516 research outputs found
Neural networks and early fast Doppler for prediction in meteor-burst communications systems.
Thesis (Ph.D.)-University of Natal, Durban, 1994.In meteor-burst communications systems, the channel is bursty with a continuously
fluctuating signal-to-noise ratio. Adaptive data rate systems attempt to use
the channel more optimally by varying the bit rate. Current adaptive rate systems
use a method of closed-loop decision-feedback to control the transmitted data rate.
It is proposed that an open-loop adaptive data rate system without a decision feedback
path may be possible using implicit channel information carried in the first
few milliseconds of the link establishment probe signal. The system would have
primary application in low-cost half-duplex telemetry systems. It is shown that the
key elements in such a system would be channel predictors. The development of
these predictors is the focus of this research. Two novel methods of predicting
channel parameters are developed.
The first utilises early fast Doppler information that precedes many long duration,
large signal-to-noise-ratio overdense trails. The presence of early fast Doppler at
the trail commencement is used as a toggle to operate at a higher data rate. Factors
influencing the use of early fast Doppler for this purpose are also presented.
The second method uses artificial neural networks. Data measured during trail
formation is processed and presented to the neural networks for prediction of trail
parameters. Several successful neural networks are presented which predict trail
type, underdense or overdense, and peak trail amplitude from the first 50ms of the
trail's lifetime. This method allows better estimation of the developing trail. This
fact can be used to implement a multi-rate open-loop adaptive data rate system
Military Radio Communications Research in Australia
An overview of recent research by the Australian Defence Science and Technology Organisation in the field of military radio communications is presented. A philosophy for improving digital radio system performance over complex, variable channels is outlined. A key breakthrough, called PDF-directed adaptive radio, which can provide substantially greater throughput over HF channels whilst minimising bit-error rate and delay, is described. Simulation results for fast adaptive Schemes applied to both serial-tone and parallel-tone HF modems are presented and shown to significantly out-perform fixed rate modems and modems employing hybrid automatic-repeat-request schemes. A new detector scheme is discussed which has superior performance to conventional detectors for digital traffic in the presence of inter-symbol interference and impulsive noise
An adaptive protocol for use over meteor scatter channels.
Thesis (Ph.D.)-University of Natal, Durban, 1987.Modem technology has revived interest in the once popular area of meteor
scatter communications. Meteor scatter systems offer reliable
communications in the 500 to 2000 km range all day, every day. Recent
advances in microprocessor technology have made meteor scatter
communications a viable and cost effective method of providing modest data
rate communications. A return to the basic fundamentals has revealed
characteristics of meteor scatter propagation that can be used to optimize the
protocols for a meteor scatter link.
The duration of an underdense trail is bounded when its initial amplitude is
known. The upper bound of the duration is determined by maximizing the
classical underdense model. The lower bound is determined by considering
the volume of sky utilized. The duration distribution between these bounds
is computed and compared to measured values.
The duration distribution is then used to specify a fixed data rate, frame
adaptive protocol which more efficaciously utilizes underdense trails, in the
half duplex environment, than a non-adaptive protocol. The performance of
these protocols is verified by modeling
On the synergy between adaptive physical layer and multiple-access control for integrated voice and data services in a cellular wireless network
In this paper, we propose a novel design to exploit the synergy between the multiple-access control (MAC) layer and the physical layer of a cellular wireless system with integrated voice and data services. As in a traditional design, the physical layer (channel encoder and modulator) is responsible for providing error protection for transmitting the packets over the hostile radio channel, while the MAC layer is responsible for allocating the precious bandwidth to the contending users for voice or data connections. However, a distinctive feature of our proposed design is that in the physical layer, a variable-rate adaptive channel encoder is employed to dynamically adjust the amount of forward error correction according to the time-varying wireless channel state such that the MAC layer, which is a reservation-based time-division multiple-access protocol, is able to make informed decisions as to bandwidth allocation. Specifically, based on the channel state information provided by the physical layer, the MAC protocol gives higher priority to users with better channel states. This novel synergistic mechanism between the two protocol layers can utilize the system bandwidth more effectively. The multiple-access performance of the proposed scheme is compared with two baseline systems. The first baseline system consists of the same reservation-based MAC protocol but with a traditional fixed-rate physical layer. The second system consists of the same reservation-based MAC protocol and the same channel adaptive physical layer, but without interaction between the two layers. All three protocols have a request queue, which stores the previous requests that survive the contention but are not allocated information slots. Our extensive simulation results demonstrate that significant performance gains are achieved through the exploitation of the synergy between the two protocol layers.published_or_final_versio
