3,127 research outputs found
Receiver Algorithms for Single-Carrier OSM Based High-Rate Indoor Visible Light Communications
In intensity-modulation and direct-detection (IM/DD) multiple-input and multiple-output (MIMO) visible light communication (VLC) systems, spatial subchannels are usually correlated, and spatial modulation is a good choice to achieve the advantages of MIMO technology. Peak-to-average power ratio (PAPR) is a key issue in VLCs due to the limited linear dynamic range of light emitting diodes (LEDs). Single-carrier communication systems have a lower PAPR than orthogonal frequency division multiplexing (OFDM) communication systems. However, it is challenging to design a single-carrier spatial modulation for high-rate transmissions because of the time domain intersymbol interference. This paper develops an optical spatial modulation (OSM) scheme based on bipolar pulse amplitude modulation (PAM) and spatial elements for high-rate indoor VLC systems. Multiple data streams can be transmitted simultaneously in the proposed scheme. Based on the transmit strategy, we develop a low-complexity receiver algorithm that achieves better bit-error rate performance than reference schemes, and the proposed OSM scheme has a much lower PAPR than OFDM based OSM schemes. When the spatial subchannels are highly correlated, a spatial area division strategy is applied, and the receiver algorithm is investigated. The symbol-error rate expression of the proposed OSM scheme is derived, and the computational complexity is analyzed
The first tests of smartphone camera exposure effect on optical camera communication links
In this paper, we study the effect of smartphone camera exposure on the performance of optical camera communications (OCC) link. The exposure parameters of image sensor sensitivity (ISO), aperture and shutter speed are included. A static OCC link with a 8Ă8 red, green and blue (RGB) LED array employed as the transmitter and a smartphone camera as the receiver is demonstrated to verify the study. Signal-to-noise ratio (SNR) analysis at different ISO values, the effect of aperture and shutter speed on communication link quality is performed. While SNRs of 20.6 dB and 16.9 dB are measured at 1 m and 2 m transmission distance, respectively for a ISO value of 100, they are decreased to 17.4 dB and 13.32 dB for a ISO of 800. The bit error rate (BER) of a 1 m long OCC link with a cameraâs shutter speed of 1/6000 s is 1.3Ă10 â3 (i.e., below the forward error correction BER limit of 3.8Ă10 â3 ) and is dropped to 0.0125 at a shutter speed of 1/20 s. This study provides insight of the basic smartphone settings and the exposure adjustment for further complex OCC links
A Survey of Air-to-Ground Propagation Channel Modeling for Unmanned Aerial Vehicles
In recent years, there has been a dramatic increase in the use of unmanned
aerial vehicles (UAVs), particularly for small UAVs, due to their affordable
prices, ease of availability, and ease of operability. Existing and future
applications of UAVs include remote surveillance and monitoring, relief
operations, package delivery, and communication backhaul infrastructure.
Additionally, UAVs are envisioned as an important component of 5G wireless
technology and beyond. The unique application scenarios for UAVs necessitate
accurate air-to-ground (AG) propagation channel models for designing and
evaluating UAV communication links for control/non-payload as well as payload
data transmissions. These AG propagation models have not been investigated in
detail when compared to terrestrial propagation models. In this paper, a
comprehensive survey is provided on available AG channel measurement campaigns,
large and small scale fading channel models, their limitations, and future
research directions for UAV communication scenarios
A Nulling Wide Field Imager for Exoplanets Detection and General Astrophysics
We present a solution to obtain a high-resolution image of a wide field with
the central source removed by destructive interference. The wide-field image is
created by aperture synthesis with a rotating sparse array of telescopes in
space. Nulling of the central source is achieved using a phase-mask
coronagraph. The full (u,v) plane coverage delivered by the 60m, six 3-meter
telescope array is particularly well-suited for the detection and
characterization of exoplanets in the infrared (DARWIN and Terrestrial Planet
Finder (TPF) missions) as well as for other generic science observations.
Detection (S/N=10) of an Earth-like planet is achieved in less than 10 hours
with a 1 micron bandwidth at 10 micron.Comment: 18 pages, 16 figures. Accepted for publication in A&
System Design Considerations for a Low-Intensity Hyperspectral Imager of Sensitive Cultural Heritage Manuscripts
Cultural heritage imaging is becoming more common with the increased availability of more complex imaging systems, including multi- and hyperspectral imaging (MSI and HSI) systems. A particular concern with HSI systems is the broadband source required, regularly including infrared and ultraviolet spectra, which may cause fading or damage to a target. Guidelines for illumination of such objects, even while on display at a museum, vary widely from one another. Standards must be followed to assure the curator to allow imaging and ensure protection of the document. Building trust in the cultural heritage community is key to gaining access to objects of significant import, thus allowing scientists, historians, and the public to view digitally preserved representations of the object, and to allow further discovery of the object through spectral processing and analysis.
Imaging was conducted with a light level of 270 lux at variable ground sample distances (GSDâs). The light level was chosen to maintain a total dose similar to an hourâs display time at a museum, based on the United Kingdom standard for cultural heritage display, PAS 198:2012. The varying GSD was used as a variable to increase signal-to-noise ratios (SNR) or decrease total illumination time on a target. This adjustment was performed both digitally and physically, and typically results in a decrease in image quality, as the spatial resolution of the image decreases.
