Cooperative Medium Access Control Policies in Wireless Networks
Andrea Munari · Padua Research Archive (University of Padova) · 2010
Broadly speaking, wireless ad hoc networks are permeated by cooperative behaviors. In such systems, indeed, nodes have to continuously pool their resources to achieve goals that are of general interest, such as routing packets towards a destination that would otherwise be out of reach for an information source, or coordinating medium access so as to successfully share a common spectrum. Recently, however, the idea of collaboration has gathered a renewed and increasing deal of attention in the research community thanks to the development of innovative concepts, most notably the idea of cooperative relaying, that have been shown to unleash significant improvements, going beyond some intrinsic limitations that affect wireless communications systems. While these emerging solutions have been thoroughly studied from a theoretical perspective, the advantages they offer can be reaped in real world implementations only if additional coordination among nodes is provided, so that cooperating terminals are allowed to offer their help obeying the rules that control medium access, and without disrupting the normal activity of the network. Along this line of reasoning, this thesis focuses on the design and study of link layers that implement cooperation in ad hoc networks. The contribution of our work is twofold. On the one hand, we introduce novel and beneficial collaborative strategies, while on the other hand we investigate how the intrinsic nature of different medium access control policies can be more or less beneficial to cooperative behaviors. In the first part, we present an innovative approach to cooperation in networks with multi-antenna equipped nodes that rely on directional transmissions and receptions. Our solution proposes terminals to share information on ongoing communications to better coordinate medium access at the link layer, and manages to overcome issues such as node deafness that typically hamper the potential of large scale directional wireless systems. The central part of this work, conversely, concentrates on the simpler ad hoc scenario where omnidirectional communications are in place, and tackles some inefficiencies of the cooperative relaying paradigm. In particular, we introduce the novel concept of hybrid cooperative-network coded ARQ, which allows a relay to code data of its own together with a corrupted packet during a retransmission at no additional cost in terms of bandwidth. Such a solution encourages nodes to cooperate, since they are offered the possibility to pursue a goal of their interest while helping surrounding terminals. Moreover, the capability of exploiting retransmissions to serve additional traffic, achieved by smartly taking advantage of network coding techniques, triggers beneficial effects also at a network level in terms of sustainable throughput and reduced congestion. The potential of the proposed approach is first investigated by means of mathematical analysis, while subsequently extensive simulation campaigns test the effectiveness of link layers that implement it in a variety of networking environments. Taking the cue from a reasoned comparison of the results achieved by relaying schemes in different scenarios, we devote the final part of the thesis to the investigation of the impact that distinct spectrum control policies can have on collaborative behaviors. Combining once again mathematical analysis and simulations, we consider how the characteristics of completely distributed, e.g., carrier sense based, and centralized, e.g., time division based, systems influence the effectiveness of a given cooperative strategy. Not only does our study shed light on the relations that exist between cooperation and medium access, but also it provides important hints on how to efficiently design link layers capable of supporting such techniques. Finally, the appendix of this thesis reports the outcome of a research activity, carried out in collaboration with the IBM Zurich Research Laboratory (Switzerland), whose focus falls out of the topic of cooperative link layers and covers the design of energy-efficient routing protocols for wireless sensor networks.