As mentioned in an earlier post about MIMO, in Rx diveristy, the receiver needs to combine multiple streams from different antenna into a single stream. The challenge here is how to use "effectively" the information from allmthe antennas. In fact, it is a just a matter of choose the appropriate weight for each received signals (see figure below). There are multiple ways:
Selective Combining (SC):
The receiver selects the antenna with the highest received signal power and ignore observations from the other antennas. w=[1,0,0,0...].
On flat-fading Rayleigh channel, if the channels from each receiving antenna is independent, then:
- with two receive antenna the effective big energy to noise ratio is 1.5 times Eb/N0
- with three receive antennas the effective bit energy to noise ratio is 1.833 times Eb/N0
- with four receive antennas, the effective bit energy to noise ratio is 2 times Eb/N0
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