asiest to experience in the pitch of sound from a moving source. As a race car moves past the spectators, the pitch of the engine whine drops noticeably. The frequency shift is the ratio of the speed of the source to the speed of sound. The same effect occurs in light and radio. The astronomical red shift is caused by dis- tant galaxies moving away from us at speeds close to the speed of light. Similar to the audio case, the size of the Doppler shift in radio is the ratio of the speed of the source to the speed of light. The Doppler shift also occurs if the receiver is moving toward or away from the transmitter. While it is hard to visualize a moving automobile being close to the speed of light (even with some of the drivers we know), the effect can be significant at closely spaced high frequencies. A car moving at 100 km/h is moving about 30 m/s, one ten-millionth of the speed of light. At 2 GHz this works out to a 200-Hz shift, enough to affect a nar- row channel. 7 There are other causes of frequency shift, including calibration differences between radio transmitters and receivers. These have nothing to do with relative motion, but radio engineers often refer to these differences as Doppler shift anyway. 2.2 Radio Measurement Errors A dynamic radio system has to measure its performance instant by instant and figure out what to do next. The base station measures the received power level from the mo- bile unit and instructs the mobile unit to adjust its broadcast power in a process called power control. Most calls require power control most of the time, and many calls re- quire handoffs. The cellular system begins to consider handoff when the power received When one of us (Rosenberg) was doing narrowband digital channel work at Bell Laboratories, we were considering a 5-kHz channel at either 900 MHz or 2 GHz. The relative Doppler shift of two subscribers moving in opposite directions on the Garden State Parkway could easily be 500 Hz. The guard bands needed to deal with those shifts were an important design issu