Massive Systems Research Articles

Efficient Federated Recommender System with Adaptive Model Pruning and Momentum-based Batch Adjustment

With the development of 5G communication and smart devices, the prosperity of online content has boosted the research of Recommender System (RS). Due to the data scarcity problem, researchers employ knowledge transfer techniques to improve the accuracy of RS. Data sharing or data augmentation are promising methods for such a problem, but the data suffers from privacy leakage during sharing. Thus, Federated Learning (FL) has been adopted to collaboratively train recommender models while preserving data privacy. Federated Recommender System (FRS) combines FL and RS to provide distributed recommendation services to the users. However, the existing FRS suffers from massive communication and system heterogeneity, where the necessity of model transmission and the diversity of the clients bring significant communication overhead to the system. In this paper, we propose Efficient Federated Recommender System with Adaptive Model Pruning and Momentum-based Batch Adjustment ( eFRSA 2 ) to reduce the communication overhead of FRS. eFRSA 2 contains two modules. Adaptive Model Pruning utilizes magnitude pruning to reduce the communication volume and adaptively modifies the compression ratios of different clients to maintain the model accuracy. Momentum-based Batch Adjustment adjusts the local training batch number by a similar method of gradient descent with momentum to align the local computation time of the clients and reduce the communication overhead. The experimental results demonstrate that eFRSA 2 can reduce up to 90% communication volume and mitigate the system heterogeneity by over 75%, demonstrating the priority of eFRSA 2 in training efficiency. Source code can be found at https://github.com/shhjwu5/eFRSA2.

AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system

With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT\,2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT\,2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every sim 60-90 days with amplitudes of sim 0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT\,2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of sim 0.83 mpc and would merge in sim 7times 10$^4$ years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.

Investigation of the nature of the wind interaction in HD 93205 based on multi-epoch X-ray observations

The study of the X-ray emission from massive binaries constitutes a relevant approach to investigate shock physics. The case of short period binaries may turn out to be quite challenging, especially in very asymmetric systems where the primary wind may overwhelm that of the secondary in the wind interaction. Our objective consists in providing an observational diagnostic of the X-ray behavior of HD\,93205, which is a very good candidate with which to investigate these aspects. We analyzed 31 epochs of XMM-Newton X-ray data spanning about two decades to investigate its spectral and timing behavior. The X-ray spectrum is very soft along the full orbit, with a luminosity exclusively from the wind interaction region in the range of 2.3 -- 5.4\,times $. The light curve peaks close to periastron, with a rather wide pre-periastron low state coincident with the secondary's body hiding a part of the X-ray emitting region close to its surface. We determined a variability timescale of 6.0807\,pm \,0.0013\,d, in full agreement with the orbital period. Making use of a one-dimensional approach to deal with mutual radiative effects, our results point to a very likely hybrid wind interaction, with a wind photosphere occurring along most of the orbit, while a brief episode of wind-wind interaction may still develop close to apastron. Besides mutual radiative effects, the radiative nature of the shock that leads to some additional pre-shock obliquity of the primary wind flow certainly explains the very soft emission. HD\,93205 constitutes a relevant target to investigate shock physics in short period, asymmetric massive binary systems, where various mutual radiative effects and radiative shocks concur to display an instructive soft X-ray behavior. HD\,93205 should be considered as a valid, though challenging target for future three-dimensional modeling initiatives.

Input / Output Relationships for the Primary Hippocampal Circuit.

The hippocampus is the most studied brain region but little is known about signal throughput -- the simplest, yet most essential of circuit operations -- across its multiple stages from perforant path input to CA1 output. Using hippocampal slices derived from male mice, we have found that single-pulse lateral perforant path (LPP) stimulation produces a two-part CA1 response generated by LPP projections to CA3 ('direct path') and the dentate gyrus ('indirect path'). The latter, indirect path was far more potent in driving CA1 but did so only after a lengthy delay. Rather than operating as expected from the much discussed trisynaptic circuit argument, the indirect path used the massive CA3 recurrent collateral system to trigger a high frequency sequence of fEPSPs and spikes. The latter events promoted reliable signal transfer to CA1 but the mobilization time for the stereotyped, CA3 response resulted in surprisingly slow throughput. The circuit transmitted theta (5Hz) but not gamma (50Hz) frequency input, thus acting as a low-pass filter. It reliably transmitted short bursts of gamma input separated by the period of theta wave - CA1 spiking output under these conditions closely resembled the input signal. In all, the primary hippocampal circuit does not behave as a linear, three-part system but instead uses novel filtering and amplification steps to shape throughput and restrict effective input to select patterns. We suggest that the operations described here constitute a default mode for processing cortical inputs with other types of functions being enabled by projections from outside the extended hippocampus.Significance statement Despite intense interest in hippocampal contributions to behavior, surprisingly little is known about how signals are processed across the network linking cortical input to CA1 output. Here, we describe the first input/output relationship for the system with results challenging the traditional tri-synaptic circuit concept. Signal throughput requires mobilization of recurrent activity within CA3 to amplify sparse input from the dentate gyrus into an unexpectedly stereotyped composite response. Potent low-pass filters determine effective input patterns. These results open the way to new analyses of how variables such as aging affect hippocampus and its contributions to behavior while providing material needed for biologically realistic models of the structure.

