Channel Transfer Research Articles

Doppler Shift Characterization of Wideband Mobile Radio Channels

The prevailing approach for characterizing the Doppler shift (DS) of mobile radio channels assumes the transmission of an unmodulated carrier. This consideration is valid for the analysis of narrowband channels, but its pertinence is questionable in regards to the modeling of wideband channels. In this correspondence, we redefine the DS from a time-frequency analysis perspective that does not depend on the aforementioned assumption. We systematically demonstrate that the DS can be characterized by the instantaneous frequency of the channel transfer function. This generic definition makes evident a fundamental aspect of the DS that is seldom acknowledged, namely, the DS is a frequency-varying quantity. We show that the second-order statistics of wideband mobile radio channels are non-stationary due to the DS's frequency variations. In addition, we present numerical results of a case study showing that such non-stationarities can cause significant system performance degradations.

MIMO-OFDM NB-PLC Designs in Underground Medium-Voltage Networks

In this paper, we investigate the application of multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) to narrowband powerline communications (NB-PLC) over medium-voltage (MV) underground (UG) networks. We study different MIMO transmission scenarios with different injection configurations utilizing both the cable conductor and sheath phases. Multiconductor transmission line theory is used to accurately characterize the UG MV NB-PLC channel transfer function and evaluate the spatial correlation between the different conductor phases. The achievable data rates are evaluated for all configurations at different line lengths after optimizing the transmit energy allocation across the different spatial information streams subject to a transmit power spectral density constraint. The achievable data rates for MIMO configurations are shown to be significantly higher (by up to three times higher) compared to single-input single-output OFDM transmission. Moreover, we investigate the effects of practical design constraints, such as cyclic prefix length and bit caps, on the achievable data rates. Finally, we show how to reduce the complexity of both the transmitter and receiver implementations while minimizing the data rate loss.

Channel Correlation-Based Approach for Feedback Overhead Reduction in Massive MIMO

For frequency-division duplex multiple-input–multiple-output (MIMO) systems, the channel state information at the transmitter is usually obtained by sending pilots or reference signals from all elements of the antenna array. The channel is then estimated by the receiver and communicated back to the transmitter. However, for massive MIMO, this periodical estimation of the full transfer matrix can lead to prohibitive overhead. To reduce the amount of data, we propose to estimate the updated channel matrix from the knowledge of the full correlation matrix at the transmitter made during some initialization time and the instantaneous measured channel matrix of smaller size, characterizing the link between the user and a limited number of reference array elements. The proposed algorithm is validated with measured massive MIMO channel transfer functions at 3.5 GHz between a $9 \times 9$ uniform rectangular array and different user positions. Since measurements were made in static conditions, the criteria chosen for evaluating the performance of the algorithm are based on a comparison of the predicted channel capacity calculated from either the measured or estimated channel matrix.

Thermo-Hydraulic Performance of a Rectangular Duct With Staggered Inclined Discrete Rib Arrangement

Artificial roughness in the form of ribs is a beneficial strategy to improve the thermal performance of solar air heaters (SAHs). In the present research work, experimental examinations have been conducted on heat transfer and friction characteristics in the rectangular channel, which is roughened through the inclined discrete ribs. The inclined ribs were discretized by creating gaps at the different positions (not inline) on trailing and leading edges in consecutive ribs The rib roughness has relative roughness pitch as 8.0, rib combination of relative gap position is varied from 0.3 & 0.1 to 0.3 & 0.4, and mass flow rate varies between 3000 -14,000 and rib gap width as 1. The higher improvement in the Nusselt number and factor of friction coefficient is obtained to be 2.92 and 3.33 times respectively, as compared with that of the smooth duct. The higher thermo-hydraulic performance parameter (THPP) is obtained for the combination of relative gap position of 0.3 & 0.3.

