Undetected Direct Path Research Articles

Utilization of Multiple-Antenna Multicarrier Systems and NLOS Mitigation for Accurate Wireless Indoor Positioning

In this paper, we investigate the performance of wideband orthogonal multicarrier signals in conjunction with multiple antenna systems for wireless indoor positioning. The multiple output orthogonal frequency division multiplexing (OFDM) scheme is used to estimate the propagation time delay and the relative direction of arrival (DOA) of multipath signals. Besides the frequency diversity attributed to OFDM, the antenna array elements can be used in two scenarios: 1) for time delay estimation with spatial diversity and 2) for joint time delay and DOA estimation. To measure the effect of the characteristic parameters of wireless positioning system, the Cramer-Rao lower bound for both scenarios have been derived. In indoor environments, the mobile unit (MU) is often in a non-line-of-sight (NLOS) situation and the direct path could be completely blocked. This degrades the performance significantly. Therefore, we introduce properly weighted least square (WLS) and hybrid WLS estimators to mitigate the effect of undetected direct path channel profiles. An adaptive method is also presented to select the reference base station (BS) and extract certain weights to scale the observations based on the estimated channel profiles. Our experimental results using the emerging IEEE 802.11ac standard reveal that the hybrid time difference of arrival (TDOA) and DOA outperform in non-perfectly synchronized BSs and NLOS environments. However, the achieved accuracy is not improved beyond that obtained by using TDOA with spatial diversity if the BSs are perfectly synchronized. The NLOS mitigation method has been tested using different obstacles with different positions between the MU and the BS. The achieved performance is nearly similar to that of using LOS BSs.

Indoor Localization With Range-Based Measurements and Little Prior Information

We address the problem of indoor localization with networks of sensors taking range-based measurements in the presence of very little prior information. We propose several robust methods that do not require previous measurement campaigns when a network is deployed. The focus is on networks of ultrawideband sensors, but the proposed range-based methods can also be applied to other types of sensor networks. The location of a target node is estimated from measured distances to anchor nodes of known positions. We allow for the possibility of large errors in the range measurements due to undetected direct path (UDP) propagation conditions. In mitigating the UDP effect, our approach is to combine intermediate location estimates from different subsets of beacons. We propose novel criteria for identifying the combinations that produce bad estimates. These combinations are then discarded in obtaining the final estimates. Simulations reveal that the proposed methods achieve improved performance with respect to that of existing techniques that exploit the same prior information. Under many scenarios, the proposed methods reach the performance of some algorithms that exploit more prior information.

A Cooperative Localization Algorithm for UWB Indoor Sensor Networks

This paper deals with range-based localization in ultra wideband sensor networks, allowing for the possibility of large range measurement errors because of a failure to detect the direct paths between some nodes. A novel algorithm is proposed that uses only partial knowledge of the service area topology, particularly of the positions of objects which are capable of causing undetected direct path (UDP) propagation conditions. Although the spirit of the proposed approach, because of the lack of information on the range error statistics, is to remove measurements performed under UDP conditions from the computation of the location estimate, these measurements are used implicitly by the algorithm to contribute to the erroneous trial locations being discarded. A cooperative stage is included that allows the probability of localization of a target with an insufficient initial number of accurate range measurements to increase. The proposed algorithm outperforms a variety of alternative positioning techniques, and thus illustrates the capability of this topology knowledge to mitigate the UDP problem, even in the absence of any knowledge about the range error statistics.

ToA Node Distance Estimation Enhancement in MANET Localization Algorithm Based on Cooperative Trilateration

This paper provides a brief survey of the Time of Arrival (ToA) ranging method often used in the Range-based localization for node distance estimation. It is focused especially on the Non Line-of-Sight (NLoS) channel identification – one of the dominant factors that negative affect the location accuracy. It deals with the improvement of the received signal arrival time determination in the case of NLoS communication with undetected direct path. The low complexity NLoS mitigation method is proposed in this paper that estimates the time of arrival of the missing direct path as a mean of the time delays of reflected paths. This method is implemented in the cooperative positioning algorithm based on the circular trilateration. The IEEE 802.15.4a statistical channel model is used to simulate the mobile node communication.

A multiple candidate time of arrival algorithm for tracking nodes in multipath environments

Tracking systems based on the measurement of time of arrival (TOA) assume that the measured signal travels directly between transmitter and receiver. In complex radio propagation environments this direct path signal is often weak compared to multipath signals and there is a tradeoff in the TOA algorithm between sensitivity to the weak direct path and false detection due to noise, sidelobes, and other artifacts. Conventional TOA algorithms return a single TOA, which can be early due to false detection or late due to an undetected direct path, and these errors degrade tracking performance. In this paper a novel approach to this problem is proposed in which tracking performance is improved using multiple candidate TOA values. In particular a set of TOAs are extracted from the channel impulse response for each received signal and converted to a set of range values. A decision as to which among the set of range values is due to the direct path is deferred to the tracking algorithm, which uses a probabilistic soft-decision approach. Experimental studies conducted using a wireless network demonstrate that the 90% percentile absolute position error is reduced from 3.3m to 1.3m and the relative position error is reduced from 1.3m to 0.5m.

