Accurate indoor geolocation using time-of-arrival (TOA) is seriously challenged with the occurrence of undetected direct path (UDP) conditions caused by severe multipath in indoor areas. As a result, analysis of the occurrence of UDP in indoor areas is extremely important for design and performance evaluation of the emerging indoor geolocation systems. A UDP occurs when the received signal strength of the direct path (DP) between the transmitter and the receiver falls below the detection threshold of the receiver, but the total received signal power is still above the threshold. This paper provides a novel analysis for modeling the behavior of the DP and the total received power and uses the model to calculate the probability of UDP occurrence. To verify the accuracy of the analytical model, the results are compared with the empirical UWB measurements in typical indoor areas. the UDP conditions. There is nothing available in the literature to address this issue. The only available analysis is the results of empirical measurements reported in (4), in which occasional occurrence of UDP in a typical building using a system with a bandwidth of 200MHz is studied. In this paper based on the analysis of the total received signal power described in (5) and a novel analysis for calculation of the received signal in the direct path, we show that the occurrence of UDP is inevitable. In addition, using a partitioning technique, we determine the probability of occurrence of this condition in an indoor area. We validate the results of our analysis with the empirical UWB channel measurements with 3GHz bandwidth performed in a typical indoor office area. The structure of the sections of this paper is as follows: Section I introduces the importance of UDP in indoor geolocation. Section II is about the measurement system and scenario. Section III describes the concept of UDP and its different definitions. Section IV introduces a channel model that helps calculation of probability of UDP and compares the results of the model with the UWB channel measurements. Finally section V is the conclusion. II. THE CONCEPT AND DEFINITIONS OF UDP A. Existing reports of UDP
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