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Radio Communication Basics 91
Reflector
RX1
TX
Reflector
RX2
Reflector
Figure 4.8 : Space diversity
Select
strongest
signal
OUT
of nulling and of strengthening a signal is dependent on transmitter and receiver spatial
positions, on wavelength (or frequency) and on polarity. Let’s see how we can use these
parameters to create independent diverse channels.
4.8.1 Space Diversity
A signal that is transmitted over slightly different distances to a receiver may be received
at very different signal strengths. For example, in Figure 4.2 the signal at 17 meters is at
a null and at 11 meters at a peak. If we had two receivers, each located at one of those
distances, we could choose the strongest signal and use it. In a true multipath environment,
the source, receiver, or the reflectors may be constantly in motion, so the nulls and the
peaks would occur at different times on each channel. Sometimes Receiver 1 has the
strongest signal, at other times Receiver 2. Figure 4.8 illustrates the paths to two receivers
from several reflectors. Although there may be circumstances where the signals at both
receiver locations are at around the same level, when it doesn’t matter which receiver
output is chosen, most of the time one signal will be stronger than the other. By selecting
the strongest output, the average output after selection will be greater than the average
output of one channel alone. To increase even more the probability of getting a higher
average output, we could use three or more receivers. From Figure 4.7 you can find the
required fading margin using diversity reception having 2, 3, or 4 channels. Note that the
plots in Figure 4.7 are based on completely independent channels. When the channels are
not completely independent, the results will not be as good as indicated by the plots.
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