiple access (FDMA) and time-division multiple access. One main feature that sets GSM apart from everybody else is its subscriber identity module (SIM)—“a portable device in the form of a smart card or plug-in module that stores the subscriber’s identification number, the networks the subscriber is authorized to use, encryption keys, and other information specific to the subscriber.” (Stallings 346) Analog systems do not fully utilize the signal between the phone and cellular network. Analog signals cannot be compressed and manipulated as easily as a true digital signal, thereby allowing for more channels to fit within any given bandwidth. Digital phones can convert our voices into binary information to be compressed, allowing anywhere between 3-10 digital cell phone calls to occupy the space of a single analog call. In the following we will examine several three common technologies used by cell phone networks for transmitting information, namely FDMA, TDMA, and CDMA. FDMA or frequency division access method puts each call on a separate frequency. It separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. Think of radio stations; they send its data at a different frequency within the available band. FDMA is used for analog transmission. Even though FDMA can handle digital information, it however is not considered to be the most efficient method for digital transmission. In the following diagram, we can see in FDMA, each phone uses a different frequency. (www.howstuffworks.com) 
TDMA or time division multiple access is the access method used by the Electronic Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 and 136. In TDMA, a narrow band or channel that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots. Each conversation gets the radio for one-third of the time. This is made possible by the compression of our digital voice data. Compressed data takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. This system operates at either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands. In the following diagram, you will see that TDMA splits a frequency into three time slots. (www.howstuffworks.com) 
CDMA or code division multiple access has a completely different approach than the traditional TDMA. After digitizing the data, it is spread out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, where each is assigned a unique sequence code. CDMA is a form of spread spectrum—which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range. All users transmit data in the same wide-band chunk of spectrum. Each user’s signal is spread over the entire bandwidth by a unique spreading code. At the receiving end, the same unique code is used to recover the signal. Approximately 8-10 calls can be carried in the same channel space, which is equivalent to one analog call on the AMPS system. In the following diagram, you will see that in CDMA, each phone’s data has a unique code. (www.howstuffworks.com) 
Third Generation Systems The objective of the 3rd generation communication systems is to universalize the personal communications and the type of communication access. As we have seen so far, there are many technologies that have been developed that uses one of the above access methods, causing incompatibility between communication devices. The following is a list of things that need to be accomplished in order for 3G to be successful:
(Stallings 353) BibliographyStallings, William. Business Data Communications. 4th ed., Prentice-Hall Inc. Upper Saddle River, New Jersey. 2001. How Cell Phones Work. http://www.howstuffworks.com 1998-2003 Download 19.05 Kb. Share with your friends:
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