LDPC-coded modulation for transmission over AWGN and flat rayleigh fading channels

La modulation codée est une technique de transmission efficace en largeur de bande qui intègre le codage de canal et la modulation en une seule entité et ce, afin d'améliorer les performances tout en conservant la même efficacité spectrale comparé à la modulation non codée. Les codes de parité à faible densité (low-density parity-check codes, LDPC) sont les codes correcteurs d'erreurs les plus puissants et approchent la limite de Shannon, tout en ayant une complexité de décodage relativement faible. L'idée de combiner les codes LDPC et la modulation efficace en largeur de bande a donc été considérée par de nombreux chercheurs. Dans ce mémoire, nous étudions une méthode de modulation codée à la fois puissante et efficace en largeur de bande, ayant d'excellentes performances de taux d'erreur binaire et une complexité d'implantation faible. Ceci est réalisé en utilisant un encodeur rapide, un décoder de faible complexité et aucun entrelaceur. Les performances du système proposé pour des transmissions sur un canal additif gaussien blanc et un canal à évanouissements plats de Rayleigh sont évaluées au moyen de simulations. Les résultats numériques montrent que la méthode de modulation codée utilisant la modulation d'amplitude en quadrature à M niveaux (M-QAM) peut atteindre d'excellentes performances pour toute une gamme d'efficacité spectrale. Une autre contribution de ce mémoire est une méthode simple pour réaliser une modulation codée adaptative avec les codes LDPC pour la transmission sur des canaux à évanouissements plats et lents de Rayleigh. Dans cette méthode, six combinaisons de paires encodeur modulateur sont employées pour une adaptation trame par trame. L'efficacité spectrale moyenne varie entre 0.5 et 5 bits/s/Hz lors de la transmission. Les résultats de simulation montrent que la modulation codée adaptative avec les codes LDPC offre une meilleure efficacité spectrale tout en maintenant une performance d'erreur acceptable

A Comparison Study of LDPC and BCH Codes

The need for efficient and reliable digital data communication systems has been rising rapidly in recent years. There are various reasons that have brought this need for the communication systems, among them are the increase in automatic data processing equipment and the increased need for long range communication. Therefore, the LDPC and BCH codes were developed for achieving more reliable data transmission in communication systems. This project covers the research about the LDPC and BCH error correction codes. Algorithm for simulating both the LDPC and BCH codes were also being investigated, which includes generating the parity check matrix, generating the message code in Galois array matrix, encoding the message bits, modulation and decoding the message bits for LDPC. Matlab software is used for encoding and decoding the codes. The percentage of accuracy for LDPC simulation codes are ranging from 95% to 99%. The results obtained shows that the LDPC codes are more efficient and reliable than the BCH codes coding method of error correction because the LDPC codes had a channel performance very close to the Shannon limit. LDPC codes are a class of linear block codes that are proving to be the best performing forward error correction available. Markets such as broadband wireless and mobile networks operate in noisy environments and need powerful error correction in order to improve reliability and better data rates. Through LDPC and BCH codes, these systems can operate more reliably, efficiently and at higher data rates

A Novel Seed Based Random Interleaving for OFDM System and Its PHY Layer Security Implications

Wireless channels are characterized by multipath and fading that can often cause long burst of errors. Even though, to date, many very sophisticated error correcting codes have been designed, yet none can handle long burst of errors efficiently. An interleaver, a device that distributes a burst of errors, possibly caused by a deep fade, and makes them appear as simple random errors, therefore, proves to a very useful technique when used in conjunction with an efficient error correcting code. In this work, a novel near optimal seed based random interleaver is designed. An optimal interleaver scatters a given burst of errors uniformly over a fixed block of data - a property that is measured by so called 'spread'. The design makes use of a unique seed based pseudo-random sequence generator or logistic map based chaotic sequence generator to scramble the given block of data. Since the proposed design is based on a seed based scrambler, the nature of input is irrelevant. Therefore, the proposed interleaver can interleave either the bits or the symbols or the packets or even the frames. Accordingly, in this work, we analyze the suitability of interleaver when introduced before or after the modulation in single carrier communication systems and show that interleaving the bits before modulation or interleaving the symbols after modulation has same advantage. We further show that, in an orthogonal frequency division multiplexing (OFDM) systems, the position of interleaver, whether before or after constellation mapper, has no significance, and is interchangeable. However, scrambling symbols is computationally less expensive than scrambling bits. For the purpose of analyzing the performance of the proposed seed based random interleaver, simulations are carried out in MA TLAB®. Results show that our proposed seed based random interleaver has near optimal properties of 'spread' and 'dispersion'. Furthermore, the proposed interleaver is evaluated in terms of bit error rate (BER) versus length of burst error in a single carrier system both before and after modulation. The proposed interleaver out-performs the built in RANDINTLV in MA TLAB® when used in the same system. It shows that proposed inter Ieaver can convert greater amount of burst errors into simple random errors than that of MA TLAB® interleaver. The proposed interleaver is also tested in IEEE 802.16e based WiMAX system with Stanford University Interim (SUI) channels to compare the performance of average BER versus SNR for both pre modulation and post modulation interleaver. Results show that pre modulation interleaver and post modulation has same performance. There is also a side advantage of this seed based interleaver, in that it generates a variety of unique random-looking interleaving sequences. Only a receiver that has the knowledge of the input seed can generate this sequence and no one else. If the interleaving patterns are kept secure then it can possibly be used to introduce an extra layer of security at physical (PHY) layer. In that way, at PHY layer, one builds an additional entry barrier to break through and it comes with no extra cost. This property has been investigated by carrying out key sensitivity analysis to show that the attacks to guess key can be very futile, as difference at 41 h decimal place in the initial condition can lead to entirely different scrambling

A Comparison Study of LDPC and BCH Codes

The need for efficient and reliable digital data communication systems has been rising rapidly in recent years. There are various reasons that have brought this need for the communication systems, among them are the increase in automatic data processing equipment and the increased need for long range communication. Therefore, the LDPC and BCH codes were developed for achieving more reliable data transmission in communication systems. This project covers the research about the LDPC and BCH error correction codes. Algorithm for simulating both the LDPC and BCH codes were also being investigated, which includes generating the parity check matrix, generating the message code in Galois array matrix, encoding the message bits, modulation and decoding the message bits for LDPC. Matlab software is used for encoding and decoding the codes. The percentage of accuracy for LDPC simulation codes are ranging from 95% to 99%. The results obtained shows that the LDPC codes are more efficient and reliable than the BCH codes coding method of error correction because the LDPC codes had a channel performance very close to the Shannon limit. LDPC codes are a class of linear block codes that are proving to be the best performing forward error correction available. Markets such as broadband wireless and mobile networks operate in noisy environments and need powerful error correction in order to improve reliability and better data rates. Through LDPC and BCH codes, these systems can operate more reliably, efficiently and at higher data rates

NASA Tech Briefs, August 1992

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