Gustavo López-Risueño

Challenges in Indoor Global Navigation Satellite Systems: Unveiling its core features in signal processing

Accurately determining ones position has been a recurrent problem in history [1]. It even precedes the first deep-sea navigation attempts of ancient civilizations and reaches the present time with the issue of legal mandates for the location identification of emergency calls in cellular networks and the emergence of location-based services. The science and technology for positioning and navigation has experienced a dramatic evolution [2]. The observation of celestial bodies for navigation purposes has been replaced today by the use of electromagnetic waveforms emitted from reference sources [3].

Digital channelized receiver based on time-frequency analysis for signal interception

A digital channelized receiver is presented for the interception of a wide variety of signals of complex structure, including those with low probability of interception. The receiver is designed from the perspective of the time-frequency analysis. It uses an extended time-frequency representation based on the noncoherent integration of the short-time Fourier transform (STFT) on which the detection system and the encoder work. The encoder includes robust frequency estimation, automatic modulation recognition, and clustering, to handle broadband and simultaneous signals and to prevent out-of-channel detections (a typical phenomenon in channelized receivers). The receiver has been evaluated for a wide range of signals and shows a good performance in terms of detection, estimation, and processing of simultaneous signals. Signals collected from real-life systems and synthetic signals have been utilized.

Multiple signal detection and estimation using atomic decomposition and EM

An algorithm to detect and estimate a linear mixture of deterministic signals corrupted by white Gaussian noise is presented. The number of signals is assumed to be unknown, and the noise power can be either known or unknown. The algorithm is based on an information-theoretic criterion in which the probability of false alarm can be adjusted; typical information criteria, such as the Akaike (AIC) and the minimum description length (MDL) criteria, can be regarded as particular cases of it for given probabilities of false alarm. The proposed approach includes the use of the atomic decomposition and the expectation maximization (EM) algorithms to efficiently approximate the signal maximum likelihood estimate. For the first time, upper-bounds for the probabilities of underestimation and overestimation of the number of signals are obtained. In addition, the constant false-alarm rate (CFAR) characteristic is shown, and the statistical efficiency of the signal parameter estimation is discussed and illustrated by simulation. Numerical experiments show the suitability of the algorithm for signal interception by using synthetic and real-life radar signals.

Atomic decomposition for radar applications

In this paper, we explore the promising capabilities of atomic decomposition (AD) for radar-related applications from a practical point of view. Some enhancements and new approaches are proposed herein, and their implementations are fully detailed. We apply the AD algorithms in two different environments, for signal detection where high sensitivity is the main requirement, and for inverse synthetic aperture radar (ISAR) imaging where focused images, target feature extraction, and computational burden are the fundamental concerns.

Analytical Performance of GNSS Receivers using Interference Mitigation Techniques

The interest in using Global Navigation Satellite Systems (GNSS) for applications with demanding requirements for security and integrity, such as civil aviation, has focused attention on the robustness of GNSS receivers against external interferences. This issue can be even more relevant considering that GNSS modernization will share new spectral bands with other systems already in use. Some recent works address the problem through the use of an interference mitigation technique (IMT) in the receiver. Here, the focus is on theoretically predicting the time of arrival (TOA) estimation error of such receivers which incorporate either of the two most extended IMTs: time-domain blanking (TDB) and frequency-domain adaptive filtering (FDAF). These techniques are currently working and being used. The theoretical expressions provided herein are suitable for receivers based on a code-tracking loop that uses an early-late processor and are accurate for small-error conditions. Besides, the performance analysis is particularized for periodic-pulsed interferences for TDB, while it is particularized for narrowband and linear frequency modulated interferences for FDAF. The most relevant conclusion is that both techniques are able to largely maintain the performance of the receiver in the presence of the respective interferences considered. These results are also validated with computer simulations.

DINGPOS: High sensitivity GNSS platform for deep indoor scenarios

Deep indoor scenarios are one of the most challenging areas of application for Global Navigation Satellite Systems (GNSS) in personal navigation devices. Especially severe signal attenuation, as well as heavy multipath constrain the use of GNSS in this environment. The project DINGPOS is focusing on the development of a platform for pedestrian users which can acquire and track GNSS signals also under most adverse indoor signal conditions. The main idea of the concept is the extension of the coherent signal integration time of the GNSS receiver to the length of several seconds, which increases the correlation gain significantly. To facilitate this goal, a very long and very precise signal replica is needed. Therefore the system must reproduce the user motion, the navigation message data bits and the satellite constellation precisely. Hence, the system uses a sensor suite of several state of the art indoor positioning sensors and innovative fusion algorithms. The integrating element of the system is a software receiver using Ultra-Tightly Coupling (UTC) implemented by vector tracking. The presented work was performed under the ESA funded contract DINGPOS, ESTEC Ctr. No. 20834.

