Consistent Reconstruction of the Input of an Oversampled Filter Bank From Noisy Subbands

This paper introduces a reconstruction approach for the input signal of an oversampled filter bank (OFB) when the sub-bands generated at its output are quantized and transmitted over a noisy channel. This approach exploits the redundancy introduced by the OFB and the fact that the quantization noise is bounded. A maximum-likelihood estimate of the input signal is evaluated, which only considers the vectors of quantization indexes corresponding to subband signals that could have been generated by the OFB and that are compliant with the quantization errors. When considering an OFB with an oversampling ratio of 3/2 and a transmission of quantized subbands on an AWGN channel, compared to a classical decoder, the performance gains are up to 9 dB in terms of SNR for the reconstructed signal, and 3 dB in terms of channel SNR.Comment: European Signal Processing Conference (2011

Reconstruction cohérente de l'entrée d'un banc de filtres suréchantillonnés à partir de sa sortie bruitée

International audienceIn this paper we introduce a reconstruction approach for the input signal of an oversampled filter bank (OFB) when the subbands generated at its output are quantized and transmitted over a noisy channel. We exploit the redundancy introduced by the OFB and the bounded quantization noise in order to construct a consistent estimator that corrects transmission errors. A maximum-likelihood estimation of the quantization indexes transmitted over the channel is evaluated, which only considers the vectors of quantization indexes corresponding to subband signals that could have been generated by the OFB and that are compliant with the quantization errors. Neither hypothesis tests nor specific parameters need to be set or computed in advance as is the case in approaches presented in \cite{Redinbo00,LabeauChiang2004}. When considering an OFB with oversampling ratio $3/2$, a BPSK modulation of the quantized subbands and a transmission over an AWGN channel, compared to a classical decoder, the gain is about $8$ dB in terms of reconstructed signal SNR for a channel SNR of $7$ dB

Interference mitigation techniques for wireless OFDM

Orthogonal Frequency Division Multiplexing (OFDM) is a promising multicarrier wireless system for transmission of high-rate data stream with spectral efficiency and fading immunity. Conventional OFDM system use efficient IFFT and FFT to multiplex the signals in parallel at the transmitter and receiver respectively. On the other hand, wavelet based OFDM system uses orthonormal wavelets which are derived from a multistage tree-structured wavelet family. The Fourier based and wavelet based OFDM systems are studied in this dissertation. Two types of QAM schemes, circular and square modulations are used to compare the performance in both OFDM systems. A new approach of determining exact BER for optimal circular QAM is proposed. In addition, the presence of narrowband interference (NBI) degrades the performance of OFDM systems. Thus, a mitigation technique is necessary to suppress NBI in an OFDM system. Recent mitigation techniques can be broadly categorized into frequency domain cancellation, receiver windowing and excision filtering. However, none of the techniques considers wavelet based OFDM. Therefore, an interference cancelation algorithm has been proposed to work for both OFDM platforms. The performance results of two OFDM schemes applicable to digital video broadcasting (DVB)-terrestrial system and under the effect of impulsive noise interference are also studied. BER performances are obtained in all results. It has been shown that wavelet based OFDM system has outperformed Fourier based OFDM system in many cases

Strategies for changing the redundancy of sub-band signals in oversampled filter banks

