621 research outputs found
Coded Index Modulation for Non-DC-Biased OFDM in Multiple LED Visible Light Communication
Use of multiple light emitting diodes (LED) is an attractive way to increase
spectral efficiency in visible light communications (VLC). A non-DC-biased OFDM
(NDC OFDM) scheme that uses two LEDs has been proposed in the literature
recently. NDC OFDM has been shown to perform better than other OFDM schemes for
VLC like DC-biased OFDM (DCO OFDM) and asymmetrically clipped OFDM (ACO OFDM)
in multiple LEDs settings. In this paper, we propose an efficient multiple LED
OFDM scheme for VLC which uses {\em coded index modulation}. The proposed
scheme uses two transmitter blocks, each having a pair of LEDs. Within each
block, NDC OFDM signaling is done. The selection of which block is activated in
a signaling interval is decided by information bits (i.e., index bits). In
order to improve the reliability of the index bits at the receiver (which is
critical because of high channel correlation in multiple LEDs settings), we
propose to use coding on the index bits alone. We call the proposed scheme as
CI-NDC OFDM (coded index NDC OFDM) scheme. Simulation results show that, for
the same spectral efficiency, CI-NDC OFDM that uses LDPC coding on the index
bits performs better than NDC OFDM
Generalized Spatial Modulation in Indoor Wireless Visible Light Communication
In this paper, we investigate the performance of generalized spatial
modulation (GSM) in indoor wireless visible light communication (VLC) systems.
GSM uses light emitting diodes (LED), but activates only of them at
a given time. Spatial modulation and spatial multiplexing are special cases of
GSM with and , respectively. We first derive an analytical
upper bound on the bit error rate (BER) for maximum likelihood (ML) detection
of GSM in VLC systems. Analysis and simulation results show that the derived
upper bound is very tight at medium to high signal-to-noise ratios (SNR). The
channel gains and channel correlations influence the GSM performance such that
the best BER is achieved at an optimum LED spacing. Also, for a fixed
transmission efficiency, the performance of GSM in VLC improves as the
half-power semi-angle of the LEDs is decreased. We then compare the performance
of GSM in VLC systems with those of other MIMO schemes such as spatial
multiplexing (SMP), space shift keying (SSK), generalized space shift keying
(GSSK), and spatial modulation (SM). Analysis and simulation results show that
GSM in VLC outperforms the other considered MIMO schemes at moderate to high
SNRs; for example, for 8 bits per channel use, GSM outperforms SMP and GSSK by
about 21 dB, and SM by about 10 dB at BER
Equipartition of Current in Parallel Conductors on Cooling Through the Superconducting Transition
Our experiments show that for two or more pieces of a wire, of different
lengths in general, combined in parallel and connected to a dc source, the
current ratio evolves towards unity as the combination is cooled to the
superconducting transition temperature Tc, and remains pinned at that value
below it. This re-distribution of the total current towards equipartition
without external fine tuning is a surprise. It can be physically understood in
terms of a mechanism that involves the flux-flow resistance associated with the
transport current in a wire of type-II superconducting material. It is the fact
that the flux-flow resistance increases with current that drives the current
division towards equipartition.Comment: Revised version of J.Phys. Condens.Matter; vol. 18(2006) L143-L147 14
pages including 3 figures; provided an explanation in terms of the physical
mechanism of flux flow induced resistance that is proportional to the
impressed current. We are adding a simple, physically robust derivation of
our equipartition without taking resort to the minimum dissipation principl
Electrochemical behaviour of titanium supported spinel type anodes
Studies on the cobalt based spinel compounds coated titanium anodes (for example, cobalt-nickel and cobaltzinc
systems) by galvanostatic polarisetion technique have been carried out under various experimental conditions.
The influence of the incorporation of nickel or zinc as oxidea to cobalt spinel has also been studied and
the rewh compared. In addition, data obtained from x-ray and SEM studii are presented
Probing local nonlinear viscoelastic properties in soft materials
Minimally invasive experimental methods that can measure local rate dependent
mechanical properties are essential in understanding the behaviour of soft and
biological materials in a wide range of applications. Needle based measurement
techniques such as Cavitation Rheology and Volume Controlled Cavity Expansion
(VCCE), allow for minimally invasive local mechanical testing, but have been
limited to measuring the elastic material properties. Here, we propose several
enhancements to the VCCE technique to adapt it for characterization of
viscoelastic response at low to medium stretch rates ( -
s). The proposed technique performs several cycles of
expansion-relaxation at controlled stretch rates in a cavity expansion setting
and then employs a large deformation viscoelastic model to capture the measured
material response. Application of the technique to soft PDMS rubber reveals
significant rate dependent material response with high precision and
repeatability, while isolating equilibrated states that are used to directly
infer the quasistatic elastic modulus. The technique is further established by
demonstrating its ability to capture changes in the rate dependent material
response of a tuneable PDMS system. The measured viscoelastic properties are
used to explain earlier reports of rate insensitive material response by needle
based methods: it is demonstrated that the conventional use of constant
volumetric rate cavity expansion can induce high stretch rates that lead to
viscoelastic stiffening and an illusion of rate insensitive material response.
We thus conclude with a cautionary note on possible overestimation of the
quasistatic elastic modulus in previous studies and suggest that the stretch
rate controlled expansion protocol, proposed in this work, is essential for
accurate estimation of both quasistatic and dynamic material parameters
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