Impacts of flow channel geometry, hydrodynamic and membrane properties on osmotic backwash of RO membranes—CFD modeling and simulation

The presence of the spacer and biofilm can accompany the high complexity of the mass and momentum transport in an osmotic backwash (OBW) process of reverse osmosis (RO) membranes. For more reliable simulations, it would be essential to consider such highly complex transport phenomena across the membranes and through membrane flow channels. Therefore, we simulated the effects of spacer designs, hydrodynamic, and membrane properties on OBW in the scenario of biofouling on RO membrane surfaces. To this end, the OBW process was numerically modeled in two-dimensional spacer-filled crossflow channels based on the finite element method. Subsequently, the concentration and velocity fields inside of the membrane, biofilm, and flow channels were analyzed. Results showed that the submerged spacer could enhance the water and salt mass transfer in the membrane channel, to a greater extent than the cavity and zigzag spacers, which can ultimately enhance the OBW efficiency for a foulant removal. Additionally, the OBW performance is chiefly impacted by the structure parameter of the porous support layer. Also, the existence of biofilm in the feed channel can render the feed concentration and membrane water permeability more sensitive to the OBW process than the non-fouled channel.

Epitaxial Lift-Off Technology for Large Size III–V-on-Insulator Substrate

III-V materials can be a very good channel candidate for monolithic 3D (M3D) integration due to the potential of high-performance and lower process temperature as compared with Si, since a low process temperature is crucial to avoid degradation of bottom devices. For III-V M3D integration, material transfer techniques are important, and such processes should be enabled by low process costs on a large wafer scale. In this study, we propose an InGaAs channel transfer technique by wafer bonding, epitaxial lift-off and III-V layers grown on Si substrate. Effective mobility of fabricated MOS HEMTs using transferred InGaAs layer was 1.3 times higher than that of Si MOSFETs. The proposing channel transfer technique would be useful for M3D integration because it provides wafer scalability, cost-effectiveness, back-gate biasing, and etc.

Indoor Localization for IoT Using Adaptive Feature Selection: A Cascaded Machine Learning Approach

Evolving Internet-of-Things (IoT) applications often require the use of sensor-based indoor tracking and positioning, for which the performance is significantly improved by identifying the type of the surrounding indoor environment. This identification is of high importance since it leads to higher localization accuracy. This paper presents a novel method based on a cascaded two-stage machine learning approach for highly-accurate and robust localization in indoor environments using adaptive selection and combination of RF features. In the proposed method, machine learning is first used to identify the type of the surrounding indoor environment. Then, in the second stage, machine learning is employed to identify the most appropriate selection and combination of RF features that yield the highest localization accuracy. Analysis is based on k-Nearest Neighbor (k-NN) machine learning algorithm applied on a real dataset generated from practical measurements of the RF signal in realistic indoor environments. Received Signal Strength, Channel Transfer Function, and Frequency Coherence Function are the primary RF features being explored and combined. Numerical investigations demonstrate that prediction based on the concatenation of primary RF features enhanced significantly as the localization accuracy improved by at least 50% to more than 70%.

Antenna Correlation Under Geometry-Based Stochastic Channel Models

Antenna correlation is an important measure for designing multiple-input–multiple-output (MIMO) antenna systems. A lower antenna correlation indicates a better MIMO performance that can be achieved with the underlying antenna systems. In the antenna design community, it is very common to evaluate the antenna correlation with isotropic or nonisotropic (e.g., Gaussian-distributed) angular power spectrum (APS) as baselines. On the other hand, antenna correlation can also be evaluated via channel transfer function (CTF) under the given propagation channel, e.g., drawn from the bidirectional geometry-based stochastic channel model. In this letter, the analytic forms for the antenna correlation based on the APS and the CTF are derived, respectively, with their similarities and differences explained. Moreover, a numerical example is also given with a standard channel model to support our findings.

An experimental investigation on the heat transfer and friction coefficients of a small plate heat exchanger with chevron angle

In the present study, a small brazed and commercial plate heat exchanger (PHE) with chevron angle 60° and symmetrical layout, was experimentally investigated. New correlations for calculating heat transfer and friction coefficients of hot flow channels were presented. Few studies have been done on the small PHEs and due to the industrial needs, it is necessary to conduct new researches in this field. For this purpose, an experimental model was designed to calculate the single-phase water flow, heat transfer, and friction coefficients. The previously presented correlations were not successful in predicting the mentioned coefficients for the small PHEs and showed a 34% difference with the results of this study. Chevron grooves on the PHE plate can enhance the heat transfer as well as increasing the pressure drop. Defining $$ \left(\frac{j}{f}\right) $$ factor, it was revealed that with the increase of Reynolds number, heat transfer increased more than friction. In previous studies, with increasing Reynolds, $$ \left(\frac{j}{f}\right) $$ factor is almost constant in large PHEs but the results of the present study showed that this increase may be 5-fold higher in the small PHEs as compared with the larger ones. As there was no temperature and heat transfer coefficient value in the hot flow channels of PHE, the Nusselt number was calculated by the modified Wilson plot method.

