1,721,119 research outputs found
Spatially modulated code-division multiple-access for high-connectivity multiple-access
No full textIn order to take the advantages of spatial modulation (SM) and support the multiple access (MA) communications requiring high-connectivity, we integrate the SM with DS-CDMA to propose a SM/DS-CDMA scheme, which is in favor of attaining both antenna diversity and frequency diversity. Associated with the SM/DS-CDMA scheme, a range of linear and nonlinear multiuser detectors (MUDs) are proposed and studied. Specifically, for linear MUDs, both minimum mean-square error aided joint spatial demodulation (MMSE-JSD) and MMSE-aided separate spatial demodulation (MMSE-SSD) are introduced. For nonlinear MUDs, we first present the joint maximum-likelihood MUD (JML-MUD) for demonstrating the potential of SM/DS-CDMA systems. Then, the MMSE-relied iterative interference cancellation (MMSE-IIC) is suggested, and associated with which two types of low-complexity high-efficiency reliability measurement schemes are proposed. In this paper, we mathematically analyze both the single-user average bit error rate (ABER) of the SM/DS-CDMA systems employing joint spatial demodulation (JSD), and the approximate ABER of the SM/DS-CDMA systems employing the MMSE-JSD. We investigate and compare the ABER performance of the SM/DS-CDMA systems with various detection schemes, and also with some legacy schemes. Our studies and performance results show that the SM/DS-CDMA systems in conjunction with appropriate MUD schemes are capable of providing a desirable trade-off among the number of users supportable, implementation complexity and the ABER performance
Channel estimation and user activity identification in massive grant-free multiple-access
No full textGrant-free multiple-access (GFMA) allows to significantly reduce the overhead of granted multiple-access. However, information detection in GFMA is challenging, as it has to be executed along with the activity detection of user equipments (UEs) and channel estimation. In this paper, we study the channel estimation and propose the UE activity identification (UAI) algorithms for the massive connectivity supporting GFMA (mGFMA) systems. For these purposes, the channel estimation is studied from several aspects by assuming different levels of knowledge to the access point, and based on which five UAI approaches are proposed. We study the performance of channel estimation, the statistics of estimated channels, and the performance of UAI algorithms. Our studies show that the proposed approaches are capable of circumventing some of the shortcomings of the existing techniques designed based on compressive sensing and message passing algorithms. They are robust for operation in the mGFMA systems where the active UEs and the number of them are highly dynamic
Asymptotic Spectral-Efficiency of MIMO-CDMA Systems with Arbitrary Spatial Correlation
No full textIn this contribution, we analyze the asymptotic spectral-efficiency (ASE) of multiuser MIMO-CDMA systems, when assuming communications over flat fading channels with arbitrary spatial correlation. Our analysis is built on the operator-valued free probability theory, which is applied to obtain the limit distribution of the correlation matrix's eigenvalues, as the MIMO-CDMA systems' size tends to infinity. The spectral-efficiency (SE) performance of the MIMO-CDMA systems is investigated via both analysis and simulations. Our simulation and numerical results show that the ASE is capable of providing a good measure of the SE achieved by the corresponding realistic MIMO-CDMA systems
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
Near-Optimum Iterative Multiuser Detection in Time-Frequency-Domain Spread Multicarrier DS-CDMA Systems
Full text linkIn this contribution we propose and study a novel iterative time-frequency-domain (TF-domain) multiuser detector (MUD) for the multicarrier direct-sequence code-division multiple-access system employing both T-domain and F-domain spreading, which, for brevity, is referred to as the TF/MC DS-CDMA system. The proposed iterative TF-domain MUD consists of a set of T-domain soft-input soft-output MUDs (SISO-MUDs) and a set of F-domain SISO-MUDs, which exchange information through two multiuser interference (MUI) cancellation units. Both the T-domain and F-domain SISO-MUDs are operated under the maximum {\em a-posteriori} (MAP) principles. In this contribution the complexity and bit error rate (BER) performance of the TF/MC DS-CDMA employing the proposed iterative TF-domain MUD are investigated and also compared with the other existing MUD schemes, including the optimum MUD and the joint/separate minimum mean-square error (MMSE) MUDs. Our study shows that the iterative TF-domain MUD is capable of achieving nearly the same BER performance as the optimum MUD, but at much lower complexity
