Showing papers by "University of Crete published in 2008"

Pillared graphene: a new 3-D network nanostructure for enhanced hydrogen storage.

Abstract: A multiscale theoretical approach was used to investigate hydrogen storage in a novel three-dimensional carbon nanostructure. This novel nanoporous material has by design tunable pore sizes and surface areas. Its interaction with hydrogen was studied thoroughly via ab initio and grand canonical Monte Carlo calculations. Our results show that, if this material is doped with lithium cations, it can store up to 41 g H2/L under ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.

EULAR recommendations for the management of systemic lupus erythematosus. Report of a Task Force of the EULAR Standing Committee for International Clinical Studies Including Therapeutics

Abstract: Objective: Systemic lupus erythematosus (SLE) is a complex disease with variable presentations, course and prognosis. We sought to develop evidence-based recommendations addressing the major issues in the management of SLE. Methods: The EULAR Task Force on SLE comprised 19 specialists and a clinical epidemiologist. Key questions for the management of SLE were compiled using the Delphi technique. A systematic search of PubMed and Cochrane Library Reports was performed using McMaster/Hedges clinical queries' strategies for questions related to the diagnosis, prognosis, monitoring and treatment of SLE. For neuropsychiatric, pregnancy and antiphospholipid syndrome questions, the search was conducted using an array of relevant terms. Evidence was categorised based on sample size and type of design, and the categories of available evidence were identified for each recommendation. The strength of recommendation was assessed based on the category of available evidence, and agreement on the statements was measured across the 19 specialists. Results: Twelve questions were generated regarding the prognosis, diagnosis, monitoring and treatment of SLE, including neuropsychiatric SLE, pregnancy, the antiphospholipid syndrome and lupus nephritis. The evidence to support each proposition was evaluated and scored. After discussion and votes, the final recommendations were presented using brief statements. The average agreement among experts was 8.8 out of 10. Conclusion: Recommendations for the management of SLE were developed using an evidence-based approach followed by expert consensus with high level of agreement among the experts.

Exploring improved holographic theories for QCD: part I

Abstract: Various holographic approaches to QCD in five dimensions are explored using input both from the putative five-dimensional non-critical string theory as well as QCD. It is argued that a gravity theory in five dimensions coupled to a dilaton and an axion may capture the important qualitative features of pure YM theory. A part of the effects of higher α'-corrections is resummed into a dilaton potential. The potential is shown to be in one-to-one correspondence with the exact β-function of QCD, and its knowledge determines the full structure of the vacuum solution. The geometry near the UV boundary is that of AdS5 with logarithmic corrections reflecting the asymptotic freedom of QCD. We find that all relevant confining backgrounds have an IR singularity of the ``good" kind that allows unambiguous spectrum computations. Near the singularity the 't Hooft coupling is driven to infinity. Asymptotically linear glueball masses can also be achieved. The classification of all confining asymptotics, the associated glueball spectra and meson dynamics are addressed in a companion paper arXiv:0707.1349

Quality of Service and Max-Min Fair Transmit Beamforming to Multiple Cochannel Multicast Groups

Abstract: The problem of transmit beamforming to multiple cochannel multicast groups is considered, when the channel state is known at the transmitter and from two viewpoints: minimizing total transmission power while guaranteeing a prescribed minimum signal-to-interference-plus-noise ratio (SINR) at each receiver; and a "fair" approach maximizing the overall minimum SINR under a total power budget. The core problem is a multicast generalization of the multiuser downlink beamforming problem; the difference is that each transmitted stream is directed to multiple receivers, each with its own channel. Such generalization is relevant and timely, e.g., in the context of the emerging WiMAX and UMTS-LTE wireless networks. The joint problem also contains single-group multicast beamforming as a special case. The latter (and therefore also the former) is NP-hard. This motivates the pursuit of computationally efficient quasi-optimal solutions. It is shown that Lagrangian relaxation coupled with suitable randomization/cochannel multicast power control yield computationally efficient high-quality approximate solutions. For a significant fraction of problem instances, the solutions generated this way are exactly optimal. Extensive numerical results using both simulated and measured wireless channels are presented to corroborate our main findings.

Unexpected power-law stress relaxation of entangled ring polymers.

