2,107 research outputs found
ALOHA Random Access that Operates as a Rateless Code
Various applications of wireless Machine-to-Machine (M2M) communications have
rekindled the research interest in random access protocols, suitable to support
a large number of connected devices. Slotted ALOHA and its derivatives
represent a simple solution for distributed random access in wireless networks.
Recently, a framed version of slotted ALOHA gained renewed interest due to the
incorporation of successive interference cancellation (SIC) in the scheme,
which resulted in substantially higher throughputs. Based on similar principles
and inspired by the rateless coding paradigm, a frameless approach for
distributed random access in slotted ALOHA framework is described in this
paper. The proposed approach shares an operational analogy with rateless
coding, expressed both through the user access strategy and the adaptive length
of the contention period, with the objective to end the contention when the
instantaneous throughput is maximized. The paper presents the related analysis,
providing heuristic criteria for terminating the contention period and showing
that very high throughputs can be achieved, even for a low number for
contending users. The demonstrated results potentially have more direct
practical implications compared to the approaches for coded random access that
lead to high throughputs only asymptotically.Comment: Revised version submitted to IEEE Transactions on Communication
Slotted Aloha for Networked Base Stations
We study multiple base station, multi-access systems in which the user-base
station adjacency is induced by geographical proximity. At each slot, each user
transmits (is active) with a certain probability, independently of other users,
and is heard by all base stations within the distance . Both the users and
base stations are placed uniformly at random over the (unit) area. We first
consider a non-cooperative decoding where base stations work in isolation, but
a user is decoded as soon as one of its nearby base stations reads a clean
signal from it. We find the decoding probability and quantify the gains
introduced by multiple base stations. Specifically, the peak throughput
increases linearly with the number of base stations and is roughly
larger than the throughput of a single-base station that uses standard slotted
Aloha. Next, we propose a cooperative decoding, where the mutually close base
stations inform each other whenever they decode a user inside their coverage
overlap. At each base station, the messages received from the nearby stations
help resolve collisions by the interference cancellation mechanism. Building
from our exact formulas for the non-cooperative case, we provide a heuristic
formula for the cooperative decoding probability that reflects well the actual
performance. Finally, we demonstrate by simulation significant gains of
cooperation with respect to the non-cooperative decoding.Comment: conference; submitted on Dec 15, 201
Link-Layer Coding for GNSS Navigation Messages
In this paper, we face the problem of ensuring reliability of Global Navigation Satellite Systems (GNSSs) in harsh channel conditions, where obstacles and scatter cause long outage events that cannot be counteracted with channel coding only. Our novel approach, stemming from information-theoretic considerations, is based on link-layer coding (LLC). LLC allows us to significantly improve the efficiency in terms of time-to-first-fix with respect to current operational GNSSs, which adopt carousel transmission. First, we investigate the maximum theoretical LLC gain under different Land Mobile Satellite channel conditions. Then, some practical LLC coding schemes, namely, fountain codes and a novel low-density parity-check plus low-rate repetition coding, are proposed and tested in realistic single-satellite and multi-satellite Land Mobile Satellite scenarios, considering the Galileo I/NAV message as study case. Simulation results show that our designed schemes largely improve on carousel transmission and achieve near-optimal performance with limited increase in complexity. Also, back-compatibility of LLC is assessed with respect to present-time GNSS specifications. © 2018 Institute of Navigation
Frameless ALOHA with Reliability-Latency Guarantees
One of the novelties brought by 5G is that wireless system design has
increasingly turned its focus on guaranteeing reliability and latency. This
shifts the design objective of random access protocols from throughput
optimization towards constraints based on reliability and latency. For this
purpose, we use frameless ALOHA, which relies on successive interference
cancellation (SIC), and derive its exact finite-length analysis of the
statistics of the unresolved users (reliability) as a function of the
contention period length (latency). The presented analysis can be used to
derive the reliability-latency guarantees. We also optimize the scheme
parameters in order to maximize the reliability within a given latency. Our
approach represents an important step towards the general area of design and
analysis of access protocols with reliability-latency guarantees.Comment: Accepted for presentation at IEEE Globecom 201
