Resource Allocation in OFDM Based Wireless Relay Networks ...

5.6 Summary
Lifetime (S)
150
100
50
OPT−SOL
EP−SOL
end−to−end rate (bits/s/Hz)
0
0 5 10 15 20 25 30
SNR (dB)
150
100
50
OPT−SOL
EP−SOL
R−Req
0
0 5 10 15 20 25 30
SNR (dB)
Figure 5.6: Lifetime vs SNR
by the two schemes. We see that at low SNR both the schemes failed to provide
the required rate. This means that there exists no feasible solution under the given
set of constraints. Therefore, we consider only the high SNR region. We see that
from SNR= 23 dB onwards, the R-min converges to the R-Req. At SNR= 30 dB,
lifetime is 138.6 seconds which is quite high as compared to the EP-SOL.
5.6 Summary
In this chapter, we defined and solved lifetime maximization problem in sensor relay
networks. To suppress the disadvantages of using pure AF or DF relay modes, we
considered a mixed AF-DF multi-hop network. We formulated the optimization
problem considering both the initial energy and the power constraint at each node.
With change of variables, the problem was reformulated to the standard convex
optimization problem. We solved the problem using dual decomposition approach
and presented the algorithm based on sub-gradient method. Convergence of the
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