34,172 research outputs found
Relaying systems with reciprocity mismatch : impact analysis and calibration
Cooperative beamforming can provide significant performance improvement for relaying systems with the help of the channel state information (CSI). In time-division duplexing (TDD) mode, the estimated CSI will deteriorate due to the reciprocity mismatch. In this work, we examine the impact and the calibration of the reciprocity mismatch in relaying systems. To evaluate the impact of the reciprocity mismatch for all devices, the closed-form expression of the achievable rate is first derived. Then, we analyze the performance loss caused by the reciprocity mismatch at sources, relays, and destinations respectively to show that the mismatch at relays dominates the impact. To compensate the performance loss, a two-stage calibration scheme is proposed for relays. Specifically, relays perform the intra-calibration based on circuits independently. Further, the inter-calibration based on the discrete Fourier transform (DFT) codebook is operated to improve the calibration performance by cooperation transmission, which has never been considered in previous work. Finally, we derive the achievable rate after relays perform the proposed reciprocity calibration scheme and investigate the impact of estimation errors on the system performance. Simulation results are presented to verify the analytical results and to show the performance of the proposed calibration approach
NOMA-enhanced computation over multi-access channels
Massive numbers of nodes will be connected in future wireless networks. This brings great difficulty to collect a large amount of data. Instead of collecting the data individually, computation over multi-access channels (CoMAC) provides an intelligent solution by computing a desired function over the air based on the signal-superposition property of wireless channels. To improve the spectrum efficiency in conventional CoMAC, we propose the use of non-orthogonal multiple access (NOMA) for functions in CoMAC. The desired functions are decomposed into several sub-functions, and multiple sub-functions are selected to be superposed over each resource block (RB). The corresponding achievable rate is derived based on sub-function superposition, which prevents a vanishing computation rate for large numbers of nodes. We further study the limiting case when the number of nodes goes to infinity. An exact expression of the rate is derived that provides a lower bound on the computation rate. Compared with existing CoMAC, the NOMA-based CoMAC not only achieves a higher computation rate but also provides an improved non-vanishing rate. Furthermore, the diversity order of the computation rate is derived, which shows that the system performance is dominated by the node with the worst channel gain among these sub-functions in each RB
Optimal Controller and Filter Realisations using Finite-precision, Floating- point Arithmetic.
The problem of reducing the fragility of digital controllers and filters
implemented using finite-precision, floating-point arithmetic is considered.
Floating-point arithmetic parameter uncertainty is multiplicative, unlike
parameter uncertainty resulting from fixed-point arithmetic. Based on first-
order eigenvalue sensitivity analysis, an upper bound on the eigenvalue
perturbations is derived. Consequently, open-loop and closed-loop eigenvalue
sensitivity measures are proposed. These measures are dependent upon the filter/
controller realization. Problems of obtaining the optimal realization with
respect to both the open-loop and the closed-loop eigenvalue sensitivity
measures are posed. The problem for the open-loop case is completely solved.
Solutions for the closed-loop case are obtained using non-linear programming.
The problems are illustrated with a numerical example
Conductance of a single-atom carbon chain with graphene leads
We study the conductance of an interconnect between two graphene leads formed
by a single-atom carbon chain. Its dependence on the chemical potential and the
number of atoms in the chain is qualitatively different from that in the case
of normal metal leads. Electron transport proceeds via narrow resonant states
in the wire. The latter arise due to strong reflection at the junctions between
the chain and the leads, which is caused by the small density of states in the
leads at low energy. The energy dependence of the transmission coefficient near
resonance is asymmetric and acquires a universal form at small energies. We
find that in the case of leads with the zigzag edges the dispersion of the edge
states has a significant effect on the device conductance.Comment: 9 pages, 4 figure
Quantum Oscillations in Magnetic Field Induced Antiferromagnetic Phase of Underdoped Cuprates : Application to Ortho-II YBa2Cu3O6.5
Magnetic field induced antiferromagnetic phase of the underdoped cuprates is
studied within the t-t'-J model. A magnetic field suppresses the pairing
amplitude, which in turn may induce antiferromagnetism. We apply our theory to
interpret the recently reported quantum oscillations in high magnetic field in
ortho-II YBa2Cu3O6.5 and propose that the total hole density abstracted from
the oscillation period is reduced by 50% due to the antiferromagnetism.Comment: 5 pages, 3 figure
Interlayer couplings and the coexistence of antiferromagnetic and d-wave pairing order in multilayer cuprates
A more extended low density region of coexisting uniform antiferromagnetism
and d-wave superconductivity has been reported in multilayer cuprates, when
compared to single or bilayer cuprates. This coexistence could be due to the
enhanced screening of random potential modulations in inner layers or to the
interlayer Heisenberg and Josephson couplings. A theoretical analysis using a
renormalized mean field theory, favors the former explanation. The potential
for an improved determination of the antiferromagnetic and superconducting
order parameters in an ideal single layer from zero field NMR and infrared
Josephson plasma resonances in multilayer cuprates is discussed.Comment: 6 pages, 2 figure
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