In localization with wireless sensor network [1], received signal strength indicator (RSSI)-based method uses a prerequisite statistical relationship on the RSSI and the distance between a target node and an anchor node in a location estimation area [2]. It is true that RSSI-based method performs poor even with a sophisticated estimation method such as maximum likelihood (ML) method, because in indoor environments, in addition to near/far effect, multipath fading as well as shadowing gives large variations to measured RSSIs. Although RSSI-based method shows poor location estimation accuracy, it is advantageous in terms of cost and energy consumption. This is because most current wireless communication standards which define physical (PHY) and medium access control (MAC) protocols support a function of measuring RSSI, so if introducing one of such standards to wireless communication nodes for location estimation, information on measured RSSIs can be unconsciously obtained through communications among the nodes, without any special device to measure the RSSIs. For instance, the IEEE 802.15.4 standard [3] has the function in its protocol. Through several experiments on RSSI-based localization in various environments, we have observed that there is a large difference in achievable location estimation performance between computer simulation result and experimental result, and finally we have found that the use of multiple transmit antenna at a target node is effective to improve the location estimation accuracy in realistic environments. To the authors' best knowledge, there has been no proposal on the use of multiple transmit antennas in RSSI-based location estimation problem.
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