Performance Analysis of Multiuser Detectors for Uplink Quasi-synchronous MC-CDMA Systems

This paper analyses and compares the average bit error rate (BER) of different multiuser detectors (MUD) in the uplink of a multicarrier code- division multiple access (MC-CDMA) system. In particular, maximum likelihood, zero-forcing, minimum mean-square error and interference cancellation-based multiuser detectors have been analysed for the special case of uncorrelated subcarriers. The derived BER expressions are based upon previous results on diversity combining and also on recent findings on multiple input multiple output (MIMO) architectures. The subcarrier correlation is considered in the context of physical parameters currently under discussion for future wireless systems to give an indication up to what extent the assumption of uncorrelated subcarrier fading is plausible.     

Subsolidus Phase Relations in the Pseudoternary System SrO·6M2O3 (M = Fe, Cr, Al) in Air

Crystalline powder samples belonging to the pseudoternary system SrO·6M₂O₃ (M = Fe, Al, Cr) were synthesized in air. The limit of the magnetoplumbite-type (M-type) phase solid solution is governed by the presence of Cr³⁺ in the structure and is accompanied by the formation of a chromate and a sesquioxide. The extension of the solid solution in the Cr-containing pseudobinaries depends to a large extent on the crystallographic site preference of the host cation. The extension of the M-type solid solution is also narrowed at a lower temperature.     

BER analysis of group-orthogonal multicarrier code-division multiplex systems

Group-orthogonal multi-carrier code division multiple access (GO-MC-CDMA) has recently been proposed as a promising technique for the uplink segment of wireless systems. In this paper we propose and analyze a related scheme, group-orthogonal multi-carrier code division multiplexing (GO-MC-CDM), suitable for the downlink segment of the future generation of wireless systems. The proposed system is shown to offer a similar bit error rate (BER) performance as the downlink version of GO-MC-CDMA at a fraction of its computational complexity. An analytical expression for the BER when using maximum likelihood (ML) detection is derived providing valuable insight into the parameters affecting the system performance and providing a basis for its optimization. Simulation results using parameters and (correlated) channel models aiming at the next generation of wireless systems are provided confirming the analytically derived results.     

Self-organized Ce(1-x)Gd(x)O(2-y) nanowire networks with very fast coarsening driven by attractive elastic interactions

Assembling arrays of ordered nanowires is a key objective for many of their potential applications. However, a lack of understanding and control of the nanowires' growth mechanisms limits their thorough development. In this work, an appealing new path towards self-organized epitaxial nanowire networks produced by high-throughput solution methods is reported. Two requisites are identified to generate the nanowires: a thermodynamic driving force for an unrestricted elongated equilibrium island shape, and a very fast effective coarsening rate. These requirements are met in anisotropically strained Ce(1-x)Gd(x)O(2-y) nanowires with the (011) orientation grown on the (001) surface of LaAlO(3) substrates. Nanowires with aspect ratios above ≈100 oriented along two mutually orthogonal axes are obtained leading to labyrinthine networks. A very fast effective nanowire growth rate (≈60 nm min(-1)) for ex-situ thermally annealed nanostructures derives from simultaneous kinetic processes occurring in a branched network. Ostwald ripening and anisotropic dynamic coalescence, both promoted by strain-driven attractive nanowire interaction, and rapid recrystallization, enabled by fast atomic diffusion associated with a high concentration of oxygen vacancies, contribute to such an effective growth rate. This bottom-up approach to self-organized nanowire growth has a wide potential for many materials and functionalities.     

On the design of uplink and downlink group-orthogonal multicarrier wireless systems

Group-orthogonal multicarrier code-division multiple access (GO-MC-CDMA) has been proposed as an attractive multiplexing technique for the uplink segment of wireless systems. More recently, a variant of this scheme has also been proposed for the downlink. This paper presents a unified bit error rate (BER) performance analysis of group-orthogonal wireless systems when using maximum likelihood (ML) multiuser/multisymbol detection covering both link directions. Valuable design rules regarding the number of subcarriers per group and the selection of spreading codes are derived. Simulations results using realistic system parameters and ETSI BRAN channel models are also presented which serve to validate the analytical results.     

A branching heuristic for SAT solvers based on complete implication graphs

Science China Information Sciences - Given a set of radio broadcast programs, the radio broadcast scheduling problem is to allocate a set of devices to transmit the programs to achieve the optimal...     

Improved linear group detection for combined spatial multiplexing/STBC systems

Wireless architectures combining the use of spatial division multiplexing (SDM) and space-time block coding (STBC) have begun to appear in the latest communications standards. In recent years, different detection strategies for these schemes have been proposed which can be broadly categorised as groupbased or direct-detection techniques. While the former class aims at separating the different STBC streams and then apply conventional simple Alamouti decoding, the latter type directly estimates the transmitted symbols without fully exploiting the Alamouti structure. Previous publications have shown that directdetection outperform group-based detectors with the penalty of a higher computational complexity. This letter presents a unified view of the detection strategies for hybrid SDM/STBC systems and presents two new families of group-based detectors. It is shown that linear group detection, when properly implemented, attains the same performance as linear direct detectors at a significant lower complexity.     

Modeling fast link adaptation-based 802.11n distributed coordination function

This paper presents a comprehensive performance study of closed-loop fast link adaptation (FLA) in the context of IEEE 802.11n, spanning the physical (PHY) and medium-access control (MAC) layers. In particular, a semi-analytical model is derived for Basic and request to send/clear to send (RTS/CTS) access schemes of the distributed coordination function (DCF), that applies to both, open- and closed-loop strategies. Numerical results serve to demonstrate the accuracy of the proposed model and the superiority of FLA, in terms of MAC goodput, in comparison to open-loop policies. Realistic operating conditions such as outdated feedback information and the use of statistical packet length distributions, issues not treated in previous studies, have also been considered. Moreover, it is shown that incorporating a time-out mechanism in the FLA scheme, weighing down the influence of channel information as this becomes outdated, is a useful strategy to counteract its deleterious effects.