Image Processing Reference
In-Depth Information
8.2.1 Related Work
Cooperative communication has gained considerable interest from both academia
and standardization bodies. The existing literature and standards on cooperative
communication can be classified into two categories: multihop communication and
cooperative relaying. In this section, we elaborate these two categories along with
reviewing the related literature. The first form of cooperative communication is
multihop communication, primarily employed in ad-hoc networks to ensure con-
nectivity in the absence of a communication infrastructure. In this form of com-
munication, wireless nodes cooperate by forwarding data packets for each other. As
such, two nodes that are out of their radio coverage are still able to communicate
through some intermediate relay nodes acting as routers (see Fig.
8.2
). The IETF
Mobile Ad-hoc Networks (MANET) working group develops standards for IP
routing protocol suitable for wireless routing application within both static and
dynamic topologies [
24
]. The second form of cooperative communication is coop-
erative relaying. In contrast to multihop communication, the main motivation for
this form of cooperation is not connectivity. Rather, it is employed to exploit the
underlying spatial diversity of the wireless channel to enhance the system perfor-
mance including the link reliability, data rate and energy efficiency.
Figure
8.2
depicts the two above-mentioned types of cooperative communica-
tion. The main difference between these two types is: in multihop relaying (the left
diagram), there is no direct link between the source node S and the destination
node D, so the communication between these two nodes has to be performed through
the relay node R. In contrast, in cooperative relaying (the right diagram), there exists
a direct link between the source and the destination nodes, and the relay path serves
as an alternative path to add more degrees-of-freedom to the channel (spatial
diversity). Figure
8.3
shows a snapshot of the signal-to-noise ratio (SNR) variations
of the direct path
. The variations are mainly
due to the shadowing and the multipath fading impairments. As seen in the figure,
there are time intervals that the received SNR drops below the threshold for a reliable
detection; these time intervals have been illustrated by the light-shaded regions in
the figure. As the variations of the perceived SNR from the two alternative paths
<
S-D
and the relay path
S-R-D
<
>
<
>
at the destination node D are statistically independent, the
probability that both links are in deep fading is very low, illustrated by the dark-
shaded regions in the figure. Therefore, exploiting the relay path besides the direct
path can enhance the reliability of the wireless channel considerably.
S-D
>
and
<
S-R-D
>
