GSM localization

GSM localization is the use of multilateration to determine the location of GSM mobile phones, usually with the intent to locate the user.

Types of LBS Technology
Localization-Based Systems can be broadly divided into:
 * Network based
 * Handset based
 * Hybrid

Network Based
Network-based techniques utilize the service provider's network infrastructure to identify the location of the handset. The advantage of network-based techniques is that they can be implemented non-intrusively, without affecting the handsets.

The accuracy of network-based techniques varies, with cell identification as the least accurate and triangulation as the most accurate. The accuracy of network-based techniques is closely dependent on the concentration of base station cells, with urban environments achieving the highest possible accuracy.

One of the key challenges of network-based techniques is the requirement to work closely with the service provider, as it entails the installation of hardware and software within the operator's infrastructure. Often, a legislative framework, such as E911, would need to be in place to compel the cooperation of the service provider as well as to safeguard the privacy of the information.

Handset Based
Handset-based technology requires the installation of client software on the handset to determine its location. This technique determines the location of the handset by computing its location by cell identification, signal strengths of the home and neighboring cells or the latitude and longitude, if the handset is equipped with a GPS module. The calculated location is then sent from the handset to a location server.

The key disadvantage of this technique is the necessity of installing software on the handset. It requires the active cooperation of the mobile subscriber as well as software that must be able to handle the different operating systems of the handsets. Typically, only a smart phone, such as one based on Symbian or Windows Mobile or iPhone or Android, would be able to run such software.

One of the proposed work-arounds is the installation of embedded hardware or software on the handset by the manufacturers. However, the obvious difficulty of convincing different manufacturers to cooperate on a common mechanism and to address the cost issue means that this avenue has not made any significant headway.

Another difficulty would be to address the issue of foreign handsets that are roaming in the network.

Well, let us find an example to demonstrate the Network based location tracking algorithm: According to global GSM structure and ETSI, the GSM service providers information flows through the control channel and the control channel is free to access. Interestingly, all the present GSM modem/mobiles (Telit, SIMCOM, HTC, Nokia etc) are coming with some extra feature to monitor the neighboring cells and its RSSI value. Theoretically you should get 1+6=7 cell information(1 home cell ID, 7 BCCH info+ 7 RSSI). If you can prepare the database of GPS location of the 7 cells, it is not impossible to locate your location with very high accuracy (<100 mtr).

Hybrid
Hybrid-based techniques use a combination of network-based and handset-based technologies for location determination. One example would be Assisted GPS, which uses both GPS and network information to compute the location. Hybrid-based techniques give the best accuracy of the three but inherit the limitations and challenges of network-based and handset-based technologies.

Examples of LBS technologies

 * Cell Identification - The accuracy of this method can be as good as a few hundred meters in urban areas, but as poor as 32 km in suburban areas and rural zones. The accuracy depends on the known range of the particular network base station serving the handset at the time of positioning.
 * Enhanced Cell Identification - With this method, one can get a precision similar to Cell Identification, but for rural areas, with circular sectors of 550 meters.
 * U-TDOA - Uplink-Time difference of arrival - The network determines the time difference and therefore the distance from each base station to the mobile phone.
 * TOA - Time of arrival - Same as U-TDOA, but this technology uses the absolute time of arrival at a certain base station rather than the difference between two stations.
 * AOA - Angle of arrival - AOA mechanism locates the mobile phone at the point where the lines along the angles from each base station intersect.
 * E-OTD - E-OTD is similar to U-TDOA, but the position is estimated by the mobile phone, not by the base station. The precision of this method depends on the number of available LMUs in the networks, varying from 50 to 200 m.
 * Assisted-GPS - A largely GPS-based technology, which uses an operator-maintained ground station to correct for GPS errors caused by the atmosphere/topography. Assisted-GPS positioning technology typically falls back to cell-based positioning methods when indoors or in an urban canyon environment.