Cooperative Strategies for End-to-End Energy Saving and QoS Control - Novel 3D Media Technologies - page 143

Fig. 8.6 Battery lifetime extension of the UEs adopting cooperative communication [ 74 ]
relays, namely 20, 50 and 80 %. As seen from the figure, the ESG increases when
the coalition size or the relay density increases. This is due to the fact that the
cooperation opportunity increases among UEs as the coalition size or the relay
density increases. We also observe that when the relay density increases from 20 to
50 %, the ESG increases significantly, but it increases marginally when the density
of relays saturates, while further increasing from 50 to 80 %. Finally, as seen from
the figure even with a moderate coalition size of 10 UEs and the relay density of
50 %, it is possible to reduce the energy consumption of the coalition by 30 %.
To study the impact of cooperative communication on the battery lifetime of the
UEs, we conduct a simulation with 50 UEs, 25 UEs acting as source nodes and the
rest 25 UEs acting as relay nodes. Every UE starts with an initial battery level of
2 Joules and an initial credit level of 0.1. The simulation lasts for 600 s, during
which the role of the sources and the relays are switched around every 5 s. To
benchmark, we repeat the simulation for the case when there is no cooperation and
all the short range interfaces are switched off and all communications are
performed over conventional direct links. Figure 8.6 shows the battery level of
the UE that has the minimum battery level among the UEs. The figure contrasts
the battery lifetime extension of this UE against the benchmark case, where there is
no cooperation. As seen from the figure, cooperation can extend the battery lifetime
of the UEs up to 166 %—more than 2.5 times. Further discussion on the battery
lifetime extension of the UEs using short range cooperation technique is available
in [ 59 , 74 ].
To incentivize the UEs to cooperate, we apply a credit scheme based on
cooperative game theory. We assume that there is a virtual central bank (VCB)
that maintains an account for every UE where the history of its cooperation can be
traced and recorded. We model the problem as a cooperative game and award
cooperative nodes energy credit. Specifically, we distribute the achieved energy
saving in the coalition, using the core solution of the cooperative game [ 75 ].