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In a wireless system, the term noise refers to an electromagnetic field that usually has large bandwidth; that is, it occurs over a wide range of frequencies and wavelengths. Noise does not convey information. It can be either natural or human-made.
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Noise never helps, and often degrades, the performance of a wireless system. It is a concern in any device or system in which data is sent from one place to another. The higher the noise level, the stronger a signal must be if it is to be received error-free. At any given signal power level, higher noise levels translate into more errors and reduced communications range. Figure 32-9 is a spectral display of signals and noise, with amplitude as a function of frequency. The device that generates this display is called a spectrum analyzer. The horizontal axis shows frequency; the vertical axis shows amplitude. The background noise level is called the noise floor. Signals above the noise floor appear in the display and can be received. The strongest signals are received with the fewest errors; weak signals are subject to the most errors. Signals below the noise floor are not displayed and cannot be retrieved unless a more sophisticated receiving system is used, or the transmitter power output is increased, or both.
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620 Wireless and personal communications systems
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32-9 A spectrum-analyzer display. The vertical axis shows amplitude, and the horizontal axis shows frequency.
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Signals below noise floor do not appear in display
Minimizing noise
The noise level in any electronic system can be minimized by using components that draw the least possible current. Noise can also be kept down by lowering the temperature tremendously. Some experimentation has been done at extremely cold temperatures; this is called cryotechnology. The narrower the bandwidth of the signal, the better the signal-to-noise ratio will be, if all other factors remain constant. But this improvement takes place at the expense of data speed. When the noise originates mainly in sources outside the wireless equipment (for example, atmospheric static ), reducing the bandwidth of the receiving equipment is generally necessary. This means that, in wireless communications with high external noise levels, the data speed must usually be slower than it would be if there were little external noise. A noise limiter or noise blanker can sometimes provide improved communications without reducing data speed. Noise limiters chop off high-amplitude noise peaks, and blankers in effect turn the receiver off during noise pulses. Such circuits can be effective against human-made impulse noise, characterized by high-amplitude peaks of very short duration. But thermal, atmospheric, solar, and galactic noise, which are essentially random, are not significantly affected by limiters and blankers.
Quiz
Refer to the text in this chapter if necessary. A good score is 18 correct. Answers are in the back of the book. 1. A network that employs one powerful central computer and several PCs is called: A. A wireless network.
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Quiz 621 B. A local-area network. C. A client-server network. D. A peer-to-peer network. 2. Infrared and optical wireless links work best: A. Over distances exceeding 1000 miles. B. On a line of sight. C. At low radio frequencies. D. In situations with high levels of noise. 3. Which of the following devices or systems is not generally considered wireless A. A remote-control garage-door opener. B. An amateur radio station. C. A beeper/pager. D. A telephone set that requires a wall jack. 4. A noise blanker can improve the quality of: A. Radio reception. B. Radio transmission. C. Infrared reception. D. Infrared transmission. 5. In the United States, a license is required for: A. Receiving on amateur-radio frequencies. B. Transmitting on amateur-radio frequencies. C. Using infrared wireless devices. D. Using a wireless automobile security system. 6. Noise in a wireless receiver can be minimized by: A. Raising the temperature to very high values. B. Reducing the temperature to very low values. C. Maximizing the amplification. D. Minimizing the relative humidity. 7. As the data speed increases in a wireless system, all other factors being equal: A. The signal bandwidth increases. B. The signal bandwidth decreases. C. The overall noise level increases. D. The overall noise level decreases. 8. The term shortwave, in reference to radio, refers to signals having wavelengths of approximately: A. 10 to 100 millimeters.
622 Wireless and personal communications systems B. 100 millimeters to 1 meter. C. 1 to 10 meters. D. 10 to 100 meters. 9. In general, as the noise level in a wireless system increases: A. Stronger incoming signals are needed to overcome it. B. Weaker signals can be received. C. The temperature of the system rises. D. The bandwidth of the system increases. 10. In a spectrum analyzer, the horizontal axis shows: A. Voltage. B. Power. C. Frequency. D. Time. 11. A device consisting of a receiver and transmitter in the same box is called a: A. Modem. B. Transverter. C. Transceiver. D. Transponder. 12. The use of direction-finding equipment to determine the latitude and longitude of a radio transmitter is an example of: A. Radiolocation. B. IR wireless. C. The Global Positioning System. D. Packet radio. 13. A LAN in which each user s computer stores its own data is called: A. A wireless LAN. B. A wide-area LAN. C. LAN topology. D. A peer-to-peer LAN. 14. Which of the following constitutes illegal use of ham radio A. Selling cars by broadcasting prices to other hams. B. Talking about the weather and exchanging local forecasts. C. Talking about who might be the next president of the United States. D. Connecting a radio to the Internet.
Quiz 623 15. An electromagnetic wave can be considered to fall in the shortwave band if its free-space wavelength is: A. 55 kilometers. B. 55 meters. C. 55 centimeters. D. 55 millimeters. 16. In a cellular network, a base station is sometimes called a: A. Transceiver. B. Cell. C. Repeater. D. Cell phone. 17. An advantage of conventional hard-wired telephone over cellular is: A. Privacy. B. Portability. C. Ease of use in a car. D. LAN topology. 18. An advantage of cellular over conventional hard-wired telephone is: A. Security. B. Lower cost. C. Mobility. D. Data speed. 19. Infrared waves are: A. Longer than radio waves. B. Longer than visible-light waves. C. Shorter than visible-light waves. D. Inaccurately named; they are really heat rays. 20. The GPS might be useful: A. For improving the performance of a LAN. B. For increasing the data speed in a wireless system. C. For minimizing noise in a wireless system. D. To a motorist who is lost.
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