Wireless Cooperation Issues (Wireless Networking Protocols)

Networking traditionally defines the sequence of intermediate node transmissions as a path or route. One way to think about cooperative communication methods is to view a set of n nodes participating in delivering packets from a source to a destination as a multi-terminal link. We refer to this generalization as a cooperative link, or simply a link (the terminology feedforward flowgraph is used in [65]). A cooperative link L is a set of nodes employing coordinated actions to deliver messages reliably from a source to a set of one or more destinations. Suppose that a destination node d is the final recipient of these packets. If node d is a relay for another link, we assume that delivery of a packet for this second link to node d represents a "renewal" point in the packet delivery process. That is, node d may be the source of another cooperative link L’ in order to deliver the packet to yet another destination d’. However, delivery of the packet to node d’ on link L might not employ the transmissions associated with the link L. In this case, a route becomes a sequence of one or more (cooperative) links.

Calling such a collection of coordinated nodes a link is consistent with the embedding of cooperative protocols in the PHY layer of the simplified IP protocol stack. This design choice would preserve the current model of mobile networks in which the PHY layer is an interface queue for IP packets and routing at the network layer is based on IP packets. On the other hand, this approach does imply an expansion in the activities of the link layer and perhaps the MAC sublayer. Now the link layer communicates with multiple network nodes in establishing a link. Moreover, a specification of the cooperative link indicates the sequence of transmissions by nodes participating in the cooperative link. That is, cooperative link establishment includes much of the functionality of routing, albeit just for nodes in a local neighborhood.


An alternate approach is to make the link layer responsible for identifying packet transmissions and whether those packets are received reliably. In this case, the network layer takes responsibility for coordinating reliable and unreliable transmissions of multiple nodes. The sequencing of such transmissions generalizes traditional store-and-forward routing.

The above approaches, cooperation via the link or network layers, differ in the network architecture to implement the following tasks:

• Message Decoding: using all received packets to decode,

• Cooperative Link Establishment: specifying a procedure to coordinate the transmissions of multiple nodes with the objective of reliable message decoding at the destination(s),

• Cooperative Routing: configuring a sequence of cooperative links as a route.

Message decoding is a link layer task while cooperative routing is a job for the network layer. Link establishment, however, could be executed in the link layer, the network layer, or even the MAC sublayer. A cross-layer implementation might be appropriate. What is certain is that the control signaling must have low rate relative to the gains of cooperation.

When we examine some proposed cooperative protocols in Section 6.6, we will see that overhead raises a number of implementation issues.

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