Cell-free massive multiple-input multiple-output (MIMO) system is a promising architecture for next generation wireless systems by deploying a very large number of distributed access points (APs), ...which simultaneously serve a smaller number of user equipments (UEs) over the same time-frequency resources. It guarantees uniformly good service at high spectral efficiency with simple linear precoding techniques and max-min power control. In this article, we propose a new joint maximum-ratio and zero-forcing (JMRZF) precoding scheme, where part of APs are combined to perform centralized zero-forcing (ZF), while other APs apply simple maximum-ratio transmission (MRT). Our proposed precoder offers an adaptable trade-off between the spectral efficiency and front-haul signalling overhead. A corresponding AP subset selection scheme is also proposed which is based on large-scale fading coefficients. A closed-form expression for the achievable spectral efficiency of our proposed scheme is derived, which represents a generalized result including both fully distributed MRT and fully centralized ZF cases. Based on this closed-form expression, max-min power control is formulated and solved via the second order cone and first order methods. The former can obtain the global optimal solution, but its computational complexity is very high. On the other hand, the latter technique is sub-optimal, yet, it has very low computational complexity. Hence, it is suitable for large-scale cell-free massive MIMO systems with hundreds or thousands of APs and users. Numerical results show that our proposed JMRZF scheme can substantially outperform the local precoding schemes, even when a small part of APs are combined to deploy ZF and is implementable even when each AP has very few antennas. In addition, it is shown that our max-min power controls improves the spectral efficiency significantly, compared to the uniform power control scheme.
Pilot Power Control for Cell-Free Massive MIMO Mai, Trang C.; Ngo, Hien Quoc; Egan, Malcolm ...
IEEE transactions on vehicular technology,
11/2018, Letnik:
67, Številka:
11
Journal Article
Recenzirano
Odprti dostop
In this correspondence paper, we consider a cell-free massive Multiple-Input Multiple-Output (MIMO) system where access points (APs) serve a much smaller number of users under the time-division ...duplex operation. The APs first estimate the channels via the uplink training phase. Then, these channel estimates are used to detect desired symbols in the uplink and precode the transmit symbols in the downlink. Nonorthogonality of pilot sequences and AP selection (e.g., received-power-based selection or largest-large-scale-fading based selection schemes) are taken into account. To reduce the effect of pilot contamination, we propose a pilot power control design, which chooses the pilot power control coefficients to minimize the mean-squared error of the channel estimation. This is achieved via the sequential convex approximation method. By using pilot power control in training phase, the system performance is considerably improved. In addition, we derive closed-form expressions for the uplink and downlink achievable rates with arbitrary power data/pilot control coefficients and any AP selection schemes.
This paper considers the fundamental power allocation problem in cell-free massive mutiple-input and multiple-output (MIMO) systems which aims at maximizing the total energy efficiency (EE) under a ...sum power constraint at each access point (AP) and a quality-of-service (QoS) constraint at each user. Existing solutions for this optimization problem are based on solving a sequence of second-order cone programs (SOCPs), whose computational complexity scales dramatically with the network size. Therefore, they are not implementable for practical large-scale cell-free massive MIMO systems. To tackle this issue, we propose an iterative power control algorithm based on the frame work of an accelerated projected gradient (APG) method. In particular, each iteration of the proposed method is done by simple closed-form expressions, where a penalty method is applied to bring constraints into the objective in the form of penalty functions. Finally, the convergence of the proposed algorithm is analytically proved and numerically compared to the known solution based on SOCP. Simulations results demonstrate that our proposed power control algorithm can achieve the same EE as the existing SOCPs-based method, but more importantly, its run time is much lower (one to two orders of magnitude reduction in run time, compared to the SOCPs-based approaches).
Realization of topological superconductors (TSCs) hosting Majorana fermions is a central challenge in condensed-matter physics. One approach is to use the superconducting proximity effect (SPE) in ...heterostructures, where a topological insulator contacted with a superconductor hosts an effective p-wave pairing by the penetration of Cooper pairs across the interface. However, this approach suffers a difficulty in accessing the topological interface buried deep beneath the surface. Here, we propose an alternative approach to realize topological superconductivity without SPE. In a Pb(111) thin film grown on TlBiSe
, we discover that the Dirac-cone state of substrate TlBiSe
migrates to the top surface of Pb film and obtains an energy gap below the superconducting transition temperature of Pb. This suggests that a Bardeen-Cooper-Schrieffer superconductor is converted into a TSC by the topological proximity effect. Our discovery opens a route to manipulate topological superconducting properties of materials.
