Wireless Localization Algorithm Based on Noise Reduction Method for Performance Improvement in Indoor Environments

Abstract

Recently, indoor localization has drawn the attention of many researchers for increasing demand of location-awareness applications in indoor environments. Although there have been many conventional localization systems based on satellites and cellular networks, the noises caused by the indoor environmental factors and the hardware impairment for cheap devices make them difficult to provide accurate localization in indoor environments. The solutions for indoor environmental factors, such as signal attenuation because of walls and indoor obstacles, mainly aim for ultra-wide band or orthogonal frequency division multiplexing systems. In particular, the clock offset, which is representative of the hardware impairment, has not been examined entirely. This dissertation proposes wireless localization algorithm based on noise reduction method for performance improvement in indoor environments. In this dissertation, the noises due to the indoor environmental factors and the hardware impairment for cheap devices are reduced in two levels. First, the ranging algorithm, which resolves the effects of the signal attenuation and the clock offset, is proposed. The clock offset causes sampling time offset in time and carrier frequency offset in frequency; as well as, it also generates symbol time offset when multiple symbols are employed. These various effects of the clock offset in time and frequency domains are approximated to the linear phase between adjacent samples in a symbol and that between adjacent symbols. After the elimination of the linear phase through the differential multiplication, the signal power is increased through the average of multiple symbols. As a result, the increment of the signal power enables the improvement of the ranging performance. Second, an effective time difference of arrival (TDOA)-based localization algorithm is proposed without time synchronization among base stations. Assuming that the mobile station periodically transmits the localization signal, a new TDOA model is provided. The TDOA model is based on the mobility of the mobile station, i.e. the time difference between the signal transmitted from the mobile station at the previous location to the base station and the signal transmitted from the mobile station at the present location to the base station. From this TDOA model, the proposed TDOA-based localization algorithm estimates the location of the mobile station through two phases: the initialization phase and the tracking phase. Through the reduction of the effects of the clock offset and the increment of the signal power, the mean square error of the proposed ranging algorithm is M times mitigated when M symbols are employed. In conclusion, accurate localization system can be achieved by resolving the indoor environmental factors and the hardware impairment due to low cost devices in the ranging and localization levels.