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Indoor Wi-Fi Positioning: Techniques and Systems

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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Remote Sensors".

Deadline for manuscript submissions: closed (31 October 2022) | Viewed by 32077

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Prof. Dr. Günther Retscher

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Department of Geodesy and Geoinformation, Technische Universitat Wien, 1040 Vienna, Austria
Interests: positioning and navigation with GNSS; location-based services; indoor and pedestrian navigation; applications of multi-sensor systems; smartphone positioning and sensor fusion
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Special Issue Information

Dear Colleagues,

Wi-Fi (Wireless Fidelity) is one of the most widely used signals of opportunity for positioning and tracking mobile users. It is widely adopted for smartphone-based indoor positioning systems due to the availability of infrastructure already deployed for communications. A high number of Access Points (APs) of public and private networks exist, guaranteeing a high signal ubiquity. The Wi-Fi signals have therefore a high potential to be employed for numerous applications for localization and guidance. Localization is based either on direct measurements of the Received Signal Strength (RSS) of the surrounding Wi-Fi Access Points or on the measurement of the Round-Trip Time (RTT) between the mobile device and the Access Points. Localization methods, therefore, include lateration and fingerprinting algorithms. Depending on the positioning technique adopted, different levels of positioning accuracy are achievable. This Special Issue is addressed to researchers on all types of localization approaches and algorithms. A special emphasis will be given to novel approaches for RSS- and RTT-based positioning and their combination and integration. Moreover, sensor integration and fusion, especially in the case of smartphone positioning, is a major research direction. Original contributions focused on systems and technologies to enable a variety of indoor localization applications are welcome. This Special Issue will provide the opportunity to disseminate among the scientific community relevant and new contributions related to the widespread use of Wi-Fi, such as for mobile devices, for the localization of mobile users in indoor and transitional environments, and for Location-Based Services (LBS), as well as to algorithm developments for these applications and the use of new and traditional technologies based on Wi-Fi for indoor spaces.

Prof. Dr. Günther Retscher
Guest Editor

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Keywords

Wireless Fidelity Wi-Fi
Received Signal Strength RSS
Round Trip Time RTT Measurements
Lateration
Location Fingerprinting
Combination of Techniques
Sensor Integration and Fusion
Positioning Algorithms
Indoor User Localization
Seamless Transition between Different Environments

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Related Special Issue

Indoor Wi-Fi Positioning: Techniques and Systems—2nd Edition in Sensors (1 article)

Published Papers (9 papers)

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30 pages, 2990 KiB

Open AccessArticle

Testing and Evaluation of Wi-Fi RTT Ranging Technology for Personal Mobility Applications

by Manos Orfanos, Harris Perakis, Vassilis Gikas, Günther Retscher, Thanassis Mpimis, Ioanna Spyropoulou and Vasileia Papathanasopoulou

Sensors 2023, 23(5), 2829; https://doi.org/10.3390/s23052829 - 5 Mar 2023

Cited by 9 | Viewed by 3024

Abstract

The rapid growth in the technological advancements of the smartphone industry has classified contemporary smartphones as a low-cost and high quality indoor positioning tools requiring no additional infrastructure or equipment. In recent years, the fine time measurement (FTM) protocol, achieved through the Wi-Fi round trip time (RTT) observable, available in the most recent models, has gained the interest of many research teams worldwide, especially those concerned with indoor localization problems. However, as the Wi-Fi RTT technology is still new, there is a limited number of studies addressing its potential and limitations relative to the positioning problem. This paper presents an investigation and performance evaluation of Wi-Fi RTT capability with a focus on range quality assessment. A set of experimental tests was carried out, considering 1D and 2D space, operating different smartphone devices at various operational settings and observation conditions. Furthermore, in order to address device-dependent and other type of biases in the raw ranges, alternative correction models were developed and tested. The obtained results indicate that Wi-Fi RTT is a promising technology capable of achieving a meter-level accuracy for ranges both in line-of-sight (LOS) and non-line-of-sight (NLOS) conditions, subject to suitable corrections identification and adaptation. From 1D ranging tests, an average mean absolute error (MAE) of 0.85 m and 1.24 m is achieved, for LOS and NLOS conditions, respectively, for 80% of the validation sample data. In 2D-space ranging tests, an average root mean square error (RMSE) of 1.1m is accomplished across the different devices. Furthermore, the analysis has shown that the selection of the bandwidth and the initiator–responder pair are crucial for the correction model selection, whilst knowledge of the type of operating environment (LOS and/or NLOS) can further contribute to Wi-Fi RTT range performance enhancement. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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26 pages, 8580 KiB

