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The angle of arrival (AoA) technique estimates the location of the desired target by analyzing the intersection of several pairs of angle direction lines, each formed by the circular radius from a base station or a beacon station to the mobile target. Figure 37.2 shows how AoA methods may use at least two known reference points (A, B), and two measured angles (θ₁, θ₂) to derive the 2D location of the subject P. The actual estimation of AoA can be accomplished with directional antennas or an array of antennas. The AoA between a UWB pulse arriving at multiple sensors has been used for real‐time 3D location positioning in [28]. The advantages of the AoA approach are that a location estimate may be determined with just three measuring units for 3D positioning or two measuring units for 2D positioning, and that no time synchronization between measuring units is required [29]. The disadvantages are primarily due to the large and complex hardware needed (e.g. Quuppa’s HAIP system [30] uses AoA for indoor localization but requires a specific hardware device including 16 array antennas with a transmitter as nearby anchors and special tags for positioning), and location estimate degradation as the mobile subject moves farther from the measuring units [31]. The angle measurements need to be accurate for accurate positioning, but this is challenging with wireless signals due to limitations imposed by shadowing, multipath reflections arriving from misleading directions, or by the directivity of the measuring aperture [32].

Figure 37.2 Positioning based on angle of arrival (AoA) measurement [27].

Source: Reproduced with permission of IEEE.

AoA‐based methods have been used in several light‐based localization solutions. PIXEL [33] is an indoor localization solution that uses AoA methods to determine localization and orientation of mobile devices. The system consists of beacons that periodically send out their identity via visible light communication, which are captured by the mobile devices, followed by AoA‐based post‐processing. Luxapose [34] also uses visible light and employs AoA techniques for indoor localization. In [35], an AoA‐based localization solution was proposed based on passive thermal IR sensors to detect thermal radiation of the human skin. The system is passive as it uses natural infrared radiation without any active IR signal emitters. The approach used thermophiles (a series of thermocouple‐based temperature sensor elements) with a lower resolution compared to IR cameras. Multiple sensors were placed in the corners of a room from where the angles relative to the radiation source were measured. The position of human subjects was then roughly estimated via the principle of AoA, using triangulation from multiple thermophile arrays. However, the effects of dynamic background radiation need to be carefully considered before the method is considered for use in real‐world environments.

A somewhat different technique from AoA that also exploits angular information was proposed in [36]. The system uses a fixed beacon composed of an active infrared (IR) light source and an optical polarizing filter, which only passes light through that oscillates along a single plane. The mobile receiver consists of a photo detector and a rotating polarizer that causes attenuation of the signal intensity depending on the horizontal angle. The phase of the time‐varying signal is then translated into the angle of the polarizing plane. This allows estimation of the absolute azimuth angle with an accuracy of 2% (or a few degrees).