15,877 research outputs found
Degrees of Freedom of the 3-User Rank-Deficient MIMO Interference Channel
We provide the degrees of freedom (DoF) characterization for the -user
multiple-input multiple-output (MIMO) interference channel (IC)
with \emph{rank-deficient} channel matrices, where each transmitter is equipped
with antennas and each receiver with antennas, and the interfering
channel matrices from each transmitter to the other two receivers are of ranks
and , respectively. One important intermediate step for both the
converse and achievability arguments is to convert the fully-connected
rank-deficient channel into an equivalent partially-connected full-rank MIMO-IC
by invertible linear transformations. As such, existing techniques developed
for full-rank MIMO-IC can be incorporated to derive the DoF outer and inner
bounds for the rank-deficient case. Our result shows that when the interfering
links are weak in terms of the channel ranks, i.e., , zero forcing is sufficient to achieve the optimal DoF. On the other
hand, when , a combination of zero forcing and
interference alignment is in general required for DoF optimality. The DoF
characterization obtained in this paper unifies several existing results in the
literature.Comment: 28 pages, 7 figures. To appear in IEEE transactions on wireless
communication
Anomalous light cones and valley optical selection rules of interlayer excitons in twisted heterobilayers
We show that, because of the inevitable twist and lattice mismatch in
heterobilayers of transition metal dichalcogenides, interlayer excitons have
six-fold degenerate light cones anomalously located at finite velocities on the
parabolic energy dispersion. The photon emissions at each light cone are
elliptically polarized, with major axis locked to the direction of exciton
velocity, and helicity specified by the valley indices of the electron and the
hole. These finite-velocity light cones allow unprecedented possibilities to
optically inject valley polarization and valley current, and the observation of
both direct and inverse valley Hall effects, by exciting interlayer excitons.
Our findings suggest potential excitonic circuits with valley functionalities,
and unique opportunities to study exciton dynamics and condensation phenomena
in semiconducting 2D heterostructures.Comment: Including the Supplemental Material
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