AoD Based Joint User-Antenna Scheduling Schemes with Limited Feedback for Multi-User Massive MIMO FDD Systems

AoD Based Joint User-Antenna Scheduling Schemes with Limited Feedback for Multi-User Massive MIMO FDD Systems

The Mass and Redshift Dependence of Halo Star Clustering

We adopt the two-point correlation function (2PCF) as a statistical tool to quantify the spatial clustering of halo stars for galaxy systems spanning a wide range in host halo virial mass ( 11.25<log10M200c/M⊙<15 ) and redshifts (0 < z < 1.5) from the IllustrisTNG simulations. Consistent with a previous study, we identify clear correlations between the strength of the 2PCF signals and galaxy formation redshifts, but over a much wider mass range. We find that such correlations are slightly stronger at higher redshifts and get weakened with the increase of host halo mass. We demonstrate that the spatial clustering of halo stars is affected by two factors: (1) the clustering gets gradually weakened as time passes (phase mixing); (2) newly accreted stars at more recent times would increase the clustering. For more massive galaxy systems, they assemble late, and the newly accreted stars would increase the clustering. The late assembly of massive systems may also help to explain the weaker correlations between the 2PCF signals and the galaxy formation redshifts in massive halos, as their 2PCFs are affected more by recently accreted stars, while formation redshift characterizes mass accretion on a much longer timescale. We find that the orbits of satellite galaxies in more massive halos maintain larger radial anisotropy, reflecting the more active accretion state of their hosts while also contributing to their stronger mass loss rates.

Overview of Tensor-Based Cooperative MIMO Communication Systems—Part 2: Semi-Blind Receivers

Cooperative MIMO communication systems play an important role in the development of future sixth-generation (6G) wireless systems incorporating new technologies such as massive MIMO relay systems, dual-polarized antenna arrays, millimeter-wave communications, and, more recently, communications assisted using intelligent reflecting surfaces (IRSs), and unmanned aerial vehicles (UAVs). In a companion paper, we provided an overview of cooperative communication systems from a tensor modeling perspective. The objective of the present paper is to provide a comprehensive tutorial on semi-blind receivers for MIMO one-way two-hop relay systems, allowing the joint estimation of transmitted symbols and individual communication channels with only a few pilot symbols. After a reminder of some tensor prerequisites, we present an overview of tensor models, with a detailed, unified, and original description of two classes of tensor decomposition frequently used in the design of relay systems, namely nested CPD/PARAFAC and nested Tucker decomposition (TD). Some new variants of nested models are introduced. Uniqueness and identifiability conditions, depending on the algorithm used to estimate the parameters of these models, are established. Two families of algorithms are presented: iterative algorithms based on alternating least squares (ALS) and closed-form solutions using Khatri–Rao and Kronecker factorization methods, which consist of SVD-based rank-one matrix or tensor approximations. In a second part of the paper, the overview of cooperative communication systems is completed before presenting several two-hop relay systems using different codings and configurations in terms of relaying protocol (AF/DF) and channel modeling. The aim of this presentation is firstly to show how these choices lead to different nested tensor models for the signals received at destination. Then, by capitalizing on these models and their correspondence with the generic models studied in the first part, we derive semi-blind receivers to jointly estimate the transmitted symbols and the individual communication channels for each relay system considered. In a third part, extensive Monte Carlo simulation results are presented to compare the performance of relay systems and associated semi-blind receivers in terms of the symbol error rate (SER) and channel estimate normalized mean-square error (NMSE). Their computation time is also compared. Finally, some perspectives are drawn for future research work.

