Diversity is the simultaneous operation of two or more systems or parts of a
system. Diversity is used to improve system reliability. Multipath fading can
cause temporary failures in even the best-designed paths. Diversity is a
possible solution to this problem. Two types of diversity are as follows:
Space Diversity
Frequency Diversity
With space diversity, the receiver of a microwave radio accepts signals
from two or more antennas that are spaced apart by many wavelengths. This is
illustrated in Figure
. The signal from
each antenna is received and then simultaneously connected to a diversity
combiner. Depending upon the design, the function of the combiner is either to
select the best signal from its inputs or to add the signals together. Space
diversity is usually the first choice for system protection, because it does
not require extra bandwidth.
With frequency diversity, the information signal is simultaneously
transmitted by two transmitters operating at two different frequencies, as
shown in Figure
. If the
separation in frequencies of the two transmitters is large, the frequency
selective fading will have low probability of affecting both paths to the same
extent. This will improve the system performance
Access points can have
two antennas attached to them. These two antennas are for diversity in signal
reception, not to increase coverage. They help eliminate the null path and RF
being received, out of phase. Only one antenna at a time is active. For optimal
signal, the active antenna is selected on a per-client basis. The selection
only applies to that specific client. The access point can hop back and forth
between antennas when talking to different clients. PCMCIA cards also have
antenna diversity built into the card. It is possible to turn the diversity off
through the configuration of the devices, by using an access point or a PCMCIA
card.
The student will test the
effects of the Cisco Aironet AP with various antenna diversity settings. The
student will configure the AP radio antennas using a GUI and IOS command
line.