Fernando D. Nunes

Gating Functions for Multipath Mitigation in GNSS BOC Signals

A new multipath mitigation technique is proposed for binary offset carrier (BOC) signals in global navigation satellite systems (GNSS) using the concept of gating function originally conceived for the GPS coarse-acquisition (C/A) code. Specially-tailored pulses are utilized to diminish the number of false-lock points of the code discriminator response and to improve the multipath mitigation capability. The code loop includes only four real correlators (two extra correlators are required for the simplified bump-jumping algorithm with BOC(n,n) signals). Results obtained with BOC(n,n) and BOC(2n,n) signals show that this technique eliminates the multipath ranging errors for reflected rays with relative delays typically above twenty percent of the spreading code chip duration, thus comparing favorably with the conventional receiver correlation techniques.

Satellite Selection Method for Multi-Constellation GNSS Using Convex Geometry

This paper proposes a methodology in the context of convex geometry theory aiming to reduce the number of satellites among all visible satellites used for the position computation by a navigation receiver. This reduction is based on the approximate equivalence between geometric dilution of precision minimization and maximization of the size of the polytope (polyhedron in 3-D or polygon in 2-D) expanded by the user-to-satellite unit vector endpoints. The polytope is contained in the convex hull of all vectors endpoints for the set of satellites in view. An optimal solution for convex hull computation with lower bound computational burden is proposed for both static and time-varying scenarios.

Microwave reflectometry diagnostic for density profile and fluctuation measurements on ASDEX Upgrade

The broadband microwave reflectometry system on ASDEX Upgrade has been recently upgraded with new channels. It has now 12 channels that probe simultaneously the high and low fields plasma, with ultrafast sweeping (20–100 μs). X mode (33–75 GHz) is used for the scrape off layer, and the O mode (16–110 GHz) is reflected from densities between 0.3×1019 and 1.5×1019 m−3. A fast sweeping heterodyne system was developed for the highest frequency channels (50–110 GHz) to cope with higher losses and lower incident power. Fixed frequency operation is employed for fluctuation measurements. A dedicated channel (33–55 GHz) operating in fixed frequency provides a signal to continuously monitor the level of density fluctuations (e.g., L–H transition), during the whole discharge. The diagnostic is fully operated by remote control. The acquisition system is based on specially developed VME boards with up to 1 Gsamp/s sampling rates. Automatic profile inversion was recently implemented using data validation and rejection ...

Joint turbo equalisation and carrier synchronisation for SC-FDE schemes

In this paper we consider the use of single carrier (SC) modulations combined with frequency-domain equalisation (FDE) in future broadband wireless systems. We propose iterative receiver structures with joint equalisation and carrier synchronisation. The proposed receivers can be regarded as modified frequency-domain turbo equalisers where we perform a decision-directed frequency offset estimation within each iteration of the turbo equaliser. Our performance results show that the proposed receiver structure has good bit error rate (BER), even with moderate frequency offsets and in severely time-dispersive channels. Moreover, our receiver has a relatively low implementation complexity, due to its fast Fourier transform (FFT)-based frequency-domain implementation. Copyright

Strobe Pulse Design for Multipath Mitigation in BOC GNSS Receivers

Binary offset carrier (BOC) modulations have been considered for the new GNSS signals since they achieve better tracking performance than PSK in the presence of channel noise and multipath. Besides, the concept of delay lock-loop based on symmetrical strobe pulses can be extended to BOC signals with significant advantage in the close-in multipath region. Herein, a new approach to the design of asymmetrical strobe pulses for BOC signals is proposed. A target code discriminator response with desirable characteristics is defined allowing to determine the strobe pulse as the solution of an integral equation. The resulting pulse provides good multipath mitigation capability, extended code tracking range, and lack of false code lock points.

