415 research outputs found
Carbon Nanotube Fibers Prepared by Activating Deactivated Iron Particles in Floating Catalyst Chemical Vapor Deposition Tail Gas
Catalysts can determine the structure and properties of carbon nanotube (CNT) fibers fabricated using the floating catalyst chemical vapor deposition (FCCVD) method. The tail gas left over when CNT fibers are fabricated by the FCCVD method has been proven to contain deactivated iron nanoparticles, as well as carbide gas and hydrogen. This study demonstrates that the deactivated iron nanoparticles in tail gas can be successfully activated in a double furnace system under certain conditions. CNT fibers can be successfully prepared using the activated iron nanoparticles by adding the precursor without the catalyst. These CNT fibers are composed of double-walled carbon nanotubes (DWNTs) and have low density, high strength, and electrical conductivity
Robust Beamforming and Rate-Splitting Design for Next Generation Ultra-Reliable and Low-Latency Communications
The next generation ultra-reliable and low-latency communications (xURLLC)
need novel design to provide satisfactory services to the emerging
mission-critical applications. To improve the spectrum efficiency and enhance
the robustness of xURLLC, this paper proposes a robust beamforming and
rate-splitting design in the finite blocklength (FBL) regime for downlink
multi-user multi-antenna xURLLC systems. In the design, adaptive rate-splitting
is introduced to flexibly handle the complex inter-user interference and thus
improve the spectrum efficiency. Taking the imperfection of the channel state
information at the transmitter (CSIT) into consideration, a max-min user rate
problem is formulated to optimize the common and private beamforming vectors
and the rate-splitting vector under the premise of ensuring the requirements of
transmission latency and reliability of all the users. The optimization problem
is intractable due to the non-convexity of the constraint set and the infinite
constraints caused by CSIT uncertainties. To solve it, we convert the infinite
constraints into finite ones by the S-Procedure method and transform the
original problem into a difference of convex (DC) programming. A constrained
concave convex procedure (CCCP) and the Gaussian randomization based iterative
algorithm is proposed to obtain a local minimum. Simulation results confirm the
convergence, robustness and effectiveness of the proposed robust beamforming
and rate-splitting design in the FBL regime. It is also shown that the proposed
robust design achieves considerable performance gain in the worst user rate
compared with existing transmission schemes under various blocklength and block
error rate requirements.Comment: 12 pages, 9 figure
Cooperative Beamforming Design for Multiple RIS-Assisted Communication Systems
Reconfigurable intelligent surface (RIS) provides a promising way to build
programmable wireless transmission environments. Owing to the massive number of
controllable reflecting elements on the surface, RIS is capable of providing
considerable passive beamforming gains. At present, most related works mainly
consider the modeling, design, performance analysis and optimization of
single-RIS-assisted systems. Although there are a few of works that investigate
multiple RISs individually serving their associated users, the cooperation
among multiple RISs is not well considered as yet. To fill the gap, this paper
studies a cooperative beamforming design for multi-RIS-assisted communication
systems, where multiple RISs are deployed to assist the downlink communications
from a base station to its users. To do so, we first model the general channel
from the base station to the users for arbitrary number of reflection links.
Then, we formulate an optimization problem to maximize the sum rate of all
users. Analysis shows that the formulated problem is difficult to solve due to
its non-convexity and the interactions among the decision variables. To solve
it effectively, we first decouple the problem into three disjoint subproblems.
Then, by introducing appropriate auxiliary variables, we derive the closed-form
expressions for the decision variables and propose a low-complexity cooperative
beamforming algorithm. Simulation results have verified the effectiveness of
the proposed algorithm through comparison with various baseline methods.
Furthermore, these results also unveil that, for the sum rate maximization,
distributing the reflecting elements among multiple RISs is superior to
deploying them at one single RIS
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