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can be tracked. A UE gains credit when cooperates and spends credit to receive help
from other UEs; the spent credit is transferred to the account of the helping UE
(i.e. the UE that is acting as a relay). Therefore, cooperative UEs increase their
credit level over time while the selfish ones fail to accumulate any credit in their
account. If a selfish node, which has no credit in its account, requests for a packet
forwarding service from the cooperative UEs, they will refuse the request. As a
result, selfish nodes are gradually excluded from the cooperative groups. In contrast
to the credit-based schemes, which are based on a centralized approach, the
reputation-based schemes are based on a distributed trust management approach,
where every mobile user maintains a reputation record of its neighbouring UEs. The
UEs also disseminate the reputation image of their immediate neighbour UEs to the
other UEs located far away to accelerate the detection and isolation of the selfish
UEs. It is worth pointing out that there is an intricate difference between the credit-
based schemes and the cooperative game-theoretic techniques: in the former,
normally players receive flat credit regardless of their effort for or their impact on
the achieved energy saving, while, in the later, the players receive credit propor-
tional to their influence and effort.
8.2.2 Proof-of-Concept for Energy Saving
For proof-of-concept, we perform a simulation to evaluate the impact of cooperative
communication on the battery lifetime of the UEs. Figure
8.4
illustrates the consid-
ered scenario, where several UEs under the radio coverage of a WiFi Access Point
(AP) make a coalition and cooperate to save energy. We assume that every UE is
equipped with two radio interfaces, namely WiFi (IEEE 802.11g) and WiMedia.
Fig. 8.4 A coalition of UEs cooperating to save energy [
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]