Parametric study and optimization on heat transfer and flow characteristics in a rectangular channel with longitudinal vortex generators

In this study, effects of various geometrical parameters of a new winglet longitudinal vortex generator (LVG) on heat transfer and flow characteristics in a rectangular channel were investigated by a numerical method, and the influence of various parameters was analyzed. A comparative study of effects of elliptical pole parameters, the thickness, the length, the height, the attack angle, the transverse pitch and the longitudinal pitch of LVGs on heat transfer and pressure loss performance was conducted. The results showed that the intensity of heat transfer could be greatly increased by the increase of the length, the height, the attack angle and the transverse pitch of LVGs, accompanied with an increase of pressure drop. The Nusselt number decreased by increasing the longitudinal pitch of LVGs. The short axis and major axis of the elliptical poles and the thickness of LVGs had a small influence on the average heat transfer coefficient and average friction factor at the present condition. The design parameters of this configuration were optimized by the Taguchi method. Sixteen kinds of models were made by compounding levels on each factor, and heat transfer and flow characteristics of each model were analyzed. The results allowed us to quantitatively estimate the various parameters affecting heat transfer performance, and the main factors for optimal design of a rectangular channel with LVGs were selected. The optimal condition was also acquired by two analytical results.

Inexpensive Computations Using Computational Fluid Dynamics Combined With Asymptotics Applied to Laminar Mixed Convection in a Vertical Channel

Abstract In this work, a solution technique is proposed by synergistically combining asymptotics and computational fluid dynamics to ascertain a problem of laminar mixed convection heat transfer in a vertical channel. First, numerical simulation is carried out on a vertical channel that consists of an aluminum heater plate assembly at the center of the channel. The numerical model is treated as a conjugate heat transfer problem, and the concept of perturbation and blending is incorporated wherein the limiting solution of natural and forced convection is obtained in terms of average Nusselt number. These correlations are then blended to find a unified composite correlation that work very well for extreme limits of mixed convection. The Richardson number is chosen as an independent variable in the present analysis; as a result, the Nusselt number correlation is cogent for the mixed convection region. Upon performing the numerical simulations, the results of the mixed convection are then compared with experimental results available in the literature for the purpose of validation of the numerical solution. The results of the present work emphasize that, with minimum computational fluid dynamics (CFD) solutions, one can obtain a reasonably good composite correlation for the Nusselt number for mixed convection and also a substantial reduction of computations is possible ensuing an asymptotically flawless solution.

Natural Convection Heat and Mass Transfer in the Vertical Cylindrical Porous Channel Under the Effects of Time-Periodic Boundary Condition

Abstract Heat and mass transfer are investigated numerically with steady-state laminar natural convection through a vertical cylindrical enclosure filled with a liquid-saturated porous medium. The vertical wall is under a constant magnetic field and various durations of periodic heating boundary condition; the top and bottom surfaces are kept at a constant cold temperature. Continuity, momentum, and energy equations are transformed to dimensionless equations. The finite difference approach with the line successive over-relaxation (LSOR) method is used to obtain the computational results. This study covers the heat transfer, the temperature distribution, and the velocity field in the domain under the variation of different parameters. The code used is validated by modifying it to analyze the Nusselt number in the existing experimental literature of Izadpanah et al. (1998, “Experimental and Theoretical Studies of Convective Heat Transfer in a Cylindrical Porous Medium,” Int. J. Heat Fluid Flow, 19(6), pp. 629–635). This work shows that Nusselt number decreases (with varying gradient) as the aspect ratio increases, and that it increases as the Rayleigh number increases. The centerline temperature has a proportional relationship with the heating amplitude and the heating period (as the system receives more heat) and is inversely proportional with Rayleigh number. Increasing the Rayleigh number causes increased convective velocity, which affects the position of the hot region, and causes a decrease in the temperature field. Increasing the aspect ratio results in a warm stream at the center of the cylinder, and when the time period of the heating increases, the circulation becomes faster and the intensity of the temperature contour layers decreases. In this work, a correlation for Nu as a function of the mentioned parameters is developed.

