Multi-target indoor localization and tracking on video monitoring system in a wireless sensor network

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Abstract

The development of wireless sensor networks (WSNs) has greatly encouraged the use of sensors for multi-target tracking. The high efficiency detection and location monitoring are critical requirements for multi-target tracking in a WSN. In this paper, we present an indoor tracking model using IEEE 802.15.4 compliant radio frequency and video monitoring system to monitor targets in a special way. Our motivation is to manipulate the erratic or unstable received signal strength indicator (RSSI) signals to deliver the stable and precise position information in the indoor environment. We propose a localization algorithm based on statistical uncorrelated vectors and develop a smoothing algorithm to minimize the noise in RSSI values. We also present a solution combining the WSN with the Ethernet technology to decrease the RSSI interference by buildings. The developed system can realize the functions of multi-target detection and tracking, and specific target inquiries, alarms and monitoring. The system architecture, hardware and software organization, as well as the solutions for multiple targets tracking, RSSI interference and localization accuracy have been introduced in details.

Highlights

► We present an indoor tracking model to monitor targets. ► We propose localization algorithm based on statistical uncorrelated vectors. ► A smoothing algorithm is developed for minimizing noise in RSSI. ► We present a solution to decrease the RSSI interference by buildings.

Introduction

The popularity of wireless communication technologies and embedded micro-sensing technologies has greatly promoted the development of wireless sensor networks (WSNs). The basic function of a WSN (Patwari et al., 2005, Hyo-Sung and Wonpil, 2009, Dai et al., 2011) is to connect a series of spatially distributed sensor nodes through the self-organizing wireless network, transfer the collected data, and carry out the appropriate data analysis and processing. So, WSNs can be used to perform the collaborative monitoring and controlling of the physical or environmental situations in a large space.

There are two major advantages of WSNs: the ease of configuration and the flexibility of deployment. WSNs have been widely used in various fields such as transportation (Khanafer et al., 2010), manufacturing (Delen et al., 2011), health care (Huanjiaet al., 2011), environmental protection (Mittal and Bhatia, 2010), and military applications (Lowell, 2011). Many researchers have studied WSNs, including design issues related to the physical and media access layers, routing and transporting protocols (Freris et al., 2010, Baronti et al., 2007).

Localization of objects and tracking of moving objects are essential to many location-based services (Ni et al., 2011; Hao et al., 2009). Tracking objects is basically consisted of detecting and monitoring the location of real-world objects, where different technologies such as WiFi, radio frequency identification (RFID), ultrasound, infrared, and video tracking, may be used. Active Bat (Andy and Alan, 1997) and Cricket (Priyantha et al., 2000) are well-known in-building mobile locating and tracking approaches that estimate the position of a target from the time difference between sent and received ultrasonic signals and offer high tracking precision. Active Badge (Want et al., 1992) employs the infrared technology for locating people in an office environment. However, these technologies often require collateral mechanism and some parts are expensive and highly susceptible to damage.

As an inexpensive localization solution, the received signal strength indicator (RSSI) model predicts positions using radio frequency (RF) signals. Researchers have attempted to adapt the existing system to the estimation of location in numerous ways. LANDMARC (Ni et al., 2003) is one of the most popular RF-based localization technologies using active RFID tags. It adopts the coordinates of the K nearest reference tags to compute the coordinates of the tracking tag. However, since the RSSI is easily influenced by environment, the chosen K nearest reference tags may not be close to the target. Therefore, if this method is applied in a large field, the tracking accuracy drops dramatically. One prominent solution is the WiFi-based positioning system (Lassabe et al., 2006), which utilizes the existing wireless local area network to predict locations. However, the WiFi-based positioning system is usually not reliable in spaces containing obstacles. Transceiver-free technologies (Zhang et al., 2007) can be used to detect the object's trajectory pattern without requiring the target to carry any device. It is based on a simple observation that wireless signals are quite stable in a static environment. When links are disturbed (for example, signal strength changes dramatically), it is very likely there are moving objects nearby. Although the dynamics of RSSI is robust to the environmental change, the densely deployed sensors cause heavy communication overhead and may introduce more interference. As a result, its localization accuracy is also limited. Cocktail (Zhang et al., 2010) is a hybrid RFID/WSN implementation. Sensors use the dynamic performance of RSSI to choose a cluster of reference tags as candidates. The final target location is estimated according to the RSSI relationship between the target tag and candidate reference tags. WSNs provide a new opportunity for objects localization and tracking. Boon-Giin and Wan-Yong (2011) have introduced an indoor tracking model using IEEE 802.15.4 compliant radio frequency and immersive 3-D graphics to present data. They proposed an accuracy refinement algorithm to filter out noise in radio-frequency transmission between sensors, and the locations of targets can be displayed on a mobile device like PDA.

Considering a WSN of a set of wireless sensor nodes deployed in a closed field, a sensor node can detect the movement of an object and report the received signal strength information when the object enters or remains in the sensing area of this node. Since WSN is a self-organizing network, we can use the mutual information connection between nodes to achieve the functions of detection, localization and tracking. In this work, detection, localization and tracking of targets are realized based on WSNs, and the real-time indoor monitoring is achieved by integrating WSNs and video monitoring systems. When the wireless signals are seriously weakened by the building walls, it becomes difficult to locate and track a target continuously in different rooms and different floors. We combine the WSN with the Ethernet technology, and design routing methods to solve this problem. Since the layout of rooms and corridors can be quite different, we propose different location algorithms for different scenarios. In addition, since the wireless signal is usually disturbed by various noises in the indoor environment, we develop a smoothing algorithm to minimize the noise in the RSSI and locate the target by our localization algorithm based on statistical uncorrelated vectors.

Section snippets

System architecture

The purposes of this research are to build a RSSI-based real-time in-building detecting and tracking system, to construct the network topology to address the serious interference on the RSSI by different floors and walls, to propose a new localization algorithm and a smoothing method to reduce the influence of the ranging error on the positioning accuracy, and to combine with video monitoring systems to monitor targets intelligently.

After comparing different available chips, we select

Indoor location

The commercial CC2431 module has a basic built-in algorithm for calculating location by the trilateration method. This localization algorithm is inadequate since it is based on RSSI values containing noises from the environment, and it does not provide reliable output for use in a real-time situation. Although the CC2431 module only uses relatively strong signals received from reference nodes, the signals are appropriate for computation. The real-time location prediction should be robust and

Experimental results

The objective of the experiments is to detect the number of targets in the networks and further observe the precise locations of multiple targets based on RSSI values. Experiments were conducted in an office building. Various indoor conditions that may bring interference to RSSI values during transmission were carefully taken into consideration, for example, the human activity in particular spaces, different objects such as chairs, desktop computers and tables (typically around 70% of spaces

Conclusions

We propose a novel localization algorithm based on statistical uncorrelated vectors to increase the precision of real-time multi-target tracking in the indoor environment. We apply the RSSI smoothing method to removes noise and abrupt changes in the received RSSI values before they are used for distance computation. We design the nearest neighbor algorithm for locating and tracking targets in the corridor. By combining the WSN with the Ethernet technology, and designing routing methods to solve

Acknowledgments

This work is supported by the Fundamental Research Funds for the Central Universities (JUSRP20914, JUDCF10031), the 111 Project (B12018) and PAPD of Jiangsu Higher Education Institutions.

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