Digital Signal Processing Reference
In-Depth Information
scheme, while noting that those for the SECps-based scheme can be easily obtained by
using the appropriate CDF and PDF instead.
Average spectral efficiency:
N
NF
SEC
( )
()
γ
−
T
N
γ
,
foroption1;
T
γ
n
c
n
=
3
η
=
(6.24)
N
∑
SEC
( )
()
γ
NF
−
T
N
γ
,
foroption2.
T
γ
n
c
n
=
2
Average BER:
∫
γ
SEC
∑
N
∫
γ
SEC
( )
( )
T
T
n
3
γ
+
1
γ
BERp
()
γ
T
N
()
γ γ
d
+
n
BE
RRp
()
γ
T
N
( )
γ γ
d
2
n
γ
γ
c
c
0
n
=
3
γ
foroption1;
T
n
,
∑
N
SEC
( )
()
γ
N
−
F
T
N
γ
T
γ
c
BER
=
n
=
3
∑
N
∫
γ
SEC
( )
T
n
+
1
γ
T
n
BER
()
γ
p
( )
γγ
d
2
n
γ
c
γ
n
=
2
foroption2.
T
n
,
∑
2
N
SEC
( )
()
γ
T
N
N
−
F
γ
T
γ
n
c
(6.25)
n
=
Average number of path estimation:
L
−
( )
−
( )
−
∑
L
1
1
P
γ
i
=+
( )
=
γ
T
SECSECps
N
N
1
P
γ
.
(6.26)
E
γ
T
N
1
P
γ
γ
T
i
=
1
N
6.4.4 Numerical Examples
In this section, we examine the different design trade-offs involved in the joint adap-
tive modulation and switched combining schemes through several selected numerical
examples.
Figure 6.4
plots the average spectral efficiency of the SEC-based minimum estima-
tion schemes. It is interesting to see that the average spectral efficiency of this scheme
for both options and different numbers of receive antennas overlaps for the high-SNR
region. This is because when the channel condition is favorable, the first path examined
will always be acceptable and will be used for modulation mode selection. As a result,
the system becomes equivalent to the no-diversity case. We also observe from Figure 6.4
that over the low- to medium-SNR region, the system can benefit from an increasing
number of receive antennas, while this benefit is more significant for option 2 than for
option 1. Note that the probability of no transmission for option 2 is reduced when the
number of receive antennas increases. Figure 6.4 also shows that option 1 has a consid-
erable spectral efficiency advantage over option 2 in the low-SNR region. This advantage
Search WWH ::
Custom Search