Performance enhancement in a proton exchange membrane fuel cell with a novel 3D flow field

Flow field plays a vital role in the design and application of a proton exchange membrane fuel cell (PEMFC). Combined with the enhancement of the abilities of mass transfer and water removal in the flow channel, a new optimized three-dimensional (3D) flow field is proposed to investigate the water transport and cell operating characteristics. The 3D flow field is composed of several straight flow units arranged in a staggered manner with an inclination at the end, and the transition areas and subchannels between the flow units are subjected to hydrophilic treatment. The result indicates that the new type of flow field can effectively separate liquid water from the reactant flow and water can be partially removed through the subchannel. Compared with the conventional flow field, the 3D flow field could enhance the mass transfer ability and improve the PEMFC performance, especially at high current densities. Based on the field synergy principle, we prove that the synergic degree between the velocity vector and concentration gradient agrees with the performance changes under the turbulence of the staggered flow units and the effective mass transfer coefficient (EMTC) in the direction of electrochemical reaction of the 3D flow field is also enhanced.

EFFECT OF ARTIFICIAL CAVITIES ON HEAT TRANSFER AND FLOW CHARACTERISTICS MICROCHANNEL

An experimental investigation was conducted to study single-phase fluid flow and heat transfer in a copper micro channel. To investigate the effect of artificial cavities on fluid flow and single phase heat transfer in micro channel heat sink, two model of straight micro channel recognized as two models (model -1and model -2) were designed and manufactured ,where model-1 have smooth bottom surface while Model-2 have 47 artificial cavities distributed uniformly at the bottom surface along the micro channel length. The two models having the same nominal dimension of 300?m height and 300?m depth while the real dimension value are 367 ?m for width and 296 ?m for depth .De-ionized water was used as the working fluids. Experimental test was conducted at 30?C inlet temperature with Reynolds numbers range from 700 to 2200 covering laminar flow conditions. The experiments were conducted with horizontal micro channel under both adiabatic (for friction factor calculation) and diabatic (for Nusselt number calculation) conditions. The results indicated that the experimental Darcy friction factor can be predicted well with conventional scale fanning friction factor correlations for developing flow in laminar region by shah and London (1978) correlation for two models. Also, the experimental Nusselt number Agree well with each correlation of shah and London (1978) and Mirmanto correlation in laminar region.

Numerical investigation of heat transfer of laminar and turbulent pulsating Al2O3/water nanofluid flow

PurposeThe purpose of this paper is to investigate the heat transfer of laminar and turbulent pulsating Al203/water nanofluid flow in a two-dimensional channel. In the laminar flow range, with increasing Reynolds number (Re), the velocity gradient is increased. Also, the Nusselt number (Nu) is increased, which causes increase in the overall heat transfer rate. Additionally, in the change of flow regime from laminar to turbulent, average thermal flux and pulsation range are increased. Also, the effect of different percentage of Al2O3/water nanofluid is investigated. The results show that the addition of nanofluids improve thermal performance in channel, but the using of nanofluid causes a pressure drop in the channel.Design/methodology/approachThe pulsatile flow and heat transfer in a two-dimensional channel were investigated.FindingsThe numerical results show that the Al2O3/Water nanofluid has a significant effect on the thermal properties of the different flows (laminar and turbulent) and the average thermal flux and pulsation ranges are increased in the change of flow regime from laminar to turbulent. Also, the addition of nanofluid improves thermal performance in channels.Originality/valueThe originality of this work lies in proposing a numerical analysis of heat transfer of pulsating Al2O3/Water nanofluid flow -with different percentages- in the two-dimensional channel while the flow regime change from laminar to turbulent.

NT Vortex enhancement of heat transfer and flow in the narrow channel with a dense packing of inclined one-row oval-trench dimples

In the present study it is found that enhancement of air turbulent flow and heat transfer at Re = 104 is influenced by a set of one-row oval-trench dimples of depth 0.25 inclined at an angle of 65° to the incoming flow and located at the heated wall of the rectangular (9 × 1) narrow channel when the dimple step H is varied. The periodic section of the dimpled channel 8 in length is considered. The dimple step H is varied from 8 to 2. Abnormal enhancement of flow and heat transfer at H = 2 is followed by both a 4-fold decrease in relative negative friction and a 6.5-fold increase in relative heat transfer in the separated flow zone. A maximum secondary flow velocity in the dimple exceeds by 10% a maximum flow velocity in the plane-parallel channel, reaching 1.27 of bulk flow velocity in the channel. A reason for this phenomenon is associated with forming a very large (up to 1.2) pressure drop between closely spaced zones of high pressure on the windward side of a dimple and of low pressure on its entrance spherical portion. The phenomenon of turbulent flow acceleration with a 1.39-fold increase in the maximum flow velocity (at H = 2) in the dimpled narrow channel in comparison to the plane-parallel channel is discovered.

