23,717 research outputs found
Broadband MC DS-CDMA Using Space-Time and Frequency-Domain Spreading
No full textIn this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS) assisted transmit diversity and frequency-domain (F-domain) spreading is investigated in the context of broadband communications over frequency-selective Rayleigh fading channels. We consider the attainable capacity extension of broadband MC DS-CDMA with the advent of using Time-Frequency-domain (TF-domain) spreading. The BER performance of STS assisted broadband MC DS-CDMA using Binary Phase Shit Keying (BPSK) modulation and TF-domain spreading is investigated by simulation for a range of parameter values. Both the correlation based single-user detector and the decorrelating multiuser detector are considered. Our study shows that the number of users supported by the broadband MC DS-CDMA system is determined by the product of the T-domain spreading factor and the F-domain spreading factor, while it is independent of the frequency diversity order. Furthermore, when multiuser detection assisted F-domain spreading is considered, the broadband MC DS-CDMA system is capable of supporting a substantially increased number of users, while maintaining a similar bit error ratio (BER) performance to that of the broadband MC DS-CDMA system using no F-domain spreading
Partial Equalization for MC–CDMA Systems in Non-Ideally Estimated Correlated Fading
No full textMulticarrier code-division multiple access (MC–CDMA) can support high data rates in next-generation multiuser wireless communication systems. Partial equalization (PE) is a low-complexity technique for combining the signals of subcarriers to improve the achievable performance of MC–CDMA systems in terms of their bit error probability (BEP) and bit error outage (BEO) in comparison with maximal ratio combining, orthogonality restoring combining, and equal-gain combining techniques. We analyze the performance of the multiuser MC–CDMA downlink and derive the optimal PE parameter expression, which minimizes the BEP. Realistic imperfect channel estimation and frequency-domain (FD) block-fading channels are considered. More explicitly, the analytical expression of the optimum PE parameter is derived as a function of the number of subcarriers, number of active users (i.e., the system load), mean signal-to-noise ratio (SNR), and variance of the channel-estimation errors for the aforementioned FD block-fading channel. We show that the choice of the optimal PE technique significantly increases the achievable system load for the given target BEP and BEO
Ant-Colony-Based Multiuser Detection for MC DS-CDMA Systems
No full textIn this contribution we present a novel ant colony optimization (ACO) based multi-user detector (MUD) designed for synchronous multi-carrier direct sequence code division multiple access (MC DSCDMA) systems. The operation of the ACO-based MUD is based on the behaviour of the ant colony in nature. The ACO-based MUD aims for achieving the same bit-error-rate (BER) performance as the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that the system is capable of supporting almost as many users as the number of chips in the spreading sequence, while searching only a small fraction of the entire ML search space. It will also be demonstrated that the number of floating point operations per second is a factor of 108 lower for the proposed ACO-based MUD than that of the ML MUD, when supporting K = 32 users in a MC DS-CDMA system employing 31-chip Gold codes as the T-domain spreading sequence
Sample-Spaced and Fractionally-Spaced CIR Estimation Aided Decision Directed Channel Estimation for OFDM and MC-CDMA
No full textThe achievable performance of decision-directed channel estimation (DDCE) is analysed in the context of both OFDM and MC CDMA systems. Two different estimation methods suitable for both OFDM and MC-CDMA systems are considered, which employ sample-spaced (SS) as well as fractionally-spaced (FS) a posteriori channel impulse response (CIR) estimators. The performance of both estimation methods is compared and it is shown that the DDCE scheme employing the FS-CIR estimator outperforms its SS-CIR estimator-based counterpart. Furthermore, the FS-CIR estimator-based method exhibits higher robustness to various channel parameters such as the channel’s root mean square delay spread. Finally, it is shown that the MC-CDMA system employing the channel estimation scheme advocated outperform its OFDM counterpart
Ant-colony-based multiuser detection for multifunctional-antenna-array-assisted MC DS-CDMA systems