Our study examines how the relationships with various human ecological systems influence the transition by rural students from home communities to postsecondary institutions. Using phenomenological ...interview data with 14 students who are from rural Alaska and 14 postsecondary student service staff, our analysis identified characteristics of the postsecondary transition processes that seem particular to rural students--in navigating urban places and cultural norms, seeking institutional supports, advocating for self, and forming new social relationships. In all of these experiences, rural identities and community-oriented values emerged as a guiding force for students' goal-setting, decision-making, interactions, and adjustment. Our data suggest opportunities to reinforce connections and cooperation among human ecological systems to improve the postsecondary transition experience for rural students.
We have performed angle-resolved photoemission spectroscopy on HfSiS, which has been predicted to be a topological line-node semimetal with square Si lattice. We found a quasi-two-dimensional Fermi ...surface hosting bulk nodal lines, alongside the surface states at the Brillouin-zone corner exhibiting a sizable Rashba splitting and band-mass renormalization due to many-body interactions. Most notably, we discovered an unexpected Dirac-like dispersion extending one dimensionally in k space-the Dirac-node arc-near the bulk node at the zone diagonal. These novel Dirac states reside on the surface and could be related to hybridizations of bulk states, but currently we have no explanation for its origin. This discovery poses an intriguing challenge to the theoretical understanding of topological line-node semimetals.
In this descriptive analysis, we consider the experiences of students who prefer face-to-face (F2F) classes but, due to the COVID-19 pandemic, had no alternative other than taking their introductory ...biology class online during the 2020–2021 academic year. We conducted focus group interviews with 12 college students who enrolled in an asynchronous online introductory biology course for non-majors. We interpret their experiences through the theoretical framing of student engagement, which generally centers students as the directors of their learning experiences. However, when reflecting on their online, technologically mediated experience, our participants regarded their instructors as the hub or convener of their interactions with content, technology, and other learners. We explore the implications of these findings for engaging other students who may find themselves involuntarily online, and make recommendations for pedagogy and communication around the culture of online learning.
In this work, Ni-doped lead-free ferroelectric Ba(Ti
0.8
Zr
0.2
)O
3
materials were well synthesized by a simple chemical route. The complex magnetic behavior of the materials was explained by the ...random distribution of Ni cations into the Ba(Ti
0.8
Zr
0.2
)O
3
host lattice. As increasing the Ni concentration to 9 mol%, nonlinear electric polarization behavior remained unchanged in the Ba(Ti
0.8
Zr
0.2
)O
3
materials. The observations in nonlinear magnetization and electric polarization in Ni-doped Ba(Ti
0.8
Zr
0.2
)O
3
materials suggested an extension of new material functions to the development of advanced materials for electronic devices.
A key problem in nanomachine networks is how information from sensors is to be transmitted to a fusion center. In this letter, we propose a molecular communication-based event detection network. In ...particular, we develop a detection framework that can cope with scenarios where the molecules propagate according to anomalous diffusion instead of the conventional Brownian motion. We propose an algorithm for optimizing the network throughput by exploiting tools from reinforcement learning. Our algorithms are evaluated with the aid of numerical simulations, which demonstrate the trade-offs between the performance and complexity.
In some settings, rotavirus vaccines have been associated with a low-level risk of intussusception, the most common cause of bowel obstruction in infants. As Vietnam prepares to introduce rotavirus ...vaccine into the national immunization program, we sought to better characterize the epidemiology of recurrent intussusception. We enrolled children <2 years of age who were hospitalized for intussusception retrospectively from January 2013 through December 2014 and prospectively from January 2015 through December 2016 at 2 hospitals in Vietnam. We enrolled 2477 children. Nearly all children were successfully treated by enema with low surgery rate (1%). We found 10% of children (n = 254) experienced at least once recurrence (range: 1–6) and 57% of first recurrences happened within the first 12 weeks after treatment of the first episode. The median age at first intussusception was 13 months for children without a recurrent episode and 10 months for children with a recurrence. The symptoms of the recurrent cases were milder with less vomiting (67%), bloody stool (7%) and fever (10%) compared to the initial cases (p < 0.01). We found the rate of recurrences following enema reduction of intussusception to be similar to that reported from other countries. Due to the high rate of intussusception and recurrent episodes in Vietnam, a better understanding of the cause of recurrent intussusception will be critical in assessing intussusception cases after rotavirus introduction.
PDF
