Abstract
The provision of high-data rates leads the advances of new technologies in mobile communications. One of these advances is the use of multicarrier signals that allow a flexible allocation of resources in time and frequency, thus the spectrum can be efficiently shared for different applications. This feature is used by several systems to combine communications and positioning capabilities, due to the increasing demand of data and location services. However, the presence of mobile devices in harsh environments, such as indoor or urban scenarios, prevents these systems to achieve the required accuracy with conventional ranging techniques. The main impairment in these conditions is the effect of the multipath channel, which induces a considerable bias on the ranging estimation. Thus, countermeasures against multipath are necessary to achieve the ultimate positioning performance.This thesis deals with the ranging capabilities of multicarrier signals in mobile communications over harsh environments, characterized by dense multipath. For this purpose, the practical case of the Long Term Evolution (LTE) mobile communications standard is considered. The LTE standard is of special interest because its downlink transmission is based on the orthogonal frequency-division multiplexing (OFDM), which is a multicarrier format. In addition, LTE specifies a multicarrier signal dedicated to support the observed time difference of arrival (OTDoA) positioning, which is based on ranging estimates with respect to the reference base stations. This pilot signal is called positioning reference signal (PRS), and it is used for time-delay estimation (TDE) in the procedure to locate the mobile device. Thus, the first part of the thesis is aimed to assess the achievable localization capabilities of LTE conventional receivers using the PRS. These conventional receivers are based on the matched filter or correlation-based techniques. The study focuses on two main impairments for TDE in LTE networks: inter-cell interference and multipath. The inter-cell interference can be mostly removed by the coordinated transmission of the PRS. However, multipath notably degrades the positioning accuracy of these conventional estimators, as it could be expected.
The main contribution of this thesis is provided in the second part, by introducing the joint estimation of time delay and channel response. This is an optimum solution for multicarrier signals, due to the straightforward implementation of the channel estimation in the frequency domain. However, most of the joint estimation algorithms are focused on communication applications, without considering the extreme accuracy of the TDE required for positioning. Typically, multipath appears close to the line-on-sight ray in urban and indoor environments. Thus, a novel channel parameterization is proposed in this thesis to characterize close-in multipath. This channel estimation model is based on the time delay and equi-spaced taps together with an arbitrary-tap with variable position between the first two equi-spaced taps. This new hybrid approach is adopted in the joint maximum likelihood (JML) time-delay estimator to improve the ranging performance in the presence of short-delay multipath. The optimality of this estimator is confirmed because its variance attains the Cramér-Rao bound. The ranging performance of this estimator is then compared to conventional estimators in realistic navigation conditions. These conditions are characterized by standard channel models adopted in LTE, additive white Gaussian noise (AWGN) and the LTE signal bandwidths. Considering the resulting time-delay estimations, the cumulative density function (CDF) in the absence of noise is used to determine the optimum model order of the estimators, and the root-mean-square error (RMSE) and bias is used to assess the achievable ranging accuracy. A notable improvement is shown by the JML estimator proposed in close-in multipath scenarios.
In the last part of the thesis, the goal is to validate the ranging performance of the proposed estimator using real LTE signals. For this purpose, a software-defined radio (SDR) receiver is developed for OTDoA positioning in LTE. A preliminary scenario with four synchronized base stations is used to validate the positioning engine. Then, the multipath error envelope (MPEE) of the JML estimators is obtained for the emulated and simulated signal cases. The work is completed with the validation of the ranging performance of the new JML time-delay and channel estimator, by using the SDR receiver in an emulated urban channel. The results obtained show the improvement on the ranging accuracy of the new JML estimator over realistic navigation channels.
| Date of Award | 13 May 2014 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jose Antonio Lopez Salcedo (Director), Gonzalo Seco Granados (Director) & Francesca Zanier (Director) |