# WIPP: Wi-Fi Compass for Indoor Passive Positioning with Decimeter Accuracy
## 1 Introduction  
With the significantly growing interest in Location-Based Services (LBSs), positioning applications start to play an important role in our daily life. In outdoor environments, people have adopted Global Navigation Satellite Systems (GNSSs) such as the Global Positioning System (GPS) and BeiDou System to conduct the positioning [1; 2; 3]. However, these systems fail to work in many indoor or underground environments due to the fact that the signal from the satellites is generally blocked by the buildings or ground. In addition, the complex environmental noise and interference make the indoor positioning challenging. At the same time, people have begun to have an urgent demand for indoor positioning. For example, they want to have the navigation information for each room in an unknown building. As another example, they hope to get local coupons when shopping in a large-scale shopping mall. Nowadays, much of the concern of outdoor positioning has been shifted to indoor positioning. According to the role of the target in positioning, the indoor positioning systems can be classified into the active and passive positioning systems. For the active positioning systems such as the Received Signal Strength Indicator (RSSI) based [4; 5; 6; 7; 8; 9; 10; 11] and motion sensors based [12; 13] positioning systems, the target scans the RSSI from the nearby Access Points (APs) or gathers the data from the sensors to locate itself. On the contrary, in the passive positioning systems, the target, as the signal source, does not participate in the positioning process. Instead, several APs with known locations are deployed to detect the signal from the target. In addition, a server is generally required to gather the positioning information such as RSSI from different APs, and then run the positioning algorithm. Compared with the active positioning systems, the passive positioning systems are more attractive for the third-party providers of LBSs. Up to now, a large number of systems have been proposed to achieve the indoor positioning [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. The systems based on the propagation modeling are susceptible to the environmental change and the interference from other networks [21]. The systems using the Radio Frequency Identification (RFID) or Bluetooth signal require special module to collect the RSSI measurement. For the systems using the location fingerprinting, the fingerprint database is required to be updated with the time going on to guarantee the reliability of the systems. Some other systems using the Time of Arrival (TOA) measurement require the dedicated hardware for time synchronization. In addition, the non-line-of-sight (NLOS) propagation property also poses a challenge for the implementation of these systems. Different from the approaches above, we propose the Wi-Fi compass for indoor passive positioning system (WIPP), an Angle of Arrival (AOA) based passive positioning system using the existing commodity Wi-Fi network, with decimeter accuracy for indoor positioning. The WIPP is robust, which can achieve high positioning accuracy even under the serious multi-path fading. Specifically, the WIPP consists of a batch of three-antenna APs with known locations. The APs are used to collect the signal from the target, and then send it to the server to conduct the positioning. In general, the WIPP is a promising system for indoor passive positioning, which combines the remarkable features of robustness and generality, and meanwhile achieves high positioning accuracy. The main contribution of this paper is to propose a new algorithm for the joint estimation of AOA and TOA measurements based on the fine-grained Channel State Information (CSI), which is collected by an off-the-shelf Wi-Fi device equipped with only three antennas. Once the AOA and TOA measurements with respect to each AP are accurately estimated, the WIPP can achieve high positioning accuracy by using the simple triangulation approach.