2,833 research outputs found
Scalable and Energy-Efficient Millimeter Massive MIMO Architectures: Reflect-Array and Transmit-Array Antennas
Hybrid analog-digital architectures are considered as promising candidates
for implementing millimeter wave (mmWave) massive multiple-input
multiple-output (MIMO) systems since they enable a considerable reduction of
the required number of costly radio frequency (RF) chains by moving some of the
signal processing operations into the analog domain. However, the analog feed
network, comprising RF dividers, combiners, phase shifters, and line
connections, of hybrid MIMO architectures is not scalable due to its
prohibitively high power consumption for large numbers of transmit antennas.
Motivated by this limitation, in this paper, we study novel massive MIMO
architectures, namely reflect-array (RA) and transmit-array (TA) antennas. We
show that the precoders for RA and TA antennas have to meet different
constraints compared to those for conventional MIMO architectures. Taking these
constraints into account and exploiting the sparsity of mmWave channels, we
design an efficient precoder for RA and TA antennas based on the orthogonal
matching pursuit algorithm. Furthermore, in order to fairly compare the
performance of RA and TA antennas with conventional fully-digital and hybrid
MIMO architectures, we develop a unified power consumption model. Our
simulation results show that unlike conventional MIMO architectures, RA and TA
antennas are highly energy efficient and fully scalable in terms of the number
of transmit antennas.Comment: submitted to IEEE ICC 201
Spin noise spectroscopy in GaAs (110) quantum wells: Access to intrinsic spin lifetimes and equilibrium electron dynamics
In this letter, the first spin noise spectroscopy measurements in
semiconductor systems of reduced effective dimensionality are reported. The
non-demolition measurement technique gives access to the otherwise concealed
intrinsic, low temperature electron spin relaxation time of n-doped GaAs (110)
quantum wells and to the corresponding low temperature anisotropic spin
relaxation. The Brownian motion of the electrons within the spin noise probe
laser spot becomes manifest in a modification of the spin noise line width.
Thereby, the spatially resolved observation of the stochastic spin polarization
uniquely allows to study electron dynamics at equilibrium conditions with a
vanishing total momentum of the electron system
Governance Structures, Cultural Distance, and Socialization Dynamics: Further Challenges for the Modern Corporation
This paper relates cultural distance and governance structures. We suggest a model of cultural evolution that captures the idiosyncratic socialization dynamics taking place in groups of communicating and interacting agents. Based on these processes, cultural distance within and between groups or organizational units develops. Transaction cost theorists associate higher cultural distance with higher transaction costs. Therefore, one problem of economic organization is assessing alternative governance structures in terms of the socialization dynamics they enable that entail different intraorganizational transaction costs. We assume that transaction can be organized within governance structures that allow transaction cost economizing socialization processes
GHz Spin Noise Spectroscopy in n-Doped Bulk GaAs
We advance spin noise spectroscopy to an ultrafast tool to resolve high
frequency spin dynamics in semiconductors. The optical non-demolition
experiment reveals the genuine origin of the inhomogeneous spin dephasing in
n-doped GaAs wafers at densities at the metal-to-insulator transition. The
measurements prove in conjunction with depth resolved spin noise measurements
that the broadening of the spin dephasing rate does not result from thermal
fluctuations or spin-phonon interaction, as previously suggested, but from
surface electron depletion
Efficient Data Averaging for Spin Noise Spectroscopy in Semiconductors
Spin noise spectroscopy (SNS) is the perfect tool to investigate electron
spin dynamics in semiconductors at thermal equilibrium. We simulate SNS
measurements and show that ultrafast digitizers with low bit depth enable
sensitive, high bandwidth SNS in the presence of strong optical background shot
noise. The simulations reveal that optimized input load at the digitizer is
crucial for efficient spin noise detection while the bit depth influences the
sensitivity rather weakly
S-cone signals invisible to the motion system can improve motion extraction via grouping by color
Peer reviewedPublisher PD
Intelligent Surface-Aided Transmitter Architectures for Millimeter Wave Ultra Massive MIMO Systems
In this paper, we study two novel massive multiple-input multiple-output
(MIMO) transmitter architectures for millimeter wave (mmWave) communications
which comprise few active antennas, each equipped with a dedicated radio
frequency (RF) chain, that illuminate a nearby large intelligent
reflecting/transmitting surface (IRS/ITS). The IRS (ITS) consists of a large
number of low-cost and energy-efficient passive antenna elements which are able
to reflect (transmit) a phase-shifted version of the incident electromagnetic
field. Similar to lens array (LA) antennas, IRS/ITS-aided antenna architectures
are energy efficient due to the almost lossless over-the-air connection between
the active antennas and the intelligent surface. However, unlike for LA
antennas, for which the number of active antennas has to linearly grow with the
number of passive elements (i.e., the lens aperture) due to the
non-reconfigurablility (i.e., non-intelligence) of the lens, for IRS/ITS-aided
antennas, the reconfigurablility of the IRS/ITS facilitates scaling up the
number of radiating passive elements without increasing the number of costly
and bulky active antennas. We show that the constraints that the precoders for
IRS/ITS-aided antennas have to meet differ from those of conventional MIMO
architectures. Taking these constraints into account and exploiting the
sparsity of mmWave channels, we design two efficient precoders; one based on
maximizing the mutual information and one based on approximating the optimal
unconstrained fully digital (FD) precoder via the orthogonal matching pursuit
algorithm. Furthermore, we develop a power consumption model for IRS/ITS-aided
antennas that takes into account the impacts of the IRS/ITS imperfections,
namely the spillover loss, taper loss, aperture loss, and phase shifter loss.Comment: Journal version of arXiv:1811.0294
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