Base Station positioning principle

As Google shuts down the base station locator API.

Location services have become increasingly hot technology and will become the standard for all future mobile devices (smartphones, handheld computers, etc.). With the rapid growth of the BLS (Based location serices, position-based services) demand, wireless positioning technology is also getting more and more attention. GSM Cellular base station positioning, with its fast positioning speed, low cost (no need to add additional hardware on the mobile terminal), less power consumption, indoor availability and other advantages, as a lightweight positioning method, is also more and more commonly used. This paper briefly introduces the location method and basic principle of GSM cellular base station, and gives the developers a reference. I'll try to describe the problem in a way that developers are familiar with.

Preliminary knowledge: GSM cellular network Infrastructure We know that the infrastructure of GSM networks is made up of a series of cellular base stations that divide the entire communication area into a cellular cell as shown in the figure (of course, a base station does not always correspond to just one cell, But this has little to do with the subject we are discussing, and we do not delve into it. These small cells are dozens of meters in size and thousands of meters large. As shown in the figure below, we use the mobile device in the GSM network communication, in fact, through a certain cellular base station to access the GSM network, and then through the GSM network data (voice data, text data, multimedia data, etc.) transmission. In other words, when we communicate in GSM, we always need to connect with a cellular base station, or in a cellular cell. GSM positioning, then, is the use of these cellular base stations to locate.

1. COO (Cell of Origin) Positioning COO positioning is a single-base station positioning, which determines the location of the device based on the location of the cellular base station currently connected to the device. It is clear, then, that the accuracy of the positioning depends on the radius of the cell. In the base station dense city center area, usually will use the multi-storey community, the cell division is very small, at this time the localization precision can reach 50M within, but in other regions, may base station distribution is relatively scattered, the cell radius is larger, may reach thousands of meters, also means the localization precision only roughly to thousands of meters. In the current Google Maps mobile version, through the cellular base station to determine "my location", basically use this method.

From the principle we can see that the COO positioning its accuracy is not very certain. However, this is the fastest and most convenient way to locate mobile devices in GSM networks, as there is no need for any additional hardware input on the GSM network side and on the device side. As long as carrier support, the GSM network devices can be programmatically obtained to the current base station a unique code, we can call the base station ID, or cellid. In general devices, there may be an interface similar to the following Getcurrentcellid () method to provide the current GSM cellular base Station ID:CELLID = Getcurrentcellid (), after acquiring the cellid through this interface, We also need to find out the specific geographic coordinates of the cellular base station based on this cellid. At this point, we may need to invoke some external data that contains the [Cellid, geographic coordinates] correspondence to determine the corresponding geographic coordinates. This external data can usually be provided by some third-party web services. The interfaces of these Web services may resemble the following:
Position=getposition (cellid); Of course, again, the above Getcurrentcellid method, GetPosition method is I fictitious, just to illustrate the logical relationship, and does not necessarily exist.

2. Signaling location seventh the technology is based on signaling monitoring, which enables the filtering and analysis of specific signaling processes in mobile networks, such as roaming, switching, and circuit-related signaling processes, and provides the monitoring results to the business Center for personalized service to specific users. The technology enables real-time monitoring of signaling, which can be targeted to a cell or region. Therefore, the application of the positioning accuracy requirements of the business, such as roaming user greeting service, remote design services, Ping An messenger and cargo tracking. At present, the domestic provinces and regions of the mobile company SMS Welcome system is the use of this technology.

The 3.toa/tdoa positioning TOA (time of arrival, arrival times), TDOA (timing difference of arrival, the time difference of arrival) are based on the method of radio wave propagation timing. It is also a three-base station positioning method, both of which need to have three locations known base station cooperation can be carried out.

As shown in the figure above, the Toa/dtoa positioning method is determined by three pairs [Positioni,ti] (i=1,2,3) to determine the location of the device. The difference between the two is only the specific algorithm of the getlocation () function.
Toa Wave arrival time positioning principle is obtained ti (i=1,2,3), from the device to the base station I ti*c the distance ri, and then based on the geometry is only set up equations and solve, so as to obtain the location value. As shown in the following figure.

Since the calculation of distance in the graph is completely dependent on time, the TOA algorithm has a high demand for the time synchronization of the system, and any small time error will be magnified many times, and the effect of multipath effect will bring great error, so the simple Toa is seldom used in practice.
Dtoa Radio Wave arrival time difference positioning is the improvement of Toa positioning, and toa difference is that TI is not immediately after the use of TI to seek the distance RI, but first to the T1,t2,t3 22 poor, and then through some clever mathematical algorithm set up equations and solve, thereby obtaining location value. As shown in the following figure.

Dtoa due to the ingenious design of the differential process will offset a large part of the time error and the multipath effect caused by the error, and thus can greatly improve the accuracy of positioning.
Because the DTOA is relatively low in network and has high precision, it has become the hotspot of research.

4.AOA positioning AoA (Angle of arrival, arrival angle) Positioning is a two-base station positioning method, which is based on the incident angle of the signal.

As shown in the figure above, know the base station 1 to the connection between the device and the reference direction of the angle α1, you can draw a ray L1; also know that the base station 2 to the device between the connection and the reference direction of the angle α2, you can draw a ray L2. Then the intersection of L1 month L2 is the location of the device. This is the basic mathematical principle of AOA positioning. This is expressed by a function call.
Location=getlocation ([pisition1,α1],[position2,α2]); AOA positioning is determined by the intersection of two straight lines, it is impossible to have multiple intersections, which avoids the ambiguity of the location. However, in order to measure the incidence angle of electromagnetic wave, the receiver must be equipped with a directional antenna array.

5. Based on the field strength of the method is measured by the received signal field strength and the known channel fading model and the field strength of the transmitting signal to estimate the short distance, according to a number of three distance values can get the location of the device. From the mathematical model, similar to the TOA algorithm, only the way to get distance is different. Although the field strength algorithm is simple, the location accuracy is poor due to the influence of multipath effect.

6. Hybrid positioning is the simultaneous use of two or more positioning methods to locate. Through the combination of various positioning methods, complementary and short, in order to achieve higher positioning accuracy.
A-GPS Positioning (auxiliary GPS positioning) is a hybrid positioning, is the combination of GPS positioning technology and GSM network. A-GPS has high positioning accuracy and is now being used more and more widely.