Communication channels are always vulnerable to noise sources. Thereby, any kind of data transmitted over these channels suffer from errors that happen during transmission which degrades the quality of the signal on the receiver side. To tackle this problem, some amount of redundancy should be added to the input signal so on the receiver side, this redundancy could be exploited to remove the errors. However, most of the existing schemes for adding redundancy to the signal have two main drawbacks. First, they are designed to work decently as long as the channel quality is always above a certain threshold. But when the channel quality falls under that threshold, the system tends to break down completely and is no longer capable of correcting the errors (threshold effect). Second, most of the current error correction schemes lack the inherent adaptability and flexibility for changing the added redundancy which is necessary due to the unavoidable change in channel characteristics.In this thesis, we will explore the methods of signal representation that are naturally amenable to transmission at flexible rates and offer graceful degradation in signal quality with degrading link quality. For such applications, we will consider the insertion of soft, structured redundancy in signals to be transmitted, through the use of Oversampled Filter Banks (OFBs). These codes enable a joint compression and protection of signals through the application of a redundant transform (or frame operator) to the signal, which serves both for signal conditioning for compression and for insertion of redundancy in the signal so as to be able to combat adverse channel effects. More specifically, in this thesis, we extend the framework of redundant expansions to the case where the amount of injected redundancy can be adjusted on the fly. This will be accomplished through two main approaches. The first method is called Adaptive Downsampling. Here, the idea is to change the downsampling rate of the OFB as a response to the changes in channel quality. We investigate the effect of such downsampling variation on filtering the input signal vectors on analysis side and propose suitable signal reconstruction method on the synthesis side of the OFB. In the second method, we introduce the notion of Instantaneous Erasures in OFBs which accounts for a situation when samples of sub-band vectors are erased arbitrarily in different time instances. We find the conditions under which the OFB maintains its perfect reconstruction property using time-domain analysis and we propose structures in order to be able to design implementable OFBs.Les canaux de communications sont vulnérables aux sources de bruit. De ce fait, n'importe quelle donnée transmise sur ces canaux peut être victime d'erreurs survenant lors de la transmission, dégradant ainsi la qualité du signal à la réception. Afin de résoudre ce problème, une certaine redondance doit être apportée au signal d'entrée pour qu'à la réception elle aide à corriger les erreurs. Cependant, la plupart des schémas ajoutant de la redondance aux signaux ont deux défauts majeurs. Premièrement, ces schémas sont conçus pour fonctionner décemment aussi longtemps que la qualité du canal reste au dessus d'un certain seuil. Mais dés que la qualité du canal tombe en dessous de ce seuil, le système est complètement perturbé et n'est plus capable de corriger les erreurs (effet de seuil). Deuxièmement, la plupart des schémas de correction d'erreurs courantes manquent d'adaptabilité et de flexibilité propres pour modifier la redondance ajoutée, ce qui est nécessaire au vue des changements inévitables des caractéristiques du canal. Dans cette thèse, nous explorons les méthodes de représentation du signal qui se prêtent naturellement à la transmission à des taux flexibles et qui offrent une dégradation graduelle (graceful degradation) en termes de qualité de signal par rapport à la détérioration du canal. Pour de telles applications, nous considérons l'insertion d'une redondance de type soft et structurée dans les signaux à transmettre en utilisant les bancs de filtres sur-échantillonnés (Oversampled Filter Bank : OFB). Ces codes offrent à la fois une compression et une protection des signaux grâce à l'application d'une transformation redondante (ou opérateur de frame) au signal, ce qui sert au prétraitement des signaux pour la compression et pour l'insertion de redondance dans le signal, permettant ainsi de contrer les effets indésirables du canal. Plus précisément, dans cette thèse, nous approfondissons le domaine des expansions redondantes de signal au cas où la quantité de redondance injectée peut être ajustée à la volée. Deux approches principales permettront cette entreprise. La première méthode est appelée sous-échantillonnage adaptatif. L'idée est de changer le taux de sous-échantillonnage de l'OFB en réponse aux variations de la qualité du canal. Nous étudions l'effet d'un tel sous-échantillonnage sur le filtrage des vecteurs du signal d'entrée du coté analyse et nous proposons une méthode de reconstruction du signal appropriée du coté synthèse de l'OFB. Pour la deuxième méthode, nous introduisons la notion d'effacement instantané dans les OFB, qui correspond au cas où les échantillons des vecteurs sous-bande sont effacés avec une distribution arbitraire à différents moments. Nous trouvons les conditions pour lesquelles l'OFB maintient sa propriété de reconstruction parfaite en utilisant une analyse temporelle et nous proposons des structures afin de permettre la conception d'OFB implémentables