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How do cell phones work?It's Friday, and Elmer and his car are parked on the freeway in the middle of rush hour traffic. "How can they call it rush hour when we never move?" he ponders. Elmer's in a bad mood. He's hot, he's tired, and earlier some numskull who was jabbering into a phone while driving nearly ran him off the road. He glances to his left, spies another motorist with phone in hand, and muses, "I wonder how those darn things work?" While Elmer idles in gridlock, let's get an SwRI Whizard to explain cell phone technology (rush hour traffic requires no explanation). "If Elmer looks out his car window along the freeway, he may be able to see one of the most visible features of cellular technology, a cell tower with a triangular array of antennas on top. Those towers are everywhere. "There are four variants of cellular technology in the United States; three are newer digital techniques -- Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Global System for Mobile Communications (GSM). The fourth is the older analog AMPS (Advanced Mobile Phone System) that has been in use since the early 1980s. More than 90 percent of the cellular users in the U.S. have AMPS phones. For the sake of brevity, let's look at a typical AMPS system. "The essential idea exploited in cellular telephony is to assign different operating frequencies in each cell so that the transmitted signal in one cell does not interfere with the signal in an adjacent cell. Typical cellular geometry consists of a cluster of seven cells -- one in the middle, surrounded by six adjacent cells. Each cell has a tower with transmitting and receiving antennas. "An important concept in cellular telephony is 'frequency reuse.' The ring of cells acts as a buffer to prevent outside interference in the middle cell, permitting the center cell frequencies to be 'reused' outside the ring. Frequencies (or channels) can be used over and over again in similar seven cell patterns. Each cellular provider has 416 channels spaced 30 kHz apart. There are 21 control channels and 395 voice channels. To get a sense of the leverage gained by frequency reuse, eight clusters of seven cells (with cell sites five miles apart) typically support 140,000 subscribers. Channels are allocated in pairs so that the mobile can use one channel for transmitting and the other for receiving. When the mobile handset power is turned on, it finds the strongest control channel signal and send out the mobile identification numbers to the cell site. The numbers are checked to see if the mobile is a valid subscriber, and then the location of the mobile is registered in the system database. This permits the network to route a call intended for the mobile telephone. If the mobile wants to call, then the called number is sent over the control channel to the cell site. As the call is processed, the mobile is assigned transmit and receive voice channels for conversation. "Each cell site is controlled by a mobile switching center (or 'switch'). The switch is the brain in the network and may control as many as 100 to 200 cell sites. The voice channels at the cell sites are generally routed to the switch through land lines, and the switch connects the cellular network to the local telephone company. The switch is responsible for keeping track of the mobile call as the handset moves from one cell coverage area to another. This process is known as hand-off, and the switch tells one cell site to drop the call, tells the mobile phone to change voice channel frequencies, and tells the new cell site to pick up the call. All of this is going on without the caller being aware of the transfer (unless it doesn't work!). "Currently, there are five cellular providers in San Antonio and three more are expected to launch services within the next year or so. That means more towers, more competition, and more numskulls for Elmer to encounter on the road." Thanks to this month's Whizard, Dr. Richard Johnson, an Institute scientist in the Signal Exploitation and Geolocation Division. Johnson is an electrical engineer who specializes in electromagnetic wave propagation and statistical communication theory. The Lighter Side
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