CELLULAR AND PCS RADIO SYSTEMS
separate paths can be developed by having two channels separated in 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.
. 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
= (antenna height)/(antenna separation) = h/d.
In Figure 18.6 we relate the correlation coefficient
() with , where is the orientation
of the antenna regarding the incoming signal from the mobile unit. Lee recommends
a value of
= 0.7. Lower values are unnecessary because of the law of diminishing
returns. There is much more fading advantage achieved from
= 1.0 to = 0.7 than
= 0.7 to = 0.1.
= 0.7 and = 11, from Figure 18.6 we can calculate antenna separation
values (for 850-MHz operation). For example, if
h = 50 ft (16 m), we can calculate d
using formula (18.4):
d = h/ = 50/11 = 4.5 ft (1.36 m).
The space diversity concept.