The Rake Receiver (Cont.)
The signal
not only passes through a multipathed channel but also is attenuated.
So in order
to receive the signal appropriately we must use some statistic modeling of the
channel’s
noise and amplify the signal.
There are
three major diversity signal-processing techniques:
Selection diversity - we receive N multipathed signals, the
signal with the highest SNR is
chosen and all other are discarded. This scheme is used in the downlink
of IS-95.
Equal gain combining - the receiver corrects the phase rotation
of the received signals caused
by the fading channel and combines the received signals of different
paths with equal weight.
Maximum ratio combining - the optimum way (in the sense of
the least BER) to use information
from different paths to achieve decoding in an additive white gaussian
channel (AWGN).
The receiver corrects the phase rotation caused by a fading
channel and then combines the
received signals of different paths proportionally to the
strength of each path.
Since each path undergoes different attenuations, combining
them with different weights yield
an optimum solution under an AWGN channel.