1,892 research outputs found
๊ฐ์ญ ํ๊ฒฝ์์ ์ ์ ๋ ฅ ๋ฌด์ ์ผ์ ๋คํธ์ํน์ ๊ดํ ์ฐ๊ตฌ
ํ์๋
ผ๋ฌธ (๋ฐ์ฌ)-- ์์ธ๋ํ๊ต ๋ํ์ : ์ ๊ธฐยท์ปดํจํฐ๊ณตํ๋ถ, 2017. 2. ์ด์ฉํ.The demand for commercial deployment of large-scale wireless sensor networks (WSNs) has rapidly been increasing over the past decade. However, conventional WSN technologies may not be feasible for commercial deployment of large-scale WSNs because of their technical flaws, including limited network scalability, susceptibility to co-channel interference and large signaling overhead. In practice, low-power WSNs seriously suffer from interference generated by coexisting radio systems such as IEEE 802.11 wireless local area networks (WLANs). This interference problem seriously hampers commercial deployment of low-power WSNs. Few commercial WSN chips can provide secure and reliable networking performance in practical operation environments.
In this dissertation, we consider performance improvement of low-power WSNs in the presence of co-channel interference. We first investigate the effect of co-channel interference on the transmission of low-power WSN signal, and then design a low-power WSN transceiver that can provide stable performance even in the presence of severe co-channel interference, while providing the backward compatibility with IEEE 802.15.4.
We also consider the network connectivity in the presence of co-channel interference. The connectivity of low-power WSNs can be improved by transmitting synchronization signal and making channel hand-off in a channel-aware manner. A beacon signal for the network synchronization is repeatedly transmitted in consideration of channel condition and signaling overhead. Moreover, when the channel is severely interfered, all devices in a cluster network make communications by means of temporary channel hopping and then seamlessly make channel hand-off to the best one among the temporary hopping channels. The performance improvement is verified by computer simulation and experiment using IEEE 802.15.4 motes in real operation environments.
Finally, we consider the signal transmission in the presence of co-channel interference. The throughput performance of low-power WSN transceivers can be improved by adjusting the transmission rate and the payload size according to the interference condition. We estimate the probability of transmission failure and the data throughput, and then determine the payload size to maximize the throughput performance. It is shown that the transmission time maximizing the normalized throughput is little affected by the transmission rate, but rather by the interference condition. The transmission rate and the transmission time can independently be adjusted in response to the change of channel and interference condition, respectively. The performance improvement is verified by computer simulation.Chapter 1 1
Chapter 2 11
2.1. ZigBee/IEEE 802.15.4-based cluster-tree networks 11
2.2. Performance of IEEE 802.15.4 transceiver 14
Chapter 3 17
3.1. System model 18
3.2. Previous works 21
3.3. Proposed interference management scheme 28
3.4. Performance evaluation 37
Chapter 4 51
4.1. System model 52
4.2. Transmission in the presence of interference 56
4.3. Proposed transmission scheme 60
4.4. Performance evaluation 65
Chapter 5 82
Appendix 85
A. Average synchronization time during frequency hopping 85
B. Derivation of (4.2) 86
References 88
Korean Abstract 97Docto
Double resonance of Raman transitions in a degenerate Fermi gas
We measure momentum-resolved Raman spectra of a spin-polarized degenerate
Fermi gas of Yb atoms for a wide range of magnetic fields, where the
atoms are irradiated by a pair of counterpropagating Raman laser beams as in
the conventional spin-orbit coupling scheme. Double resonance of first- and
second-order Raman transitions occurs at a certain magnetic field and the
spectrum exhibits a doublet splitting for high laser intensities. The measured
spectral splitting is quantitatively accounted for by the Autler-Townes effect.
We show that our measurement results are consistent with the spinful band
structure of a Fermi gas in the spatially oscillating effective magnetic field
generated by the Raman laser fields.Comment: 7 pages, 6 figure
Perfect Basis Theory for Quantum Borcherds-Bozec Algebras
In this paper, we develop the perfect basis theory for quantum
Borcherds-Bozec algebras and their irreducible highest
weight modules . We show that the perfect graph (resp. dual perfect
graph) of every perfect basis (resp. dual perfect basis) of
(resp. ) is isomorphic to
(resp. ). For this purpose, we define a new class of Kashiwara
operators which is different from the one given by Bozec and prove all the
interlocking inductive statements in Kashiwara's grand loop argument, which
shows the existence and the uniqueness of crystal bases for quantum
Borcherds-Bozec algebras
Two distinct red giant branch populations in the globular cluster NGC 2419 as tracers of a merger event in the Milky Way
Recent spectroscopic observations of the outer halo globular cluster (GC) NGC
2419 show that it is unique among GCs, in terms of chemical abundance patterns,
and some suggest that it was originated in the nucleus of a dwarf galaxy. Here
we show, from the Subaru narrow-band photometry employing a calcium filter,
that the red giant-branch (RGB) of this GC is split into two distinct
subpopulations. Comparison with spectroscopy has confirmed that the redder RGB
stars in the [=(Ca] index are enhanced in [Ca/H] by 0.2
dex compared to the bluer RGB stars. Our population model further indicates
that the calcium-rich second generation stars are also enhanced in helium
abundance by a large amount (Y = 0.19). Our photometry, together with
the results for other massive GCs (e.g., Cen, M22, and NGC 1851),
suggests that the discrete distribution of RGB stars in the index might be
a universal characteristic of this growing group of peculiar GCs. The planned
narrow-band calcium photometry for the Local Group dwarf galaxies would help to
establish an empirical connection between these GCs and the primordial building
blocks in the hierarchical merging paradigm of galaxy formation.Comment: 4 pages, 4 figures, 1 table, accepted for the publication in ApJ
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