29,836 research outputs found
Wireless Software Synchronization of Multiple Distributed Cameras
We present a method for precisely time-synchronizing the capture of image
sequences from a collection of smartphone cameras connected over WiFi. Our
method is entirely software-based, has only modest hardware requirements, and
achieves an accuracy of less than 250 microseconds on unmodified commodity
hardware. It does not use image content and synchronizes cameras prior to
capture. The algorithm operates in two stages. In the first stage, we designate
one device as the leader and synchronize each client device's clock to it by
estimating network delay. Once clocks are synchronized, the second stage
initiates continuous image streaming, estimates the relative phase of image
timestamps between each client and the leader, and shifts the streams into
alignment. We quantitatively validate our results on a multi-camera rig imaging
a high-precision LED array and qualitatively demonstrate significant
improvements to multi-view stereo depth estimation and stitching of dynamic
scenes. We release as open source 'libsoftwaresync', an Android implementation
of our system, to inspire new types of collective capture applications.Comment: Main: 9 pages, 10 figures. Supplemental: 3 pages, 5 figure
Software Defined Radio Implementation of Carrier and Timing Synchronization for Distributed Arrays
The communication range of wireless networks can be greatly improved by using
distributed beamforming from a set of independent radio nodes. One of the key
challenges in establishing a beamformed communication link from separate radios
is achieving carrier frequency and sample timing synchronization. This paper
describes an implementation that addresses both carrier frequency and sample
timing synchronization simultaneously using RF signaling between designated
master and slave nodes. By using a pilot signal transmitted by the master node,
each slave estimates and tracks the frequency and timing offset and digitally
compensates for them. A real-time implementation of the proposed system was
developed in GNU Radio and tested with Ettus USRP N210 software defined radios.
The measurements show that the distributed array can reach a residual frequency
error of 5 Hz and a residual timing offset of 1/16 the sample duration for 70
percent of the time. This performance enables distributed beamforming for range
extension applications.Comment: Submitted to 2019 IEEE Aerospace Conferenc
Cross-Bifix-Free Codes Within a Constant Factor of Optimality
A cross-bifix-free code is a set of words in which no prefix of any length of
any word is the suffix of any word in the set. Cross-bifix-free codes arise in
the study of distributed sequences for frame synchronization. We provide a new
construction of cross-bifix-free codes which generalizes the construction in
Bajic (2007) to longer code lengths and to any alphabet size. The codes are
shown to be nearly optimal in size. We also establish new results on Fibonacci
sequences, that are used in estimating the size of the cross-bifix-free codes
Optimal Sequential Frame Synchronization
We consider the `one-shot frame synchronization problem' where a decoder
wants to locate a sync pattern at the output of a channel on the basis of
sequential observations. We assume that the sync pattern of length N starts
being emitted at a random time within some interval of size A, that
characterizes the asynchronism level between the transmitter and the receiver.
We show that a sequential decoder can optimally locate the sync pattern, i.e.,
exactly, without delay, and with probability approaching one as N tends to
infinity, if and only if the asynchronism level grows as O(exp(N*k)), with k
below the `synchronization threshold,' a constant that admits a simple
expression depending on the channel. This constant is the same as the one that
characterizes the limit for reliable asynchronous communication, as was
recently reported by the authors. If k exceeds the synchronization threshold,
any decoder, sequential or non-sequential, locates the sync pattern with an
error that tends to one as N tends to infinity. Hence, a sequential decoder can
locate a sync pattern as well as the (non-sequential) maximum likelihood
decoder that operates on the basis of output sequences of maximum length A+N-1,
but with much fewer observations.Comment: 6 page
Double-Frame Current Control with a Multivariable PI Controller and Power Compensation forWeak Unbalanced Networks
The handling of weak networks with asymmetric loads and disturbances implies
the accurate handling of the second-harmonic component that appears in an
unbalanced network. This paper proposes a classic vector control approach using
a PI-based controller with superior decoupling capabilities for operation in
weak networks with unbalanced phase voltages. A synchronization method for weak
unbalanced networks is detailed, with dedicated dimensioning rules. The use of
a double-frame controller allows a current symmetry or controlled imbalance to
be forced for compensation of power oscillations by controlling the negative
current sequence. This paper also serves as a useful reminder of the proper way
to cancel the inherent coupling effect due to the transformation to the
synchronous rotating reference frame, and of basic considerations of the
relationship between switching frequency and control bandwidth.Comment: 17 pages, contribution to the 2014 CAS - CERN Accelerator School:
Power Converters, Baden, Switzerland, 7-14 May 201
EC-GSM-IoT Network Synchronization with Support for Large Frequency Offsets
EDGE-based EC-GSM-IoT is a promising candidate for the billion-device
cellular IoT (cIoT), providing similar coverage and battery life as NB-IoT. The
goal of 20 dB coverage extension compared to EDGE poses significant challenges
for the initial network synchronization, which has to be performed well below
the thermal noise floor, down to an SNR of -8.5 dB. We present a low-complexity
synchronization algorithm supporting up to 50 kHz initial frequency offset,
thus enabling the use of a low-cost +/-25 ppm oscillator. The proposed
algorithm does not only fulfill the 3GPP requirements, but surpasses them by 3
dB, enabling communication with an SNR of -11.5 dB or a maximum coupling loss
of up to 170.5 dB.Comment: Wireless Communications and Networking Conference (WCNC), 201
Towards a new generation of transport services adapted to multimedia application
Une connexion d'ordre et de fiabilité partiels (POC, partial order connection) est une connexion de transport autorisée à perdre certains objets mais également à les délivrer dans un ordre éventuellement différent de celui d'émission. L'approche POC établit un lien conceptuel entre les protocoles sans connexion au mieux et les protocoles fiables avec connexion. Le concept de POC est motivé par le fait que dans les réseaux hétérogènes sans connexion tels qu'Internet, les paquets transmis sont susceptibles de se perdre et d'arriver en désordre, entraînant alors une réduction des performances des protocoles usuels. De plus, on montre qu'un protocole associé au transport d'un flux multimédia permet une réduction très sensible de l'utilisation des ressources de communication et de mémorisation ainsi qu'une diminution du temps de transit moyen. Dans cet article, une extension temporelle de POC, nommée TPOC (POC temporisé), est introduite. Elle constitue un cadre conceptuel permettant la prise en compte des exigences de qualité de service des applications multimédias réparties. Une architecture offrant un service TPOC est également introduite et évaluée dans le cadre du transport de vidéo MPEG. Il est ainsi démontré que les connexions POC comblent, non seulement le fossé conceptuel entre les protocoles sans connexion et avec connexion, mais aussi qu'ils surpassent les performances des ces derniers lorsque des données multimédias (telles que la vidéo MPEG) sont transportées
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