# Acoustic TDOA Measurement and Accurate Indoor Positioning for Smartphone
###### Abstract  
The global satellite navigation signal works well in open areas outdoors. However, due to its weakness, it is challenging to position continuously and reliably indoors. In this paper, we developed a hybrid system that combines radio signals and acoustic signals to achieve decimeter-level positioning indoors. Specifically, acoustic transmitters are synchronized with different codes. At the same time, our decoding scheme only requires a simple cross-correlation operation without time-frequency analysis. Secondly, acoustic signals will be reflected by glass, walls and other obstacles in the indoor environment. Time difference of arrival (TDOA) measurement accuracy is seriously affected. We developed a robust first path detection algorithm to obtain reliable TDOA measurement values. Finally, we combine the maximum likelihood (ML) algorithm with the proposed TDOA measurement method to obtain the location of the smartphone. We carried out static positioning experiments for smartphones in two scenes. The experimental results show that the average positioning error of the system is less than 0.5 m. Our system has the following advantages: (1) smartphone access. (2) an unlimited number of users. (3) easily deployed acoustic nodes. (4) decimeter-level positioning accuracy. acoustic signal; TDOA estimation; ML; indoor positioning +
Footnote †: journal: _Future Internet_  
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signal is the signal source [12] in ultra-wide-band (UWB) positioning technology. The positioning process of the UWB positioning technology is as follows: (1) estimating the arrival time of the signal. (2) Calculating the distance information. (3) Using the triangle positioning method to obtain the location information. However, the cost of the UWB module is high, and it is difficult to promote UWB positioning technology in mobile phone manufacturers. There are more metal structures, cables and wires in special scenes such as underground mine tunnels and underground cable tunnels [13], which means that the UWB positioning technology with an electromagnetic wave as the carrier faces the problems of metal shielding effect and complex channel environment [14]. So there are certain application limitations in the UWB positioning technology. Visible light positioning technology can roughly include three categories [15; 16]: The first is to determine the receiver position by using the intersection positioning method. Such a method can be implemented by obtaining one of information on optical signal strength [16], time of arrival (TOA), time difference of arrival (TDOA [17]) and angle of arrival (AOA [18]). The second is the fingerprint database method [19]. In the offline phase, the optical fingerprint database is established by measuring basic information (such as optical intensity information). In the online phase, the optical intensity at the test point is matched with the fingerprint database to determine the location of the test point. The third is the proximity method [20]. The optical node grid needs to be established in the proximity method. When the optical receiver collects the coding information of a specific optical node, it is considered that the optical receiver and the optical node are in the same position. These three types of methods are affected by other indoor light sources and occlusion [21]. At the same time, the propagation distance of visible light sources is limited. In a word, the effect of optical positioning technology is not particularly outstanding. In geomagnetic positioning technology [22], magnetic field strength can be used as a fingerprint feature. Mobile devices can be located by matching the indoor magnetic field characteristics without the help of other devices [23]. In geomagnetic positioning technology, the disadvantage is that it is necessary to collect data in advance to establish a magnetic field strength distribution map.