JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission
Section: Diversity

Maximum Ratio Combining Diversity

Various techniques are known to combine the signals from multiple diversity 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.



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