# Risk-Aware Wireless Positioning in
## 1 Introduction  
In our daily lives, we can observe positioning (i.e., location tracking) techniques that have been widely adopted in many applications for various purposes, which include location-based service (LBS) [1] and emergency location alarms [2]. One of the most popular positioning systems is the global positioning system (GPS) [3], which utilizes global satellites. The GPS can accurately estimate the location of an object in an outdoor environment but it requires high power consumption, a long delay time and high maintenance costs. Moreover, the GPS is generally not applicable for indoor environments. The infrared ray based positioning is also widely used in short-range applications when a line-of-sight (LoS) is present [4]. As wireless devices have become very common, wireless positioning techniques that utilizes radio frequency (RF) for localization have been intensively studied [5]. The wireless positioning schemes estimate the location of an object using wireless signals, so it can be utilized in both outdoor and indoor environments at various ranges. There are various wireless positioning schemes based on various measures such as received signal strength (RSS), the angle of arrival (AoA), the time of arrival (ToA), the time difference of arrival (TDoA) and fingerprinting (i.e., database). Wireless positioning schemes can be classified in many ways, for example, into range-based and range-free techniques [5]. Range-based techniques estimate the position using some metrics such as RSS, AoA, ToA and TDoA so that the metrics are directly mapped into ranges. In range-free techniques, on the other hand, the measurement cannot be directly converted into a range and the position is obtained with the aid of additional information such as a database. There are two major range-freetechniques: the fingerprinting scheme and the hop count scheme. These techniques will be briefly explained in Section 2. The fingerprinting scheme is widely used in both outdoor and indoor environments thanks to its simple and cost-effective implementation [6]. The fingerprinting scheme also shows accurate positioning performance against multipath fading and non-line-of-sight (NLoS) condition in challenging environments [7]. There are various fingerprinting schemes according to the measurement databases used, such as visual fingerprinting, motion fingerprinting, signal fingerprinting and hybrid fingerprinting, for which each scheme has pros and cons under different situations [8]. In this paper, we propose an efficient fingerprinting-based wireless positioning scheme that is aware of user safety conditions in the maritime environment. There is a mobile user in a maritime area comprising safe and unsafe zones and the base stations (BSs) monitor the user position to prevent the user from getting into an unsafe zone. In this case, the safe zone is divided into many cells and the BSs have a received signal strength (RSS) map consisting of the average RSS sequences for each cell. Thus, once the BSs measure the RSS values from the user, the BSs can jointly estimate the user's current cell (i.e., user position) from the RSS map. Thus, our positioning scheme adaptively controls the user's positioning signal frequency with respect to the risk at the current position, whereas the conventional scheme maintains the same signaling frequency oblivious to the risk. As a result, with our scheme the positioning signal the user transmits becomes more frequent as the user approaches more dangerous locations to allow BSs to obtain more accurate user position. Meanwhile, the mobile user has a limited power budget, so our proposed scheme reduces the user's average risk under the same total power consumption. We model the risk of the user considering many factors including geometry (i.e., water depth) and the positioning signal frequency. Then, we optimize the user positioning signal frequency to minimize the user's overall risk, which is the average risk of the user over all positions. Our paper is organized as follows.