Research On IR-UWB Propagation Mechanism For Wireless Indoor Positioning System

As the development of short range wireless communications technology, the demands of accuracy positioning in complicated environments were also increased dramatically in recent years. The impulse radio ultra wideband (IR-UWB) technology can fully satisfied the requirements of high accuracy, low power consumption and low complexity in indoor localization system, based on the time-of-arrival (TOA) ranging algorithm, the IR-UWB positioning system can achieve centimeter degree position accuracy, which rapidly became a research focuses in short range localization.Different from the telecommunication applications which interested in the behavior the multipath spread of the wireless channel, the indoor positioning applications place greater emphasis on the behavior of the direct path (DP) between the transmitter and the receiver node, the correspondence performance criteria is defined as the distance error. In complicated indoor environment, ranging measurements are typically corrupted by the multipath and non-line-of-sight (NLOS) blockage. IR-UWB signal have the capability to resolve multipath components in nano-seconds, which greatly reduce the multipath interference to DP in correlation receiver, however, considered the limitation of sampling rate in actual system realization, there are also multipath error in IR-UWB positioning system due to different system bandwidth. On the other hand, the NLOS blockage makes only reflection or transmission signals can reach to the IR-UWB receiver, which lead to the TOA ranging estimation larger than the true distances, and consequently cause a positive ranging error. Apparently, the propagation characteristics of IR-UWB signal determined the performance of indoor positioning system. Although the IEEE 802.15.4a channel model has already given the statistical information of multipath in IR-UWB propagation, it can not effectively reflected the interaction mechanism between IR-UWB propagation properties and the specific environments, which are very important for the localization system concerning the DP chanracteristics. This paper mainly focuses on investigate the propagation characteristics of IR-UWB per-path signal, explore the IR-UWB positioning mechanism in completely NLOS envronments, establish the ranging error model caused by IR-UWB propagation in indoor environments, then give a system framework about the error mitigation in TOA ranging. The contents of this dissertation are summarized as follows.Firstly, the basic principles of IR-UWB positioning system are studied. Three types of pulse waveforms designed to fit the FCC spectrum mask in IR-UWB system are analyzed under the restrictions of high time resolution for short range localization. The statistical channel models of IEEE 802.15.3a and 4a proposed by IEEE 802.15TG are comprehensively introduced, with its characteristics in CM1~CM4 channel environments. The two-step positioning receiver structure was chose to achieve low complexity, and the ranging algorithms based on received-signal-strength (RSS), TOA and angle-of-arrival (AOA) are described and compared with its CRLB. Also the feasibility studies of these three algorithms in IR-UWB ranging are carried out.Secondly, the research schemes of IR-UWB per-path signal propagation is proposed and IR-UWB propagated in indoor line-of-sight (LOS) environments are analyzed. Due to the huge bandwidth of IR-UWB signal, the classical central frequency theory in narrowband system was no longer suit for the study of IR-UWB transient pulse, here a deterministic method in both time domain (TD) and frequency domain (FD) is raised to cope with the frequency dependence of IR-UWB signal in propagation. As the wavelength of IR-UWB signal is comparable to the stuffs commonly in indoor environment, which makes the relatively easier reflection phenomenon for IR-UWB signal compared with the 2G/3G mobile signals, the IR-UWB propagated in LOS situation is modeled by the LOS direct path and reflected paths from walls in a 3D environment. The reflection mechanism of IR-UWB signal is emphasized with closed form expression in both TD and FD methods, the reflected waveforms in different conditions are analyzed. In order to make sure the relationship between the multipath ranging error and environments/system parameters (transceiver height, transceiver distance and pulse duration time), the two-ray model is used to study the impacts of reflected path on DP in TOA estimation.Thirdly, the IR-UWB signal through wall propagation mechanism in NLOS environment is established in both theoretical and experiments study. The closed form of transmission coefficients in both TD/FD are derived on considering the multi-reflection phenomenon in slab. Based on the requirements of wireless localization system on IR-UWB signal, the through wall ability and range resolution of different Gaussian pulse are studied. And the IR-UWB signal waveform distortions as propagatting through common obstacles in indoor environment are investigated with its impacts on TOA ranging performance. Then the through wall propagation experiments of IR-UWB signal are accomplished using the IR-UWB system designed by the Communication Research Center of Harbin Institute of Technology Shenzhen Graduate School, a novel data process scheme was proposed to filter the transceiver antenna's responses from the experiment data. The measurement data which only contain the wireless channel response were compared with the theoretical results.Fourthly, researches on model the ranging error of IR-UWB positioning system are carried out. The principles of multipath error and NLOS error due to IR-UWB propagation are analyzed. Based on the statistical information of IEEE 802.15.4a channel model, the influences of system bandwidth on multipath ranging error in coherent TOA estimation is studied. As the recent researches can't give a uniform NLOS error model, according to the propagation charateristics of IR-UWB signal, the NLOS error are modeled by geometrical error caused by the geometrical structure of IR-UWB through wall propagation and the correlation peak biased error due to IR-UWB waveform distortions. The geometrical ranging error is derived using the equivalent source method, and the impacts of transceiver distance and wall parameters on geometrical error are discussed. Finally, based on the coherent TOA estimation and trilateration positioning algorithm, the positioning performance modification using the NLOS prior information in completely NLOS environment are studied, then a generalized distance error model is proposed based on the deterministic ranging error and the distribution characteristics of channel environments.