Hence not using them is equivalent to wasting potential performance improvement.
Note, the antenna selection can occur prior to the baseband demodulation, as shown above; this is
called predetection diversity. Also the antenna selection can occur after baseband demodulation, this
means there will be M baseband sections and we will choose one of their outputs to input to the
demodulator. If each of the M antennas performed their own demodulation and the selection was
made on their respective output symbols, then this is called postdetection diversity. In this last scenario
the received symbol sequence can be selected by received SNR and/or maximum eye opening.
The PDF of the local SNR per antenna is given as
(5.88)
Selecting one out of two antennas provides performance improvement because the probability that
both antennas are in a deep fade is very low, p2. In other words, there is a good chance that at least
one antenna would have a useful signal to be used for demodulation.
The probability that each of the local SNR values, i, are simultaneously below or equal to s is
expressed as
(5.89)
The cumulative distribution function (CDF) is written as
(5.90)
In Fig. 5.51 we plot the CDF of the SNR for an M branch selection diversity receiver, where the
x-axis has been normalized by the mean SNR.
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