3,526 research outputs found
Is the stellar system WR 11 a gamma-ray source?
Many early-type stars are in systems; some of them have been indicated as
putative high-energy emitters. The radiation is expected to be produced at the
region where two stellar winds collide. Compelling evidence of such emission
was found only for the colliding-wind binary (CWB) Eta Car, which was
associated to a GeV source. Very recently, the closest CWB, WR 11, was proposed
as a counterpart of a 6sigma emission excess, measured with the Fermi LAT
satellite. We looked for evidence to support or reject the hypothesis that WR
11 is responsible of the gamma-ray excess. Archive radio interferometric data
at 1.4 and 2.5 GHz taken with the Australia Telescope Compact Array along
sixteen different dates were reduced. The sizes of the field-of-view at 2.5 GHz
and of the central region of the Fermi LAT excess are alike. We analyzed the
emission of the field of WR 11, characterized the radio sources detected and
derived their spectral indices, to investigate their nature. Eight sources with
fluxes above 10 mJy were detected at both frequencies. All but one (WR 11)
showed negative spectral indices. Four of them were identified with known
objects, including WR 11. A fifth source, labeled here S6, could be a promising
candidate to produce gamma-ray emission, besides the CWB WR 11.Comment: The paper has been accepted for publication in PASA; 11 page
PEBO-SLAM: Observer design for visual inertial SLAM with convergence guarantees
This paper introduces a new linear parameterization to the problem of visual
inertial simultaneous localization and mapping (VI-SLAM) -- without any
approximation -- for the case only using information from a single monocular
camera and an inertial measurement unit. In this problem set, the system state
evolves on the nonlinear manifold , on which we
design dynamic extensions carefully to generate invariant foliations, such that
the problem can be reformulated into online \emph{constant parameter}
identification, then interestingly with linear regression models obtained. It
demonstrates that VI-SLAM can be translated into a linear least squares
problem, in the deterministic sense, \emph{globally} and \emph{exactly}. Based
on this observation, we propose a novel SLAM observer, following the recently
established parameter estimation-based observer (PEBO) methodology. A notable
merit is that the proposed observer enjoys almost global asymptotic stability,
requiring neither persistency of excitation nor uniform complete observability,
which, however, are widely adopted in most existing works with provable
stability but can hardly be assured in many practical scenarios
Loss Diagnosis and Indoor Position Location System based on IEEE 802.11 WLANs
Wireless local area networks (WLANs) have been widely deployed to provide short range broadband communications. Due to the fast evolvement of IEEE 802.11 based WLAN standards and various relevant applications, many research efforts have been focused on the optimization of WLAN data rate, power and channel utilization efficiency. On the other hand, many emerging applications based on WLANs have been introduced. Indoor position location (IPL) system is one of such applications which turns IEEE 802.11 from a wireless communications infrastructure into a position location network. This thesis mainly focuses on data transmission rate enhancement techniques and the development of IEEE 802.11 WLAN based IPL system with improved locationing accuracy.
In IEEE 802.11 systems, rate adaptation algorithms (RAAs) are employed to improve transmission efficiency by choosing an appropriate modulation and coding scheme accord ing to point-to-point channel conditions. However, due to the resource-sharing nature of WLANs, co-channel interferences and frame collisions cannot be avoided, which further complicates the wireless environment and makes the RAA design a more challenging task. As WLAN performance depends on many dynamic factors such as multipath fading and co-channel interferences, differentiating the cause of performance degradation such as frame losses, which is known as loss diagnosis techniques, is essential for performance enhance ments of existing rate adaptation schemes. In this thesis, we propose a fast and reliable collision detection scheme for frame loss diagnosis in IEEE 802.11 WLANs. Collisions are detected by tracking changes of the signal-to-interference-and-noise-ratio (SINR) in IEEE 802.11 WLANs with a nonparametric order-based cumulative sum (CUSUM) algorithm for rapid loss diagnosis. Numerical simulations are conducted to evaluate the effectiveness of the proposed collision detection scheme.
The other aspect of this thesis is the investigation of an IEEE 802.11 WLAN based IPL system. WLAN based IPL systems have received increasing attentions due to their variety of potential applications. Instead of relying on dedicated locationing networks and devices, IEEE 802.11 WLAN based IPL systems utilize widely deployed IEEE 802.11 WLAN infrastructures and standardized wireless stations to determine the position of a target station in indoor environments.
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Abstract
In this thesis, a WLAN protocol-based distance measurement technique is investigated, which takes advantages of existing IEEE 802.11 data/ACK frame exchange sequences. In the proposed distance measurement technique, neither dedicated hardware nor hardware modifications is required. Thus it can be easily integrated into off-the-shelf commercial, inexpensive WLAN stations for IPL system implementation. Field test results confirm the efficacy of the proposed protocol-based distance measurement technique. Furthermore, a preliminary IPL system based on the proposed method is also developed to evaluate the feasibility of the proposed technique in realistic indoor wireless environments
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