6,576 research outputs found
Non-coherent detection for ultraviolet communications with inter-symbol interference
Ultraviolet communication (UVC) serves as a promising supplement to share the responsibility for the overloads in conventional wireless communication systems. One challenge for UVC lies in inter-symbol-interference (ISI), which combined with the ambient noise, contaminates the received signals and thereby deteriorates the communication accuracy. Existing coherent signal detection schemes (e.g. maximum likelihood sequence detection, MLSD) require channel state information (CSI) to compensate the channel ISI effect, thereby falling into either a long overhead and large computational complexity, or poor CSI acquisition that further hinders the detection performance. Non-coherent schemes for UVC, although capable of reducing the complexity, cannot provide high detection accuracy in the face of ISI. In this work, we propose a novel non-coherent paradigm via the exploration of the UV signal features that are insensitive to the ISI. By optimally weighting and combining the extracted features to minimize the bit error rate (BER), the optimally-weighted non-coherent detection (OWNCD) is proposed, which converts the signal detection with ISI into a binary detection framework with a heuristic decision threshold. As such, the proposed OWNCD avoids the complex CSI estimation and guarantees the detection accuracy. Compared to the state-of-the-art MLSD in the cases of static and time-varying CSI, the proposed OWNCD can gain ∼1 dB and 8 dB in signal-to-noise-ratio (SNR) at the 7% overhead FEC limit (BER of 4.5×10 −3 , respectively, and can also reduce the computational complexity by 4 order of magnitud
Single-scatter channel impulse response model of non-line-of-sight ultraviolet communications
Previous studies on the temporal characteristics of single-scatter
transmission in non-line-of-sight (NLOS) ultraviolet communications (UVC) were
based on the prolate-spheroidal coordinate system. In this work, a novel
single-scatter channel impulse response (CIR) model is proposed in the
spherical coordinate system, which is more natural and comprehensible than the
prolate-spheroidal coordinate system in practical applications. Additionally,
the results of the widely accepted Monte-Carlo (MC)-based channel model of NLOS
UVC are provided to verify the proposed single-scatter CIR model. Results
indicate that the computational time costed by the proposed single-scatter CIR
model is decreased to less than 0.7% of the MC-based one with comparable
accuracy in assessing the temporal characteristics of NLOS UVC channels.Comment: 10 pages, 4 figure
The Three Dimensional Structure of EUV Accretion Regions in AM Herculis Stars: Modeling of EUV Photometric and Spectroscopic Observations
We have developed a model of the high-energy accretion region for magnetic
cataclysmic variables and applied it to {\it Extreme Ultraviolet Explorer}
observations of 10 AM Herculis type systems. The major features of the EUV
light curves are well described by the model. The light curves exhibit a large
variety of features such as eclipses of the accretion region by the secondary
star and the accretion stream, and dips caused by material very close to the
accretion region. While all the observed features of the light curves are
highly dependent on viewing geometry, none of the light curves are consistent
with a flat, circular accretion spot whose lightcurve would vary solely from
projection effects. The accretion region immediately above the WD surface is a
source of EUV radiation caused by either a vertical extent to the accretion
spot, or Compton scattering off electrons in the accretion column, or, very
likely, both. Our model yields spot sizes averaging 0.06 R, or the WD surface area, and average spot heights of 0.023
R. Spectra extracted during broad dip phases are softer than spectra
during the out-of-dip phases. This spectral ratio measurement leads to the
conclusion that Compton scattering, some absorption by a warm absorber,
geometric effects, an asymmetric temperature structure in the accretion region
and an asymmetric density structure of the accretion columnare all important
components needed to fully explain the data. Spectra extracted at phases where
the accretion spot is hidden behind the limb of the WD, but with the accretion
column immediately above the spot still visible, show no evidence of emission
features characteristic of a hot plasma.Comment: 30 Pages, 11 Figure
Effects of Transceiver Jitter on the Performance of Optical Scattering Communication Systems
In ultraviolet communications, the transceiver jitter effects have been
ignored in previous studies, which can result in non-negligible performance
degradation especially in vibration states or in mobile scenes. To address this
issue, we model the relationship between the received power and transceiver
jitter by making use of a moment-based density function approximation method.
Based on this relationship, we incorporate the transceiver jitter effects in
combination with Poisson distribution. The error rate results are obtained
assuming on-off key modulation with optimal threshold based detection. We
validate the error rate expressions by comparing the analytical results with
Monte-Carlo simulation results. The results show that the transceiver jitter
effects cause performance degradation especially in smaller transceiver
elevation angles or in shorter distances, which are often adopted in
short-range ultraviolet communications. The results also show that larger
elevation angle cases have a better performance with respect to anti-jitter and
may perform better compared to smaller elevation angle situations in the case
of larger standard deviation of jitter. This work studies for the first time
the transceiver jitter effects in ultraviolet communications and provides
guidelines for experimental system design.Comment: 5 pages, 2 figures, comments are welcome
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