Effects of Helical Tube Electrode Structure on Mixed Machining Product Transfer in Micro-Machining Channel during Tube Electrode High-Speed Electrochemical Discharge Machining.

In tube electrode high-speed electrochemical discharge machining (TSECDM), mixed products are constantly produced in the narrow machining gap by simultaneous discharge erosion and electrochemical dissolution. For the high-efficiency removal of these products from the machining gap, a tool electrode with an optimized helical structure was utilized in TSECDM in this study. Firstly, the concentration distributions of the processed products in the machining gap using tube electrode tools with three typical helical structures were studied through the simulation of solid–liquid coupling; this showed that a trapezoidal helical structure benefited the reduced accumulation of products in grooves and the effective removal of products from the machining gap. Secondly, the main geometric parameters of the trapezoidal helical structure, including the helical groove depth, pitch, and tooth angle, were optimized by gap flow-field simulation to enhance the removal effect on processed products. Finally, it was verified that the trapezoidal helical electrode showed a definitive and significant advantage over the ordinary cylindrical electrode in effectively removing processed products from the machining gap by the comparison of flow-field simulations and actual machining experiments.

Bismuth selenide nanocrystalline array electrodes for high-performance sodium-ion batteries

This work, for the first time, reports on the Bi2Se3 nanocrystal array wrapped by amorphous carbon layer as anode for high-performance sodium-ion batteries. In addition to a remarkable lithium storage capability, the Bi2Se3 nanocrystal array assembling onto flexible carbon cloth exhibits an outstanding cyclical stability for sodium ion batteries, which delivers a discharge capacity of 652.1 mAh g−1 at 100 mA g−1 (∼ a rate of 0.1 C) after 100 cycles. Even at 1 A g−1, a reversible capacity of 210.2 mAh g−1 can be maintained undergoing 200 charge-discharge cycles. The excellent cycle and rate capabilities of Bi2Se3 nanocrystal array could be attributed to the stable structure, fast ion migration channel and low charge transfer resistance for electrochemical processes. Detailed investigation reveals the appreciated reversibility of stepwise conversion and alloying reaction of Bi2Se3 nanocrystal array with high Coulombic efficiency and reversible capacity.

Calibration of Stochastic Radio Propagation Models Using Machine Learning

This letter proposes a machine learning-based method for the calibration of stochastic radio propagation models. Model calibration is cast as a regression problem involving mapping of the channel transfer function or impulse response to the model parameters. A multilayer perceptron is trained with summary statistics computed from synthetically generated channel realizations using the model. To calibrate the model, the trained network is used to estimate the model parameters from channel statistics obtained from measurements. The performance of the proposed method is evaluated with propagation graph and Saleh–Valenzuela models using both simulated data and in-room channel measurements. Results show accurate estimation of the parameters of both models.

Numerical simulation of turbulent flow and forced heat transfer of water/CuO nanofluid inside a horizontal dimpled fin

In the current study, turbulent flow and heat transfer in a three-dimensional channel is simulated numerically. The main purpose of this study is investigating the effect of using dimpled fin and CuO nanoparticles on the heat transfer enhancement and the improvement in cooling performance in a channel under the heat flux (100 kW m−2). Pure water flow is simulated at Reynolds numbers of 5000, 10,000, 15,000 and 20,000 with nanoparticles volume fractions 2 and 4%, and the effect of dimpled fins is investigated. To ensure the accuracy of the numerical solving procedure, the obtained results are compared with other experimental results and acceptable coincidence is observed. The results show that dimpled fins cause the improvement in heat transfer and using nanoparticles inside a dimpled fin causes better cooling performance than using pure water flow in a smooth channel. According to the obtained results at lower Reynolds numbers, the dimpled fin has more efficient performance in heat transfer enhancement than higher Reynolds numbers. Hence, using dimpled fin at lower Reynolds numbers flow is recommended.