464 research outputs found
A General Rate Duality of the MIMO Multiple Access Channel and the MIMO Broadcast Channel
We present a general rate duality between the multiple access channel (MAC)
and the broadcast channel (BC) which is applicable to systems with and without
nonlinear interference cancellation. Different to the state-of-the-art rate
duality with interference subtraction from Vishwanath et al., the proposed
duality is filter-based instead of covariance-based and exploits the arising
unitary degree of freedom to decorrelate every point-to-point link. Therefore,
it allows for noncooperative stream-wise decoding which reduces complexity and
latency. Moreover, the conversion from one domain to the other does not exhibit
any dependencies during its computation making it accessible to a parallel
implementation instead of a serial one. We additionally derive a rate duality
for systems with multi-antenna terminals when linear filtering without
interference (pre-)subtraction is applied and the different streams of a single
user are not treated as self-interference. Both dualities are based on a
framework already applied to a mean-square-error duality between the MAC and
the BC. Thanks to this novel rate duality, any rate-based optimization with
linear filtering in the BC can now be handled in the dual MAC where the arising
expressions lead to more efficient algorithmic solutions than in the BC due to
the alignment of the channel and precoder indices.Comment: Submitted to IEEE Globecom 2008; Fixed dimensions of channel matrix
H_k and covariance matrix Z_k, slightly modified conclusio
Formation of chondrules in radiative shock waves I. First results, spherical dust particles, stationary shocks
The formation of chondrules in the protoplanetary nebulae causes many
questions concerning the formation process, the source of energy for melting
the rims, and the composition of the origin material. The aim of this work is
to explore the heating of the chondrule in a single precursor as is typical for
radiation hydrodynamical shock waves. We take into account the gas-particle
friction for the duration of the shock transition and calculate the heat
conduction into the chondrules. These processes are located in the
protoplanetary nebulae at a region around 2.5 AU, which is considered to be the
most likely place of chondrule formation. The present models are a first step
towards computing radiative shock waves occurring in a particle-rich
environment. We calculated the shock waves using one-dimensional,
time-independent equations of radiation hydrodynamics involving realistic gas
and dust opacities and gas-particle friction. The evolution of spherical
chondrules was followed by solving the heat conduction equation on an adaptive
grid. The results for the shock-heating event are consistent with the
cosmochemical constraints of chondrule properties. The calculations yield a
relative narrow range for density or temperature to meet the requested heating
rates of as extracted from cosmochemical constraints.
Molecular gas, opacities with dust, and a protoplanetary nebula with accretion
are necessary requirements for a fast heating process. The thermal structure in
the far post-shock region is not fully consistent with experimental constraints
on chondrule formation since the models do not include additional molecular
cooling processes.Comment: 8 pages,5 figure
A framework for joint design of pilot sequence and linear precoder
Most performance measures of pilot-assisted multiple-input multiple-output systems are functions of the linear precoder and the pilot sequence. A framework for the optimization of these two parameters is proposed, based on a matrix-valued generalization of the concept of effective signal-to-noise ratio (SNR) introduced in the famous work by Hassibi and Hochwald. Our framework aims to extend the work of Hassibi and Hochwald by allowing for transmit-side fading correlations, and by considering a class of utility functions of said effective SNR matrix, most notably including the well-known capacity lower bound used by Hassibi and Hochwald. We tackle the joint optimization problem by recasting the optimization of the precoder (resp. pilot sequence) subject to a fixed pilot sequence (resp. precoder) into a convex problem. Furthermore, we prove that joint optimality requires that the eigenbases of the precoder and pilot sequence be both aligned along the eigenbasis of the channel correlation matrix. We finally describe how to wrap all studied subproblems into an iteration that converges to a local optimum of the joint optimization.Peer ReviewedPostprint (author's final draft
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