Iterative Frequency Domain Equalization and Carrier Synchronization for Multi-Resolution Constellations

Broadband broadcast and multicast wireless systems usually employ OFDM modulations (Orthogonal Frequency Division Multiplexing) combined with non-uniform hierarchical constellations. However, these schemes are very prone to nonlinear distortion effects and have high carrier synchronization requirements. SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an attractive alternative for OFDM, especially when an efficient power amplification is intended. In this paper we consider the use of SC-FDE schemes combined with non-uniform hierarchical constellations in broadband broadcast and multicast wireless systems. We study the impact of residual CFO (Carrier Frequency Offset) on the performance of multi-resolution schemes and we propose iterative frequency domain receivers with joint detection and carrier synchronization to cope with residual CFO estimation errors (a coarse CFO estimation and compensation is assumed before the equalization procedure). Our results show that while a very high carrier synchronization accuracy is required for the least protected bits, the most protected bits are relatively robust to the CFO. By employing the proposed receiver we increase significantly the robustness to residual CFO estimation errors.

Relation between GDOP and the geometry of the satellite constellation

The relationship between Geometrical Dilution Of Precision (GDOP) and maximum volume1 of the polytope2 expanded by the user-satellite unit vectors endpoints has been used for long as an approach to reduce the time that a receiver devotes to the satellite selection process. Although receivers are able to track all satellites in view, a satellite selection process may still be needed for some applications or when the number of satellites available is large (i.e., due to the availability of several constellations of satellites). This paper evaluates the relationship between the determinant of the GDOP matrix and the maximum volume of the polytope expanded by the user-satellite unit vectors endpoints when any number of satellites is employed for the computation of the position.

Automatic evaluation of plasma density profiles from microwave reflectometry on ASDEX upgrade based on the time-frequency distribution of the reflected signals

A new data analysis technique has been developed for the evaluation of the density profiles from broadband reflectometry in the presence of plasma turbulence. The method is based on the spectrogram of the reflected signals and uses the complete information of the beat frequency spectrum. The application of the Floyd best path algorithm, that takes into account the history of the beat frequency curve, enables us to extract the slow component due to the plasma profile. The statistics of the group delay data points is also considered to validate (or reject) data with great confidence. The results obtained in a wide range of plasma regimes show that accurate and detailed profiles can be measured automatically. One and two dimensional moving averaging over consecutive sweeps is also available to reduce the variance of the inverted profile while retaining a good temporal resolution.

Innovations-based code discriminator for GPS/Galileo BOC signals

Binary offset carrier (BOC) modulation was considered for modernized military GPS and for the European navigation system, Galileo. Although it performs better than conventional BPSK modulation, the BOC waveforms have autocorrelation functions with multiple peaks that lead to potential tracking ambiguities. We propose to minimize this problem with a bank of extended Kalman filters (EKF) that tracks the phase/frequency from the quadrature components of the incoming signal. The code discriminator is a weighted sum of code delays with the weights being nonlinear functions of the EKF innovations. When compared to the early-late power discriminator, the proposed algorithm is much less prone to lose the tracking of the BOC autocorrelation main peak.

Bayesian Approach for the Estimation of Phase Noise in SC-FDE Schemes

A solution for the problem of estimating the PN (Phase Noise) from the observation of the channel output in burst communications is to establish a state-model for the PN and determine the a posteriori probability density function (pdf) of the state conditioned on all measurement data, thus providing the means to compute an optimal estimate with respect to any criterion, e.g., Minimum Mean-Squared Error (MMSE). However, except in the Gaussian case, it is extremely difficult to determine and propagate this density function. As a result,and since the observation model of the PN is non-linear, the a posteriori pdf becomes non-Gaussian, and a sub-optimal solution for the problem must be found. In this paper we approximate the non-Gaussian pdf by a weighted sum of Gaussians. Additionally, we compare the performance results obtained when modeling the density function by a weighted sum of Gaussians with those obtained with a single Gaussian using a SC-FDE (Single Carrier-Frequency Domain Equalizer) scheme.