1,721,039 research outputs found
Analysis of challenge-response authentication with reconfigurable intelligent surfaces
Get PDFPhysical-layer authentication (PLA) mechanisms exploit
signals exchanged at the physical layer of communication
systems to confirm the sender of a received message. In this
paper, we propose a novel challenge-response PLA (CR-PLA)
mechanism for a cellular system that leverages the reconfigurability
property of a reconfigurable intelligent surface (RIS) (under
the control of the verifier) in an authentication mechanism. In
CR-PLA, the verifier base station (BS) sets a random RIS configuration,
which remains secret to the intruder, and then checks
that the resulting estimated channel is modified correspondingly.
In fact, for a message sent by an attacker in a different location
than the legitimate user equipment (UE), the BS will estimate a
different channel and the message will be rejected as fake. Such a
solution reduces the communication and computational overhead
with respect to higher-layer cryptographic authentication. We
derive the maximum a-posteriori attack when the attacker
observes a correlated channel and the reconfigurable intelligent
surface (RIS) has many elements, and the attacker transmits
to Bob either directly or through the RIS. Using a generalized
likelihood ratio test to test the authenticity at the base station
(BS), we derive approximate expressions of the false alarm
and misdetection probabilities when both the BS and the UE
have a single antenna each, while the RIS has a large number
of elements. We also evaluate the trade-off between security
and communication performance, since choosing a random RIS
configuration reduces the data rate. Moreover, we investigate
the impact of various parameters (e.g., the RIS randomness, the
number of RIS elements, and the operating signal-to-noise ratio)
on security and communication performance
A Theoretical Framework for Soft-Information-Based Synchronization in Iterative (Turbo) Receivers
Get PDFThis contribution considers turbo synchronization, that is to say, the use of soft data information to estimate parameters like carrier phase, frequency, or timing offsets of a modulated signal within an iterative data demodulator. In turbo synchronization, the receiver exploits the soft decisions computed at each turbo decoding iteration to provide a reliable estimate of some signal parameters. The aim of our paper is to show that such “turbo-estimation” approach can be regarded as a special case of the expectation-maximization (EM) algorithm. This leads to a general theoretical framework for turbo synchronization that allows to derive parameter estimation procedures for carrier phase and frequency offset, as well as for timing offset and signal amplitude. The proposed mathematical framework is illustrated by simulation results reported for the particular case of carrier phase and frequency offsets estimation of a turbo-coded 16-QAM signal.</p
The analysis of the effect of time-interleaving analog-to-digital converter mismatches on BER performance in OFDM systems
No full textAdaptive pilot allocation for estimating sparse uplink MU-MIMO-OFDM channels
No full textWe consider uplink multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication. The transmit (Tx) side of the envisaged system consists of several single-antenna users or/and several multiple-antenna users. At the receive side, a multiple-antenna access point employs compressive sensing techniques to estimate the channel impulse response from the preamble portion of the observed packets. The traditional approach is that of orthogonal pilot allocation: during a short training period, each OFDM subcarrier is assigned exclusively to a single Tx antenna. In this case, the channel state information can conveniently be acquired on a per Tx antenna basis. To the best of our knowledge, all related research imposes that all Tx antennas are allocated the same amount of pilots (which must then be tailored for the most extreme channel conditions). However, in the considered system, Tx antennas may experience totally different channel conditions. Under these circumstances, the use of a fixed number of pilots per Tx antenna results in a lot of unnecessary overhead. To tackle this problem, our work addresses the design of efficient algorithms for adaptive orthogonal pilot allocation. The following design principles are applied: orthogonal pilot allocation, constant-modulus modulation, minimum measurement matrix mutual coherence optimization, and the condition that the number of pilot subcarriers allocated to each Tx antenna is adjusted to the channel conditions experienced by that Tx antenna. The paper tackles the problem of determining the optimal number of pilot subcarriers as well as the optimal positions of the pilots. To facilitate adaptive operation, we propose a reduced-complexity method to determine the optimal pilot positions. The performance of our algorithms is demonstrated by means of computer simulations, using both theoretical channel models and results from our own channel measurement campaign
Adaptive Pilot Allocation for Estimating Sparse Uplink MU-MIMO-OFDM Channels
No full textThis work was supported in part by the Belgian Excellence of Science (EOS) Grant under Project EOS30452698; in part by the Flemish Fund for Scientific Research (FWO); in part by the National Natural Science Foundation of China under Grant 61801516, Grant 61701530, Grant 61701531, and Grant 61971273; and in part by the Flemish Government (AI Research Program)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Synchronization in digital communication systems: performance bounds and practical algorithms
Get PDFCommunication channels often transfer signals from different transmitters. To avoid interference the available frequency spectrum is divided into non-overlapping frequency bands (bandpass channels) and each transmitter is assigned to a different bandpass channel. The transmission of a signal over a bandpass channel requires a shift of its frequency-content to a frequency range that is compatible with the designated frequency band (modulation). At the receiver, the modulated signal is demodulated (frequency shifted back to the original frequency band) in order to recover the original signal. The modulation/demodulation process requires the presence of a locally generated sinusoidal signal at both the transmitter and the receiver. To enable a reliable information transfer, it is imperative that these two sinusoids are accurately synchronized.
Recently, several powerful channel codes have been developed which enable reliable communication at a very low signal-to-noise ratio (SNR). A by-product of these developments is that synchronization must now be performed at a SNR that is lower than ever before. Of course, this imposes high requirements on the synchronizer design.
This doctoral thesis investigates to what extent (performance bounds) and in what way (practical algorithms) the structure that the channel code enforces upon the transmitted signal can be exploited to improve the synchronization accuracy at low SNR
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