Open AccessArticle

Improving Fingerprint-Based Positioning by Using IEEE 802.11mc FTM/RTT Observables

by Israel Martin-Escalona and Enrica Zola

Sensors 2023, 23(1), 267; https://doi.org/10.3390/s23010267 - 27 Dec 2022

Cited by 7 | Viewed by 2816

Abstract

Received signal strength (RSS) has been one of the most used observables for location purposes due to its availability at almost every wireless device. However, the volatile nature of RSS tends to yield to non-reliable location solutions. IEEE 802.11mc enabled the use of the round trip time (RTT) for positioning, which is expected to be a more consistent observable for location purposes. This approach has been gaining support from several companies such as Google, which introduced that feature in the Android O.S. As a result, RTT estimation is now available in several recent off-the-shelf devices, opening a wide range of new approaches for computing location. However, RTT has been traditionally addressed to multilateration solutions. Few works exist that assess the feasibility of the RTT as an accurate feature in positioning methods based on classification algorithms. An attempt is made in this paper to fill this gap by investigating the performance of several classification models in terms of accuracy and positioning errors. The performance is assessed using different AP layouts, distinct AP vendors, and different frequency bands. The accuracy and precision of the RTT-based position estimation is always better than the one obtained with RSS in all the studied scenarios, and especially when few APs are available. In addition, all the considered ML algorithms perform pretty well. As a result, it is not necessary to use more complex solutions (e.g., SVM) when simpler ones (e.g., nearest neighbor classifiers) achieve similar results both in terms of accuracy and location error. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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12 pages, 15907 KiB

Open AccessArticle

WiFi FTM and UWB Characterization for Localization in Construction Sites

by Carlos S. Álvarez-Merino, Emil J. Khatib, Hao Qiang Luo-Chen, Joel Llanes Michel, Sebastián Casalderrey-Díaz, Jesus Alonso and Raquel Barco

Sensors 2022, 22(14), 5373; https://doi.org/10.3390/s22145373 - 19 Jul 2022

Cited by 2 | Viewed by 2690

Abstract

A high-precision location is becoming a necessity in the future Industry 4.0 applications that will come up in the near future. However, the construction sector remains particularly obsolete in the adoption of Industry 4.0 applications. In this work, we study the accuracy and penetration capacity of two technologies that are expected to deal with future high-precision location services, such as ultra-wide band (UWB) and WiFi fine time measurement (FTM). For this, a measurement campaign has been performed in a construction environment, where UWB and WiFi-FTM setups have been deployed. The performance of UWB and WiFi-FTM have been compared with a prior set of indoors measurements. UWB seems to provide better ranging estimation in LOS conditions but it seems cancelled by reinforcement concrete for propagation and WiFi is able to take advantage of holes in the structure to provide location services. Moreover, the impact of fusion of location technologies has been assessed to measure the potential improvements in the construction scenario. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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16 pages, 9616 KiB

