# An Accurate Direction Finding Scheme Using Virtual Antenna Array via Smartphones
## 1 Introduction  
Nowadays, localization technology has been widely applied in mobile social networking, augmented reality, etc. However, most of the applications are based on Global Positioning System (GPS). Due to the coarse-grained results of the GPS, these schemes cannot be used in the complex indoor environment. For instance, the GPS-based localization systems are not precise enough for users to locate a restroom in a shopping mall or an elevator in a hospital. Hence, an accurate indoor localization solution becomes an urgent need for many common users. An indoor direction finding technique is the basis of the indoor localization technology. Many prior indoor direction finding systems detect the direction of signal source using antenna arrays. They steer the array's beam to detect the direction of maximum energy using multiple signal classification. This direction corresponds to the signal's spatial angle of arrival. Although these solutions are accurate and efficient, to gain the narrow beam and achieve a precise resolution, they need a large antenna array with a number of antenna elements that would result in a cumbersome and expensive device and impose restrictions on regular users in daily life. We propose an accurate and lightweight indoor direction finding system that takes the advantages of an antenna array while avoiding its disadvantages. It utilizes the movement of the smartphone to emulate an antenna array. In brief, the smartphone samples the received signal at successive locations in space, as if we had a receive antenna at each of these points. By treating consecutive time samples as spatial samples, we can imitate an antenna array and leverage it to determine signal's spatial angle of arrival. Our research subjects are the high-frequency narrow-band acoustic signals. We set several loudspeakers emitting acoustic signals in the indoor environment. Then, we move the smartphone horizontally and keeping it steady in regular patterns with an approximate constant velocity for a period of time. During this sampling process, the acoustic samples are gathered by the microphone of the smartphone. After applying the noise reduction and signal optimizing, we imitate a virtual antenna array to process the received signals. Eventually, we leverage the multiple signal classification (MUSIC) algorithm [1] to estimate the direction of arrival of the target signals. The Uniform Linear Arrays (ULA) are widely studied. We already did a lot of pre-studies about imitating a Virtual Uniform Linear Array (VULA) by moving the smartphone in front of the user's body horizontally and steady to determine the direction of received signals [2]. However, the system based on the VULA is invalid under some particular circumstances. Through abundant experiments, we discover that the accuracy of the VULA system is limited not only by the azimuth angle of the target signal but also the elevation angle between the user and acoustic source. In practice, the smartphone and the loudspeaker are not always at the same height. In addition, during direction finding, when the user turns his/her back on the signal source will cause massive errors on the experiment results. We call it the blind-zone problem of the VULA system. To the best of our knowledge, the following properties of Uniform Circular Arrays (UCA) make them attractive to our case. The UCA provides 360\({}^{\circ}\) azimuthal coverage and also provides the information on source elevation angles. ULA, in contrast, provides only 180\({}^{\circ}\) azimuthal coverage, and no results of the elevation angles. Moreover, the signal resolution processing by ULA is high at some particular arrival angles but low at others. However, the UCA has the same high signal resolution at around the 360\({}^{\circ}\) azimuth. These advantages of UCA can solve the above problems.