branches.
In Maximum Ratio combining each signal branch is multiplied by a weight factor
that is proportional to the signal amplitude. That is, branches with strong signal are
further amplified, while weak signals are attenuated.
Figure: L-branch antenna diversity receiver (L = 5). With MRC, the attenuation/amplification
factor is proportional to the signal amplitude ai = ri for each channel i.
Derivation
Consult the
slide show to see why MRC is the optimum diversity combining method for an AWGN LTI channel.
Distribution of signal power
After MRC of i.i.d Rayleigh-fading signals, the received signal
exhibits
Nakagami fading, with a
gamma distribution.
This allows relative simple mathematical evaluation
of for instance
outage probabilities.
The same concept again
The idea to boost the strong signal components and attenuate the weak
(relatively noisy) components,
as performed in MRC diversity, is exactly the same as the type of filtering
and signal weighting used in the
matched filter
receiver.
A particularly interesting application of this concept is the
Rake receiver for detecting
direct-sequence CDMA signals over a
dispersive channel.