Open AccessArticle

Indoor Localization Using Uncooperative Wi-Fi Access Points

by Berthold K. P. Horn

Sensors 2022, 22(8), 3091; https://doi.org/10.3390/s22083091 - 18 Apr 2022

Cited by 14 | Viewed by 3798

Abstract

Indoor localization using fine time measurement (FTM) round-trip time (RTT) with respect to cooperating Wi-Fi access points (APs) has been shown to work well and provide 1–2 m accuracy in both 2D and 3D applications. This approach depends on APs implementing the IEEE 802.11-2016 (also known as IEEE 802.11mc) Wi-Fi standard (“two-sided” RTT). Unfortunately, the penetration of this Wi-Fi protocol has been slower than anticipated, perhaps because APs tend not to be upgraded as often as other kinds of electronics, in particular in large institutions—where they would be most useful. Recently, Google released Android 12, which also supports an alternative “one-sided” RTT method that will work with legacy APs as well. This method cannot subtract out the “turn-around” time of the signal, and so, produces distance estimates that have much larger offsets than those seen with two-sided RTT—and the results are somewhat less accurate. At the same time, this method makes possible distance measurements for many APs that previously could not be used. This increased accessibility can compensate for the decreased accuracy of individual measurements. We demonstrate here indoor localization using one-sided RTT with respect to legacy APs that do not support IEEE 802.11-2016. The accuracy achieved is 3–4 m in cluttered environments with few line-of-sight readings (and using only 20 MHz bandwidths). This is not as good as for two-sided RTT, where 1–2 m accuracy has been achieved (using 80 MHz bandwidths), but adequate for many applications A wider Wi-Fi channel bandwidth would increase the accuracy further. As before, Bayesian grid update is the preferred method for determining position and positional accuracy, but the observation model now is different from that for two-sided RTT. As with two-sided RTT, the probability of an RTT measurement below the true distance is very low, but, in the other direction, the range of measurements for a given distance can be much wider (up to well over twice the actual distance). We describe methods for formulating useful observation models. As with two-sided RTT, the offset or bias in distance measurements has to be subtracted from the reported measurements. One difference is that here, the offsets are large (typically in the 2400–2700 m range) because of the “turn-around time” of roughly 16 μs (i.e., about two orders of magnitude larger than the time of flight one is attempting to measure). We describe methods for estimating these offsets and for minimizing the effort required to do so when setting up an installation with many APs. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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17 pages, 1378 KiB

Open AccessArticle

WiFi FTM, UWB and Cellular-Based Radio Fusion for Indoor Positioning

by Carlos S. Álvarez-Merino, Hao Qiang Luo-Chen, Emil Jatib Khatib and Raquel Barco

Sensors 2021, 21(21), 7020; https://doi.org/10.3390/s21217020 - 23 Oct 2021

Cited by 22 | Viewed by 3730

Abstract

High-precision indoor localisation is becoming a necessity with novel location-based services that are emerging around 5G. The deployment of high-precision indoor location technologies is usually costly due to the high density of reference points. In this work, we propose the opportunistic fusion of several different technologies, such as ultra-wide band (UWB) and WiFi fine-time measurement (FTM), in order to improve the performance of location. We also propose the use of fusion with cellular networks, such as LTE, to complement these technologies where the number of reference points is under-determined, increasing the availability of the location service. Maximum likelihood estimation (MLE) is presented to weight the different reference points to eliminate outliers, and several searching methods are presented and evaluated for the localisation algorithm. An experimental setup is used to validate the presented system, using UWB and WiFi FTM due to their incorporation in the latest flagship smartphones. It is shown that the use of multi-technology fusion in trilateration algorithm remarkably optimises the precise coverage area. In addition, it reduces the positioning error by over-determining the positioning problem. This technique reduces the costs of any network deployment oriented to location services, since a reduced number of reference points from each technology is required. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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25 pages, 8207 KiB

Open AccessArticle

Deep-Learning-Based Wi-Fi Indoor Positioning System Using Continuous CSI of Trajectories

by Zhongfeng Zhang, Minjae Lee and Seungwon Choi

Sensors 2021, 21(17), 5776; https://doi.org/10.3390/s21175776 - 27 Aug 2021

Cited by 11 | Viewed by 3995

Abstract

In a Wi-Fi indoor positioning system (IPS), the performance of the IPS depends on the channel state information (CSI), which is often limited due to the multipath fading effect, especially in indoor environments involving multiple non-line-of-sight propagation paths. In this paper, we propose a novel IPS utilizing trajectory CSI observed from predetermined trajectories instead of the CSI collected at each stationary location; thus, the proposed method enables all the CSI along each route to be continuously encountered in the observation. Further, by using a generative adversarial network (GAN), which helps enlarge the training dataset, the cost of trajectory CSI collection can be significantly reduced. To fully exploit the trajectory CSI’s spatial and temporal information, the proposed IPS employs a deep learning network of a one-dimensional convolutional neural network–long short-term memory (1DCNN-LSTM). The proposed IPS was hardware-implemented, where digital signal processors and a universal software radio peripheral were used as a modem and radio frequency transceiver, respectively, for both access point and mobile device of Wi-Fi. We verified that the proposed IPS based on the trajectory CSI far outperforms the state-of-the-art IPS based on the CSI collected from stationary locations through extensive experimental tests and computer simulations. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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21 pages, 1443 KiB

