42,749 research outputs found
Pilot Decontamination Through Pilot Sequence Hopping in Massive MIMO Systems
This work concerns wireless cellular networks applying massive multiple-input
multiple-output (MIMO) technology. In such a system, the base station in a
given cell is equipped with a very large number (hundreds or even thousands) of
antennas and serves multiple users. Estimation of the channel from the base
station to each user is performed at the base station using an uplink pilot
sequence. Such a channel estimation procedure suffers from pilot contamination.
Orthogonal pilot sequences are used in a given cell but, due to the shortage of
orthogonal sequences, the same pilot sequences must be reused in neighboring
cells, causing pilot contamination. The solution presented in this paper
suppresses pilot contamination, without the need for coordination among cells.
Pilot sequence hopping is performed at each transmission slot, which provides a
randomization of the pilot contamination. Using a modified Kalman filter, it is
shown that such randomized contamination can be significantly suppressed.
Comparisons with conventional estimation methods show that the mean squared
error can be lowered as much as an order of magnitude at low mobility
Log Skeletons: A Classification Approach to Process Discovery
To test the effectiveness of process discovery algorithms, a Process
Discovery Contest (PDC) has been set up. This PDC uses a classification
approach to measure this effectiveness: The better the discovered model can
classify whether or not a new trace conforms to the event log, the better the
discovery algorithm is supposed to be. Unfortunately, even the state-of-the-art
fully-automated discovery algorithms score poorly on this classification. Even
the best of these algorithms, the Inductive Miner, scored only 147 correct
classified traces out of 200 traces on the PDC of 2017. This paper introduces
the rule-based log skeleton model, which is closely related to the Declare
constraint model, together with a way to classify traces using this model. This
classification using log skeletons is shown to score better on the PDC of 2017
than state-of-the-art discovery algorithms: 194 out of 200. As a result, one
can argue that the fully-automated algorithm to construct (or: discover) a log
skeleton from an event log outperforms existing state-of-the-art
fully-automated discovery algorithms.Comment: 16 pages with 9 figures, followed by an appendix of 14 pages with 17
figure
Atomically thin dilute magnetism in Co-doped phosphorene
Two-dimensional dilute magnetic semiconductors can provide fundamental
insights in the very nature of magnetic orders and their manipulation through
electron and hole doping. Despite the fundamental physics, due to the large
charge density control capability in these materials, they can be extremely
important in spintronics applications such as spin valve and spin-based
transistors. In this article, we studied a two-dimensional dilute magnetic
semiconductors consisting of phosphorene monolayer doped with cobalt atoms in
substitutional and interstitial defects. We show that these defects can be
stabilized and are electrically active. Furthermore, by including holes or
electrons by a potential gate, the exchange interaction and magnetic order can
be engineered, and may even induce a ferromagnetic-to-antiferromagnetic phase
transition in p-doped phosphorene.Comment: 7 pages, 4 colorful figure
Massive MIMO for Crowd Scenarios: A Solution Based on Random Access
This paper presents a new approach to intra-cell pilot contamination in
crowded massive MIMO scenarios. The approach relies on two essential properties
of a massive MIMO system, namely near-orthogonality between user channels and
near-stability of channel powers. Signal processing techniques that take
advantage of these properties allow us to view a set of contaminated pilot
signals as a graph code on which iterative belief propagation can be performed.
This makes it possible to decontaminate pilot signals and increase the
throughput of the system. The proposed solution exhibits high performance with
large improvements over the conventional method. The improvements come at the
price of an increased error rate, although this effect is shown to decrease
significantly for increasing number of antennas at the base station
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