399 research outputs found
Millimeter Wave Beam Alignment: Large Deviations Analysis and Design Insights
In millimeter wave cellular communication, fast and reliable beam alignment
via beam training is crucial to harvest sufficient beamforming gain for the
subsequent data transmission. In this paper, we establish fundamental limits in
beam-alignment performance under both the exhaustive search and the
hierarchical search that adopts multi-resolution beamforming codebooks,
accounting for time-domain training overhead. Specifically, we derive lower and
upper bounds on the probability of misalignment for an arbitrary level in the
hierarchical search, based on a single-path channel model. Using the method of
large deviations, we characterize the decay rate functions of both bounds and
show that the bounds coincide as the training sequence length goes large. We go
on to characterize the asymptotic misalignment probability of both the
hierarchical and exhaustive search, and show that the latter asymptotically
outperforms the former, subject to the same training overhead and codebook
resolution. We show via numerical results that this relative performance
behavior holds in the non-asymptotic regime. Moreover, the exhaustive search is
shown to achieve significantly higher worst-case spectrum efficiency than the
hierarchical search, when the pre-beamforming signal-to-noise ratio (SNR) is
relatively low. This study hence implies that the exhaustive search is more
effective for users situated further from base stations, as they tend to have
low SNR.Comment: Author final manuscript, to appear in IEEE Journal on Selected Areas
in Communications (JSAC), Special Issue on Millimeter Wave Communications for
Future Mobile Networks, 2017 (corresponding author: Min Li
Design and Analysis of Transmit Beamforming for Millimetre Wave Base Station Discovery
In this paper, we develop an analytical framework for the initial access
(a.k.a. Base Station (BS) discovery) in a millimeter-wave (mm-wave)
communication system and propose an effective strategy for transmitting the
Reference Signals (RSs) used for BS discovery. Specifically, by formulating the
problem of BS discovery at User Equipments (UEs) as hypothesis tests, we derive
a detector based on the Generalised Likelihood Ratio Test (GLRT) and
characterise the statistical behaviour of the detector. The theoretical results
obtained allow analysis of the impact of key system parameters on the
performance of BS discovery, and show that RS transmission with narrow beams
may not be helpful in improving the overall BS discovery performance due to the
cost of spatial scanning. Using the method of large deviations, we identify the
desirable beam pattern that minimises the average miss-discovery probability of
UEs within a targeted detectable region. We then propose to transmit the RS
with sequential scanning, using a pre-designed codebook with narrow and/or wide
beams to approximate the desirable patterns. The proposed design allows
flexible choices of the codebook sizes and the associated beam widths to better
approximate the desirable patterns. Numerical results demonstrate the
effectiveness of the proposed method.Comment: 30 pages, 13 figures, submitte
Non-Gaussianity, Isocurvature Perturbation, Gravitational Waves and a No-Go Theorem for Isocurvaton
We investigate the isocurvaton model, in which the isocurvature perturbation
plays a role in suppressing the curvature perturbation, and large
non-Gaussianity and gravitational waves can be produced with no isocurvature
perturbation for dark matter. We show that in the slow roll non-interacting
multi-field theory, the isocurvaton mechanism can not be realized. This result
can also be generalized to most of the studied models with generalized kinetic
terms. We also study the implications for the curvaton model. We show that
there is a combined constraint for curvaton on non-Gaussianity, gravitational
waves and isocurvature perturbation. The technique used in this paper can also
help to simplify some calculations in the mixed inflaton and curvaton models.
We also investigate possibilities to produce large negative non-Gaussianity and
nonlocal non-Gaussianity in the curvaton model.Comment: 23 pages, 1 figur
Existence of Solutions for a Variable Exponent System without PS Conditions
In this article, we study the existence of solution for the following elliptic system of variable exponents with perturbation terms − div |∇u| p(x)−2∇u) + |u| p(x)−2u = λa(x)|u| γ(x)−2u + Fu(x, u, v) in R N , − div |∇v| q(x)−2∇v) + |v| q(x)−2 v = λb(x)|v| δ(x)−2 v + Fv(x, u, v) in R N , u ∈ W1,p(·) (R N ), v ∈ W1,q(·) (R N ), where the corresponding functional does not satisfy PS conditions. We obtain a sufficient condition for the existence of solution and also present a result on asymptotic behavior of solutions at infinity
Multiple Solutions With Constant Sign of a Dirichlet Problem for a Class of Elliptic Systems With Variable Exponent Growth
We present here, in the system setting, a new set of growth conditions under which we manage to use a novel method to verify the Cerami compactness condition. By localization argument, decomposition technique and variational methods, we are able to show the existence of multiple solutions with constant sign for the problem without the well-known Ambrosetti--Rabinowitz type growth condition. More precisely, we manage to show that the problem admits four, six and infinitely many solutions respectively
Screening effects on field emission from arrays of (5,5) carbon nanotubes: Quantum-mechanical simulation
The simulation of field electron emission from arrays of micrometer-long
open-ended (5, 5) carbon nanotubes is performed in the framework of quantum
theory of many electrons. It is found that the applied external field is
strongly screened when the spacing distance is shorter than the length of the
carbon nanotubes. The optimal spacing distance is two to three times of the
nanotube length, slightly depending on the applied external fields. The
electric screening can be described by a factor that is a exponential function
of the ratio of the spacing distance to the length of the carbon nanotubes. For
a given length, the field enhancement factor decreases sharply as the screening
factor larger than 0.05. The simulation implies that the thickness of the array
should be larger than a value but it does not help the emission much by
increasing the thickness a great deal
The Role of Paragus quadri-fasciatus Meigen on Aphid Control and the Observations of its Biological Characteristics
Four-strip small syrphid fly, Paragus quadri-fasciatus Meigen is the important natural enemy of aphids in our region. A fly can eat about 800 aphids during its whole life. There are more than 10 kinds of aphids can be food of this fly, such as soybean aphid, Chinese sorghum aphid and radish aphid etc. The fly has 3~4 generations each year in Tonghua county, Jilin province. The adult of the first generation appears after the last ten-day period of April each year. It takes 30~35 days to complete one generation. The fly can oviposit 84~124 eggs in its whole life. Major natural enemies of the fly are ichneumon wasps, spiders, lacewings and etc.Originating text in Chinese.Citation: Gao, Junfeng, Jiang, Lianfeng, Zhang, Guangxin, Li, Chunshan, Zhao, Guangquan. (1996). The Role of Paragus quadri-fasciatus Meigen on Aphid Control and the Observations of its Biological Characteristics. Journal of Jilin Agricultural Sciences, 5(2), 60-61
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