475 research outputs found
Performance evaluation of electromagnetically large antenna arrays for 6G communications
No full textModern multiple-input multiple-output (MIMO)
communication systems are almost exclusively designed under the
assumption of locally plane wavefronts over antenna arrays. This
is known as the far-field approximation and is soundly justified
at sub-6-GHz frequencies at most relevant transmission ranges.
However, when higher frequencies and shorter transmission
ranges are used, the wave curvature over the array is no longer
negligible, and arrays operate in the so-called radiative near-field
region. This work aims at showing that the classical far-field
approximation may significantly underestimate the achievable
spectral efficiency of multi-user MIMO communications operating
in the 30-GHz bands and above, even at ranges beyond the
Fraunhofer distance. For planar arrays with typical sizes, we
show that computing combining schemes based on the far-field
model significantly reduces the channel gain and spatial multiplexing
capability. When the radiative near-field model is used,
interference rejection schemes, such as the optimal minimum
mean-square-error combiner, appear to be very promising, when
combined with electrically large arrays, to meet the stringent
requirements of next-generation networks
Spherical Wavefronts Improve MU-MIMO Spectral Efficiency When Using Electrically Large Arrays
No full textModern MIMO communication systems are almost exclusively designed assuming locally plane wavefronts over antenna arrays. This is known as the far-field approximation and is soundly justified at sub-6-GHz frequencies at most relevant transmission ranges. However, when higher frequencies and shorter transmission ranges are used, the wave curvature over the array is no longer negligible, and arrays operate in the so-called radiative near-field region. This letter aims to show that the classical far-field approximation may significantly underestimate the achievable spectral efficiency of multi-user MIMO communications operating in the 30-GHz bands and above, even at ranges beyond the Fraunhofer distance. For planar arrays with typical sizes, we show that computing combining schemes based on the far-field model significantly reduces channel gain and spatial multiplexing capability. When the radiative near-field model is used, interference rejection schemes, such as the optimal minimum mean-square-error combiner, appear to be very promising, when combined with electrically large arrays, to meet the requirements of next-generation networks
MMSE Channel Estimation in Large-Scale MIMO: Improved Robustness with Reduced Complexity
No full textLarge-scale MIMO systems with a massive number N of individually controlled antennas pose significant challenges for minimum mean square error (MMSE) channel estimation, based on uplink pilots. The major ones arise from the computational complexity, which scales with N 3 , and from the need for accurate knowledge of the channel statistics. This paper aims to address both challenges by introducing reduced-complexity channel estimation methods that achieve the performance of MMSE in terms of estimation accuracy and uplink spectral efficiency while demonstrating improved robustness in practical scenarios where channel statistics must be estimated. This is achieved by exploiting the inherent structure of the spatial correlation matrix induced by the array geometry. Specifically, we use a Kronecker decomposition for uniform planar arrays and a well-suited circulant approximation for uniform linear arrays. By doing so, a significantly lower computational complexity is achieved, scaling as N √ N and N log N for squared planar arrays and linear arrays, respectively
Distributed Energy-Efficient Power and Subcarrier Allocation for OFDMA-Based Small Cells
No full textIn this work, we derive a distributed resource allocation scheme for the uplink of an OFDMA-based small- cell network. The mobile terminals are modeled as utility-driven rational agents that aim at maximizing the number of bits correctly delivered at destination per unit of energy consumed, under minimum-rate constraints. The theoretical analysis of the underlying game equilibrium is exploited to derive an iterative and distributed algorithm that allows each terminal to select its optimal power allocation over subcarriers. Extensive simulations show that the proposed technique is able to properly allocate the resources across the network in a scalable and adaptive manner, while improving the performance of each user in terms of energy efficiency compared to an iterative waterfilling criterion
Energy-efficient power control for drone communications
No full textThis work aims at developing an energy-efficient power control algorithm for a swarm of drones that simultaneously transmit data to the serving base station while moving around a given coverage area. A stochastic mobility model based on random walk is developed, which guarantees boundedness and continuity of the movement, and used in a non-cooperative stochastic differential game, with utility functions defined by an Hamilton-Jacobi-Bellman (HJB) equation for each drone. We analyze the existence and uniqueness of the equilibrium point, and develop a distributed algorithm whose convergence to such equilibrium point is guaranteed. Numerical results are used to validate the performance of the proposed solution
DFT-based channel estimation for holographic MIMO
No full textHolographic MIMO (hMIMO) systems with a massive number N of individually controlled antennas make minimum mean square error (MMSE) channel estimation particularly challenging, due to its computational complexity that scales as N 3 . This paper investigates uniform linear arrays and proposes a low-complexity method based on the discrete Fourier transform approximation, which follows from replacing the covariance matrix by a suitable circulant matrix. Numerical results show that, already for arrays with moderate size (in the order of tens of wavelengths), it achieves the same performance of the optimal MMSE, but with a significant lower computational load that scales as N log N. Interestingly, the proposed method provides also increased robustness in case of imperfect knowledge of the covariance matrix
PROBLEMI ORGANIZZATIVI DEL PRONTO SOCCORSO IN UN OSPEDALE DI MEDIE DIMENSIONI IN MERITO ALLA PATOLOGIA PROVOCATA
No full textFlapless Dental Implant Surgery in Bleeding Disorders
No full textHemostasis disorders require particular attention in dental treatment. Dental implants are a very widespread and valid method for total rehabilitation. Flapless dental implant surgery is a minimally invasive treatment that allows the implants to be placed in the jaw bones with minimal surgical trauma. The aim of this study is to report the bleeding incidence in a group of patients with bleeding disorders treated with flapless implants. A total of 52 patients with bleeding disorders (46 in anticoagulant therapy; 4 with hemophilia; 2 with von Willebrandt disease) were treated with 188 flapless implant surgeries Anticoagulants were not discontinued. Patients with hemophilia and VWD were treated following specific protocols. Four late, easy to treat bleeding complications were reported (three mild bleeding, one ecchymosis). No additional sutures or other hemostatic measures were taken, no further infusions or transfusions were reported, and no severe bleeding complications requiring more than easy on-chair treatment, were reported. In conclusion, with adequate knowledge of the procedure and the pathology, dental implantology can be safely performed in patients with bleeding disorders
Valutazione dell'efficacia dell'apicectomia eseguita con il microscopio: Studio retrospettivo
No full textOBJECTIVES Apical post-treatment pathology can jeopardize the success of endodontic treatment. Various alternatives must be taken into account to promote periapical healing: orthograde retreatment, which is considered the treatment of choice, surgical endodontic retreatment and tooth extraction, in case of failure of the previous treatments. The aim of the present study is to evaluate the efficacy of periapical surgery under operating microscope, at two years follow up time. MATERIALS AND METHODS A retrospective cross-sectional study was conducted on 42 teeth from 36 patients. All surgical procedures were performed using an operating microscope. The criteria to state success or failure were both clinical (swelling, pain, tooth tender to palpation) and radiographical (healing and ossification) and all the data have been obtained by consulting clinical folders. Sixteen teeth (drop out rate of 38%) were lost during the follow- up period. RESULTS At the 2-year follow-up examination, 25 tee..
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