Opportunistic Cooperations: A New Communication Approach for MANETs

Opportunistic Cooperations: A New Communication Approach for MANETs Renato M. de Moraes, Hamid R. Sadjadpour J.J. Garcia-Luna-Aceves Department of Electrical Engineering University of California at Santa Cruz (UCSC) Santa Cruz, CA 95064, USA Email: {renato,hamid}@soe.ucsc.edu Department of Computer Engineering at UCSC and Palo Alto Research Center 3333 Coyote Hill Road, Palo Alto, CA 94304, USA Email: jj@soe.ucsc.edu Abstract—We introduce a collaboration-driven approach to the sharing of the available bandwidth in wireless ad hoc networks, which we call opportunistic cooperation. Transmissions are divided in frequency and codes according to nodal locations, and succes- sive interference cancellation (SIC) is used at receivers to allow them to decode and use all transmissions from strong interfer- ing sources. We show that both the link’s Shannon capacity and the per source-destination throughput scale like O(n 2 ) (upper- bound) and Ω[f (n)] (lower-bound), for n nodes in the network, a path loss parameter α > 2, and 1 ≤ f (n) < n 2 . I. I NTRODUCTION Communication protocols used in wireless ad hoc networks today are meant to support reliable communication among senders and receivers that are competing with one another for the use of the shared bandwidth. This “competition-driven” view of bandwidth sharing has had profound implications on network architectures and methods used to access the channel and disseminate information. Gupta and Kumar [1] showed that, in a wireless connected network with static nodes, the throughput for each node degrades as the number of nodes in- creases under the competition-driven view of networking. That p is, it scales as Θ(1/ n log(n)), 1 where n is the number of nodes in the network. Grossglauser and Tse [2] analyzed a two-hop, single-relay forwarding scheme for MANETs in which a source passes a packet to a relay that in turn delivers it to the destination when the two nodes are close to each other. This and many subse- quent studies on how to make MANETs scale by using mobil- ity [2], [3], [4], consider each transmission as competing with all the other concurrent transmissions in the network. However, because a relay cooperates with a source by storing the source’s packet until it is close enough to the intended destination, the throughput of MANETs can be increased. 2 Recently, Toumpis and Goldsmith [5] have shown that the capacity regions for ad hoc networks are significantly increased when multiple access schemes are combined with spatial reuse (i.e., multiple simultaneous transmissions), multihop routing (i.e., packet relaying), and SIC, even without performing power This work was supported in part by CAPES/Brazil, by the US Army Re- search Office under grants W911NF-04-1-0224 and W911NF-05-1-0246, by the Basking Chair of Computer Engineering, and by UCOP CLC under grant SC-05-33. 1 Ω, Θ and O are the standard order bounds. log(·) is the natural logarithm. 2 In [2], the per source-destination throughput scales as Θ(1). control. Also, SIC circuits with simple implementation and low complexity have been introduced recently [6], and code divi- sion multiple access (CDMA) [7] and global positioning sys- tem (GPS) [8] technologies have been already integrated into a single IC chip [9]. In this paper, we present an integrated approach to coopera- tive bandwidth sharing in MANETs and propose what we call opportunistic cooperation. 3 We show that with opportunistic cooperation, nodes access the available channel(s) and forward information across a MANET in such a way that concurrent transmissions become useful at destinations or relays. Hence, sender-receiver pairs collaborate, rather than compete, with oth- ers. Therefore, a better network performance is possible. Section II summarizes the basic network model that has been used recently to analyze the capacity of wireless networks [1], [2], [3], [4], [10]. Section III describes the opportunistic co- operation implementation. Section IV presents the the link’s Shannon capacity, the per source-destination throughput, and the bandwidth requirement. Section V concludes the paper. II. N ETWORK M ODEL The term cell denotes the set of nodes located inside a defined area of the network. The receiver range of a node is defined as the radius, measured from the node, which contains all other nodes of the same cell. The cluster associated with a given node is the set of cells reached by the receiver range of this node. Our assumptions are consistent with prior work [1], [2], [10]. Also, in this paper, nodes are considered to have SIC capability. The modeling problem we address is that of a MANET in which n mobile nodes move in a unit square area. To simplify our analysis, we assume that cells have square shapes, each with area equal to a(n) = φn , in which φ ∈ (0, 1) is the cell area pa- rameter of the network. We consider that the communication occurs only among those nodes that are close enough (i.e., in same cell), so that interference caused by farther nodes is low, allowing reliable communication. In other words, the receiver chooses the closest nodes because they present the best chan- nel, in a respective order, due to the assumption of the simple path propagation model, i.e., the receiver takes advantage of 3 The term “opportunistic” is used here to indicate that the number of nodes cooperating with one another in a cell during a communication session is a random variable.