Experimental results from an ultra-tightly coupled GPS/Galileo/WiFi/ZigBee/MEMS-IMU indoor navigation test system featuring coherent integration times of several seconds

This paper contains selected test results of an indoor positioning prototype system making use of GPS/Galileo signals on L1/E1 and L5/E5a in addition to WiFi data, ZigBee data and a MEMS-based dead reckoning algorithm called pedestrian navigation system (PNS). The system uses a partially coherent ultra-tightly coupled integration scheme. The GNSS signal processing (acquisition and tracking) supports coherent integration times up to several seconds, which reduces the squaring loss and mitigates multipath by the high Doppler selectivity of a few hertz. The system works in post processing and is intended to be an indoor analysis system allowing to get a better understanding of the indoor positioning problem and to act as a reference system. GNSS signal acquisition is demonstrated at ∼ 3 dBHz and tracking well below 0 dBHz.

Assessment of the Feasibility of GNSS in C-Band

There is an interest in exploring the possible use for GNSS applications of the frequency band 5000 – 5030 MHz. Considering the increasing number of GNSS systems sharing the Lband spectrum and the power flux density limitations affecting the GNSS reserved frequency bands, there is an interest to look at new frequency bands. C-band GNSS frequency band is attractive because of the expected less polluted spectrum, the reduced ionospheric errors and the possibility to transmit with an higher PFD from the satellite as well the possible utilisation of higher gain receiver antennas. The main drawback is represented by the higher path losses and the smaller bandwidth available for GNSS services. The paper reports on analyses made on the type of coverage (global vs. regional) and on the possible transmission schemes (continuous vs. burst transmission coupled with beam-hopping/steering). Innovative directions are explored and compared to current LBand capabilities to allow achieving the required performances within the reasonable onboard payload and user terminal constraints. To this purpose, we investigate potential advantages deriving from future GNSS satellites carrying combined L & C-Band payloads. Signal design trade-offs consider Cramer Rao lower bound, multipath and out-of-band emission performance. Detailed link budgets for the different configurations are used as a design/analysis tool, encompassing, among others, receiver architecture performance.

Detection and Mitigation of Cross-Correlation Interference in High-Sensitivity GNSS Receivers

This paper presents the architecture of a GPS receiver suitable for indoor operation with extremely weak signals. The receiver works in acquisition-only mode, and combines coherent and long non-coherent correlations in order to achieve low sensitivity. It is confirmed by processing simulated and live GPS signals that this kind of receivers must have the capability to detect and mitigate near-far interference. Novel near-far detection and mitigation techniques are proposed. The new detector is based on the different statistics of the two largest cross-correlation peaks in the presence and in the absence of near-far interference, and it outperforms previously proposed detectors in terms of (mis)detection probability and complexity. The mitigation technique is based on the concepts of successive interference cancellation and subspace projection. It is also novel the fact that, according to the proposed receiver architecture, the near- far mitigation technique is only applied selectively, when the detector has found some interfered signals and the interfering signals have been identified. This reduces the computational complexity without any performance penalty, compared to previously proposed techniques that eliminate the contribution of all potentially (but not necessarily) interfering signals.

Unambiguous tracking of high-order BOC signals in urban environments: Channel considerations

The usage of BOC modulations in GNSS provides a better code tracking accuracy than BPSK modulation at the cost of new correlation peaks appearing in the autocorrelation function. For high-order BOC modulations side peaks might be only few meters away from the main correlation peak to be tracked, so there is a risk to lock into one of those peaks, inducing therefore a bias in the pseudorange estimation. The probability of false lock can be in particular important in harsh channel conditions, where the unambiguous tracking can be difficult due to the low C/No and the presence of multipath, which impact can be relevant during fading periods. This paper presents typical channel conditions that should be considered in the design and assessment of unambiguous tracking techniques of high-order BOC signals, focusing in urban environments where the receiver might need to track the signal even at low C/No. A preliminary assessment of some state-of-the-art techniques in urban environments is presented, showing the limitations of some of the techniques, which have been designed considering very mild propagation conditions typically present in open sky environments. The multi-correlator approach is then presented as a possible way to overcome the limitations of techniques based on legacy GNSS receiver architectures. In this case the unambiguous tracking problem is presented as the minimization of a non-linear Least Squares cost function subject to restrictions and some design alternatives are discussed.