Abstract: After many years of intense research, most aspects of the motion of entangled polymers have been understood. Long linear and branched polymers have a characteristic entanglement plateau and their stress relaxes by chain reptation or branch retraction, respectively. In both mechanisms, the presence of chain ends is essential. But how do entangled polymers without ends relax their stress? Using properly purified high-molar-mass ring polymers, we demonstrate that these materials exhibit self-similar dynamics, yielding a power-law stress relaxation. However, trace amounts of linear chains at a concentration almost two decades below their overlap cause an enhanced mechanical response. An entanglement plateau is recovered at higher concentrations of linear chains. These results constitute an important step towards solving an outstanding problem of polymer science and are useful for manipulating properties of materials ranging from DNA to polycarbonate. They also provide possible directions for tuning the rheology of entangled polymers.

Ultra-low shrinkage hybrid photosensitive material for two-photon polymerization microfabrication.

Abstract: Investigations into the structuring by two-photon polymerization of a nonshrinking, photosensitive, zirconium sol−gel material are presented This hybrid material can be photostructured even when it contains up to 30 mol % of zirconium propoxide (ZPO); by varying the material’s inorganic content, it is possible to modify and tune its refractive index The introduction of ZPO significantly increases the photosensitivity of the resulting photopolymer The fabricated three-dimensional photonic crystal structures demonstrate high resolution and a clear band-stop in the near-IR region In contrast to common practice, no additional effort is required to precompensate for shrinkage or to improve the structural stability of the fabricated photonic crystals; this, combined with the possibility of tuning this material’s optical, mechanical, and chemical properties, makes it suitable for a variety of applications by two-photon polymerization manufacturing

Prognostic Value of Mature MicroRNA-21 and MicroRNA-205 Overexpression in Non–Small Cell Lung Cancer by Quantitative Real-Time RT-PCR

Abstract: Background: microRNA (miRNA) expression profiles are being intensively investigated for their involvement in carcinogenesis. We evaluated the prognostic value of mature microRNA-21 (miR-21) and mature microRNA-205 (miR-205) overexpression in non–small cell lung cancer (NSCLC). Patients and methods: We studied 48 pairs of NSCLC fresh frozen tissue specimens collected at time of surgery and before chemotherapy. Highly specific amplification and quantification of mature miR-21 and mature miR-205 was achieved using looped real time RT-PCR. Results: miRNA expression, determined by real time RT-PCR, was defined by ΔΔCt measurements. We detected overexpression of mature miR-21 in 25 (52.0%) of the 48 NSCLC paired specimens and overexpression of miR-205 in 31 (64.6%). Overexpression was assessed after comparison of miRNA expression in NSCLC tissues and in their corresponding noncancerous tissues with respect to U6 expression. During the follow-up period, 29 of 48 (60.4%) patients relapsed, and 23 of 48 died (47.9%). Mature miR-21 was upregulated in 16 of 29 (55.2%) patients who relapsed and 15 of 23 (65.2%) patients who died. Mature miR-205 was overexpressed in 19 of 29 patients who relapsed (65.5%) and 15 of 23 patients who died (65.2%). Mature miR-21 overexpression correlated with overall survival (OS) of the patients ( P = 0.027), whereas overexpression of mature miR-205 did not. Conclusions: Our results suggest that overexpression of mature miR-21 is an independent negative prognostic factor for OS in NSCLC patients.

Design and implementation of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study.

Abstract: To provide an overview of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study (HELENA-CSS) design, with particular attention to its quality control procedures. Other important methodological aspects are described in detail throughout this supplement. Description of the HELENA-CSS sampling and recruitment approaches, standardization and harmonization processes, data collection and analysis strategies and quality control activities. The HELENA-CSS is a multi-centre collaborative study conducted in European adolescents located in urban settings. The data management systems, quality assurance monitoring activities, standardized manuals of operating procedures and training and study management are addressed in this paper. Various quality controls to ensure collection of valid and reliable data will be discussed in this supplement, as well as quantitative estimates of measurement error. The great advantage of the HELENA-CSS is the strict standardization of the fieldwork and the blood analyses, which precludes to a great extent the kind of immeasurable confounding bias that often interferes when comparing results from isolated studies.

The motivating role of positive feedback in sport and physical education: evidence for a motivational model.