The IceCube Neutrino Observatory: Instrumentation and Online Systems
The IceCube Neutrino Observatory is a cubic-kilometer-scale high-energy
neutrino detector built into the ice at the South Pole. Construction of
IceCube, the largest neutrino detector built to date, was completed in 2011 and
enabled the discovery of high-energy astrophysical neutrinos. We describe here
the design, production, and calibration of the IceCube digital optical module
(DOM), the cable systems, computing hardware, and our methodology for drilling
and deployment. We also describe the online triggering and data filtering
systems that select candidate neutrino and cosmic ray events for analysis. Due
to a rigorous pre-deployment protocol, 98.4% of the DOMs in the deep ice are
operating and collecting data. IceCube routinely achieves a detector uptime of
99% by emphasizing software stability and monitoring. Detector operations have
been stable since construction was completed, and the detector is expected to
operate at least until the end of the next decade.Comment: 83 pages, 50 figures; updated with minor changes from journal review
and proofin
Satellite-Based Communications Security: A Survey of Threats, Solutions, and Research Challenges
Satellite-based Communication systems are gaining renewed momentum in
Industry and Academia, thanks to innovative services introduced by leading tech
companies and the promising impact they can deliver towards the global
connectivity objective tackled by early 6G initiatives. On the one hand, the
emergence of new manufacturing processes and radio technologies promises to
reduce service costs while guaranteeing outstanding communication latency,
available bandwidth, flexibility, and coverage range. On the other hand,
cybersecurity techniques and solutions applied in SATCOM links should be
updated to reflect the substantial advancements in attacker capabilities
characterizing the last two decades. However, business urgency and
opportunities are leading operators towards challenging system trade-offs,
resulting in an increased attack surface and a general relaxation of the
available security services. In this paper, we tackle the cited problems and
present a comprehensive survey on the link-layer security threats, solutions,
and challenges faced when deploying and operating SATCOM systems.Specifically,
we classify the literature on security for SATCOM systems into two main
branches, i.e., physical-layer security and cryptography schemes.Then, we
further identify specific research domains for each of the identified branches,
focusing on dedicated security issues, including, e.g., physical-layer
confidentiality, anti-jamming schemes, anti-spoofing strategies, and
quantum-based key distribution schemes. For each of the above domains, we
highlight the most essential techniques, peculiarities, advantages,
disadvantages, lessons learned, and future directions.Finally, we also identify
emerging research topics whose additional investigation by Academia and
Industry could further attract researchers and investors, ultimately unleashing
the full potential behind ubiquitous satellite communications.Comment: 72 page
Channel-predictive link layer ARQ protocols in wireless networks
Communication performance over a wireless channel should be considered according to two main parameters: energy and throughput. The reliable data transfer is a key to these goals. The reliable node-to-node data transfer is performed by link layer protocols. One prominent approach is Automatic Repeat Request (ARQ) protocol. The traditional ARQ protocols attempt to recover the erroneously transmitted frames by retransmitting those frames, regardless of the channel state. Since this channel state unaware behaviour may cause unnecessary retransmissions, traditional ARQ protocols are expected to be energy inefficient. Some ideas have been proposed such as stochastic learning automaton based ARQ, and channel probing based ARQ. However, these algorithms do not attempt to estimate the channel\u27s existing condition. Instead, the retransmission decision is made according to a simple feedback, on whether the previous frame was successful.
This thesis presents four proposed algorithms, which incorporates the channel state estimate in the feedback process to judiciously select a frame (re)transmission timing instant. Algorithms have been applied on Stop-and-Wait (S-W) ARQ, and the performance have been compared with respect to simple S-W ARQ, and probing based S-W ARQ. In probing based ARQ, when the channel deteriorates, transmitter starts probing channel periodically, but the periodicity is chosen arbitrarily, regardless of the fading state. In contrast, the proposed algorithms estimate the channel\u27s existing condition by using feedbacks, and the probing interval is chosen according to the Average Fading Duration (AFD) of received signal. Simulations are performed with Rayleigh Fading Channel. The performance results show that at the cost of some additional delay, significiant gain on energy saving and throughput performance can be achieved when AFD based intelligent probing is done
NASA/MSFC FY-85 Atmospheric Processes Research Review
The two main areas of focus for the research program are global scale processes and mesoscale processes. Geophysical fluid processes, satellite doppler lidar, satellite data analysis, atmospheric electricity, doppler lidar wind research, and mesoscale modeling are among the topics covered
- …