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International audience; A novel intra prediction algorithm is proposed to improve the coding performance of screen content for the emerging Versatile Video Coding (VVC) standard. The algorithm, called in-loop residual coding with scalar quantization, employs in-block pixels as reference rather than the regular out-block ones. To this end, an additional in-loop residual signal is used to partially reconstruct the block at the pixel level, during the prediction. The proposed algorithm is essentially designed to target high detail textures, where deep block partitioning structure is required. Therefore, it is implemented to operate on 4×4 blocks only, where further block split is not allowed and the standard algorithm is still unable to properly predict the texture. Experiments in the Joint Exploration Model (JEM) reference software show that the proposed algorithm brings a Bjontegaard Delta (BD)-rate gain of 13% on synthetic content, with a negligible computational complexity overhead at both encoder and decoder sides.

International audience; This paper considers the analysis of communication protocols in wireless networks implementing both cooperation and Hybrid Automatic Repeat reQuest (HARQ) for Type I decoder and Type II decoder with Chase Combining. Using an example of a three-node network, we show that the commu- nication protocol can be modeled using Finite State Markov Chains. This model efficiently predicts the performance of the system. However, the complexity depends on the number of states, which increases very fast as the protocol gets more sophisticated. We then derive a simplified model using state aggregation, and obtain a compact description which can be used to predict the performance with a reduced complexity. Moreover, we show that the simplified model describes a probabilistic communication protocol on the same network. Monte Carlo simulations show that the theoretical predictions match the simulated performance.

International audience; Network Coded Cooperation (NCC) is known to provide full diversity order and high spectral efficiency for uncoded cooperative networks. However, the understanding of NCC applied to signals that have been protected by some Forward Error Correction (FEC) codes is still limited. This letter analyzes the diversity order attainable by NCC with channel coding (Coded-NCC) in a network topology with multiple sources, one relay and in the presence of fast Rayleigh fading. Due to the difficulty of characterizing the exchange of information between the network decoder and the channel decoder, iterative network and channel decoding algorithms are usually studied with the aid of simulations. In this letter, we overcome this limitation by proposing a near-optimal receiver that performs network decoding and channel decoding in a single decoding step of an equivalent super code. An upper bound and a tight approximation of the Bit Error Rate (BER) for all sources are derived. Based on the upper bound, we analytically show that Coded-NCC achieves a diversity order equal to 2f , where f is the minimum distance of the FEC code. This result generalizes those available for cooperative networks in the absence of channel coding (Uncoded-NCC), where the diversity order is equal to 2, as well as those available for coded transmission but without cooperation, where the diversity order is equal to f .

In this paper, we study outage probability (OP) and diversity order of a $M$ -source and $N$ -relay wireless network that combines network coding (NC) and relay selection (RS). More specifically, a decode-and-forward (DF) relaying protocol is considered and the network-encoding vectors at the relays are assumed to constitute a maximum distance separable (MDS) code. Single relay selection (SRS) and multiple relay selection (MRS) protocols are investigated, where the best relay and the $L$ best relays forward the network-coded packets to the destination, respectively. An accurate mathematical framework for computing the OP is provided and from its direct inspection the following conclusions on the achievable diversity are drawn: 1) the SRS protocol achieves diversity order equal to two regardless of $M$ and $N$ and 2) the MRS protocol achieves diversity order equal to $L+1$ if $L and equal to $N+1$ if $L\geq M$ . These analytical findings are substantiated with the aid of Monte Carlo simulations, which also show that RS provides a better OP than NC based on repetition coding if $L\geq M$ .

International audience; Relaying protocols are usually deterministic, i.e. they commute from one action to the next one in a predefined manner, depending on the success of the transmission. This paper demonstrates the advantages that probabilistic protocols (i.e. the next action to be undertaken depends on some probability) may have in this context. The whole study is performed on a simple source-relay-destination network, the relay working in demodulate and forward mode, in order to obtain insights on the interaction between relaying and HARQ. So far, probabilistic protocols have been mainly proposed for higher layers of communication systems, but are applied here to physical and MAC layers. Since it contains only two parameters, we demonstrate that our probabilistic protocol can easily be tuned for best performance using a Finite State Markov Chain (FSMC), and that it brings improvement over deterministic protocols in the same scenario.

