Cooperative Strategies for End-to-End Energy Saving and QoS Control - Novel 3D Media Technologies - page 142

The WiFi interface is used to communicate with the AP, while the WiMedia
interface is used for short range communications. As illustrated by the figure,
there are three source nodes ( S 1 , S 2 and S 3 ) and three relay nodes ( R 1 , R 2 and R 3 ).
Source nodes communicate with the AP over either a conventional direct link or a
cooperative short range link. A cooperative short range link is a two-hop link—with
only one intermediate relay node between the source and the destination. Commu-
nication over the first hop (from the source to the relay) is performed over a short
range WiMedia link, while the communication over the second hop (from the relay
to the AP) is performed over a WiFi link.
Exploiting their multiple radio interfaces, the relays operate in a full-duplex
mode. They receive a packet through their short range WiMedia interface and
decode it. Then, they encode and forward it through their long range WiFi interface
to the AP. In the scenario shown by Fig. 8.4 , source node S 1 , which is experiencing
a bad channel quality to the AP, cooperates with relay node R 1 that has good
channel quality to the AP; source node S 2 , which is running on low battery, finds
a nearby relay R 1 with good battery level to cooperate with; and finally source node
S 3 , which has a good channel quality to the AP, communicates with the AP directly.
Relay node R 3 , despite willing to cooperate, is left unused as its cooperation brings
no additional energy saving to the coalition. We conduct the simulation for different
coalition sizes (ranging from 10 to 100 nodes) and different relay densities (low
density of 20 %, medium density of 50 % and high density of 80 %). For example, if
the coalition size is ten, a relay density of 20 % means that only two of the UEs in
the coalition are relay nodes, while the remaining eight UEs are acting as source
nodes. During the simulation, which lasts for 100 s, every source node generates a
constant bit rate (CBR) traffic with the rate of 10 packets per second and the packet
size of 1024 bytes. We use channel models and system parameters summarized in
[ 74 ] as well as random way point (RWP) mobility model with the maximum speed
of 3 m/s and the pause time of 5 s for the mobile nodes.
Figure 8.5 illustrates the average energy saving gain (ESG) of the coalition when
the coalition size varies between 10 and 100 UEs for three different densities of the
Fig. 8.5 Energy saving gain (ESG) for different coalition sizes and relay densities [ 59 ]