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Wireless sensor networks have been attracting increasing research interest given the recent advances in microelectronics, array processing, and wireless networking. Consisting of a large collection of small, wireless, low-cost, integrated sensing, computing, and communicating nodes capable of performing various demanding collaborative space-time processing tasks, wireless sensor network technology poses various unique design challenges, particularly for real-time operation. In this paper, we review the Approximate Maximum-Likelihood (AML) method for source localization and direction-of-arrival (DOA) estimations. Then, we consider the use of least-squares (LS) method applied to DOA bearing crossings to perform source localization. A novel virtual array model applicable to the AML-DOA estimation method is proposed for reverberant scenarios. Details on the wireless acoustical testbed are given. We consider the use of Compaq iPAQ 3760s, which are handheld, battery-powered device normally meant to be used as personal organizers (PDAs), as sensor nodes. The iPAQ provide a reasonable balance of cost, availability, and functionality. It has a build-in StrongARM processor, microphone, codec for acoustic acquisition and processing, and a PCMCIA bus for external IEEE 802.11b wireless cards for radio communication. The iPAQs form a distributed sensor network to perform real-time acoustical beamforming. Computational times and associated real-time processing tasks are described. Field measured results for linear, triangular, and square subarrays in free-space and reverberant scenarios are presented. These results show the effective and robust operation of the proposed algorithms and their implementations on a real-time acoustical wireless testbed.

Abstract-The IEEE802.11 has been devised explicitly for low mobility and single access point scenarios, so its effectiveness is impaired in conditions with not negligible interference in ad hoc and/or in infrastructure modes. Well known problems typical of these scenarios are the so called hidden and exposed terminals. In this paper we propose and test a novel MAC layer for Wireless LANs, able to improve the performance of IEEE802.11 DCF (Distributed Coordination Function) in environments with high interference levels. The key point is to insert information about received power and interference levels into MAC control packets. By computing an estimation of the interference increasing due to an eventual transmission (forbidden with IEEE802.11 because blocked by the virtual carrier sensing), the number of parallel transmissions can grow significantly.

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