International audience; Vector Quantization (VQ) design for distributed binary decision in the presence of an eavesdropper (Eve) is investigated. An encoder/quantizer (Alice) observes i.i.d. samples and communicates them via a public noiseless rate-limited chan- nel to the detector (Bob) who has also access to a correlated analog source. Bob can take advantage of both informations to perform a binary decision on the joint probability law of these observations. Eve is further assumed to have access to another correlated analog source. This paper evaluates relevant trade-offs between the error probability of the two types and the amount of tolerated information leakage, for the particular case of testing against independence. An application to illustrate our results to real-data measured from the electric consumption at houses to perform anomaly detection is also provided.

International audience; —Transmit power allocation techniques are very important to manage interference in small-cell networks. While available power allocation algorithms in the literature rely on a predefined routing protocol, we propose in this paper a power-efficient two-step algorithm that allows power allocation and routing to be performed jointly in femto-relay networks. First, we propose an interference-based partitioning method to cluster the femto-relays, then we adopt an iterative and distributed algorithm, inspired from game theory, for efficient transmit power allocation. We show that the corresponding power allocation game possesses a pure Nash equilibrium which is reached by the proposed algorithm within a number of iterations per femto-relay which can be as small as 1. Moreover, we show that our approach grants significant improvements in terms of power consumption, and permits the total consumed power to be divided by about 6 and 3 when respectively compared to the direct transmission and shortest path techniques.

International audience; The design of scalar quantization for distributed binary decision in presence of an eavesdropper (Eve) is investigated. An encoder/quantizer (Alice) observes a memoryless source and communicate via a public noiseless rate-limited channel with the detector (Bob) who has also access to a correlated analog source. Bob can take advantage of both informations to perform a binary decision on the joint probability law of these observations. Eve is further assumed to have access to a different correlated analog source and perfectly observe the information bits sent by Alice. This paper evaluates the various tradeoffs between the probabilities of error (on the decision) depending on the amount of information leakage from Alice to Eve. The Bhattacharyya distance; one of the distances measuring the difference between two probability distributions; is taken as a criterion to optimize the scalar quantizer subject to a tolerable constraint on the information leakage at the level of Eve. Numerical results for memoryless Gaussian sources demonstrate the performance of the proposed quantization method.

International audience; In many communication standards, several variable length frames generated by some source coder may be aggregated at a given layer of the protocol stack in the same burst to be transmitted. This decreases the signalization overhead and increases the throughput. However, after a transmission over a noisy channel, Frame Synchronization (FS), i.e., recovery of the aggregated frames, may become difficult due to errors affecting the bursts. This paper proposes several robust FS methods making use of the redundancy present in the protocol stack combined with channel soft information. A trellis-based FS algorithm is proposed first. Its efficiency is obtained at the cost of a large delay, since the whole burst must be available before beginning the processing, which might not be possible in some applications. Thus, a low-delay and reduced-complexity Sliding Window-based variant is introduced. Second, an improved version of an on-the-fly three-state automaton for FS is proposed. Bayesian hypothesis testing is performed to retrieve the correct FS. These methods are compared in the context of the WiMAX MAC layer when bursts are transmitted over Rayleigh fading channels.

International audience; This paper presents a MAP estimation method allowing the robust decoding of compressed video streams by exploiting the bitstream structure (i.e., information about the source, related to variable-length codes and source characteristics) together with the knowledge of the MAC layer CRC (here considered as additional redundancy on the MAC packet). This method is implemented via a sequential decoding algorithm in which the branch selection metric in the decoding trellis incorporates a CRC-dependent factor, and the paths which are not compatible with the source constraints are pruned. A first implementation of the proposed algorithm performs exact computations of the metrics, and is thus computationally expensive. Therefore, we also introduce a suboptimal (with tunable complexity) version of the proposed metric computation. This technique is then applied to the robust decoding of sequences encoded using the H.264/AVC standard based on CAVLC, and transmitted using aWiFi-like packet structure. Significant link budget improvement results are demonstrated for BPSK modulated signals sent over AWGN channels, even in the presence of channel coding.

For the case of imperfect channel estimation, we propose an improved detector for multiple-input-multiple-output (MIMO) systems using an iterative receiver. We consider two iterative detectors based on maximum a posteriori and soft parallel-interference cancellation and propose, for each case, modifications to the MIMO detector by taking into account the channel-estimation errors. Considering training-based channel estimation, we present some numerical results for the case of Rayleigh block fading, which demonstrate an interesting performance improvement, compared with the classically used mismatched detectors. This improvement is obtained without a considerable increase in receiver complexity. It is more advantageous when few pilots are used for channel estimation. Thus, the proposed improved detectors are of particular interest under relatively fast-fading conditions, where it is important to reduce the number of pilots to avoid a significant loss in data rate.

