Handbook of Propagation Effects for Vehicular and ... - Courses

6-10
6.5.3 Diversity Gain
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
Diversity gain is a concept defined by Hodge [1978] for an earth-satellite
communications system involving two spaced antennas operating in a diversity mode in
the presence of precipitation. This concept may also be applied to antennas separated
atop a vehicle for LMSS scenarios. The diversity gain is defined as the fade reduction
experienced while operating in the diversity mode at a given fade margin. It is equal to
the difference in fades between the single terminal and joint probability distributions at a
fixed probability level. For example, from Figure 6-6 we note that the diversity gain at a
probability of 1% for a 1 m antenna separation is 4 dB. Hence, while the single terminal
operation at 1% probability will experience a 12 dB fade, the diversity pair for a 1 m
separation will experience only an 8 dB fade.
In Figure 6-8 are plotted the diversity gains versus antenna separations for a family
of single terminal fade levels. Each single terminal fade uniquely defines a probability
level. For example, an 8 dB fade occurs at a probability level of 3% as is noted from
Figure 6-6 (for d = 0). Figure 6-8 shows that for any given fade margin, the effect of the
antenna separation is dramatic the first 2 meters, whereas at larger spacing, relatively
little additional fade reduction ensues.
Diversity Gain (dB)
8
7
6
5
4
3
2
1
0
14 dB
12 dB
10 dB
8 dB
6 dB
4 dB
3 dB
2 dB
1 dB
1 2 3 4 5 6 7 8 9 10
Antenna Separation, d (m)
Figure 6-8: Diversity gain versus antenna separation distance for a family of single
terminal fade levels.