Abstract: Based on self-determination theory (Deci & Ryan, 2000), an experimental study with middle school students participating in a physical education task and a correlational study with highly talented sport students investigated the motivating role of positive competence feedback on participants’ well-being, performance, and intention to participate. In Study 1, structural equation modeling favored the hypothesized motivational model, in which, after controlling for pretask perceived competence and competence valuation, feedback positively predicted competence satisfaction, which in turn predicted higher levels of vitality and greater intentions to participate, through the mediation of autonomous motivation. No effects on performance were found. Study 2 further showed that autonomous motivation mediated the relation between competence satisfaction and well-being, whereas amotivation mediated the negative relation between competence satisfaction and ill-being and rated performance. The discussion focuses on the motivational role of competence feedback in sports and physical education settings.

Ontology change: Classification and survey

Abstract: Ontologies play a key role in the advent of the Semantic Web. An important problem when dealing with ontologies is the modification of an existing ontology in response to a certain need for change. This problem is a complex and multifaceted one, because it can take several different forms and includes several related subproblems, like heterogeneity resolution or keeping track of ontology versions. As a result, it is being addressed by several different, but closely related and often overlapping research disciplines. Unfortunately, the boundaries of each such discipline are not clear, as the same term is often used with different meanings in the relevant literature, creating a certain amount of confusion. The purpose of this paper is to identify the exact relationships between these research areas and to determine the boundaries of each field, by performing a broad review of the relevant literature.

Spermidine Exodus and Oxidation in the Apoplast Induced by Abiotic Stress Is Responsible for H2O2 Signatures That Direct Tolerance Responses in Tobacco

Abstract: Polyamines (PAs) exert a protective effect against stress challenges, but their molecular role in this remains speculative. In order to detect the signaling role of apoplastic PA-derived hydrogen peroxide (H2O2) under abiotic stress, we developed a series of tobacco (Nicotiana tabacum cv Xanthi) transgenic plants overexpressing or downregulating apoplastic polyamine oxidase (PAO; S-pao and A-pao plants, respectively) or downregulating S-adenosyl-l-methionine decarboxylase (samdc plants). Upon salt stress, plants secreted spermidine (Spd) into the apoplast, where it was oxidized by the apoplastic PAO, generating H2O2. A-pao plants accumulated less H2O2 and exhibited less programmed cell death (PCD) than did wild-type plants, in contrast with S-pao and samdc downregulating plants. Induction of either stress-responsive genes or PCD was dependent on the level of Spd-derived apoplastic H2O2. Thus, in wild-type and A-pao plants, stress-responsive genes were efficiently induced, although in the latter at a lower rate, while S-pao plants, with higher H2O2 levels, failed to accumulate stress-responsive mRNAs, inducing PCD instead. Furthermore, decreasing intracellular PAs, while keeping normal apoplastic Spd oxidation, as in samdc downregulating transgenic plants, caused enhanced salinity-induced PCD. These results reveal that salinity induces the exodus of Spd into the apoplast, where it is catabolized by PAO, producing H2O2. The accumulated H2O2 results in the induction of either tolerance responses or PCD, depending also on the levels of intracellular PAs.

Assessing, understanding and modifying nutritional status, eating habits and physical activity in European adolescents: the HELENA (Healthy Lifestyle in Europe by Nutrition in Adolescence) Study.

Abstract: Objectives: To identify the main knowledge gaps and to propose research lines that will be developed within the European Union-funded 'Healthy Lifestyle in Europe by Nutrition in Adolescence' (HELENA) project, concerning the nutritional status, physical fitness and physical activity of adolescents in Europe. Design: Review of the currently existing literature. Results: The main gaps identified were: lack of harmonised and comparable data on food intake; lack Of understanding regarding the role of eating attitudes, food choices and food preferences; lack of harmonised and comparable data on levels and patterns of physical activity and physical fitness; lack of comparable data about obesity prevalence and body composition; lack of comparable data about micronutrient and immunological status; and lack of effective intervention methodologies for healthier lifestyles. Conclusions: The HELENA Study Group should develop, test and describe harmonised and state-of-the-art methods to assess the nutritional status and lifestyle of adolescents across Europe; develop and evaluate an intervention on eating habits and physical activity; and develop and test new healthy food products attractive for European adolescents.

