se that is not gaussian. Noise is something that is not signal. In our CDMA models, radio noise is something introduced after the modulation phase and before demodulation. 2.1.2 Interference Noise is something that is not signal. Not only is noise not our own signal, but it is also not somebody else's signal either. When somebody else's signal becomes an impairment to our own radio link, we call it interference. It may be additive but is not white or gaussian. In wireless telephone networks, calls have interference that consists of other calls using the same frequency and modulation. Before cellular, interference was simply avoided for a long time. Systems were de- signed SO that no two broadcasters in the same area were transmitting signals at the same time at the same frequency. The cellular principle, in a nutshell, is the idea that the tradeoff between interference and capacity is something we can manage through engineering and planning. Managing interference, rather than simply avoiding it, gives us much more flexibility, allowing us to offer new kinds of services. It is a deep and powerful truth in CDMA that interfering CDMA signals can be treated as noise, statistically speaking. This is the subject of Chap. 8. 2.1.3 Distortion Both noise and interference are additive, superimposed on an unchanged underlying signal. Change in the shape of the signal itself is called distortion. A signal is not dis- torted if it is merely amplified or attentuated; the change has to be more substantial. A radio amplifier that amplifies loud and soft signals differently is called nonlinear. A nonlinear amplifier produces a waveform other than the input signal and, therefore, creates distortion. An amplifier without enough power distorts the radio wave by clip- ping it (see Fig. 4.4 in Sec. 4.3). Radio frequency (RF) filters are used to keep transmissions within allotted frequency bandwidth, but they also add distortion in narrowing the frequency distribution. 2The discussion of hum and pink noise is here as a contrast to whit