01%
0.10%
1.00%
10.00%
100.00%
??“40 ??“35 ??“30 ??“25 ??“20 ??“15 ??“10 ??“5 0 5 10
10*log(SNR)
Probability Amplitude < Abscissa
M = 1
M = 2
M = 4
M = 8
FIGURE 5.54 Output CDF of MRC diversity combining.
Let us compare the results of MRC to those given earlier for SS diversity for the M 4 antennas
case. The SS diversity curves show that for approximately 0.5% of the time the SNR is less than or
equal to 5 dB. However, for MRC diversity, the probability dramatically decreased to approximately
0.03% of the time.
A plot of all the mean SNR expressions for the SS, EGC, and MRC diversity techniques is given
in Fig. 5.55. This shows the potential gains with each additional antenna used in the receiver.
Average SNR Improvement from Diversity
0
2
4
6
8
10
12
14
1 2 3 4 5 6 7
8
91011 12 13 14 15 16 17 18 19 20 21
Number of Antennas (M)
SNR Improvement
SS
MRC
EGC
FIGURE 5.55 Mean diversity output SNR comparison.
Here we can clearly see, as M increases the performance gap between the MRC and the SS continues
to increase dramatically. While the performance gap between the MRC and EGC stays approximately
the same with slight performance improvements.
An alternative viewpoint is given below.
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