Fungal Infections Complicating Tumor Necrosis Factor α Blockade Therapy

Abstract: Tumor necrosis factor α (TNF-α) blockade has emerged as a useful therapy for collagen vascular diseases or graft-vs-host disease. Fungal infections complicating such therapy have been reported sporadically. MEDLINE and PubMed databases (from January 1, 1966, to June 1, 2007) were searched for reports of invasive fungal infections (IFIs) associated with the 3 available anti-TNF-α agents, ie, infliximab, etanercept, and adalimumab. Of the 281 cases of IFI associated with TNF-α inhibition, 226 (80%) were associated with infliximab, 44 (16%) with etanercept, and 11 (4%) with adalimumab. Fungal infections associated with infliximab occurred a median of 55 days (interquartile range [IQR], 15-140 days) after initiation of therapy and 3 infusions of the medication (IQR, 2-5), whereas those associated with etanercept occurred a median of 144 days (IQR, 46-240 days) after initiation of therapy. The median age of patients was 58 years (IQR, 44-68 years), and 62% were male. Use of at least 1 other immunosuppressant medication, typically a systemic corticosteroid, was reported during the course of the fungal infection in 102 (98%) of the 104 patients for whom data were available. The most prevalent IFIs were histoplasmosis (n=84 [30%]), candidiasis (n=64 [23%]), and aspergillosis (n=64 [23%]). Pneumonia was the most common pattern of infection. Of the 90 (32%) of 281 cases for which outcome information was available, 29 fatalities (32%) were recorded. Tumor necrosis factor α blockade is associated with IFI across a range of host groups. A high index of suspicion in patients treated with TNF-α antagonists is recommended because the course of such infections can be serious or fulminant, and rapid access to health care should be provided. Surveillance of IFIs complicating TNF-α blockade and other biologic therapies is warranted through well-organized prospective patient registries.

Deconfinement and gluon plasma dynamics in improved holographic QCD.

Abstract: The thermodynamics of 5D dilaton gravity duals to confining gauge theories is analyzed. We show that they exhibit a first order Hawking-Page type phase transition. In the explicit background of improved holographic QCD of [U. Gursoy and E. Kiritsis, J. High Energy Phys. 02 (2008) 03210.1088/1126-6708/2008/02/032] [U. Gursoy, E. Kiritsis, and F. Nitti, J. High Energy Phys. 02 (2008) 01910.1088/1126-6708/2008/02/019], we find T_{c}=235 MeV. The temperature dependence of various thermodynamic quantities such as the pressure, entropy, and speed of sound is calculated. The results are in agreement with the corresponding lattice data.

Sleeve Gastrectomy—A “Food Limiting” Operation

Abstract: Background Sleeve gastrectomy (SG), which, thus far, is showing good resolution of comorbidities and good weight loss, shows increasing popularity among bariatric surgeons. The aim of this study was to evaluate clinical outcome and the gastric emptying of solid foods, 24 months after SG.

Mediterranean diet in pregnancy is protective for wheeze and atopy in childhood

Abstract: Introduction: Dietary intake of specific nutrients or food groups during pregnancy could play a role in the risk of asthma and atopy in offspring, but specific dietary patterns have not been implicated. We evaluated the impact of maternal (during pregnancy) and child adherence to the Mediterranean Diet on asthma and atopy in childhood. Methods: Women presenting for antenatal care at all general practices in Menorca, a Mediterranean island in Spain, over a 12-month period starting in mid-1997 were recruited. Four hundred and sixty children were included in the analysis after 6.5 years of follow-up. Maternal dietary intake during pregnancy and children’s dietary intake at age of 6.5 years were assessed by food frequency questionnaires and adherence to a Mediterranean Diet was evaluated through a priori defined scores. During follow-up, parents completed questionnaires on the child’s respiratory and allergic symptoms. Children underwent skin prick tests with 6 common aeroallergens. Results: The prevalence of persistent wheeze, atopic wheeze, and atopy at age 6.5 years were 13.2%, 5.8%, and 17.0% respectively. One third (36.1%) of the mothers had low quality of Mediterranean Diet during pregnancy according to the Mediterranean Diet Score, while the rest had a high score. A high Mediterranean Diet Score during pregnancy (in two levels, using "low" score as reference) was found to be protective for persistent wheeze (OR, 0.22; 95% CI, 0.08-0.58), atopic wheeze (OR, 0.30; 95% CI, 0.10-0.90), and atopy (OR, 0.55; 95% CI, 0.31-0.97) at age 6.5 years after adjusting for potential confounders. Childhood adherence to the Mediterranean Diet was negatively associated with persistent wheeze and atopy though the associations did not reach statistical significance. Conclusion: Our results support a protective effect of a high level of adherence to the Mediterranean Diet during pregnancy against asthma-like symptoms and atopy in childhood.

Improving hydrogen storage capacity of MOF by functionalization of the organic linker with lithium atoms.

