15,464 research outputs found
PAR-Aware Large-Scale Multi-User MIMO-OFDM Downlink
We investigate an orthogonal frequency-division multiplexing (OFDM)-based
downlink transmission scheme for large-scale multi-user (MU) multiple-input
multiple-output (MIMO) wireless systems. The use of OFDM causes a high
peak-to-average (power) ratio (PAR), which necessitates expensive and
power-inefficient radio-frequency (RF) components at the base station. In this
paper, we present a novel downlink transmission scheme, which exploits the
massive degrees-of-freedom available in large-scale MU-MIMO-OFDM systems to
achieve low PAR. Specifically, we propose to jointly perform MU precoding, OFDM
modulation, and PAR reduction by solving a convex optimization problem. We
develop a corresponding fast iterative truncation algorithm (FITRA) and show
numerical results to demonstrate tremendous PAR-reduction capabilities. The
significantly reduced linearity requirements eventually enable the use of
low-cost RF components for the large-scale MU-MIMO-OFDM downlink.Comment: To appear in IEEE Journal on Selected Areas in Communication
Lifting 1/4-BPS States on K3 and Mathieu Moonshine
The elliptic genus of K3 is an index for the 1/4-BPS states of its sigma
model. At the torus orbifold point there is an accidental degeneracy of such
states. We blow up the orbifold fixed points using conformal perturbation
theory, and find that this fully lifts the accidental degeneracy of the 1/4-BPS
states with h=1. At a generic point near the Kummer surface the elliptic genus
thus measures not just their index, but counts the actual number of these BPS
states. We comment on the implication of this for symmetry surfing and Mathieu
moonshine.Comment: 29+5 pp, a sign mistake corrected in eqs. (3.14) and (4.20), footnote
6 added to clarify this point, references adde
Genus Two Partition Functions and Renyi Entropies of Large c CFTs
We compute genus two partition functions in two dimensional conformal field
theories at large central charge, focusing on surfaces that give the third
Renyi entropy of two intervals. We compute this for generalized free theories
and for symmetric orbifolds, and compare it to the result in pure gravity. We
find a new phase transition if the theory contains a light operator of
dimension . This means in particular that unlike the second
Renyi entropy, the third one is no longer universal.Comment: 28 pages + Appendice
Extracting the light quark mass ratio m_u/m_d from bottomonia transitions
We propose a new method to extract the light quark mass ratio using
the bottomonia transitions. The decay
amplitudes are dominated by the light quark mass differences, and the
corrections from other effects are rather small, allowing for a precise
extraction. We also discuss how to reduce the theoretical uncertainty with the
help of future experiments. As a by-product, we show that the decay
is expected to be a nice channel for searching for
the state.Comment: 4 pages, 1 figure. Version slightly shortened to match the published
version in PR
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