UDP identification and error mitigation in toa-based indoor localization systems using neural network architecture

Time-of-Arrival (ToA) based localization has attracted considerable attention for solving the very complex and challenging problem of indoor localization, mainly due to its fine range estimation process. However, ToA-based localization systems are very vulnerable to the blockage of the direct path (DP) and occurrence of undetected direct path (UDP) conditions. Erroneous detection of other multipath components as DP, which corresponds to the true distance between transmitter and receiver, introduces substantial ranging and localization error into ToA-based systems. Therefore, in order to enable robust and accurate ToA-based indoor localization, it is important to identify and mitigate occurrence of DP blockage. In this paper we present two methodologies to identify and mitigate the UDP conditions in indoor environments. We first introduce our identification technique which utilizes the statistics of radio propagation channel metrics along with binary hypothesis testing and then we introduce our novel identification technique which integrates the same statistics into a neural network architecture. We analyze each approach and the effects of neural network parameters on the accuracy of the localization system. We also compare the results of the two approaches in a sample indoor environment using both real-time measurement and ray tracing simulation. The identification metrics are extracted from wideband frequency-domain measurements conducted in a typical office building with a system bandwidth of 500 MHz, centered around 1 GHz. Then we show that with the knowledge of the channel condition, it is possible to improve the localization performance by mitigating those UDP-induced ranging errors. Finally, we compare the standard deviation of localization error of traditional localization system and UDP identification-enhanced localization system with their respective lower bound.

Introduction to the IJWIN Special Issue: ‘Physical Layer Engineering for Emerging Wireless Applications’

In 2007 we had the honor and responsibility to serve as chairs of the Technical Programme Committee (TPC) of the prestigious IEEE PIMRC (Personal Indoor and Mobile Radio Communications) conference. The conference attracted more than 1,200 participants and included several excellent contributions covering all aspects of the contemporary wireless telecommunications landscape. We personally had the chance to read almost all papers, and attend to a large number of interesting talks during the conference. Furthermore, we came directly in touch with many authors that excelled in the conference. In the scope of this special issue we extended an invitation to excellent authors, with a view to publishing the next steps of their works in this special issue. The five papers of this issue cover a number of different topics ranging from channel characterization and localization to smart antennas. Specifically, the first paper is titled ‘Analysis of Scattering in Mobile Radio Channels based on Clustered Multipath Estimates’ is co-authored by Maurice Kwakkernaat and Matti Herben from the Department of Electrical Engineering of the Eindhoven University of Technology. The paper deals with the accurate estimation of the propagation effect in smart-antenna and MIMO systems. The authors capitalize on a 3-D high-resolution channel sounder, which was developed based on a 3-D antenna array in combination with a multi-dimensional version of the Unitary ESPRIT algorithm. This system is capable of accurately measuring the angle-delay characteristics in azimuth conditions, as well as under mobile conditions. The authors also compare the results from deterministic propagation models to the clustered highresolution angle-of-arrival (AoA) estimates. Comparison results show that: (a) a significant amount of multipath components (MPCs) was missing and (b) the spreads in delay and angle are much larger in the measurements, (c) The MPCCs (MPCs clusters) tend to evolve rather slow along the trajectory, which may allow for interpolation techniques to reduce simulation time. The title of the second paper of the issue is ‘Identification of the Absence of Direct Path in ToA-Based Indoor Localization Systems’. The authors of the paper (namely Mohammad Heidari, Ferit Ozan Akgul, Nayef Alsindi and Kaveh Pahlavan from the Worcester Polytechnic Institute) introduce a methodology to identify and mitigate the undetected direct path (UDP) conditions, which can substantially improve the overall indoor positioning accuracy of Ultra-wideband (UWB) positioning systems based on time of arrival (ToA) techniques. The methodology relies on a novel UDP identification and mitigation technique that is based on analyzing the statistics of the propagation channel. The paper introduces the fundamentals of ToAbased localization and describes the metrics being used for UDP identification. Accordingly it presents results of binary likelihood ratio tests for UDP identification. In deriving these results the outcome of the hypothesis tests are compared to a certain threshold, in order to determine if a receiver is in DDP or UDP condition. The contribution is validated based on simulation results for a sample indoor environment. J. Soldatos (&) Athens Information Technology, 0.8 km Markopoulou Ave., P.O. Box 68, 19002 Peania, Athens, Greece e-mail: jsol@ait.edu.gr

Identification of the Absence of Direct Path in ToA-Based Indoor Localization Systems

Ultra-wideband (UWB) indoor positioning systems based on time of arrival (ToA) techniques are considered to be the high precision alternatives to those employing received signal strength (RSS) or angle of arrival (AoA) due to their superior time-domain resolution. However, the performance of such systems may easily be degraded by the blockage of the direct path (DP) and occurrence of undetected direct path (UDP) condition. By erroneous detection of the other multipath components (MPCs) as DP, which is the indicator of the true distance between the transmitter and the receiver, substantial localization errors will be introduced into the system. Hence, detection of DP categorizes the receiver locations into two main classes of detected direct path (DDP) and undetected direct path (UDP). The real challenge is to be able to identify the class of the receiver location; therefore, to remedy the ranging measurement in UDP condition. In this paper we propose a methodology to identify and mitigate the UDP conditions, which can substantially improve the overall indoor positioning accuracy.

