1,721,037 research outputs found
Short codes for telecommand space link
No full textLo scopo di questa tesi ´e studiare l’impatto di diversi algoritmi di decodifica sulle performance dei codici a correzione d’errore proposti per l’aggiornamento dello standard del telocommand link nelle missioni spaziali, che sono rappresentati dai codici Low Density Parity Check (LDPC) binari e non
binari.
Per la valutazione delle performance si sono considerati tre diversi modelli di canale di grande rilevanza nello comunicazioni con lo spazio, ossia: Additive White Gaussian Noise (AWGN), canale AWGN con pulsed jamming e canale AWGN con scintillazione solare; e tre diverse metriche, ossia:
Codeword Error Rate (CER), Undetected Codeword Error Rate (UCER) e
complessità.
La lista degli algoritmi considerati include algoritmi iterativi, come: il Sum Product Algorithm (SPA), l’algoritmo Min-Sum (MS) e l’algoritmo Normalized Min-Sum (NMS); ma anche una procedura non iterativa, ossia l’algortimo Most Reliable Basis (MRB). Infine per i codici non binari consideriamo una specifica implementazione del metodo Belief Propagation (BP).
Oltre a questi schemi di decodifica classici, viene proposto anche un approccio alternativo, chiamato decoder ibrido. Il decoder ibrido è costituito dalla combinazione parallela di un algoritmo iterativo e di un algoritmo basato sull’affidabilità. Vengono mostrati i vantaggi che questo schema ´è in grado di fornire rispetto agli algoritmi di decodifica classici. Nello specifico, il decoder ibrido combina le eccellenti performance in termini di error
rate dell’algoritmo MRB con una complessità media lievemente maggiore di
quella degli algoritmi iterativi.
Gran parte del lavoro è stato sviluppato tramite analisi teorica e simulazioni. Tuttavia, vengono anche fornite alcune considerazioni preliminari sull’implementazione pratica dei più efficienti algoritmi di decodifica. In par-
ticolare, viene discussa la realizzazione dell’algoritmo MRB su Field Programmable Gate Array (FPGA) proponendo, come possibile soluzione, una implementazione combinata hardware-software
Telecommand Rejection Probability in CCSDS-Compliant LDPC-Coded Space Transmissions with Tail Sequence
Full text linkAccording to the Consultative Committee for Space Data Systems (CCSDS) recommendation for TeleCommand (TC) synchronization and coding, the Communications Link Transmission Unit (CLTU) consists of a start sequence, followed by coded data, and a tail sequence, which might be optional depending on the employed coding scheme. With regard to the latter, these transmissions traditionally use a modified Bose–Chaudhuri–Hocquenghem (BCH) code, to which two state-of-the-art Low-Density Parity-Check (LDPC) codes were later added. A low-complexity approach classically used to detect CLTU termination is to choose a non-correctable string as the tail sequence, and then exploit the decoder failure on that sequence as termination detection. This works very well with the BCH code, for which bounded-distance decoders are employed. Instead, when the same approach is employed with LDPC codes and probabilistic belief propagation iterative decoders, the scenario becomes more challenging. In this paper, we study CCSDS-compliant space communications in which LDPC codes are employed, and analyze the TC rejection probability both theoretically and through intensive numerical simulations. Such a performance figure, being the rate at which CLTUs are discarded, should clearly be minimized. Our numerical analysis considers many different choices of the system parameters (e.g., length of the CLTU, decoding algorithm, maximum number of decoding iterations). Particular attention is devoted to the probability of not-acknowledged termination, i.e., the probability that the tail sequence is not recognized
AONT-LT: A data protection scheme for cloud and cooperative storage systems
No full textWe propose a variant of the well-known AONT-RS
scheme for dispersed storage systems. The novelty consists in
replacing the Reed-Solomon code with rateless Luby transform
codes. The resulting system, named AONT-LT, is able to improve
the performance by dispersing the data over an arbitrarily large
number of storage nodes while ensuring limited complexity.
The proposed solution is particularly suitable in the case of
cooperative storage systems. It is shown that while the AONT-RS
scheme requires the adoption of fragmentation for achieving
widespread distribution, thus penalizing the performance, the
new AONT-LT scheme can exploit variable length codes which
allow to achieve very good performance and scalability
A physical layer secured key distribution technique for IEEE 802.11g wireless networks
No full textKey distribution and renewing in wireless local area
networks is a crucial issue to guarantee that unauthorized users
are prevented from accessing the network. In this paper, we
propose a technique for allowing an automatic bootstrap and
periodic renewing of the network key by exploiting physical
layer security principles, that is, the inherent differences among
transmission channels. The proposed technique is based on
scrambling of groups of consecutive packets and does not need
the use of an initial authentication nor automatic repeat request
protocols. We present a modification of the scrambling circuits
included in the IEEE 802.11g standard which allows for a suitable
error propagation at the unauthorized receiver, thus achieving
physical layer security
A hybrid decoding scheme for short non-binary LDPC codes
No full textIn this paper, an iterative soft-decision hybrid decoding
algorithm for non-binary low-density parity-check (LDPC)
codes with short codeword lengths is proposed. The rationale of
the approach is to combine the classical belief propagation (BP)
iterative LDPC decoding algorithm with the most reliable basis
(MRB) decoding algorithm. This allows to achieve significant performance
improvements, with a complexity that, for medium/low
error rates, is only slightly higher than that of the BP algorithm
alone. The performance improvement with respect to pure BP
decoding is up to 0.7 dB at codeword error rate (CER) ≈ 10−5.
Notably, for a fixed MRB order, hybrid decoding achieves a gain
up to 0.5 dB at CER ≈ 10−5 with respect to BP decoding and
MRB decoding used alon
Improving the efficiency of the LDPC code-based McEliece cryptosystem through irregular codes
No full textLDPC coded modulation schemes with largely unequal error protection
No full textCoding and modulation schemes able to achieve
unequal error protection are of interest for many applications
in which parts of the payload must be differently protected
against the noise. They are also useful for physical layer security
of transmissions over the broadcast channel with confidential
messages. Classical design approaches aim at optimizing the
performance over all the protection classes, independently of
the separation between them. We instead propose a solution to
improve the performance over the most protected bits, at the
expense of performance over the least protected ones. This allows
to design coded modulation schemes with largely unequal error
protection. We also consider the use of high order modulations,
and propose a technique to study the performance over each
protection class in the asymptotic regime of infinite code length
Practical LDPC coded modulation schemes for the fading broadcast channel with confidential messages
No full textThe broadcast channel with confidential messages
is a well studied scenario from the theoretical standpoint, but
there is still lack of practical schemes able to achieve some
fixed level of reliability and security over such a channel. In this
paper, we consider a quasi-static fading channel in which both
public and private messages must be sent from the transmitter
to the receivers, and we aim at designing suitable coding and
modulation schemes to achieve such a target. For this purpose,
we adopt the error rate as a metric, by considering that reliability
(security) is achieved when a sufficiently low (high) error rate is
experienced at the receiving side. We show that some conditions
exist on the system feasibility, and that some outage probability
must be tolerated to cope with the fading nature of the channel.
The proposed solution exploits low-density parity-check codes
with unequal error protection, which are able to guarantee two
different levels of protection against noise for the public and the
private information, in conjunction with different modulation
schemes for the public and the private message bits
A practical viewpoint on the performance of LDPC codes over the fast Rayleigh fading wire-tap channel
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