Basic Principle of GSM cell base station positioning

 

Location service has become a popular technology and will become a standard for all mobile devices (such as smartphones and handheld computers) in the future. With the rapid growth of users' demand for Location-Based services, wireless Location technology is becoming more and more important. The positioning of GSM cellular base stations is characterized by the advantages of fast positioning speed, low cost (no additional hardware needs to be added to mobile terminals), low power consumption, and indoor availability, as a lightweight Positioning method, it is increasingly common. This article briefly introduces the positioning methods and basic principles of GSM-based cellular base stations for developers. I will try to describe the problem in a way that developers are familiar.

Prerequisites: the basic structure of the GSM cellular network. We know that the basic structure of the GSM network is composed of a series of cellular base stations, these cell Base Stations divide the entire communication area into cells (of course, in fact, a base station often does not only correspond to a cell, but this has little to do with the topic we discuss, ). These cells are several dozen meters in size and several thousand meters in size. As shown in, we use mobile devices to communicate in the GSM network. In fact, we access the GSM network through a cell base station, then, data (voice data, text data, multimedia data, etc.) is transmitted through the GSM network. In other words, we always need to connect to a cell Base Station in GSM communication, or be in a cell zone. Then, GSM positioning is based on these cellular base stations.

1. COO (Cell of Origin) Positioning COO positioning is a single base station positioning, that is, the device location is determined based on the location of the current connected cellular base station of the device. Obviously, the positioning accuracy depends on the radius of the cell. In base station-intensive urban central areas, multi-layer residential areas are usually used, and the cell division is very small. In this case, the positioning accuracy can reach 50 m. In other areas, the distribution of base stations may be relatively scattered, the cell radius is large and may reach several thousand meters, which means that the positioning accuracy can only be roughly several thousand meters. Currently, in the Google map mobile edition, this method is basically used to determine "My location" through the cell base station.

From the principle, we can see that COO positioning is not accurate. However, this is the quickest and most convenient method for locating mobile devices in the GSM network, because neither the GSM network end nor the device end requires any additional hardware investment. As long as the carrier supports it, devices in the GSM network can obtain a unique code of the current base station programmatically. We can call it the base station ID or CellID. A common device may have an interface similar to the following GetCurrentCellID () method to provide the current GSM Cell ID: CellID = GetCurrentCellID (); after obtaining the CellID through this interface, we also need to identify the specific geographic coordinates of the cell Base Station Based on this CellID. In this case, we may need to call some external data that contains the [CellID, geographic coordinate] ing to determine the corresponding geographic coordinates. This external data can usually be provided by some third-party Web services. These Web service interfaces may be in the following form:
Position = GetPosition (CellID); of course, again, the above GetCurrentCellID and GetPosition methods are all fictitious, just to describe the logical relationship, not necessarily actually exist.

2. 7 signaling positioning this technology is based on order monitoring and can filter and analyze specific signaling processes in mobile communication networks, such as roaming, switching, and circuit-Related Signaling processes, the monitoring results are provided to the business center to personalize services for specific users. Through real-time monitoring of signaling, this technology can locate a residential area or a region. Therefore, it is applicable to businesses that do not require high positioning accuracy, such as roaming user greetings service, remote design service, Ping An Reporting Service, and cargo tracking. At present, mobile companies in various provinces and regions in China use this technology in their text message welcome systems.

 

3. TOA/TDOA Location TOA (Time of Arrival, Arrival Time) and TDOA (Time Difference of Arrival, Arrival Time Difference) are all based on the Time of radio wave propagation. At the same time, it is also a three-base station positioning method. Both of them must be located at the same time with a base station with three locations known for cooperation.

 

As shown in, the TOA/DTOA locating method uses three pairs of [Positioni, Ti] (I =, 3) to determine the device Location. The difference between the two lies in the specific algorithm difference of the GetLocation () function.
The basic principle of TOA radio wave arrival time location is that after Ti (I = 1, 2, 3) is obtained, Ti * c obtains the distance Ri from the device to the base station I, then, based on the ry, only the equations are established and solved to obtain the Location Value. As shown in.

 

Since the distance calculation in the figure is completely dependent on time, the TOA algorithm has a high requirement on system time synchronization, and any small time error will be magnified many times, at the same time, due to the influence of the multi-path effect, there will be great errors. Therefore, TOA is rarely used in practice.
The difference in arrival time of DTOA radio waves is an improvement in TOA positioning. The difference is that after Ti is obtained, the distance Ri is not obtained by Ti immediately, but first for T1, T2, calculate the difference between two T3. then, use some clever mathematical algorithms to establish and solve the equations and obtain the Location Value. As shown in.

 

Due to the clever design of the difference process, DTOA can offset a large part of the time error and the error caused by the multi-path effect, thus greatly improving the positioning accuracy.
Because DTOA has relatively low network requirements and high precision, it has become a hot topic of research.

4. AOA positioning AOA (Angle of Arrival, Arrival Angle) positioning is a two-station positioning method, which is based on the incident Angle of the signal.

 

As shown in, you can draw a ray L1 when you know the angle α 1 between the connection between the base station 1 and the device and the baseline direction; now that we know the angle between the connection between the base station 2 and the device and the baseline direction α 2, we can draw a ray L2. The intersection of L2 in January is the location of the device. This is the basic mathematical principle of AOA positioning. Function call is expressed as follows.
Location = GetLocation ([Pisition1, α 1], [Position2, α 2]); the Location of AOA is determined by the intersection of two straight lines. It is impossible to have multiple intersections to avoid ambiguity in positioning. However, to measure the electromagnetic wave incident angle, the receiver must be equipped with an antenna array with a strong direction.

5. location Based on Field Strength this method is to estimate the distance between the received signal field strength and the known channel fading model and the field strength of the transmitted signal, you can obtain the device Location Based on Multiple distance values. From the mathematical model, it is similar to the TOA algorithm, but the method for obtaining distance is different. Although the field strength algorithm is simple, the positioning accuracy is poor due to the influence of the multi-path effect.

6. Hybrid positioning: Two or more positioning methods are used at the same time. Through the combination of various positioning methods, complementary short and long, to achieve higher positioning accuracy.
A-GPS positioning (assisted GPS positioning) is a kind of hybrid positioning, which is the combination of GPS Positioning Technology and GSM network. A-GPS has a high positioning accuracy, and is now more and more widely used.