1,720,967 research outputs found
Impact of GPS acquisition strategy on decision probabilities
No full textThe first stage of processing within a GPS receiver consists of the signal acquisition process, the output of which provides a rough estimation of code delay and Doppler frequency. The strong dependence of the acquisition performance on the decision strategy is shown, establishing the role of decision probabilities. Three acquisition strategies are analyzed and a new model describing the performance of a hybrid acquisition system is developed. The theoretical models are validated by simulations, and secondary phenomena, generally neglected in the literature, are also discusse
DSP emulator for OFDM communication via HAP with beamforming algorithms
No full textOne of the main limitations in testing beamforming algorithms for OFDM, using traditional simulation tools like Matlab, is represented by the difficulties in taking into account DAC/ADC degradations, sampling clock errors and other aspects specific of a true communication system. The use of a real smart antenna is normally constrained by high costs relative to the front-end implementation. In order to overcome these problems we have realized a prototype that exploits the audio interface of commercial DSPs to simulate an OFDM communication on a SIMO channel, allowing the use of beamforming algorithms at the receiver side. Projects involving DSPs and their audio interface have already been developed, but they do not use a smart antenna at the reception: this simulator is a low cost solution to study the feasibility of an OFDM transmission and to test beamforming algorithms in presence of impairments due to analog and digital conversions. The realized emulator represents a flexible and powerful tool that can be easily adapted to other contexts thanks to its modular architecture and its parametric natur
Two-pole and multi-pole notch filters: a computationally effective solution for GNSS interference detection and mitigation
No full textIn a global navigation satellite system (GNSS) receiver, the presence of detection and mitigation units, capable of reducing the impact of disturbing signals, can extremely enhance the position accuracy. However, the presence of such units is usually limited to professional receivers that dispose of additional computational power that can be used for interference detection and mitigation. In this paper, the two-pole notch filter, that is the natural extension of the one-pole notch filter, is proposed as computationally effective solution for interference detection and mitigation. The notch filter structure and the adaptive algorithm employed for tracking the disturbing signal are analyzed, and an interference detection unit, based on the adaptive algorithm convergence, is proposed. The two-pole notch filter coupled with the detection unit is used as elementary block for the design of a multi-pole notch filter that can efficiently mitigate more than one CW interference. Theoretical and simulative analyses show the feasibility and the good performance of the proposed metho
DTFT-based frequency lock loop for GNSS applications
No full textA frequency lock loop (FLL) for Global Navigation Satellite System (GNSS) applications is described here. The core element is the discrete time Fourier transform (DTFT), that leads to a maximum likelihood (ML) frequency estimation and can be conveniently implemented in a software defined radio (SDR) HW platform. The algorithm is based on the iterative evaluation of DTFTs in a single frequency until the incoming frequency is locke
DTFT-based frequency lock loop for GNSS applications
No full textPrecise frequency estimation is one of the most stringent constraints for Global Navigation Satellite System (GNSS) receivers, that generally uses a Frequency Locked Loop (FLL) to provide a reliable evaluation of the Doppler frequency of the received signal. The proposed FLL is a two step algorithm based on an initial coarse estimation followed by an iterative procedure providing frequency refinements. The coarse estimation stage is implemented by finding the sampled frequency that maximizes the envelop of the FFT of a windowed portion of the input signal. The iterative procedure is based on the phase difference of two Discrete Time Fourier Transforms (DTFTs) evaluated over two overlapping data segments at the previous frequency estimate. The FLL has been integrated with denoising blocks, and median filters have been added to reduce the effect of outliers in the frequency estimation: simulations have proved the effectiveness of the system and the obtained results are very encouraging and show the feasibility of an FLL based on DTFT
Beamforming and synchronization algorithms integration for OFDM HAP-based communications
No full textHigh Altitude Platforms (HAPs) are currently experiencing a great interest as potential
solutions for the growing demand of wireless broadband applications, but their rapid motion
and the characteristics of the stratospheric channel, which may cause strong attenuation,
multipath fading and Doppler shift, can strongly impact their performances. The described
scenario requires the use of beamforming techniques, able to provide medium to high gains in
a mobile environment. Given these premises, this paper discusses the conjugation of OFDM
techniques with smart antennas for HAP-based communications, stressing the need for integration
of the different functional blocks of an OFDM receiver. In fact, in order to face a time
varying scenario, adaptive techniques are needed, and only a careful joint design of time and
frequency synchronization with beamforming algorithms can guarantee reliable communication.
The detailed and integrated receiver design described in the paper is based on the OFDM
version of the IEEE 802.16a transmission standard
Acquisition analysis for Galileo BOC modulated Signals: theory and simulation
No full textThis paper is about the performance of a Galileo acquisition stage where
the noise reduction and signal preservation is achieved by means of non-coherent
summations. In fact, the presence of a secondary code, planned for many Galileo
modulations and which further modulates the primary pseudo-random sequence,
does not easily allow a coherent integration on more than one code period.
Moreover, coherent integration is made much more complex by the longer codes
employed for Galileo and by the use of the Binary Offset Carrier (BOC)
modulations.
In a mass-market receiver, where the complexity burdens are extremely
important, these problems must be taken into account. A very easy solution is to
combine the correlator outputs in a non-coherent process after the squaring
operation in the acquisition block with an effective noise reduction without
increasing dramatically the processing and memory capabilities of the receiver
hardware.
An analytical model of the acquisition block, starting from the well known
statistical analysis carried out for GPS has been extended to Galileo. A complete
digital acquisition system is considered and parametric expressions, depending on
the number of terms coherently and non-coherently integrated, on the impact of the
sampling frequency, code time and Doppler step resolution have been derived for
the threshold settings and detection probability. The derived model for the noncoherent
acquisition is validated by means of Monte Carlo simulations.
Finally the effects of additional impairments over the correlation function
have been considered: an incomplete code delay recovery and slight mistakes in the
Doppler frequency estimation, which can potentially reduce the acquisition
performance, have been taken into account
A Time-Frequency Excisor for GNSS Interference
No full textInterference detection and mitigation in Global Navigation Satellite Systems (GNSSs) are important issues for both military and civilian applications. In this paper, a novel time-frequency algorithm for GNSS application is proposed. The use of infinite impulse-response notch filters for the interference excision is introduced and analytical formulas for the detection of the disturbing signals are derived. The proposed method is tested by simulations and compared with time-frequency excision algorithms reported in literature, proving its effectiveness for interference removal
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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