Abstract: A combination of quantum and classical calculations have been performed in order to investigate hydrogen storage in metal-organic frameworks (MOFs) modified by lithium alkoxide groups. Ab initio calculations showed that the interaction energies between the hydrogen molecules and this functional group are up to three times larger compared with unmodified MOF. This trend was verified by grand canonical Monte Carlo (GCMC) simulations in various thermodynamic conditions. The gravimetric capacity of the Li-modified MOFs reached the value of 10 wt % at 77 K and 100 bar, while our results are very promising at room temperature, too, with 4.5 wt %.

Using singlet oxygen to synthesize polyoxygenated natural products from furans.

Abstract: Singlet oxygen is a powerful tool in the armament of the synthetic organic chemist and possibly in that of nature itself. In this Account, we illustrate a small selection of the many ways singlet oxygen can be harnessed in the laboratory to aid in the construction of the complex molecular motifs found in natural products. A more philosophical question is also addressed: namely, how much do singlet oxygen oxidations influence the biogenesis of these natural products? All the synthetic examples surveyed in this Account can be characterized as belonging to the same class because they all involve the oxidation of a substituted furan nucleus by singlet oxygen. Readily accessible and relatively simple furans can be transformed into a host of complex motifs present in a diverse range of natural products by the action of singlet-oxygen-mediated reaction sequences. These reactions are highly advantageous because they frequently deliver a rapid and dramatic increase in molecular complexity in high yield. Furthermor...

Particle Size Distribution of Airborne Microorganisms and Pathogens during an Intense African Dust Event in the Eastern Mediterranean

Abstract: In the last decade, the increase of desertification has resulted in a concomitant intensification of atmospheric dust loadings (Moulin and Chiapello 2006). Furthermore, El Nino events have coincided with increased flux of Saharan dust across the Atlantic (Prospero and Lamb 2003). Moulin et al. (1997) have estimated that dust flux from the Saharan-Sahel region to the atmosphere is approximately 1 billion tons/year. In addition to the effect of airborne dust on visibility and Earth’s climate through the processes of atmospheric radiation balance, photochemistry, and cloud formation, these dusts can also exert a direct impact on human health (Taylor 2002). The World Health Organization (WHO) has identified drought and dust storm activity in the sub-Saharan region of Africa as causing regional outbreaks of meningococcal meningitis (in 1996 there were ~ 250,000 cases and 25,000 deaths) (Griffin 2007; WHO 2003). Recently, dust events have been shown to introduce a significant pulse of microorganisms (Griffin 2007) and other microbiological materials (i.e., cellular fragments, fungal spores) into the atmosphere (Jaenicke 2005). However, to a large extent, the distribution of microorganisms in different particle sizes has been ignored, although it could have significant implications regarding the dispersion of microorganisms around the world. Study of the particle size distribution of microorganisms will enhance our knowledge a) of the species of microbes able to be transported over long distances and thus affect remote areas, and b) how climate change could increase the health risk from microbial pathogens. Traditionally, the detection and enumeration of airborne microorganisms has been conducted using light microscopy and/or culture-based methods. However, these analyses are time-consuming and laborious, lack sensitivity and specificity (Stetzenbach et al. 2004), and offer just a glimpse of the biological agents present (< 1% of environmental bacteria can be cultivated) (Amann et al. 1995; Hugenholtz et al. 1998; Pace 1997). The development of various techniques based on community molecular analysis has freed researchers from culturing biases and allowed characterization of community structure [e.g., 16S and 18S ribosomal RNA (rRNA) genes for bacteria and microeukaryotes, respectively] (Amann et al. 1995). In the present study, we used molecular-based methods to analyze the microbial components of bioaerosol samples collected from the atmosphere of a coastal city (Heraklion, Crete) of the eastern Mediterranean Sea during an intense African dust event. The study area is subject to frequent and severe Saharan dust events (Gerasopoulos et al. 2006). Air sampling was carried out during 24–25 February 2006 when large quantities of dust were exported from northern Africa to southern Europe (NASA 2006). The aim of the study was to investigate the microbial quality of size-distributed aerosol particles during a dust storm by using a high-volume pump equipped with a five-stage cascade impactor for efficient genomic DNA extraction (Radosevich et al. 2002). The composition of the airborne microorganisms was determined by cloning and sequencing the 16S rRNA genes. To the best of our knowledge, this is the first report of size-distributed airborne bacteria during a Saharan storm using molecular-based methods.