International audience; This paper presents an efficient MAP estimator for the joint source-channel decoding of data encoded with a context adaptive binary arithmetic coder (CABAC). The decoding process is compatible with realistic implementations of CABAC in standards like H.264, i.e., handling adaptive probabilities, context modeling and integer arithmetic coding. Soft decoding is obtained using an improved sequential decoding technique, which allows to obtain various tradeoffs between complexity and efficiency. The algorithms are simulated in a context reminiscent of H264. Error detection is realized by exploiting on one side the properties of the binarization scheme and on the other side the redundancy left in the code string. As a result, the CABAC compression efficiency is preserved and no additional redundancy is introduced in the bit stream. Simulation results outline the efficiency of the proposed techniques for encoded data sent over AWGN and UMTS-OFDM channels.

Ultrawideband (UWB) communications involve very sparse channels, because the bandwidth increase results in a better time resolution. This property is used in this paper to propose an efficient algorithm that jointly estimates the channel and the transmitted symbols. More precisely, this paper introduces an expectation-maximization (EM) algorithm within a wavelet-domain Bayesian framework for semiblind channel estimation of multiband orthogonal frequency division multiplexing based UWB communications. A prior distribution is chosen for the wavelet coefficients of the unknown channel impulse response to model a sparseness property of the wavelet representation. This prior yields, in maximum a posteriori estimation, a thresholding rule within the EM algorithm. We particularly focus on reducing the number of estimated parameters by iteratively discarding ldquoinsignificantrdquo wavelet coefficients from the estimation process. Simulation results using UWB channels that were issued from both models and measurements show that, under sparseness conditions, the proposed algorithm outperforms pilot-based channel estimation in terms of the mean square error (MSE) and bit error rate (BER). Moreover, the estimation accuracy is improved, whereas the computational complexity is reduced compared with traditional semiblind methods.

Assuming imperfect channel estimation, we propose an improved detector for orthogonal frequency-division multiplexing (OFDM) systems over a frequency-selective fading channel. By adopting a Bayesian approach involving the statistics of the channel estimation errors, we formulate an improved maximum-likelihood (ML) detection metric taking into account the characteristics of the channel estimates. As a first step, we propose a modified iterative detector based on a maximum a posteriori receiver formulation which reduces the impact of channel uncertainty on the decoder performance, by an appropriate use of this metric. The results are compared to those obtained by using a detector based on a mismatched ML metric, which uses the channel estimate as if it was the perfect channel. In a second step, we calculate the information rates achieved by both the improved and mismatched ML detectors, in terms of achievable outage rates. These outage rates are compared to those provided by a theoretical (not practical) decoder defined as the best decoder in the presence of channel estimation errors. Numerical results over both uncorrelated Rayleigh fading channels and realistic ultra wideband channels show that the improved detector outperforms the classically-used mismatched approach in terms of bit error rate and achievable outage rates, without any increase in the receiver complexity.

International audience; Ultra wideband (UWB) communications involve very sparse channels, since the bandwidth increase results in a better time resolution. This property is used here to propose an efficient algorithm jointly estimating the channel and the transmitted symbols. More precisely, this paper introduces an expectation-maximisation (EM) algorithm within a wavelet domain Bayesian framework for semi-blind channel estimation of multiband orthogonal frequency-division multiplexing (MB-OFDM) based UWB communications. A prior distribution is chosen for the wavelet coefficients of the unknown channel impulse response (CIR) in order to model a sparseness property of the wavelet representation. This prior yields, in maximum a posteriori (MAP) estimation, a thresholding rule within the EM algorithm. We particularly focus on reducing the number of estimated parameters by iteratively discarding 'insignificant' wavelet coefficients from the estimation process. Simulation results using UWB channels issued from both models and measurements show that under sparsity conditions, the proposed algorithm outperforms pilot based channel estimation in terms of mean square error (MSE) and bit error rate (BER). Moreover, the estimation accuracy is improved, while the computational complexity is reduced, when compared to traditional semi-blind methods. Copyright © 2008 John Wiley & Sons, Ltd.

In this paper, we propose a minimum mean squared error (MMSE) decision feedback equalizer (DFE) for pulse position modulated (PPM) signals in the presence of intersymbol interference (ISI). While traditional uses of PPM may not have had ISI, PPM is increasingly being considered for use in situations where ISI is an issue, such as high-performance optical communication systems and ultrawideband communications. First, we review previous work on the subject which used the zero-forcing criterion under strict assumptions about the channel and equalizer lengths. Then, we derive a computationally efficient MMSE equalizer which removes these restrictions, and is suitable for use with training-based stochastic gradient-descent algorithms. Finally, we demonstrate the performance of the proposed equalizer with simulations.

