“In recent years, with the increase of user demand, wireless positioning technology has received more and more attention, which has promoted the research on wireless positioning technology and the development of ranging technology. How to use the existing network resources as much as possible and realize the precise positioning of users at low cost has always been the focus of research. In general, there are two main types of solutions to realize wireless positioning. The first type is the positioning technology dominated by the mobile station (MS).
Authors: Zhou Jiang, Sun Xianpu
Application and classification of positioning technology
In recent years, with the increase of user demand, wireless positioning technology has received more and more attention, which has promoted the research on wireless positioning technology and the development of ranging technology. How to use the existing network resources as much as possible and realize the precise positioning of users at low cost has always been the focus of research. In general, there are two main types of solutions to realize wireless positioning. The first type is the positioning technology dominated by the mobile station (MS). From a technical point of view, this technology is easier to provide more accurate user positioning information, it can use some existing positioning systems, for example, integrating GPS receivers in mobile stations, so as to use existing GPS signals to achieve user positioning. accurate locating. However, such techniques require adding new hardware to the mobile station, which adversely affects the size and cost of the mobile station. The second category is the positioning technology dominated by the base station (BS). This solution requires a certain degree of improvement on the existing base stations and switching centers, but it is compatible with existing terminal equipment. The optional specific implementation technologies mainly include: positioning technology for measuring signal direction (angle of arrival of signal, AOA for short), positioning technology for measuring signal power, measuring signal propagation time characteristics (time of arrival, abbreviated as TOA; time difference of arrival, abbreviated as TDOA) ) positioning technology. In order to improve the positioning accuracy, a combination of the above techniques can also be used. Since the second type of solution can make better use of the existing network and its terminal equipment, it has wider application prospects, so this article will take it as the main content of the discussion.
Positioning Techniques to Measure Received Signal Power
A common method for implementing wireless positioning is based on the power of the received wireless signal. By measuring the signal power received by the base station (BS) from the mobile station (MS), and the transmission model of the wireless channel between them, the approximate distance between the mobile station and the base station can be estimated as d. In this way, for a base station BS(i), the mobile station must be on a circle with BS(i) as the center and d as the radius. When three or more base stations are used to measure the distance of the same mobile station, the location of the mobile station can be measured.
In this method, the multipath effect in the process of wireless signal transmission and the shadow effect when passing through obstacles are the main reasons for the positioning error. In the transmission direction of the signal, the multipath effect sometimes causes the signal strength to differ by about 30 to 40 dB at two points that are only 0.5 wavelengths apart. In order to overcome the influence of multi-path effect on ranging, the accuracy of positioning can be improved by obtaining the average value of signal power for wireless users in high-speed movement, but the effective average power value for slow-moving or even stationary wireless users is Hard to measure. The shadow effect is another major cause of positioning error, and the most important method to overcome the shadow effect is to measure the signal power loss contours around each base station in advance.
In practical CDMA systems, in order to reduce the interference of short-range users to long-distance users, power control technology must be used. In some TDMA systems, power control is also applied to reduce the power consumption of mobile stations (MS). . In such a power-controlled cellular system, to implement a positioning technique based on measured signal power, the mobile station must inform the base station of the power of its transmitted signal with sufficient accuracy, and the base station then calculates the signal from the received signal power. The loss in the transmission process is calculated, and the distance estimate from the mobile station to the base station is calculated to realize the positioning of the wireless user.
Positioning technique to measure the direction of received signal (AOA)
Measuring the angle of arrival of a signal (Angle Of Arrival, AOA for short) is also a positioning technology commonly used in cellular networks. This method requires a dedicated antenna array at the base station to measure the source direction of a particular signal. For a base station, the AOA measurement can obtain the direction of a specific mobile station. When two base stations measure the signal sent by the same mobile station at the same time, the focus of the direction line obtained by the two base stations respectively measuring the AOA is the position of the mobile station. . Although the principle of this positioning method is very simple, there are some insurmountable shortcomings in practical applications. First, AOA positioning requires that the RF signal is line-of-sight (LOS) between the mobile station being measured and all base stations participating in the measurement. Non-line-of-sight transmission (NLOS) will bring unpredictable errors to AOA positioning, see Figure 1.
Even in the case of LOS transmission, the multipath effect of the RF signal will still interfere with the AOA measurement. Secondly, due to the limitation of the angular resolution of the antenna device, the measurement accuracy of AOA decreases with the increase of the distance between the base station and the mobile station.