Design of FPGA-Implemented Reed-Solomon Erasure Code (RS-EC) Decoders With Fault Detection and Location on User Memory
Reed–Solomon erasure codes (RS-ECs) are widely used in packet communication and storage systems to recover erasures. When the RS-EC decoder is implemented on a field-programmable gate array (FPGA) in a space platform, it will suffer single-event upsets (SEUs) that can cause failures. In this article, the reliability of an RS-EC decoder implemented on an FPGA when there are errors in the user memory is first studied. Then, a fault detection and location scheme is proposed based on partial reencoding for the faults in the user memory of the RS-EC decoder. Furthermore, check bits are added in the generator matrix to improve the fault location performance. The theoretical analysis shows that the scheme could detect most faults with small missing and false detection probability. Experimental results on a case study show that more than 90% of the faults on user memory could be tolerated by the decoder, and all the other faults can be detected by the fault detection scheme when the number of erasures is smaller than the correction capability of the code. Although false alarms exist (with probability smaller than 4%), they can be used to avoid fault accumulation. Finally, the fault location scheme could accurately locate all the faults. The theoretical estimates are very close to the experiment results, which verifies the correctness of the analysis done.This work was supported in part by the National Natural Science Foundation of China under Grant 61501321, in part by the China Postdoctoral Science Foundation and Luoyang Newvid Technology Company, Ltd., and in part by the ACHILLES Project PID2019-104207RB-I00 funded by the Spanish Ministry of Science and Innovation
Optical Time-Frequency Packing: Principles, Design, Implementation, and Experimental Demonstration
Time-frequency packing (TFP) transmission provides the highest achievable
spectral efficiency with a constrained symbol alphabet and detector complexity.
In this work, the application of the TFP technique to fiber-optic systems is
investigated and experimentally demonstrated. The main theoretical aspects,
design guidelines, and implementation issues are discussed, focusing on those
aspects which are peculiar to TFP systems. In particular, adaptive compensation
of propagation impairments, matched filtering, and maximum a posteriori
probability detection are obtained by a combination of a butterfly equalizer
and four 8-state parallel Bahl-Cocke-Jelinek-Raviv (BCJR) detectors. A novel
algorithm that ensures adaptive equalization, channel estimation, and a proper
distribution of tasks between the equalizer and BCJR detectors is proposed. A
set of irregular low-density parity-check codes with different rates is
designed to operate at low error rates and approach the spectral efficiency
limit achievable by TFP at different signal-to-noise ratios. An experimental
demonstration of the designed system is finally provided with five
dual-polarization QPSK-modulated optical carriers, densely packed in a 100 GHz
bandwidth, employing a recirculating loop to test the performance of the system
at different transmission distances.Comment: This paper has been accepted for publication in the IEEE/OSA Journal
of Lightwave Technolog
Application of advanced on-board processing concepts to future satellite communications systems: Bibliography
Abstracts are presented of a literature survey of reports concerning the application of signal processing concepts. Approximately 300 references are included
Real-time transmission of digital video using variable-length coding
Huffman coding is a variable-length lossless compression technique where data with a high probability of occurrence is represented with short codewords, while 'not-so-likely' data is assigned longer codewords. Compression is achieved when the high-probability levels occur so frequently that their benefit outweighs any penalty paid when a less likely input occurs. One instance where Huffman coding is extremely effective occurs when data is highly predictable and differential coding can be applied (as with a digital video signal). For that reason, it is desirable to apply this compression technique to digital video transmission; however, special care must be taken in order to implement a communication protocol utilizing Huffman coding. This paper addresses several of the issues relating to the real-time transmission of Huffman-coded digital video over a constant-rate serial channel. Topics discussed include data rate conversion (from variable to a fixed rate), efficient data buffering, channel coding, recovery from communication errors, decoder synchronization, and decoder architectures. A description of the hardware developed to execute Huffman coding and serial transmission is also included. Although this paper focuses on matters relating to Huffman-coded digital video, the techniques discussed can easily be generalized for a variety of applications which require transmission of variable-length data
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