Reducing Successive Interference Cancellation Iterations in Hybrid Beamforming Multiuser Massive Multiple Input Multiple Output Systems Through Grouping Users with Symmetry Channels

This paper presents a comprehensive exploration of advanced beamforming techniques tailored for millimeter-wave (mm-Wave) communication systems. In response to the burgeoning demand for higher data rates, coupled with the constraints of power consumption and hardware complexity, this study focuses on developing a hybrid beamforming framework optimized for downlink scenarios, specifically targeting groups of users based on the approximate symmetry of their channels. The primary innovation of this research lies in leveraging the symmetry of channels among near users to develop a group-based successive interference cancellation (SIC) algorithm. Unlike traditional approaches that address interference on a per-user basis, this algorithm utilizes channel symmetry within clusters of users to reduce computational complexity and improve the efficiency of SIC. By grouping users with symmetrical channel characteristics, the algorithm simplifies the interference management process while maintaining system performance. The proposed system demonstrates notable advantages over existing non-linear algorithms through extensive simulations and performance evaluations, particularly in terms of spectral efficiency and computational complexity. In this study, we further emphasize the importance of balancing spectral efficiency improvements with reduced computational demands, offering a nuanced trade-off that accommodates various operational requirements. The flexible optimization framework provided showcases the system’s adaptability to diverse deployment scenarios and network configurations.

Outflow from the very massive Wolf-Rayet binary Melnick 34

Melnick 34 (Mk\,34) is one of the most massive binary systems known and is one of the brightest X-ray point sources in the 30 Doradus region. We investigated the impact of this massive system on the surrounding interstellar medium (ISM) using the optical spectroscopic capabilities of the narrow-field mode (NFM) of the Multi-Unit Spectroscopic Explorer (MUSE). MUSE-NFM spatially resolved the ISM in the vicinity of Mk\,34 with a resolution comparable to that of the HST. The analysis of the N ii \,lambda \,6583 and S ii \,lambda \,6717 emission lines reveals a cone-like structure apparently originating from Mk\,34 and extending southeast. Electron density maps and radial velocity measurements of the ISM lines further support an outflow scenario traced by these emissions. While no clear northwestern counterpart to this outflow was observed, we note increased extinction in that direction, towards the R136 cluster. The ISM material along the projected diagonal of the outflow on both sides of Mk\,34 shows similar properties in terms of the emission line ratios seen in the Baldwin-Phillips-Terlevich diagram. These results are consistent across two observational epochs. Additionally, we examined the residual maps within a 0.5" radius of Mk\,34 after modeling and subtracting the point spread function. The observed variations in the residuals could potentially be linked to Mk\,34's known periodic behavior. However, further observations with appropriate cadence are needed to fully monitor the 155\,day periodicity of Mk\,34's X-ray emissions.

Long-Term Comparative Life Cycle Assessment, Cost, and Comfort Analysis of Heavyweight vs. Lightweight Construction Systems in a Mediterranean Climate

Massive construction systems have always characterized traditional architecture and are currently the most prevalent, straightforward, and cost-effective in many Mediterranean countries. However, in recent years, the construction industry has gradually shifted towards using lightweight, dry construction techniques. This study aims to assess the effects on energy consumption, comfort levels, and environmental sustainability resulting from the adoption of five high-performance construction systems in a multi-family residential building: (i) reinforced concrete structure with low-transmittance thermal block infill; (ii) reinforced concrete structure with light-clay bricks and outer thermal insulation; (iii) steel frame; (iv) cross-laminated timber (CLT); (v) timber-steel hybrid structure. To achieve this goal, a multidisciplinary approach was employed, including the analysis of thermal parameters, the evaluation of indoor comfort through the adaptive model and Fanger’s PMV, and the quantification of environmental and economic impacts through life cycle assessment and life cycle cost applied in a long-term analysis (ranging from 30 to 100 years). The results highlight that heavyweight construction systems are the most effective in terms of comfort, cost, and long-term environmental impact (100 years), while lightweight construction systems generally have higher construction costs, provide lower short-term environmental impacts (30 years), and offer intermediate comfort depending on the thermal mass.