Since the positioning method for measuring AOA has the above-mentioned characteristics, for a small cell in an urban area, there are many obstacles that cause RF signal reflection, and the distance from the mobile station to the mobile station can be compared with the cell radius, which will cause a relatively large angle measurement error. In this case, AOA-based localization methods have no practical significance. For a macro cell, because its base station is generally located at a relatively high position, compared with the radius of the cell, the obstacles that cause RF signal reflection are mostly located near the mobile station, and the angle measurement error caused by NLOS transmission is relatively small. Therefore, the positioning method of measuring the angle of arrival of the signal is mostly used in macro cells, or mixed with other positioning technologies to improve the positioning accuracy.
Positioning Techniques for Measuring Signal Propagation Time Characteristics
The last major type of positioning technology is achieved by measuring the time characteristics of the radio frequency signal transmission between the base station and the mobile station. This type of positioning technology is easier to implement. Compared with AOA, it can provide higher positioning accuracy under the same conditions (the same number of base stations participating in positioning), so it is most widely used in practice. It mainly has two specific implementation means. The first is to measure the time of arrival (TOA) of the signal, that is, the base station sends a specific ranging command or command signal to the mobile station, and the mobile station is required to respond to the command. The base station will record the time T from sending the ranging command to receiving the confirmation signal from the mobile station, which is mainly determined by the propagation delay of the radio frequency signal on the loop, the response delay and processing delay of the mobile station, and the processing delay composition. If the response and processing delay of the mobile station and the base station can be accurately obtained, the loop propagation delay Td of the radio frequency signal can be calculated. Since radio waves travel in air at the speed of light c, the distance estimate between the base station and the mobile station is dm=c*Td/2. When three base stations participate in the measurement, the area where the mobile station is located can be determined according to the triangulation method, as shown in Figure 2.
Since this positioning method is based on time, the increase in transmission delay caused by multipath effects and non-line-of-sight transmission (NLOS) is the main reason for ranging and positioning errors. As a result, dm is generally larger than the actual distance d between the base station and the mobile station. In order to overcome the adverse effects of NLOS and multipath effects and improve the positioning accuracy, the number N of base stations participating in the same positioning is generally greater than 3, which can reduce the area of the shaded area in Figure 2. In addition, some positioning algorithms should be applied to the results of each measurement to minimize the error of the positioning estimation value under a certain criterion. For example, T is the TOA measured by each base station, and i is the number of the base station participating in the measurement. In a certain coordinate system, the position estimate of the mobile station is (x, y), and the position of the base station i is (xi, yi). The function fi=c*Ti- is used as the performance measure of the ranging of the base station BS, that is, the ranging error of the base station BS. In the ideal state, that is, when (x, y) is the actual position of the mobile station, and the wireless signal from the mobile station to each base station is line-of-sight transmission (LOS), then for each base station participating in the measurement, fi should be zero. However, in practice, due to the influence of NLOS transmission and multipath effects, it is generally impossible to obtain (x, y) so that fi=0 (i=1, 2, …, N) are all established. Therefore, in terms of the entire positioning system, the weighted sum of squares F of the ranging errors of the base stations participating in the positioning can be used as the system performance measurement function, and the (x, y) that minimizes F can be used as the result of a positioning measurement. where ai is the weighting coefficient of the base station BS in the measurement result, and its size reflects the accuracy and reliability of the ranging from the BS to the MS.
Another positioning method based on the characteristics of signal transmission time is to measure the time difference (TDOA) of the positioning signal received by different base stations from the same mobile station, and calculate the distance difference between the mobile station and different base stations. The distance difference d between the mobile station and any two base stations can give a hyperbola between the two base stations, and the mobile station must be on the curve. When there are N base stations participating in ranging (N ≥ 3) at the same time, the intersection area between multiple hyperbolas is the estimation of the user’s location, as shown in Figure 3. This method requires that the clocks of all base stations participating in the measurement are strictly synchronized. Compared with TOA, its main advantage is that it does not need to accurately obtain the response and processing delay of the base station and mobile station. Like TOA, the positioning error of TDOA mainly comes from non-line-of-sight transmission and multipath effects of RF signals. The main way to solve this problem is to increase the number of base stations involved in positioning and to use high-precision estimation algorithms.
This paper mainly introduces several basic methods of using existing cellular systems to provide users with location services. These methods do not need to change the existing mobile communication terminals, but they all require a certain degree of improvement on the existing network equipment. . In an actual system, one or a combination of technologies can be selected to achieve user positioning according to the user’s requirements for positioning accuracy, the environment of wireless transmission, and changes in cost. Generally speaking, TOA/TDOA can provide relatively high positioning accuracy, and is also relatively easy to implement, so it is widely used, and the existing CDMA systems mostly use this method.