1,720,976 research outputs found
On Coherent and Non-coherent Receiver Structures for Impulse Radio UWB Systems [Elektronisk resurs]
No full textThe demand and growth of short-range wireless communications has been tremendous in the past few years for a diverse range of applications. There is significant interest in ultra wideband (UWB) as a physical layer technology for both high-data-rate (HDR), and low power, low-data-rate (LDR) short-range communications. The unique qualities of UWB such as large bandwidth, very low power spectral density (PSD) and fine time-resolution provide higher channel capacity, reduced fading effects and position location capability in a UWB system. The extremely wide bandwidth of UWB systems also poses many system design challenges to achieve low cost and low-complexity UWB devices. The UWB systems have difficulties in using digital signal processing (DSP) technology, require high sampling frequencies and also face frequency dependent signal distortion. The propagation characteristics of UWB signals also require consideration as it differs significantly from traditional narrowband systems. In this thesis, the issue of design and performance evaluation of coherent and non-coherent receivers for detection of impulse radio (IR) UWB signals is addressed. The coherent RAKE and non-coherent transmitted reference (TR) receiver structures are investigated for low power and low-data-rate wireless sensor network applications. First, the performance of coherent RAKE receivers for a single-user system operating in non-line-of-sight (NLOS) scenarios in an industrial environment is evaluated. The results and performance comparison is presented for partial RAKE (PRake) and selective RAKE (SRake) using maximal ratio combining (MRC) and equal gain combining (EGC). Secondly, the recursive structures of conventional TR and averaged TR schemes are presented to decrease the performance loss associated with the low-complexity TR scheme for IR-UWB systems. Further, a doublet-shift TR (DSTR) signaling and detection scheme is presented for IR-UWB systems. The simulation results validate that the doublet-shift TR signaling scheme improves the performance over conventional TR signaling scheme. Finally, dual-doublet TR (DDTR) signaling and detection schemes are proposed. The proposed dual-doublet TR schemes recover 25 to 50% energy/rate loss of conventional TR scheme. The performance of receiver structures is evaluated in terms of uncoded bit-error-rate (BER) over the channels measured in a medium-sized industrial environment and standard IEEE 802.15.4a multipath channels.</p
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Performance Evaluation of RAKE Receiver for Low Data Rate UWB Systems using Multipath Channels for Industrial Environments [Elektronisk resurs]
No full textSince US FCC passed a resolution in 2002 allowing ultra wideband (UWB) transmissions within a specified unlicensed spectral mask, the interest in UWB technology has grown tremendously. The large bandwidth, low power spectral density (PSD), high multiple access capability and high resolution are some qualities of UWB technology. For UWB communication systems, the industrial environments are an important scenario. However, due to large number of metallic scatterers in the environment, the multipath offered by UWB channels is dense and many multipath components have significant energy. In this report, the performance evaluation of RAKE receivers for a single user system operating in non-line-of-sight (NLOS) scenarios in industrial environments is presented. The channels used for the evaluation are measured in a medium-sized industrial environment. In addition, the standard IEEE 802.15.4a channel model for NLOS industrial environment is used for comparison with the results of the measured channels. The performance is compared for partial RAKE (PRake) and selective RAKE (SRake) in terms of uncoded bit-error-rate (BER) with the assumption that the channel is known. The effect of different number of fingers on BER of PRake and Srake is studied. Moreover, the performance of maximal ratio combining (MRC) and equal gain combining (EGC) is compared for PRake and Srake receiver. The results also provide a performance comparison between different Tx-Rx separations. Finally, based on the simulation results, conclusions are presented considering the performance and complexity issues.</p
On Modulation and Detection Schemes for Low-Complexity Impulse Radio UWB Communications
No full textDue to wealth of advantages offered by short range ultra wideband (UWB)
technology, such as capacity improvement, fading reduction and localization, it
has gathered a considerable attention. Distinct UWB qualities also pose many
system design challenges like difficulties in using digital processing, complex
channel estimation and different propagation characteristics. The main
objective of the thesis is to develop and evaluate efficient modulation and
detection schemes for impulse radio (IR) UWB with a focus on wireless sensor
networks characterized by low cost and low power consumption. The content of
the thesis comprises of five parts.
In Part I, a coherent RAKE and non-coherent energy detector (ED) and
transmitted reference (TR) receivers are examined and their bit-error-rate
(BER) performance is evaluated using channels measured in an industrial
environment. In specific, selective RAKE (SRake) and partial RAKE (PRake) for
both maximal ratio combining (MRC) and equal gain combining (EGC) are compared.
