versity, switched di- versity and selection diversity. In switched diversity, we use one receiver until its per- formance goes below some threshold and then switch to the other one. In selection di- versity, we monitor both receivers and select the better one instant by instant. Switched diversity can be designed with two antennas and just one receiver, whereas selection diversity requires two complete receiver systems so that it is possible to com- pare the two signals and select one. When the two receivers are in geographically distinct locations, we call it macro- scopic diversity. The CDMA soft handoff described in Sec. 8.7 has two cells in two dif- ferent places receiving the mobile telephone's signal; this is a form of macroscopic di- versity. Macroscopic diversity combats multipath fading as well as larger sources signal fading such as buildings, trees, and small hills. While the fading phenomenon depends on the multipath environment, broader bandwidth always offers more protection from multipath fading. Resistance from mul- tipath fading is a tremendous advantage for spread spectrum. The 25- or 30-kHz chan- nel of analog cellular is very susceptible, the 200-kHz channel of the Global System for Mobility (GSM) is more resistant, and the 1.25-MHz of cdmaOne is better still. The 5-MHz or greater wideband CDMA is almost immune to multipath fading. We will now turn to the second issue created by multipath, which is the spreading out of the signal across time at the receiver. We call the range of time delay containing most of the received power the delay spread. 5 The frequency autocorrelation verse of the delay spread, so a delay spread of 1 us requires about 1 MHz of bandwidth to allow us to be reasonably comfortable that most of the bandwidth will not be affected. Spreading of the signal across time is not an important issue for analog transmission and audio frequencies. 6 AM and FM signals with sufficient amplitude should see no ill effects from delay spreads less than 20 us, a very long path length differe