In this paper, a new still image coding scheme is presented. In contrast with standard tandem coding schemes, where the redundancy is introduced after source coding, it is introduced before source coding using real BCH codes. A joint channel model is first presented. The model corresponds to a memoryless mixture of Gaussian and Bernoulli-Gaussian noise. It may represent the source coder, the channel coder, the physical channel, and their corresponding decoder. Decoding algorithms are derived from this channel model and compared to a state-of-art real BCH decoding scheme. A further comparison with two reference tandem coding schemes and the proposed joint coding scheme for the robust transmission of still images has been presented. When the tandem scheme is not accurately tuned, the joint coding scheme outperforms the tandem scheme in all situations. Compared to a tandem scheme well tuned for a given channel situation, the joint coding scheme shows an increased robustness as the channel conditions worsen. The soft performance degradation observed when the channel worsens gives an additional advantage to the joint source-channel coding scheme for fading channels, since a reconstruction with moderate quality may be still possible, even if the channel is in a deep fade.

Quantization-based schemes, such as scalar distortion compensated quantization index modulation (DC-QIM), have demonstrated some performance merits in data hiding, which is mainly a transmission problem. However, a number of applications can be stated in terms of the watermark detection problem (also named zero-bit watermarking), and this situation has been seldom addressed in the literature for quantization-based techniques. In this context, we carry out a complete performance analysis of dithered uniform quantizer-based QIM with DC under additive white Gaussian noise. Using large deviation theory, performance is evaluated according to the receiver operating characteristic (ROC) and the total probability of detection error. Under the white Gaussian host signal assumption, scalar DC-QIM is compared with other existing watermarking methods, including quantized projection (QP), spread spectrum (SS) and improved SS (ISS, for which zero-bit watermarking performances with correlation detector are also proposed). Among the compared methods, it is shown that dithered scalar DC-QIM is the more relevant choice to address zero-bit watermarking, due to its host independent performance. A short comparison is also provided with respect to the corresponding transmission problem, thus evaluating the loss in performance due to the detection. We conclude in measuring the performance gain that could be provided by the use of more sophisticated lattice quantizers than the cubic structure

Multiple user information embedding is concerned with embedding several messages into the same host signal. This paper presents several implementable dirty-paper-coding (DPC)-based schemes for multiple user information embedding, through emphasizing their tight relationship with conventional multiple user information theory. We first show that depending on the targeted application and on whether the different messages are asked to have different robustness and transparency requirements or not, multiple user information embedding parallels one of the well-known multiuser channels with state information available at the transmitter. The focus is on the Gaussian broadcast channel (GBC) and the Gaussian multiple access channel (GMAC). For each of these channels, two practically feasible transmission schemes are compared. The first approach consists in a straightforward-rather intuitive-superimposition of DPC schemes. The second consists in a joint design of these DPC schemes. This joint approach heavily relies on a recent work by Kim in which the authors extend the single-user Costa's DPC to the multiple user case. The results in this paper extend the practical implementations quantization index modulation (QIM), distortion-compensated QIM (DC-QIM), and scalar Costa scheme (SCS) that have been originally conceived for one user to the multiple user case. After presenting the key features of the joint design within the context of structured scalar codebooks, we broaden our view to discuss the framework of more general lattice-based (vector) codebooks and show that the gap to full performance can be bridged up using finite dimensional lattice codebooks. Performance evaluations, including bit error rates (BERs) and achievable rate region curves are provided for both methods, illustrating the improvements brought by a joint design

Motivated by increasing interest in energy efficient modulations, we provide the first look at adaptive equalization of biorthogonal signaling. While this modulation has historically been considered only for use in narrowband systems without intersymbol interference (ISI), recent attention has been given to its use in ISI channels. Due to the fact that biorthogonal modulation (BOM) results in a source that is not i.i.d., however, classical blind adaptive equalization techniques cannot be directly applied to equalization of BOM signals. We first examine minimum mean-squared error (MMSE) and least mean squares (LMS)-based equalizers, and identify some peculiarities that arise in equalization of BOM signals when compared to more traditional modulations like binary phase shift keying (BPSK). Next, we present two novel blind algorithms for the adaptive equalization of BOM signals: LTBOMB and TROMBONE. We discuss the convergence properties of these algorithms, and demonstrate their performance with numerical simulations

