Assessment of Anchors Constellation Features in RSSI-Based Indoor Positioning Systems for Smart Environments  
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1 Department of Information Engineering (DINFO), University of Florence, 50139 Florence, Italy;  
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1 Department of Information Engineering (DINFO), University of Florence, 50139 Florence, Italy;  
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1 Department of Information Engineering (DINFO), University of Florence, 50139 Florence, Italy;  
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In this work, the problem is addressed by following the paradigm of increasing the amount of information exchanged by a number of wireless nodes, while assuming a limited use of additional ad-hoc hardware to that already implemented in the established wireless infrastructure. The reason is that positioning should be offered as an application layer on top of the data networking layers, with a minimal impact on the already existing networks. In this context, several positioning approaches relying on wireless infrastructures based on IEEE 802.11x/802.15.4, have been presented in recent literature [12; 13]. The adoption of commercial radio transceivers makes the use of the Received Signal Strength Indicator (RSSI) a common approach. Typically, an RSSI enables fingerprinting, which has its main drawback in its requirement of a periodical and lengthy on-site calibration phase, to overcome environment characterization uncertainties. Alternatively, a dense distribution of specific beacons can be used. This type of solutions is not feasible in IoT environments, especially in typical smart environment applications, which include integrations of positioning systems and autonomous machines. For instance, in smart home environments a robot can use data from a positioning system to improve the context awareness of interacting objects, and not only to move into the specific scenario [14]. This requires a localization system characterized by low cost, low power, ease-of-installation, and ease-of-maintenance. In [15] and [16], the authors proposed an indoor localization system based on a mesh network of IEEE 802.15.4 compliant anchor nodes. In those works, as well as hereinafter, the mesh is associated to the location of the positioning system anchors, which define the mesh vertexes. The technology is based on the installation of a set of switched anchor nodes equipped with Switched Beam Antennas (SBA) [15; 16; 17], each one capable of IEEE 802.15.4 communications. After having installed a proper number of anchors working together as a \"distributed\" router, the network is able to offer wireless connectivity to its nodes, while collecting a long set of distributed RSSI values. The various received signal power levels obtained with this procedure are then exploited using proper algorithms to obtain a position estimation, based only on received signals magnitude (i.e., phase-less). A high level of accuracy is normally achieved after an on-site calibration phase (as opposed to [16]), or additional tracking based on odometric readings (as opposed to [13; 18]). Using only RSSI (as defined in the IEEE 802.11x and 802.15.4 standards) readings as input data for positioning algorithms, this method is suitable to any IEEE compliant wireless communication protocol, making its implementation in any pre-existent wireless network infrastructure easily achievable (i.e., [19; 20]). In this paper we further extend the concept already presented in [15; 16], by introducing and validating a comprehensive assessment about the trade-off between system positioning complexity and positioning accuracy in a typical office room site. The paper is organized as it follows.