6,390 research outputs found
Beamforming and Multiuser Detection in CDMA Systems with External Interferences
Multiuser detection has been investigated to mitigate the near-far effect in CDMA systems. Antenna arrays have been shown to provide spatial diversity and cancel undesired signals. In this paper we consider the synergy of both multiuser detection and antenna arrays for the base station of a CDMA system. The receiver we proposed consists of the known multiuser decorrelator, which cancels multiple-access interferences followed by a beamformer for each user, which cancels the external interferences. This receiver adds an extra branch to the decorrelator. This additional branch, corresponding to a fictitious user with an unused code and zero power, allows to estimate the external interference signal subspace and compute a suitable beamforming weight-vector that cancels the external interferences. The receiver is also extended to the asynchronous case and all of this without any training signal or any a priori spatial information.Peer ReviewedPostprint (published version
Two-Dimensional Phononic Crystals: Disorder Matters
The design and fabrication of phononic crystals (PnCs) hold the key to
control the propagation of heat and sound at the nanoscale. However, there is a
lack of experimental studies addressing the impact of order/disorder on the
phononic properties of PnCs. Here, we present a comparative investigation of
the influence of disorder on the hypersonic and thermal properties of
two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of
circular holes with equal filling fractions in free-standing Si membranes.
Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman
thermometry based on a novel two-laser approach are used to study the phononic
properties in the gigahertz (GHz) and terahertz (THz) regime, respectively.
Finite element method simulations of the phonon dispersion relation and
three-dimensional displacement fields furthermore enable the unique
identification of the different hypersonic vibrations. The increase of surface
roughness and the introduction of short-range disorder are shown to modify the
phonon dispersion and phonon coherence in the hypersonic (GHz) range without
affecting the room-temperature thermal conductivity. On the basis of these
findings, we suggest a criteria for predicting phonon coherence as a function
of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201
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