Radio Transmission

Radio encompasses the electromagnetic spectrum in the range of 3 kHz to 300 GHz. In radio communications the signal is transmitted into the air or space, using an antenna that radiates energy at some carrier frequency. For example, in QAM modulation the information sequence determines a point in the signal constellation that specifies the amplitude and phase of the cosine wave that is transmitted. Depending on the frequency and the antenna, this energy can propagate in either a unidirectional or omnidirectional fashion. In the unidirectional case a properly aligned antenna receives the modulated signal, and an associated receiver in the direction of the transmission recovers the original information. In the omnidirectional case any receiver with an antenna in the area of coverage can pick up the signal.

Radio communication systems are subject to a variety of transmission impairments. We indicated earlier that the attenuation in radio links varies logarithmically with the distance. Attenuation for radio systems also increases with rainfall. Radio systems are subject to multipath fading and interference. Multipath fading refers to the interference that results at a receiver when two or more versions of the same signal arrive at slightly different times. If the arriving signals differ in polarity, then they will cancel each other. Multipath fading can result in wide fluctuations in the amplitude and phase of the received signal. Interference refers to energy that appears at the receiver from sources other than the transmitter. Interference can be generated by other users of the same frequency band or by equipment that inadvertently transmits energy outside its band and into the bands of adjacent channels. Interference can seriously affect the performance of radio systems, and for this reason regulatory bodies apply strict requirements on the emission properties of electronic equipment.

Figure 3.48 gives the range of various frequency bands and their applications. The frequency bands are classified according to wavelengths. Thus the low frequency (LF) band spans the range 30 kHz to 300 kHz, which corresponds to a

Frequency (Hz)

104 105 106 107 108 109 1010 1011 1012

Frequency (Hz)

104 105 106 107 108 109 1010 1011 1012

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