Joint Power Allocation and User Scheduling in Integrated Satellite–Terrestrial Cell-Free Massive MIMO IoT Systems

Joint Power Allocation and User Scheduling in Integrated Satellite–Terrestrial Cell-Free Massive MIMO IoT Systems

An Adaptive CSI Feedback Model Based on BiLSTM for Massive MIMO-OFDM Systems

An Adaptive CSI Feedback Model Based on BiLSTM for Massive MIMO-OFDM Systems

Performance Optimization on Cell-Free Massive MIMO-Aided URLLC Systems With User Grouping

Performance Optimization on Cell-Free Massive MIMO-Aided URLLC Systems With User Grouping

Sum rate optimization in STAR-RIS assisted multiuser massive MIMO-OFDM VLC systems

Visible Light Communication (VLC) offers a promising solution for future networks, leveraging existing lighting infrastructure in indoor environments. However, VLC requires a direct line of sight to function which can be limiting. Reconfigurable Intelligent Surface (RIS) is a new technology that can bend light and radio waves, addressing this limitation in VLC. RIS come in three types: passive which reflects signals, active that boosts and reflects signals, and Simultaneous Transmission and Reflection (STAR)-RIS which can both reflect and transmit signals simultaneously. STAR-RIS offers the most control over the signal. In this paper, we propose a new multi-user VLC system with a massive Multiple-Input, Multiple-Output Orthogonal Frequency-Division Multiplexing (MIMO-OFDM) architecture, leveraging STAR-RIS to optimize data rates and improve coverage, particularly in non-line-of-sight scenarios. We formulate a system model and solve a convex optimization problem to determine the optimal transmission and reflection coefficients for STAR-RIS elements, aiming to maximize the total sum rate for all users. By employing maximum ratio transmission precoding, we minimize interference among users and demonstrate significant performance gains. Simulation results show that our proposed energy splitting-based STAR-RIS configuration outperforms traditional mode selection and time switching approaches with fixed or random coefficients, yielding substantial improvements in data rates and user experience. This study offers the first detailed exploration of STAR-RIS in VLC systems, highlighting its potential for future high-speed, multi-user communication networks. Our findings set the stage for further research into optimizing VLC systems using STAR-RIS, particularly in complex environments with non-line-of-sight users and massive MIMO-OFDM configurations.

Power Allocation for Massive MIMO-ISAC Systems

Power Allocation for Massive MIMO-ISAC Systems

Block-term tensor decomposition in array signal processing

The ability of tensor decomposition (TD)-based methods to recover latent information, including channel parameters and transmitted symbols in the array signal processing (ASP) context, in a deterministic manner and with little or no training overhead, fits well with the data efficiency needs of future-generation multi-user massive multiple-input multiple-output systems. However, such methods have mostly relied on a few classical TD models, notably the canonical polyadic decomposition (CPD), which can be quite restrictive in realistic scenarios. In this paper, a generalized CPD model, known as block-term decomposition (BTD), is re-visited in the context of ASP, and shown to be the natural choice in sensor arrays of increased dimensionality that also involve channel multipath. Semi-blind joint channel estimation/data detection (JCD) is addressed in this context via efficient algorithms that wed existing JCD schemes with BTD approximation. Robust to sensor failures and recursive versions are also developed. The special yet important case of the uniform rectangular array (URA) configuration is adopted to illustrate the ideas and results. The signal detection performance of the BTD-inspired semi-blind JCD schemes is evaluated under various conditions with the aid of simulations and seen to be favorably compared with that of the training-only-based solution.

The effects of stellar and AGN feedback on the cosmic star formation history in the simba simulations

ABSTRACT Using several variants of the cosmological simba simulations, we investigate the impact of different feedback prescriptions on the cosmic star formation history. Adopting a global-to-local approach, we link signatures seen in global observables, such as the star formation rate density (SFRD) and the galaxy stellar mass function (GSMF), to feedback effects in individual galaxies. We find a consistent picture: stellar feedback mainly suppresses star formation below halo masses of $M_{\rm H} = 10^{12} \rm \, {\rm M}_{\odot }$ and before $z = 2$, whereas AGN feedback quenches the more massive systems after $z = 2$. Among simba’s AGN feedback modes, AGN jets are the dominant quenching mechanism and set the shape of the SFRD and the GSMF at late times. AGN-powered winds only suppress the star formation rate in intermediate-mass galaxies ($M_{\rm \star } = 10^{9.5 - 10} \rm \, {\rm M}_{\odot }$), without affecting the overall stellar mass-assembly significantly. At late times, the AGN X-ray feedback mode mainly quenches residual star formation in massive galaxies. Our analysis reveals that this mode is also necessary to produce the first fully quenched galaxies before $z=2$, where the jets alone are inefficient. These initially highly star-forming galaxies contain relatively large black holes, likely strengthening the X-ray-powered heating and ejection of gas from the dense, central region of galaxies. Such extra heating source quenches the local star formation and produces a more variable accretion rate. More generally, this effect also causes the break down of correlations between the specific star formation rate, the accretion rate and the black hole mass.

