Y. Q. FEI ET AL.
Copyright © 2013 SciRes. CN
larger. And finally, smaller constellation sizes turn out to
be more energy efficient when d = 200 m. Generally,
larger constellation sizes require higher Pt to satisfy the
PEmax constraint. However, Pt is small in short distance,
in other words, Pc dominates the total power consump-
tion. In this case, larger constellation sizes significantly
shorten transmission time and reduce the bit energy con-
sumption. On the contrary, Pt rises and dominates the
total power consumption as d increases, leading to the
result that smaller constellation sizes perform better than
larger ones. This is similar to former research results de-
rived in no-retransmission cases.
By appropriately designing the AMC, the low-bond bit
energy consumption of all constellation sizes, marked as
Ebmin, can be reached. For instance, we choose 1024QAM
when d is smaller than 20 m, while 16QAM is applied
when d ranges fr om 80 m to 180 m.
Figures 3 and 4 illustrate the impact of ARQ em-
ployment. In Figure 3, the most energy efficient Nmax is 1
when PEmax is 10−3. However, with a stricter PEmax con-
straint, 10−7, larger Nmax leads to smaller Ebmin, as shown
in Figure 4. This is due to the fact that larger Nmax re-
duces Pt requirement, but inevitably increases
at the
same time. Under a looser constraint PEmax, the impact of
dominates and smaller Nmax makes better trade-off.
When PEmax constraint becomes stronger, the impact of Pt
is dominant, and larger Nmax performs better.
The largest energy saving achieved by ARQ changes
from about 0.5 dBmJ to 0.8 dBmJ when PEmax varies from
10−3 to 10−7. We can also observe that Ebmin of Nmax = 4 is
larger than that of no-retransmission with PEmax = 10−3,
which emphasizes the importance of ARQ design. With
proper Nmax setting we can acquire energy saving, while
improper Nmax may cause lower energy efficiency.
5. Conclusion
In this paper, a cross-layer scheme combining AMC and
Figure 3. Energy Consumption per Bit under Different Re-
transmission Time with PEmax = 10−3.
Figure 4. Energy Consumption per Bit under Different Re-
transmission Time with PEmax = 10−7.
ARQ is proposed in order to minimize the bit energy
consumption under certain delay and performance con-
straints. We analyzed the energy efficiency of the system,
considering dynamic transmission power and duration.
Numerical results are presented to verify the validity of
the cross-layer design, which brings a significant energy
consumption improvement. More channel model and other
retransmission protocol will be included in future work.
6. Acknowledgements
This work is supported by 863 National Science and
Technology Project of the Ministry of Science and Tech-
nology of China, Number: SS2012AA011701.
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