5.2.3 Maximal Ratio Combining (MRC)
In the previous section, EGC made use of all the M antennas to present a signal to the demodulator.
This essentially weighted all antennas equally, regardless of their respective SNR. So what this next
operation accomplishes is to emphasize the signals with high SNR and de-emphasizes signals with
low SNR. A block diagram of this spatial combing technique is shown in Fig. 5.53.
266 CHAPTER FIVE
Receiver
1
Receiver
Receiver
2
M
..
.
Co-
Phase
Baseband
Demodulator
C
O
M
B
I
N
E
R
..
.
SNR
X
SNR
X
SNR
X
A1
A2
AM
FIGURE 5.53 Maximal ratio combining diversity method.
The PDF of the combiner output SNR is given as
(5.94)
The CDF of the combiner output of SNR is
(5.95)
(5.96)
The mean SNR at the output of the MRC combiner is given as
(5.97) E5g6 aM
i1
M
P(g) 1 e
g
# aM
k1 Qg
Rk1
(k 1)!
P(g) 3
gA
0
p(x) dx
p(g)
1
(M 1)!
# gM1
M
# e
g
When comparing the incremental SNR improvement of MRC to that of SS and EGC, each additional
antenna has a greater impact with MRC.
A plot of the CDF of this MRC technique is shown in Fig. 5.54.
PERFORMANCE IMPROVEMENT TECHNIQUES 267
CDF of SNR for M Branch MRC Diversity System
0.
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