4,766 research outputs found
Fast growing tree species in alley cropping systems and their influence on microclimate in Germany
Paper presented at the 13th North American Agroforesty Conference, which was held June 19-21, 2013 in Charlottetown, Prince Edward Island, Canada.In Poppy, L., Kort, J., Schroeder, B., Pollock, T., and Soolanayakanahally, R., eds. Agroforestry: Innovations in Agriculture. Proceedings, 13th North American Agroforestry Conference, Charlottetown, Prince Edward Island, Canada, June 19-21, 2013.The production of energy wood on arable land has been increased in Germany during the last years. In this context, agroforestry systems keep a prominent position in agriculture, since they allow the simultaneous production of energy wood and food or feed on the same field. Fast growing trees arranged in hedge structures (alley cropping) can have positive effects on microclimate. Results of different research studies carried out in several alley cropping sites located in eastern Germany show that wind velocity can be reduced by more than 50 percent, even though tree hedgerows were not higher than four meters. The observed reduction of wind speed was depending on the distance to trees, on the orientation of tree hedges as well as on the width of the crop alleys. Potentially negative effects on crop yield were expected due to the shading the peripheries of crop alleys by trees. However, first results indicate that the reduction of the global radiation by short rotation trees did no show any negative effect on crop yield. As an exception, the crop yield on a post-mining site was evenly higher near trees compared to the center of crop alleys. In summary, the establishment of alley cropping with fast growing trees have positive effects on microclimate and hence on the yield stability of crops cultivated in between the tree hedgerows without any significantly negative impact on the recent practice of land management.Christian B�hm (1), Michael Kanzler (1) and Dirk Freese (1) ; 1. Brandenburg University of Technology, Chair of Soil Protection and Recultivation, Konrad-Wachsmann-Allee 6, D-03046 Cottbus, Germany.Includes bibliographical references
Dequantization via quantum channels
For a unital completely positive map ("quantum channel") governing the
time propagation of a quantum system, the Stinespring representation gives an
enlarged system evolving unitarily. We argue that the Stinespring
representations of each power of the single map together encode the
structure of the original quantum channel and provides an interaction-dependent
model for the bath. The same bath model gives a "classical limit" at infinite
time in the form of a noncommutative "manifold" determined by the
channel. In this way a simplified analysis of the system can be performed by
making the large- approximation. These constructions are based on a
noncommutative generalization of Berezin quantization. The latter is shown to
involve very fundamental aspects of quantum-information theory, which are
thereby put in a completely new light
Cooperative Cargo Transport by Several Molecular Motors
The transport of cargo particles which are pulled by several molecular motors
in a cooperative manner is studied theoretically. The transport properties
depend primarily on the maximal number, , of motor molecules that may pull
simultaneously on the cargo particle. Since each motor must unbind from the
filament after a finite number of steps but can also rebind to it again, the
actual number of pulling motors is not constant but varies with time between
zero and . An increase in the maximal number leads to a strong increase
of the average walking distance (or run length) of the cargo particle. If the
cargo is pulled by up to kinesin motors, e.g., the walking distance is
estimated to be micrometers which implies that seven or eight
kinesin molecules are sufficient to attain an average walking distance in the
centimeter range. If the cargo particle is pulled against an external load
force, this force is shared between the motors which provides a nontrivial
motor-motor coupling and a generic mechanism for nonlinear force-velocity
relationships. With increasing load force, the probability distribution of the
instantenous velocity is shifted towards smaller values, becomes broader, and
develops several peaks. Our theory is consistent with available experimental
data and makes quantitative predictions that are accessible to systematic in
vitro experiments.Comment: 24 pages, latex, 6 figures, includes Supporting Tex
Quantum Channels with Memory
We present a general model for quantum channels with memory, and show that it
is sufficiently general to encompass all causal automata: any quantum process
in which outputs up to some time t do not depend on inputs at times t' > t can
be decomposed into a concatenated memory channel. We then examine and present
different physical setups in which channels with memory may be operated for the
transfer of (private) classical and quantum information. These include setups
in which either the receiver or a malicious third party have control of the
initializing memory. We introduce classical and quantum channel capacities for
these settings, and give several examples to show that they may or may not
coincide. Entropic upper bounds on the various channel capacities are given.
For forgetful quantum channels, in which the effect of the initializing memory
dies out as time increases, coding theorems are presented to show that these
bounds may be saturated. Forgetful quantum channels are shown to be open and
dense in the set of quantum memory channels.Comment: 21 pages with 5 EPS figures. V2: Presentation clarified, references
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Impact of User Pairing on 5G Non-Orthogonal Multiple Access
Non-orthogonal multiple access (NOMA) represents a paradigm shift from
conventional orthogonal multiple access (MA) concepts, and has been recognized
as one of the key enabling technologies for 5G systems. In this paper, the
impact of user pairing on the performance of two NOMA systems, NOMA with fixed
power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA), is
characterized. For FNOMA, both analytical and numerical results are provided to
demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the
performance gain of F-NOMA over conventional MA can be further enlarged by
selecting users whose channel conditions are more distinctive. For CR-NOMA, the
quality of service (QoS) for users with the poorer channel condition can be
guaranteed since the transmit power allocated to other users is constrained
following the concept of cognitive radio networks. Because of this constraint,
CR-NOMA has different behavior compared to F-NOMA. For example, for the user
with the best channel condition, CR-NOMA prefers to pair it with the user with
the second best channel condition, whereas the user with the worst channel
condition is preferred by F-NOMA
Orbital frequencies in the carbonate sedimentary record: distorted by diagenesis?
The most important archive of Earth’s climate change through geologic history is the sedimentary rock record. Rhythmic sedimentary alternations are usually interpreted as a consequence of periodic variations in the orbital parameters of the Earth. This interpretation enables the application of cyclostratigraphy as a very precise chronometer, when based on the assumption that orbital frequencies are faithfully recorded in the sedimentary archive. However, there are numerous uncertainties with the application of this concept. Particularly in carbonates, sediment properties such as mineralogical composition and fossil associations are severely altered during post-depositional alteration (diagenesis). We here point out that the assumption of a 1:1 recording of orbital signals in many cases is questionable for carbonate rhythmites. We use computer simulations to show the effect of diagenetic overprint on records of orbital signals in the carbonate record. Such orbital signals may be distorted in terms of frequency, amplitude, and phase by diagenetic processes, and cycles not present in the insolation record may emerge. This questions the routine use of carbonate rhythmites for chronostratigraphic datin
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