Data Communications Networking Devices - 4th Ed.pdf

368 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.15 Quadrature amplitude modulation produces 16 signal states from a combinationof 12 angles and three amplitude levelsFigure 4.16QAM modem transmitterphase, with each point placed at the x-y coordinates representing the modulationlevels of the cosine carrier and the sine carrier.If you plot the signal points previously illustrated in Figure 4.15 which representall of the data samples possible in that particular method of QAM, the series ofpoints can be considered to be the signal structure of the modulation technique.Another popular term used to describe these points is the constellation pattern. Byan examination of the constellation pattern of a modem, it becomes possible topredetermine its susceptibility to certain transmission impairments. As anexample, phase jitter which causes signal points to rotate about the origin canresult in one signal being misinterpreted for another, which would cause four bitsto be received in error. Since there are 12 angles in the QAM method illustrated inFigure 4.15, the minimum rotation angle is 30 , which provides a reasonableimmunity to phase jitter.