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This paper proposes a new principle for designing MAC protocols for CDMA-based ad hoc networks - inducing spatial clustering in contending transmitters/receivers. We first highlight the advantages of CDMA in handling quality of service (QoS) requirements, enhancing energy efficiency, and enabling spatial multiplexing of bursty traffic. Then, based on stochastic geometric models and simulation, we show how idealized contention resolution among randomly distributed nodes results in clustering of successful transmitters and receivers, in turn leading to efficient spatial reuse. This motivates the central idea of the paper which is to explicitly induce clustering among contending nodes to achieve even better spatial reuse. We propose two distributed mechanisms to realize such clustering and show substantial capacity gains over simple random access/ALOHA-like and even RTS/CTS-based protocols. We examine under what regimes such gains can be achieved, and how clustering and contention resolution mechanisms should be optimized to do so. We propose the design of ad hoc networks supporting hop-by-hop relaying on different spatial scales. By allowing nodes to relay beyond the set of nearest neighbors using varying transmission distances (scales), one can reduce the number of hops between a source and destination so as to meet end-to-end delay requirements. To that end we propose a multi-scale MAC clustering and power control mechanism to support transmissions with different ranges while achieving high spatial reuse. The considerations, analysis and simulations included in this paper suggest that the principle of inducing spatial clustering in contention has substantial promise towards achieving high spatial reuse, QoS, and energy efficiency in CDMA ad hoc networks.
Date of Publication: Dec. 2007