A Markov Model for Dynamic Behavior of ToA-Based Ranging in Indoor Localization

The existence of undetected direct path (UDP) conditions causes occurrence of unexpected large random ranging errors which pose a serious challenge to precise indoor localization using time of arrival (ToA). Therefore, analysis of the behavior of the ranging error is essential for the design of precise ToA-based indoor localization systems. In this paper, we propose a novel analytical framework for the analysis of the dynamic spatial variations of ranging error observed by a mobile user based on an application of Markov chain. The model relegates the behavior of ranging error into four main categories associated with four states of the Markov process. The parameters of distributions of ranging error in each Markov state are extracted from empirical data collected from a measurement calibrated ray tracing (RT) algorithm simulating a typical office environment. The analytical derivation of parameters of the Markov model employs the existing path loss models for the first detected path and total multipath received power in the same office environment. Results of simulated errors from the Markov model and actual errors from empirical data show close agreement.

Analysis of Time of Arrival Estimation Using Wideband Measurements of Indoor Radio Propagations

Using time of arrival (TOA) to determine the distance between the transmitter and the receiver is the most popular technique for accurate indoor positioning. The accuracy of measuring the distance using this method is sensitive to the bandwidth of the system and the multipath condition between the wireless terminal and the access point. In general, as the bandwidth increases beyond a certain value, it is expected that the measured TOA error approaches zero. However, for the so-called undetected direct path (UDP) conditions, the system exhibits substantially high distance measurement errors that cannot be eliminated with the increase in the bandwidth of the system. In this paper, we provide an analysis of the behavior of superresolution and traditional TOA estimation algorithms in line-of-sight (LOS), non-LOS, and UDP conditions in indoor areas. The analysis is based on wideband frequency- domain measurements of the indoor radio channel propagations in several indoor areas, with special attention to the UDP conditions.

Indoor geolocation in the absence of direct path

Severe multipath in indoor areas causes undetected direct path (UDP) conditions, which pose a serious challenge to the design of robust precision indoor geolocation systems. Based on a scenario on the third floor of the Atwater Kent Laboratory at the Worcester Polytechnic Institute, we explain the reason for frequent absence of direct path, and introduce and analyze the effectiveness of two novel approaches to mitigating the large ranging errors caused by UDP conditions. The first technique exploits nondirect paths for ranging, while the second approach relies on cooperative localization for wireless sensor and ad hoc networks

USING UWB MEASUREMENTS FOR STATISTICAL ANALYSIS OF THE RANGING ERROR IN INDOOR MULTIPATH ENVIRONMENT

In this paper we use UWB measurements for bandwidths up to 3 GHz to present a framework for statistical modeling of the indoor radio channel propagation characteristics that are pertinent to precise indoor geolocation using time-of-arrival (TOA) estimations. Accuracy of indoor geolocation systems relies on the strength and TOA of the direct path (DP) in the channel profile. Based on UWB measurements in a typical office building, we introduce empirical models for the path-loss and TOA of the DP. Path-loss model for the DP is used to analyze the occurrence of the undetected direct path (UDP) conditions which cause large errors in indoor geolocation systems. We then introduce a novel statistical model for the ranging or distance measurement error (DME) which is needed for comparative performance evaluation of the indoor positioning algorithms. The DME is a function of the bandwidth of the system, occurrence of the UDP conditions, and the distance between the transmitter and the receiver.

Modeling of the TOA-based distance measurement error using UWB indoor radio measurements

Time of arrival (TOA) estimation used with ultra wideband (UWB) transmission is currently the most popular technique for accurate indoor geolocation. Due to severe indoor multipath conditions, these techniques often suffer from significant inaccuracy in location estimation. In this paper, we introduce a model for the error in estimated distance as measured from the estimated TOA of the direct path (DP) in a typical multipath indoor environment. The TOA estimation error has two components, (1) the errors caused by the multipath dispersion affecting any signal path and (2) the errors caused by undetected direct path (UDP) conditions. The statistical behavior of this error is also a function of the system bandwidth. The empirical data from UWB indoor measurements in an office building are used to design a model for the distance measurement error. This model relates the behavior of the two components of the TOA estimation error to the bandwidth of the system.