d even 30-dB fades can be expected. On digital systems, the deleterious effects of multipath fading can be even more severe. Consider a digital bit stream to a mobile terminal with a transmission rate of 1000 bps. Assuming NRZ coding, the bit period would be 1 msec (bit period = 1/bit rate). We find the typical multipath delay spread may be on the order of 10 usec. Thus delayed energy will spill into a subsequent bit (or symbol) for the first 10 usec of the bit period and will have no negative effect on the bit decision. If the bit stream is 64,000 bps, then the bit period is 1/64,000 or 15 usec. Destructive energy from the previous bit (symbol) will spill into the first two-thirds of the bit period, well beyond the midbit sampling point. This is typical intersymbol interference (ISI), and in this case there is a high probability that there will be a bit error. The bottom line is that the destructive potential of ISI increases as the bit rate increases (i.e., as the bit period decreases). 18.4.2 Diversity: A Technique to Mitigate the Effects of Fading and Dispersion 18.4.2.1 Scope. We discuss diversity to reduce the effects of fading and to mitigate dispersion. Diversity was briefly covered in Section 9.2.5 where we dealt with LOS microwave. In that section we discussed frequency and space diversity. In principle, such techniques can be employed either at the base station and/or at the mobile unit, although different problems have to be solved for each. The basic concept behind diversity is that two or more radio paths carrying the same information are relatively uncorrelated: When one path is in a fading condition, often the other path is not undergoing a fade. These 3 Diffraction is defined by the IEEE (Ref. 6) as "the deviation of the direction of energy flow of a way (ray beam), not attributable to reflection or refraction, when it passes an obstacle, a restricted aperture, or other inhomogeneities in a medium. CELLULAR AND PCS RADIO SYSTEMS separate paths can be developed by having two channels separated in 