However, a technique called âpanchromatic sharpeningâ was used to recover some of the spatial resolution. This method fuses a panchromatic image with good spatial resolution with a spectral image (either MSI or HSI) with poorer spatial resolution to construct a derivative spectral image with improved spatial resolution. Detector systems and additional methods of data capture to assist in processing of cultural heritage documents are investigated, with specific focus on preserving the physical condition of the potentially sensitive documents
Quantum Communication, Sensing and Measurement in Space
The main theme of the conclusions drawn for classical communication systems
operating at optical or higher frequencies is that there is a wellâunderstood
performance gain in photon efficiency (bits/photon) and spectral efficiency
(bits/s/Hz) by pursuing coherentâstate transmitters (classical ideal laser light)
coupled with novel quantum receiver systems operating near the Holevo limit (e.g.,
joint detection receivers). However, recent research indicates that these receivers
will require nonlinear and nonclassical optical processes and components at the
receiver. Consequently, the implementation complexity of Holevoâcapacityapproaching
receivers is not yet fully ascertained. Nonetheless, because the
potential gain is significant (e.g., the projected photon efficiency and data rate of
MIT Lincoln Laboratory's Lunar Lasercom Demonstration (LLCD) could be achieved
with a factorâofâ20 reduction in the modulation bandwidth requirement), focused
research activities on groundâreceiver architectures that approach the Holevo limit
in spaceâcommunication links would be beneficial.
The potential gains resulting from quantumâenhanced sensing systems in space
applications have not been laid out as concretely as some of the other areas
addressed in our study. In particular, while the study period has produced several
interesting highârisk and highâpayoff avenues of research, more detailed seedlinglevel
investigations are required to fully delineate the potential return relative to
the stateâofâtheâart. Two prominent examples are (1) improvements to pointing,
acquisition and tracking systems (e.g., for optical communication systems) by way
of quantum measurements, and (2) possible weakâvalued measurement techniques
to attain highâaccuracy sensing systems for in situ or remoteâsensing instruments.
While these concepts are technically sound and have very promising benchâtop
demonstrations in a lab environment, they are not mature enough to realistically
evaluate their performance in a spaceâbased application. Therefore, it is
recommended that future work follow small focused efforts towards incorporating
practical constraints imposed by a space environment.
The space platform has been well recognized as a nearly ideal environment for some
of the most precise tests of fundamental physics, and the ensuing potential of
scientific advances enabled by quantum technologies is evident in our report. For
example, an exciting concept that has emerged for gravityâwave detection is that the
intermediate frequency band spanning 0.01 to 10 Hzâwhich is inaccessible from
the groundâcould be accessed at unprecedented sensitivity with a spaceâbased
interferometer that uses shorter arms relative to stateâofâtheâart to keep the
diffraction losses low, and employs frequencyâdependent squeezed light to surpass
the standard quantum limit sensitivity. This offers the potential to open up a new
window into the universe, revealing the behavior of compact astrophysical objects
and pulsars. As another set of examples, research accomplishments in the atomic
and optics fields in recent years have ushered in a number of novel clocks and
sensors that can achieve unprecedented measurement precisions. These emerging
technologies promise new possibilities in fundamental physics, examples of which
are tests of relativistic gravity theory, universality of free fall, frameâdragging
precession, the gravitational inverseâsquare law at micron scale, and new ways of gravitational wave detection with atomic inertial sensors. While the relevant
technologies and their discovery potentials have been well demonstrated on the
ground, there exists a large gap to spaceâbased systems. To bridge this gap and to
advance fundamentalâphysics exploration in space, focused investments that further
mature promising technologies, such as spaceâbased atomic clocks and quantum
sensors based on atomâwave interferometers, are recommended.
Bringing a group of experts from diverse technical backgrounds together in a
productive interactive environment spurred some unanticipated innovative
concepts. One promising concept is the possibility of utilizing a spaceâbased
interferometer as a frequency reference for terrestrial precision measurements.
Spaceâbased gravitational wave detectors depend on extraordinarily low noise in
the separation between spacecraft, resulting in an ultraâstable frequency reference
that is several orders of magnitude better than the state of the art of frequency
references using terrestrial technology. The next steps in developing this promising
new concept are simulations and measurement of atmospheric effects that may limit
performance due to nonâreciprocal phase fluctuations.
In summary, this report covers a broad spectrum of possible new opportunities in
space science, as well as enhancements in the performance of communication and
sensing technologies, based on observing, manipulating and exploiting the
quantumâmechanical nature of our universe. In our study we identified a range of
exciting new opportunities to capture the revolutionary capabilities resulting from
quantum enhancements. We believe that pursuing these opportunities has the
potential to positively impact the NASA mission in both the near term and in the
long term. In this report we lay out the research and development paths that we
believe are necessary to realize these opportunities and capitalize on the gains
quantum technologies can offer
Laser Guide Star Design Project for the USAF John Bryan State Park Quad Axis Observatory
Atmospheric Turbulence has long remained one of the great unsolved problems in physics. Laser guide stars were invented in order for telescopes to overcome atmospheric turbulence while used in combination with adaptive optics. This study focuses on the design and implementation phase of a Rayleigh laser guide star for the John Bryan State Park Observatory, owned and operated by the United States Air Force. Atmospheric simulations, as well as optical modelling of proposed equipment, were completed to optimize the design of this laser guide star. In addition, a novel method for the implementation of the guide star onto this very unique four axis telescope platform is presented
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