frequency. The two paths can also be separated in space and in time. When the two (or more) paths are separated in frequency, we call this frequency diversity. However, there must be at least some 2% or greater frequency separation for the paths to be comparatively uncorrelated. This is because, in the cellular situation, we are SO short of spectrum, using frequency diversity (i.e., using a separate frequency with redundant information) is essentially out of the question. So it will not be discussed further except for its implicit use in CDMA. 18.4.2.2 Space Diversity. Space diversity is commonly employed at cell sites, and two separate receive antennas are required, separated in either the horizontal or vertical plane. Separation of the two antennas vertically is impractical for cellular receiving systems. Horizontal separation, however, is quite practical. The space diversity concept is illustrated in Figure 18.5. One of the most important factors in space diversity design is antenna separation to achieve the necessary signal decorrelation. There is a set of empirical rules for the cell site, and there is another set of rules for the mobile unit. Space diversity antenna separation, shown as distance D in Figure 18.5, varies not only as a function of the correlation coefficient but also as a function of antenna height, h. The wider the receive antennas are separated, the lower the correlation coefficient and the more uncorrelated the diversity paths are. Sometimes we find that, by lowering the antennas as well as adjusting the distance between them, we can achieve a very low correlation coefficient. However, we might lose some of the height-gain factor. Lee (Ref. 7) proposes a new parameter N, where n = (antenna height) / (antenna separation) = h/d. (18.4) In Figure 18.6 we relate the correlation coefficient (p) with N, where a is the orientation of the antenna regarding the incoming signal from the mobile unit. Lee recommends a value of p = 0.7. Lower values are unnecessary because of the law of diminishing ret