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The growth of the communication industry has resulted in ever-increasing demands on the radio spectrum. This, in turn, has spurred the exploitation of previously unused frequency bands and the development of techniques for making better use of those already available. In this paper, spectral utilization is examined using the information-theory techniques of Shannon. The power-bandwidth tradeoff for the ideal channel is established and the performance of practical analog and digital modulation techniques is compared with this ideal. It is shown that well-designed systems tend to operate near the "knee" of the power-bandwidth tradeoff curve for the ideal channel and that they are frequently within 10 dB or less of the performance of the ideal.