No full textA novel Ant Colony Optimization (ACO) based Multi-User Detector (MUD) is designed for the synchronous Multi-Functional Antenna Array (MFAA) assisted Multi-Carrier Direct-Sequence Code-Division Multiple-Access (MC DS-CDMA) uplink (UL), which supports both receiver diversity and receiver beamforming. The ACO-based MUD aims for achieving a bit-error-rate (BER) performance approaching that of the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that regardless of the number of the subcarriers or of the MFAA configuration, the system employing the proposed ACO based MUD is capable of supporting 32 users with the aid of 31-chip Gold codes used as the T-domain spreading sequence without any significant performance degradation compared to the single-user system. As a further benefit, the number of floating point operations per second (FLOPS) imposed by the proposed ACO-based MUD is a factor of 108 lower than that of the ML MUD. We will also show that at a given increase of the complexity, the MFAA will allow the ACO based MUD to achieve a higher SNR gain than the Single-Input Single-Output (SISO) MC DS-CDMA system. Index Terms—Ant Colony Optimization, Multi-User Detector, Multi-Functional Antenna Array, Multi-Carrier Direct-Sequence Code-Division Multiple-Access, Uplink, Near-Maximum Likelihood Detection
Generic Reduced-Complexity MMSE Channel Estimation for OFDM and MC-CDMA
No full textThe performance of a decision-directed channel estimation scheme is analyzed in the context of both OFDM and MC-CDMA systems. A difficulty associated with the employment of the Least Squares (LS) approach to the problem of a posteriori channel estimation in the context of a MC-CDMA system is described and a suitable MMSE-based estimator is proposed instead. It is demonstrated that the computational complexity associated with the MMSE estimation method proposed is relatively high in comparison to that of the conventional LS technique. Thus a low-complexity version of the MMSE estimator is proposed. The MC-CDMA system using a low-complexity MMSE estimator proposed is shown to outperform the corresponding OFDM-based scheme
Interactively using Semantic Web knowledge: Creating scalable abstractions with FacetOntology
No full textThe amount of knowledge accessible on the Semantic Web is growing, and there is a need for a scalable solution to facilitate exploring that data. Currently approaches to exploring Semantic Web data either focus on exploring resources individually, following links during exploration, and making little use of collated data, or take the approach of collating and aligning multiple sources into one store for one purpose, and hand-crafting a specific browsing interface onto it. We present an approach that provides a scalable browsing interface, which can browse knowledge from the Semantic Web at will. Our approach creates abstractions of knowledge, collated into facets, which are described using FacetOntology. FacetOntology facilitates describing facets from RDF data, suitable for use in creating datasets for faceted browsing
Christie V. Mc Donald : The Dialogue of Writing, University Press, 1984
No full textBuffat Marc. Christie V. Mc Donald : The Dialogue of Writing, University Press, 1984. In: Recherches sur Diderot et sur l'Encyclopédie, n°5, 1988. p. 165
The Pontiac Press: 1956-12-28
No full textPontiac Press newspaper scanned from microfilmMore homes threatened by 2nd brush fire ; Cancel state trucking strike: Protest move is called off by Teamsters ; Ike may ask okay to use US Army ; Car crashed take lives of 2 ; Ironwood fire claims 6 lives ; General Hospital gets four new trustees ; No immediate plan to hike bread prices ; Most persons happy to pay to use parks ; Central business area vital to every city ; Seek to abolish board authority ; Lawmakers ask reports of advisory committee ; Father, jailed for debt, will pay $600; can now enjoy his Christmas gifts ; 'Jock' Whitney appointed ambassador to Britain ; Explosion shakes chemical factory ; Find plane wreck in Allegan area ; Hunt New York 'Mad Bomber' ; Actor Sal Mineo made Pontiac honorary mayor ; Rail office to open at Grand Rapids ; Advisory group will investigate foreign aid job ; Claims find 'missing link' to man ; Jerry Lester to MC song quiz panel show ; Seek to amend diagnosis bill ; Europe offers homes for 58,000 Hungarian
Generic MMSE Decision Directed Channel Estimation for OFDM and MC-CDMA
No full textIn this letter we propose a Minimum Mean Square Error (MMSE) Channel Impulse Response (CIR) estimator-aided Decision Directed Channel Estimation (DDCE) method, which is suitable for employment in both OFDM and MC-CDMA systems. Firstly, we outline the difficulty associated with the employment of the Least Squares (LS) approach to the problem of a posteriori Frequency Domain Channel Transfer Function (FDCTF) estimation in the context of a MC-CDMA system and derive a suitable MMSE-based CIR estimator, which circumvents the LS method’s deficiencies. Secondly, we demonstrate that the computational complexity associated with the conventional MMSE estimation method is relatively high compared to that of the LS method. Thus a reduced-complexity version of the MMSE estimator is proposed, which has a complexity similar to that exhibited by the LS-aided estimator. The MC-CDMA system using the reduced-complexity MMSE estimator proposed is shown to outperform the corresponding OFDM-based scheme
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