bandwidth portion of frequency spectrum occupied by a signal baseband information signal
carrier high-frequency signal which is modulated by the baseband signal in a communication system
Citizens’ Band (CB) radio short-distance unlicensed radio communication system
demodulation recovery of a baseband signal from a modulated signal Fourier series expression showing the structure of a signal in the
frequency domain
frequency domain method of analyzing signals by observing them on a power-frequency plane
frequency-division multiplexing combining of several signals into one communication channel by assigning each a different carrier frequency
full-duplex communication two-way communication in which both terminals can transmit simultaneously
half-duplex communication two-way communication system in which only one station can transmit at a time
handoff transfer of a call in progress from one cell site to another Improved Mobile Telephone Service (IMTS) a mobile telephone service,
now obsolescent, using trunked channels but not cellular in nature intelligence information to be communicated
modulating signal the information signal that is used to modulate a carrier for transmission
network an organized system for communicating among terminals noise an unwanted random signal that extends over a considerable
frequency spectrum
noise power density the power in a one-hertz bandwidth due to a noise source
personal communication system (PCS) a cellular telephone system designed mainly for use with portable (hand-carried) telephones public switched telephone network (PSTN) the ordinary public wireline
phone system
repeater a transmitter-receiver combination used to receive and retransmit a signal
signal-to-noise ratio ratio between the signal power and noise power at some point in a communication system
simplex a unidirectional communication system; for example, broadcasting
spectrum analyzer test instrument that typically displays signal power as a function of frequency
star network a computer network topology in which each terminal is connected to a central mainframe or server
time domain representation of a signal as a function of time and some other parameter, such as voltage
white noise noise containing all frequencies with equal power in every hertz of bandwidth
( Questions
1. Why were the first radio communication systems used for telegraphy only?
2. When were the first two-way mobile radio communication systems installed, and for what purpose?
3. What characteristics of CB radio led to its great popularity?
4. Why are cellular radio systems more efficient in their use of spectrum than earlier systems?
5. What types of modulation are used with cellular phones?
6. Explain the differences among simplex, half-duplex, and full-duplex communication.
7. Identify each of the following communication systems as simplex, half-duplex, or full-duplex.
(a) cordless telephone (b) television broadcast
(c) intercom with push-to-talk bar
8. Why is it necessary to use a high-frequency carrier with a radio commu-nication system?
9. Name the three basic modulation methods.
10. Suppose that a voice frequency of 400 Hz is transmitted using a trans-mitter operating at 800 MHz. Which of these is:
(a) the information frequency?
(b) the carrier frequency?
(c) the baseband frequency?
(d) the modulating frequency?
11. What effect does doubling the bandwidth of a system have on its noise level?
12. What is the meaning of the signal-to-noise ratio for a system, and why is it important?
13. What is the difference between the kelvin and Celsius temperature scales?
14. State whether the time or frequency domain would be more appropriate for each of the following:
(a) a display of all UHF television channels (b) measuring the peak voltage of a waveform (c) measuring the bandwidth of a waveform (d) determining the rise time of a signal
15. What is meant by the term frequency-division multiplexing?
16. Why is thermal noise sometimes called white noise?
17. Give the frequency designation for each of the following systems:
(a) marine radio at 160 MHz (b) cell phones at 800 MHz
(c) direct-to-home satellite television at 12 GHz (d) CB radio at 27 Mhz
( Problems
1. Express the frequency of a 10-kHz signal in radians per second.
2. Find the noise power produced by a resistor at a temperature of 60 °C in a bandwidth of 6 MHz in
(a) watts (b) dBm (c) dBf
3. If the signal power at a certain point in a system is 2 W and the noise power is 50 mW, what is the signal-to-noise ratio, in dB?
4. Sketch the spectrum for the half-wave rectified signal in Figure 1.11, show-ing harmonics up to the fifth. Show the voltage and frequency scales and indicate whether your voltage scale shows peak or RMS voltage.
5. Sketch the frequency spectrum for the triangle wave shown in Fig-ure 1.12 for harmonics up to the fifth. Show the voltage and frequency scales.
6. A 1-kHz square wave passes through each of three communication channels whose bandwidths are given below. Sketch the output in the time domain for each case.
(a) 0 to 10 kHz (b) 2 kHz to 4 kHz (c) 0 to 4 kHz
7. Sketch the spectrum for the pulse train shown in Figure 1.13.
FIGURE1.11
FIGURE1.12
FIGURE1.13
8. Sketch the spectrum for the sawtooth waveform in Figure 1.14. Explain why this waveform has no dc component, unlike the sawtooth wave-form in Example 1.3.
9. Visible light consists of electromagnetic radiation with free-space wave-lengths between 400 and 700 nanometers (nm). Express this range in terms of frequency.
10. Equation (1.11) applies to any kind of wave. The velocity of sound waves in air is about 344 m/s. Calculate the wavelength of a sound wave with a frequency of 1 kHz.
FIGURE1.14