37.5.1.1 Angle‐Based Methods
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).
37.5.1.2 Time‐Based Methods
ToA‐based localization solutions are based on the synchronization of the arrival time of a signal transmitted from a mobile subject (P) to at least three receiving beacons, as shown in Figure 37.3. The underlying idea is that the distance from the mobile subject to the beacons is directly proportional to the propagation time. This distance between the mobile subject and beacons is calculated based on one‐way propagation time measurements [37]. Several methods have been proposed for such measurements using DSSS [38] or UWB [39, 40]. In general, short‐pulse UWB waveforms permit accurate determination of the precise ToA and time of flight of a burst transmission from a short‐pulse transmitter to a corresponding receiver, which has been utilized in UWB‐based indoor localization solutions that can achieve very high indoor location accuracy (down to 20 cm in some cases) [41, 42]. But care needs to be taken in ToA systems so that that all transmitters and receivers in the system are precisely synchronized. Also, the transmitting signal must send a timestamp for the receiver to discern the distance the signal has traveled. The Active Bat positioning system [43] uses ultrasound signaling and ToA triangulation to measure the location of a tag carried by a person. The tag periodically broadcasts a short pulse of ultrasound that is received by a matrix of ceiling‐mounted receivers at known positions. The distances between the tag and the receivers can be measured by the ToA of the ultrasonic waves. The Hexamite system [44] also uses ToA triangulation‐based localization with ultrasound signaling. A hybrid ToA/AoA approach was introduced in [45]. By utilizing the information measured from AoA and ToA, the number of beacons (anchors) required can be reduced. In [46], another hybrid approach is proposed, in which a hybrid AoA/ToA scheme is used for localization if only one Wi‐Fi AP is available; however, if more APs are available, then a multiple‐message‐based AoA scheme is used to obtain higher accuracy location. This design aims to provide accurate localization even when the number of nearby anchors (Wi‐Fi APs) is limited.
Figure 37.3 Positioning based on ToA/RToF measurements [27].
Source: Reproduced with permission of IEEE.
TDoA techniques determine the relative position of a mobile transmitter by analyzing the difference in time at which the signal arrives at multiple measuring units, rather than the absolute arrival time of ToA. A 2D target location can be estimated from the two intersections of two or more TDoA measurements, as shown in Figure 37.4. Two hyperbolas are formed from TDoA measurements at three fixed measuring units (A, B, C) to provide an intersection point and locate the subject P. The conventional methods for obtaining TDoA estimates are to use correlation techniques, for example, by the cross‐correlation between the signals received at a pair of measuring units. With TDoA, a transmission with an unknown starting time is received at various receiving nodes, with only the receivers requiring time synchronization [47]. TDoA does not need a synchronized time source of transmission in order to resolve timestamps and find the location. A delay‐measurement‐based TDoA estimation method was proposed in [38] for Wi‐Fi signals, which eliminates the requirement of initial synchronization in conventional methods. The TDoA between a UWB pulse arriving at multiple sensors has been used for high‐precision real‐time 3D location positioning [28]. Wi‐Fi‐based TDoA was proposed by [48], for indoor location estimation. The approach requires the same radio signal to be received at three or more separate points, timed very accurately (to a few nanoseconds) and processed using a TDoA algorithm to determine the location. A TDoA system with a proprietary RF signal (from