Radio-location of mobile stations in third generation networks

Abstract

Mobile station localization in mobile networks started with simple methods (e.g. Cell-ID method) which required only slight modifications of network infrastructures. Principally, it was about network localization by which a localization service became available to all types of mobile phones. Due to low precision, the initiated development of more sophisticated methods has not been finished yet. Among the advanced location-based methods are those based on the measurement of location parameters in the time domain. In this paper the general consideration of radio location methods in 3G (UMTS) radio networks is presented. The use of time based measurement methods was analysed in detail. Due to the limited article length, the use of other locating methods in 3G networks (based on power measurements, on radio direction measurement, and on cells identification – Cell ID and global positioning system - GPS) are not described.

Introduction

Mobile station localization within modern cellular networks increases the level of user security and opens wide opportunities for commercial operators who provide this service. The major obstacle for the implementation of this service, which also prevents its practical usage, is the modification of the existing network infrastructure.

In general, depending on the infrastructure used, positioning methods can be divided into two groups: integrated and independent. Integrated methods are primarily created for communication networks. A possibility to locate users represents just an additional service within a radio network. Independent methods are totally detached from the communication network in which the user whose location is being determined is.

Radio location methods

Determining the location of a mobile radio station is performed by determining the intersection of two or more lines of position. These lines represent the position of the set of points at which the mobile station may be located. These lines can be: (a) direction, if specified by a direction on an object, (b) circle, if determined by measuring the distance of an object, or (c) a hyperbola, if the difference is determined by measuring the distance between two objects. In cellular networks, the location of mobile stations can be estimated roughly by proximity sensing methods and methods based on an evaluation of the characteristics of base stations (fingerprinting).

Basic characteristics of the UMTS standard

The radio network controller (RNC) has three different roles and is therefore known as: Controlling RNC (CRNC), Serving RNC (SRNC) and Drifting RNC (DRNC). The Local Measurement Unit (LMU) performs radio measurements (measuring the delay of signals from base stations to the LMU) and forwards the data to the CRNC. The main location functions are performed within the Serving RNC which can operate in two modes: RNC central and central SAS (Standalone SMLC). The RNC in the RNC central mode controls the flow of requirements for the localization, chooses the method of localization, provides information as necessary and, finally, estimates the location of the user. The SAS mode performs the procedures based on the requirements of the service radio network controller (SRNC). The SAS executes the global location (Global Navigation Satellite System) and location on the uplink, based on measuring the time difference U-TDoA (Uplink - Time Difference of Arrival).

Methods of locating the MS in a UMTS network

In the UMTS network, depending on the used network infrastructure,  both the station and network-based method can be used, i.e. determining the MS location can be performed on the MS or on the network side. In UMTS networks, the following methods can be used: methods based on measurements in the time domain (time-based), methods based on measurements of a received power level (Received Signal Strength - RSS), methods based on measuring the angle under which the signal arrives to the receiving antenna and methods based on the global location system (Global Navigation Satellite System).

The methods based on measuring the values ​​of the parameters of radio signals propagation in the time domain, analyzed in detail in the work, are divided by the type of location parameters used into: RTT, ToA and TDoA methods.

Conclusion

This paper examines the methods of radio locating in radio networks with a detailed analysis of methods of locating mobile stations in the 3G (UMTS) cellular networks based on station and network measurements of location parameters in the time domain.