Non Line Of Sight effects in UWB indoor direct one-step selflocalization using distributed antenna system: Measurement based study

The problem of identification and mitigation of the Non Line Of Sight (NLOS) effects in Ultra Wideband (UWB) indoor localization is a very challenging ongoing topic in the research community. There are two groups of methods applied in UWB indoor localization: indirect two-step localization methods and direct one-step localization methods [1]. Two-step localization methods are widely investigated and their theoretical performances and limitations are briefly surveyed in UWB literature [1]-[15]. These methods are based on the estimation of localization parameters (such as Receiving Signal Strengths (RSS), Direction of Arrivals (DOA) or Time Difference of Arrival (TDOA)) in the first step, and location estimation in fusion center in the second step. Direct one-step localization methods are more recent [16]-[23]. Weiss and Amar proposed in [16] a method for Direct Position Determination (DPD) of multiple radio signals. Their localization method is based on calculation of localization criteria function in frequency domain, and due to widebandness of UWB signals it is not quite appropriate for UWB localization. Erić and Vučić proposed a new MUSIC based method for direct network-centric localization in UWB systems based on the concept of steered covariance matrix [17],[18]. In the proposed method calculation of localization function is performed in time domain, whereas preprocessing is performed in frequency domain. Navarro, Closas and Nájar proposed in [20] the Direct Position Estimation (DPE) algorithm, which is based on a generalization of the pseudoperiodogram approach proposed for TOA estimation, and applied it for direct positioning in Impulse Radio Ultra Wide Band (IR UWB) in IEEE 802.15.4a channels. Erić, Dukić and Vučić proposed a new MUSIC based method for direct self-localization of synchronous IR UWB Node(s) [21]. Erić, Zetik, Dukić and Vučić presented in [22,23] results of experimental verification of the self-localization method in real indoor environment, based on real measurements performed by UWB MIMO channel sounder. NLOS identification and mitigation is a big research challenge in both two-step indirect and one-step direct localization methods It is known that the existence of NLOS propagation between some of referent and user UWB nodes drastically degrades localization performances when localization is performed by two-step localization methods [26,27,28] . Key approaches for NLOS identification and mitigation in UWB indoor localization considered in literature so far are commonly based on range estimates or on the channel pulse response. On the other hand, NLOS identification and mitigation in UWB indoor one-step direct localization hasn't been considered and highlighted in literature yet. That is why there are only a few papers related to direct indoor UWB localization. During experimental verification of the method for indoor UWB self'-localization, the authors noticed that the proposed method was relatively robust to existence of NLOS time subintervals in observation time interval used for self-localization. Since the proposed self-localization method is based on calculation of localization function on the hypothetic locations in space of interest for localization, nature of such process offers quite new approaches to solving NLOS identification and mitigation problem. Therefore it will be the focus of this paper. It is based on theoretical foundation of direct one-step self-localization methods proposed by the authors in [21] and data obtained in measurement campaign realised by the use of MIMO UWB sounder at Ilmenau Technical University.