Wireless Position Methods In Cellular Network

Cellular wireless location service is a new mobile value-added service with a good market future. Its basic principle is to implement mobile user location through estimating characteristics parameters relative to position. In the cellular network,according to the position orientated and estimated, position subject and adopting different equipment, the location systems can be divided into the following several kinds: mobile based location system, network based location system, network assisted location system, mobile assisted location system, GPS assisted location system. If adopt mobile based location system or GPS assisted location system to offer LCS function to mobile subscriber in the cellular network, we must modify the mobile station properly and add the essential software and hardware equipment. So this kind of scheme is not suitable for the existing cellular network. Network based location system need only revise and expanse properly the cellular network equipment. So this kind of scheme is suitable for the existing cellular network, such as adding LCS function in GSM network. This thesis introduce the design of the mobile position service system, including system framework, the composition of system, operations of signaling and the measurement and estimation of the time parameter. Most wireless position techniques use some characteristics of MS'signal, such as Strength-of Signal (SOS), Angle-of-Arrival (AOA), Time-of-Arrival (TOA) and Time-Difference-of-Arrival (TDOA). According to these characteristics, MS's position can be determined by some geometric theorems. The disadvantage of using SOS is the large, random deviations from the mean received signal strength caused by small scale channel effects and shadowing. Determining the AOA of a signal usually requires an antenna array, However, TDOA location can be implemented using existing BS antennas. This reduces implementation complexity. Furthermore, AOA based system perform slightly worse than TDOA based ones in a multi-path environment and the accuracy of AOA system tends to degrade as the distance to the mobile increases. The TDOA method is fairly robust in multi-path channels. Multi-path reflections arrive after the first line of sight (LOS) component of the signal. This means that as long as the rising edge of the signal is used, the multi-path channel won't cause as much error as it does for others techniques. Estimating a TOA usually requires accuracy clocks both in MS and BS. TDOA and TOA method are relatively easy to realize in the cellular network, and can relatively reach high accuracy too. So, the too methods, especially TDOA, have been paid more attention. In the basic location technology that 3GPP chooses for UTRAN, no matter E-OTD method chosen for GSM network or OTDOA-IPDL method chosen for WCDMA network, they all adopt TDOA technology. The algorithm that adopted most extensively for estimating MS is LS algorithm. As soon as makes several TDOA measurement value in the cellular network, can form a hyperbolic equation group, solving this equation group can receive the estimation position of MS. It is difficult to solve this non-linear equation group. There have presented many kinds of algorithms at present to solve the equation group. This paper has introduced three location algorithms in detail: Chan algorithm, Taylor algorithm, Fang algorithm Taylor algorithm can offer priori information of error of TDOA measurement value, can apply to the problem of location estimation for TDOA measurement value based on two or more figure, and do not have special demand to the statistics characteristic of the error of TDOA. But it can not offer clear expression formula solving because that it is Pass and return algorithm. The complexity of its calculation is high, and it also needs an initial position that not quite with real position disparity. Fang algorithm needs not offer priori information of error of TDOA measurement value, and the computational process of the algorithm is simple and clear. But it can only utilize the same TDOA measurement value as figure of the position coordinate, can not utilize the redundant TDOA measurement value that offered by cellular network. Chan algorithm can utilize all the TDOA measurement value, so it can reduce the influence of the specific bigger random measurement error to make better localization result. But this algorithm needs to offer priori information of error of TDOA measurement value. Based on scientific evaluation index of position precision, this paper carries on emulation to the three algorithms under Gauss of noises and T1P1 mobile channel environments, studied the influence on location algorithm of many kinds of parameters. The simulation result shows that: the location precision of Taylor and Chan algorithms is high and easy to realize, they each have pluses and minuses under different environments and simulation conditions. The location precision of Chan algorithm is high under the circumstances that the noise obeys on Gauss distribution, and it...