文档介绍：Introduction munication Principles ? Course Name: Introduction munication Principles ? Course Code: EI211 ? Course Instructor: Dianguang Ma ? Text: Digital and munication Systems, 6 th ed. by Leon W. Couch II, ISBN 0-13-081223-4 ? Contents: Chapters 1 – 4. CHAPTER 2 SIGNALS AND SPECTRA Chapter Objectives ? Basic signal properties (dc, rms , dBm , and power) ? Fourier transform and spectra ? Linear systems and linear distortion ? Bandlimited signals and sampling ? Discrete Fourier transform ? Bandwidth of signals 2-1 PROPERTIES OF SIGNALS AND NOISE ? munication systems, the received waveform is usually categorized into the desired part containing the information and the extraneous or undesired part. The desired part is called the signal , and the undesired part is called noise . 2-1 PROPERTIES OF SIGNALS AND NOISE ? Practical waveforms that are physically realizable (., measurable in a laboratory) satisfy several conditions: – The waveform has significant (nonzero) values over posite time interval that is finite. – The spectrum of the waveform has significant values over a composite frequency interval that is finite. – The waveform is a continuous function of time. – The waveform has a finite peak value. – The waveform has only real values. That is, at any time, it cannot have plex value a + jb , where b is nonzero. ? Mathematical models that violate some or all of the conditions listed above are often used, and for one main reason – to simplify the mathematical analysis. 2-1 PROPERTIES OF SIGNALS AND NOISE 2-1 PROPERTIES OF SIGNALS AND NOISE ? The time average operator is given by ???? dtT TTT????????? 221 lim ? If the waveform involved is periodic with period T 0 , the time average operator can be reduced to ???? dtT aa TT????????? 2 02 001 2-1 PROPERTIES OF SIGNALS AND NOISE ? The dc ( “ direct current ”) value of a waveform w(t) is given by its time average, <w (t)>.dttwT twW TTT dc?????? 22)( 1 lim )(? For any physical