6
- 3 = 3 dB. The remaining data bits are used to choose a point within the selected subset, so for a data
rate of m bits/symbol, the four lattice subsets must each have 2
m
-1
points
4
.
. . . A B A B A B A B A B . . .
. . . B A B A B A B A B A . . .
. . . A B A B A B A B A B . . .
. . . B A B A B A B A B A . . .
. . . A B A B A B A B A B . . .
. . . A B A B A B A B A B . . .
. . . B A B A B A B A B A . . .
. . . A B A B A B A B A B . . .
. . . B A B A B A B A B A . . .
. . . A B A B A B A B A B . . .
0
1
0
1
0
0
1
0
1
0
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1
0
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0
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0
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0
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. . .
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. .
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
Figure 8.16: Subset Partition for an Eight-Dimensional Lattice.
Coded modulation using turbo codes has also been investigated. This work shows that turbo trellis
coded modulation can come very close to the Shannon limit for nonbinary signalling.
8.7.2
Coded Modulation with Interleaving for Fading Channels
Coded modulation for fading channels also uses the coding and interleaving approach of block and con-
volutional codes, however the interleaver is matched to the block or convolutional encoder in coded
modulation design [19, 20]. However, the minimum distance error event in a trellis code depends both on
the parallel transitions and the minimum distance error event through the trellis. Thus, the dominating
error event is not always obvious, which complicates code design. A good overview of trellis code design
for fading channels, including the impact of interleaving and channel fade information, can be found in
[19] and the references therein. There is no good rule of thumb for these code designs, and in many cases
simulations must be used to evaluate performance and choose between different code designs.
8.7.3
Adaptive Coded Modulation
As discussed in Chapter 9, adaptive modulation is a powerful technique to achieve high spectral efficiency
on fading channels. It turns out that additional coding gain can be achieved by superimposing trellis
codes or more general coset codes on top of the adaptive modulation. Specifically, by using the subset
partitioning inherent to coded modulation, trellis or lattice codes designed for AWGN channels can be
superimposed directly onto the adaptive modulation with the same approximate coding gain. The basic
idea of adaptive coded modulation is to exploit the separability of code and constellation design inherent
to coset codes, as described in Chapter 8.7.
Coded modulation is a natural coding scheme to use with the variable-rate variable-power MQAM
described in Chapter 7, since the channel coding gain is essentially independent of the modulation. We
4
This yields
m - 1 bits/symbol, with the additional bit/symbol conveyed by the channel code.
207