Sum-Rate Maximization for a Hybrid Precoding-Based Massive MIMO NOMA System with Simultaneous Wireless Information and Power Transmission

Non-orthogonal multiple access (NOMA) has emerged as a key enabling technology in the realm of millimeter-wave (mmWave) massive MIMO (mMIMO) systems for enhancing spectral efficiency (SE). Furthermore, it is believed that simultaneous wireless information and power transmission (SWIPT) will allow for the system’s energy efficiency (EE) to be maximised. The effectiveness of the mmWave mMIMO-NOMA system along with SWIPT has been examined in this article under multi-user (MU) scenarios. This paper’s major goal is to construct a low-complexity hybrid-precoder (HP) while taking into account the sub-connected (SC) architecture. The linear precoder is a computationally demanding technique as a result of the matrix inversion. The authors of this paper have suggested a symmetric sequential over-relaxation (SSOR) complex regularised zero-forcing (CRZF) linear precoder. The power distribution for the mmWave mMIMO-NOMA system and power splitting factors for SWIPT are jointly tuned to maximize the sum rate along with the suggested SSOR-CRZF precoder. In regards to complexity, SE, and EE, the SSOR-CRZF-HP surpasses conventional linear precoders.

Performance Evaluation of CF-MMIMO Wireless Systems Using Dynamic Mode Decomposition

Cell-Free Massive Multiple-Input–Multiple-Output (CF-MIMO) systems have transformed the landscape of wireless communication, offering unparalleled enhancements in Spectral Efficiency and interference mitigation. Nevertheless, the large-scale deployment of CF-MIMO presents significant challenges in processing signals in a scalable manner. This study introduces an innovative methodology that leverages the capabilities of Dynamic Mode Decomposition (DMD) to tackle the complexities of Channel Estimation in CF-MIMO wireless systems. By extracting dynamic modes from a vast array of received signal snapshots, DMD reveals the evolving characteristics of the wireless channel across both time and space, thereby promising substantial improvements in the accuracy and adaptability of channel state information (CSI). The efficacy of the proposed methodology is demonstrated through comprehensive simulations, which emphasize its superior performance in highly mobile environments. For performance evaluation, the most common techniques have been employed, comparing the proposed algorithms with traditional methods such as MMSE (Minimum Mean Squared Error), MRC (Maximum Ration Combining), and ZF (Zero Forcing). The evaluation metrics used are standard in the field, namely the Cumulative Distribution Function (CDF) and the average UL/DL Spectral Efficiency. Furthermore, the study investigates the impact of DMD-enabled Channel Estimation on system performance, including beamforming strategies, spatial multiplexing within realistic time- and delay-correlated channels, and overall system capacity. This work underscores the transformative potential of incorporating DMD into massive MIMO wireless systems, advancing communication reliability and capacity in increasingly dynamic and dense wireless environments.

Elemental Abundances in And XIX from Coadded Spectra

With a luminosity similar to that of Milky Way dwarf spheroidal systems like Sextans, but a spatial extent similar to that of ultra-diffuse galaxies, Andromeda (And) XIX is an unusual satellite of M31. To investigate the origin of this galaxy, we measure chemical abundances for And XIX derived from medium-resolution (R ∼ 6000) spectra from the Deep Extragalactic Imaging Multi-Object Spectrograph on the Keck II telescope. We coadd 79 red giant branch stars, grouped by photometric metallicity, in order to obtain a sufficiently high signal-to-noise ratio to measure 20 [Fe/H] and [α/Fe] abundances via spectral synthesis. The latter are the first such measurements for And XIX. The mean metallicity we derive for And XIX places it ∼2σ higher than the present-day stellar mass–metallicity relation for Local Group dwarf galaxies, potentially indicating it has experienced tidal stripping. A loss of gas and associated quenching during such a process, which prevents the extended star formation necessary to produce shallow [α/Fe]–[Fe/H] gradients in massive systems, is also consistent with the steeply decreasing [α/Fe]–[Fe/H] trend we observe. In combination with the diffuse structure and disturbed kinematic properties of And XIX, this suggests tidal interactions, rather than galaxy mergers, are strong contenders for its formation.