Asymptotic Performance Analysis of Time-Frequency-Domain Spread MC DS-CDMA Systems Employing MMSE Multiuser Detection
Full text linkIn this contribution the asymptotic signal-to-interference-plus-noise ratio (SINR) performance of multicarrier direct-sequence code-division multiple-access systems employing time-frequency-domain spreading, i.e., of the TF/MC DS-CDMA systems, is studied, when separate minimum mean-square error multiuser detection (MMSE-MUD) is considered. The separate MMSE-MUD detects signals first in the time (T)-domain and then in the frequency (F)-domain. Based on random matrix theory, closed-form expressions for the asymptotic SINR of the TF/MC DS-CDMA systems using separate MMSE-MUD is derived, when communicating over additive white Gaussian noise (AWGN) channels. The closed-form expressions show that the asymptotic SINR performance is only depended on the T- and F-domain user load factors as well as noise variance. Hence, they are beneficial to evaluation. Furthermore, our simulation and numerical results show that in most cases the asymptotic SINR can provide a good approximation to the SINR achieved by realistic TF/MC DS-CDMA systems employing separate MMSE-MUD
Dynamic DS-CDMA aided by successive interference cancellation for massive grant-free multiple-access
No full textWe introduce a dynamic DS-CDMA (DyDS-CDMA) scheme for supporting massive grant-free multiple-Access (mGFMA), where each base-station (BS) or access point (AP) with limited degrees of freedom is expected to control a massive number of devices of ultra densely deployed. We assume that devices become active to transmit data on the basis of time-slots, and during a time-slot, each of devices activates independently and following a common activation probability. A low-complexity detector based on minimum mean-square error and successive interference cancellation, referred to as the MMSE-SICD, is proposed for signal detection in DyDS-CDMA systems. Assuming ideal active device identification and ideal channel estimation of active device, in this paper, we focus on the potential performance achievable by the DyDS-CDMA systems employing the MMSESICD. Our studies reveal that the DyDS-CDMA aided by the MMSE-SICD is high-efficiency for operation in mGFMA environments. Near single-user performance is achievable, when the average number of active devices per time-slot reaches two times of the system's degrees-of-freedom, and hence, the number of active devices of a time-slot may be much higher than two timing the system's degrees-of-freedom.</p
Time–frequency iterative multiuser detection in time–frequency-domain spread multicarrier DS-CDMA systems over Nakagami-m fading channels
No full textIn this paper, we propose and study a novel Time–frequency-domain iterative multiuser detector (TF-IMUD) for the multicarrier direct-sequence code-division multiple-access (DS-CDMA) systems employing both time (T)-domain and frequency (F)-domain spreading, which, for brevity, are referred to as the TF/MC DS-CDMA systems. This novel TFIMUD consists of a set of T-domain soft-input soft-output MUDs (SISO-MUDs) and a set of F-domain SISO-MUDs, where information is exchanged between the T-domain and F-domain through two multiuser interference (MUI) cancellation units. The T-domain and F-domain SISO-MUDs are operated under the maximum likelihood (ML) principles, and they are facilitated to be implemented using parallel signal processing techniques. In this paper, the complexity of the TF-IMUD and the bit error rate (BER) performance of the TF/MC DS-CDMA systems employing the proposed TF-IMUD are investigated and also compared with the other existing MUD schemes, including the optimum ML-MUD and the joint/separate minimum mean-square error (MMSE)-MUDs. Our study and simulation results show that the TF-IMUD is capable of achieving nearly the same BER performance as the optimum ML-MUD, but at significantly lower complexity
Variations on the Author
Full text link“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
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