Open AccessArticle

Transfer Learning for Wireless Fingerprinting Localization Based on Optimal Transport

by Siqi Bai, Yongjie Luo and Qun Wan

Sensors 2020, 20(23), 6994; https://doi.org/10.3390/s20236994 - 7 Dec 2020

Cited by 6 | Viewed by 2589

Abstract

Wireless fingerprinting localization (FL) systems identify locations by building radio fingerprint maps, aiming to provide satisfactory location solutions for the complex environment. However, the radio map is easy to change, and the cost of building a new one is high. One research focus is to transfer knowledge from the old radio maps to a new one. Feature-based transfer learning methods help by mapping the source fingerprint and the target fingerprint to a common hidden domain, then minimize the maximum mean difference (MMD) distance between the empirical distributions in the latent domain. In this paper, the optimal transport (OT)-based transfer learning is adopted to directly map the fingerprint from the source domain to the target domain by minimizing the Wasserstein distance so that the data distribution of the two domains can be better matched and the positioning performance in the target domain is improved. Two channel-models are used to simulate the transfer scenarios, and the public measured data test further verifies that the transfer learning based on OT has better accuracy and performance when the radio map changes in FL, indicating the importance of the method in this field. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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24 pages, 3197 KiB

Open AccessArticle

Adaptive Multi-Type Fingerprint Indoor Positioning and Localization Method Based on Multi-Task Learning and Weight Coefficients K-Nearest Neighbor

by Zhengwu Yuan, Xupeng Zha and Xiaojian Zhang

Sensors 2020, 20(18), 5416; https://doi.org/10.3390/s20185416 - 21 Sep 2020

Cited by 12 | Viewed by 2908

Abstract

The complex indoor environment makes the use of received fingerprints unreliable as an indoor positioning and localization method based on fingerprint data. This paper proposes an adaptive multi-type fingerprint indoor positioning and localization method based on multi-task learning (MTL) and Weight Coefficients K-Nearest Neighbor (WCKNN), which integrates magnetic field, Wi-Fi and Bluetooth fingerprints for positioning and localization. The MTL fuses the features of different types of fingerprints to search the potential relationship between them. It also exploits the synergy between the tasks, which can boost up positioning and localization performance. Then the WCKNN predicts another position of the fingerprints in a certain class determined by the obtained location. The final position is obtained by fusing the predicted positions using a weighted average method whose weights are the positioning errors provided by positioning error prediction models. Experimental results indicated that the proposed method achieved 98.58% accuracy in classifying locations with a mean positioning error of 1.95 m. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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25 pages, 641 KiB

Open AccessSystematic Review

Comprehensive Analysis of Applied Machine Learning in Indoor Positioning Based on Wi-Fi: An Extended Systematic Review

by Vladimir Bellavista-Parent, Joaquín Torres-Sospedra and Antoni Pérez-Navarro

Sensors 2022, 22(12), 4622; https://doi.org/10.3390/s22124622 - 19 Jun 2022

Cited by 18 | Viewed by 4428

Abstract

Nowadays, there are a multitude of solutions for indoor positioning, as opposed to standards for outdoor positioning such as GPS. Among the different existing studies on indoor positioning, the use of Wi-Fi signals together with Machine Learning algorithms is one of the most important, as it takes advantage of the current deployment of Wi-Fi networks and the increase in the computing power of computers. Thanks to this, the number of articles published in recent years has been increasing. This fact makes a review necessary in order to understand the current state of this field and to classify different parameters that are very useful for future studies. What are the most widely used machine learning techniques? In what situations have they been tested? How accurate are they? Have datasets been properly used? What type of Wi-Fi signals have been used? These and other questions are answered in this analysis, in which 119 papers are analyzed in depth following PRISMA guidelines. Full article

(This article belongs to the Special Issue Indoor Wi-Fi Positioning: Techniques and Systems)

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