Based on the analysis and simulation results, it is concluded the SRake with
EGC is to be preferred, whereas the best complexity/performance trade-off is
provided by the ED based receivers. Part II presents several signaling and
detection schemes; the proposed schemes are recursive TR (R-TR), dual-doublet
TR (DDTR), doublet-shift TR (DSTR) and binary pulse position modulation
(BPPM)/DSTR. Analysis and simulations verify that the proposed schemes may be
preferred over the conventional TR in terms of BER, energy efficiency and/or
implementation complexity.
Part III presents a non-coherent kurtosis detector (KD) and a fourth-order
detector (FD), which can discriminate between Gaussian noise and non-Gaussian
IR-UWB signals by directly estimating the fourth-order moment of the received
signal. Empirical evaluations and simulations using channel measurements
conducted in a corridor, an office and a laboratory environment verify that
performance of the proposed FD receiver is slightly better than the ED in the
low SNR region and its performance improves as the SNR increases. Part IV
presents a robust weighted ED (WED) in which the weighting coefficients are
estimated adaptively based on the received stochastic data. Simulation results
confirm that performance of the proposed weight estimation method is close to
that of a data-aided (DA) scheme.
Finally, Part V focuses on a multi-user scenario and develops a weighted
code-multiplexed TR (WCM-TR) receiver employing the robust adaptive weight
estimation scheme. Secondly, a BPPM/CM-TR UWB system is presented to mitigate
inter-frame interference (IFI) and multi-user interference (MUI) from other
asynchronous users. The BPPM/CM-TR system is 3 dB energy-efficient and improves
the BER performance by mitigating MUI/IFI in the high SNR region, while for the
low SNR case and single-user scenario, a dual-mode BPPM/CM-TR system is
suggeste
On Coherent and Non-coherent Receiver Structures for Impulse Radio UWB Systems
No full textThe demand and growth of short-range wireless communications has been
tremendous in the past few years for a diverse range of applications. There is
significant interest in ultra wideband (UWB) as a physical layer technology for
both high-data-rate (HDR), and low power, low-data-rate (LDR) short-range
communications. The unique qualities of UWB such as large bandwidth, very low
power spectral density (PSD) and fine time-resolution provide higher channel
capacity, reduced fading effects and position location capability in a UWB
system. The extremely wide bandwidth of UWB systems also poses many system
design challenges to achieve low cost and low-complexity UWB devices. The UWB
systems have difficulties in using digital signal processing (DSP) technology,
require high sampling frequencies and also face frequency dependent signal
distortion. The propagation characteristics of UWB signals also require
consideration as it differs significantly from traditional narrowband systems.
In this thesis, the issue of design and performance evaluation of coherent and
non-coherent receivers for detection of impulse radio (IR) UWB signals is
addressed. The coherent RAKE and non-coherent transmitted reference (TR)
receiver structures are investigated for low power and low-data-rate wireless
sensor network applications.
First, the performance of coherent RAKE receivers for a single-user system
operating in non-line-of-sight (NLOS) scenarios in an industrial environment is
evaluated. The results and performance comparison is presented for partial RAKE
(PRake) and selective RAKE (SRake) using maximal ratio combining (MRC) and
equal gain combining (EGC). Secondly, the recursive structures of conventional
TR and averaged TR schemes are presented to decrease the performance loss
associated with the low-complexity TR scheme for IR-UWB systems.
Further, a doublet-shift TR (DSTR) signaling and detection scheme is presented
for IR-UWB systems. The simulation results validate that the doublet-shift TR
signaling scheme improves the performance over conventional TR signaling
scheme. Finally, dual-doublet TR (DDTR) signaling and detection schemes are
proposed. The proposed dual-doublet TR schemes recover 25 to 50% energy/rate
loss of conventional TR scheme. The performance of receiver structures is
evaluated in terms of uncoded bit-error-rate (BER) over the channels measured
in a medium-sized industrial environment and standard IEEE 802.15.4a multipath
channels
Recursive Transmitted Reference Receivers for Impulse Radio UWB Systems [Elektronisk resurs]
No full textFor the detection of impulse radio (IR) UWB signals, RAKE receivers or transmitted reference (TR) autocorrelation receivers can be used. The complexity of RAKE receiver increases significantly when the number of received multipath components is large. The TR scheme is a low-complexity alternative as it does not require channel estimation. However, there is a performance loss associated with the low-complexity TR scheme. The recursive structures of the conventional TR and averaged TR schemes are presented to improve the detection performance of IR-UWB signals. The performance of proposed schemes is evaluated over the standard IEEE 802.15.4a multipath channels. The performance is compared with conventional TR receivers in terms of uncoded bit-error-rate (BER), assuming that the channel is quasi-static. For averaged and recursive averaged TR schemes, the TR sequence is also slightly modified. The simulation results validate that the proposed schemes have better performance by about 2 dB than the conventional TR and averaged TR receivers.</p
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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