Holography and Thermodynamics of 5D Dilaton-gravity

Abstract: The asymptotically-logarithmically-AdS black-hole solutions of 5D dilaton gravity with a monotonic dilaton potential are analyzed in detail. Such theories are holographically very close to pure Yang-Mills theory in four dimensions. The existence and uniqueness of black-hole solutions is shown. It is also shown that a Hawking-Page transition exists at finite temperature if and only if the potential corresponds to a confining theory. The physics of the transition matches in detail with that of deconfinement of the Yang-Mills theory. The high-temperature phase asymptotes to a free gluon gas at high temperature matching the expected behavior from asymptotic freedom. The thermal gluon condensate is calculated and shown to be crucial for the existence of a non-trivial deconfining transition. The condensate of the topological charge is shown to vanish in the deconfined phase.

Genomic DNA hypomethylation as a biomarker for bladder cancer susceptibility in the Spanish Bladder Cancer Study: a case–control study

Abstract: Summary Background DNA hypomethylation has been suggested to cause genomic instability and increase cancer risk. We aimed to test the hypothesis that DNA hypomethylation is associated with increased risk of bladder cancer. Methods We measured cytosine methylation (5-mC) content in genomic DNA from blood cells from patients with bladder cancer enrolled in a large case–control study in Spain between Jan 1, 1998, and Dec 31, 2001. Cases were men and women with newly diagnosed and histologically confirmed urothelial carcinoma of the bladder. Controls were selected from patients admitted to the same hospital for diseases or conditions unrelated to smoking or other known risk factors for bladder cancer. Controls were individually matched to cases on age (within 5 years), sex, race, and area of hospital referral. 5-mC content was measured in leucocyte DNA by use of a combination of high-performance capillary electrophoresis, Hpa II digestion, and densitometry. Data on demographics, 34 polymorphisms in nine folate metabolism genes, and nutritional intake of six B vitamins (including folate), alcohol, and smoking were assessed as potential confounders. Relative 5-mC content was expressed as a percentage (%5-mC) with respect to the total cytosine content (the sum of methylated and non-methylated cytosines). The primary endpoint was median %5-mC DNA content. Findings %5-mC was measured in leucocyte DNA from 775 cases and 397 controls. Median %5-mC DNA was significantly lower in cases (3·03% [IQR 2·17–3·56]) than in controls (3·19% [2·46–3·68], p=0·0002). All participants were subsequently categorised into quartiles by %5-mC content in controls. When the highest quartile of %5-mC content was used as the reference category (Q4), the following adjusted odds ratios (OR) and 95% CI were recorded for decreasing methylation quartiles: OR (Q3) 2·05 (95% CI 1·37–3·06); OR (Q2) 1·62 (1·07–2·44); and OR (Q1) 2·67 (1·77–4·03), p for trend Interpretation For the first time, to our knowledge, we have shown in a large case–control study that leucocyte DNA hypomethylation is associated with increased risk of developing bladder cancer, and this association is independent of smoking and the other assessed risk factors. Amount of global methylation in genomic DNA could provide a useful biomarker of susceptibility to certain cancer types and further research is warranted. Funding Intramural Research Program of the National Institutes of Health, National Cancer Institute, Division of Cancer Epidemiology and Genetics, Bethesda, MD, USA, and Fondo de Investigacion Sanitaria, Spain (G03/174).

Convex approximation techniques for joint multiuser downlink beamforming and admission control

Abstract: Multiuser downlink beamforming under quality of service (QoS) constraints has attracted considerable interest in years, because it is particularly appealing from a network operator's perspective (e.g., UMTS, 802.16e). When there are many co-channel users and/or the service constraints are stringent, the problem becomes infeasible and some form of admission control is necessary. We advocate a cross-layer approach to joint multiuser transmit beamforming and admission control, aiming to maximize the number of users that can be served at their desired QoS. It is shown that the core problem is NP-hard, yet amenable to convex approximation tools. Two computationally efficient convex approximation algorithms are proposed: one is based on semidefinite relaxation of an equivalent problem reformulation; the other takes a penalized second-order cone approach. Their performance is assessed in a range of experiments, using both simulated and measured channel data. In all experiments considered, the proposed algorithms work remarkably well in terms of the attained performance-complexity trade-off, consistently exhibiting close to optimal performance at an affordable computational complexity.

A GaAs polariton light-emitting diode operating near room temperature

Abstract: The increasing ability to control light-matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons-such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity-are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.