This paper presents a new view of the Bose-Chaudhuri-Hocquengem (BCH) code through the addition of some flexibility to the syndromes distribution in the transmitted sequence. In order to get this flexibility, we derive a necessary condition (NC) on the syndromes distribution for decoding BCH codes, which includes the already known Hartmann-Tzeng proposition. This NC is essentially deduced from the decoding process of BCH code, and is related to the locator polynomial and the requested constraints to guarantee a maximal error-correction capacity. The obtained results have the advantage to be applicable for any considered field (finite or not). Furthermore, we prove that when the correction capacity is equal to 2 or 3, the obtained NC becomes also sufficient. This result is very useful in some practical transmission systems such as orthogonal frequency-division multiplexing systems. Once the pilot tones considered in such systems verify the necessary and sufficient condition, it becomes possible to both reduce the peak-to-average-power rate and correct the impulse noise, present in such multicarrier systems. The usefulness of the presented analysis and the exploitation of the derived condition on the pilot tones distribution is illustrated by simulation results in the case of the Hiperlan2 system

Many source coding standards (JPEG, H263+, H264), rely heavily on entropy coding with variable-length codes (VLC). However, bitstreams made of VLC-encoded data are particularly sensitive to transmission errors. Recent results tend to use knowledge of the VLC structure in order to perform an efficient decoding of the bitstream. These techniques use a trellis describing the structure of the VLC codebook and assume that some a priori information is available at decoder side. Significant improvements, compared with prefix decoding of bitstreams are achieved. However, the complexity of these techniques may become intractable when realistic VLC codebooks are considered. This paper presents an algorithm for compacting VLC tables. The codewords are grouped into a minimum number of classes. Decoding algorithms may then work on a reduced number of classes, instead of working on the whole set of codewords. A proof of optimality is provided for the VLC table-compaction algorithm. The algorithm is applied to the H263+ VLC codebook and merges the 204 codewords into 25 classes. The resulting compact tables are shown to be exactly equivalent to the initial ones when used with hard decoding algorithms. The properties of the associated soft decoding algorithms using these compact tables are also evaluated

This paper details a new orthogonal-frequency-division-multiplexing (OFDM) modulator based on the use of a pseudorandom postfix (PRP)-OFDM and discusses low-complexity equalization and channel estimation/tracking architectures. The main property of this new modulation scheme is the ability to estimate and track the channel variations semi-blindly using order-one statistics of the received signal. Compared with known cyclic prefix OFDM (CP-OFDM) pilot-symbol-assisted modulation (PSAM) schemes, the pilot overhead is avoided: The channel estimation is performed based on the exploitation of pseudorandomly weighted postfix sequences replacing the guard interval contents of CP-OFDM. PRP-OFDM is shown to be of advantage if the target application requires 1) a minimum pilot overhead, 2) low-complexity channel tracking (e.g., high mobility context), and 3) adjustable receiver complexity/performance trade-offs (available due to the similarities of PRP-OFDM to the zero-padded OFDM (ZP-OFDM) modulation scheme) without requiring any feedback loop to the transmitter.

Oversampled filter banks (OFBs) provide an overcomplete representation of their input signal. This paper describes how OFBs can be considered as error-correcting codes acting on real or complex sequences, very much like classical binary convolutional codes act on binary sequences. The structured redundancy introduced by OFBs in subband signals can be used to increase robustness to noise. In this paper, we define the notions of code subspace, syndrome, and parity-check polynomial matrix for OFBs. Furthermore, we derive generic expressions for projection-based decoding, suitable for the case when a simple second-order model completely characterizes the noise incurred by subband signals. We also develop a nonlinear hypotheses-test based decoding algorithm for the case when the noise in subbands is constituted by a Gaussian background noise and impulsive errors (a model that adequately describes the action of both quantization noise and transmission errors). Simulation results show that the algorithm effectively removes the effect of impulsive errors occurring with a probability of 10/sup -3/.

We first provide an analysis of available redundancy remaining in compressed video data. This residual redundancy has two origins: the variable-length code (VLC) syntax and the source constraints. We compute the two in terms of equivalent redundancy bits. Then, a VLC decoding algorithm able to exploit both VLC syntax and source constraints for improving the decoding performance is outlined. Finally, it is shown that by taking this redundancy into account, video decoding over additive white Gaussian noise channels is substantially improved.

A parallel between Reed-Solomon codes in the complex field and multicarrier transmission using orthogonal frequency-division multiplexing (OFDM) is first presented. This shows that when the signal is sent over some channel composed of Gaussian plus impulsive noise, the impulsive noise can be removed by a procedure similar to channel decoding, using information carried by the "syndrome." This result is first derived in a simple situation (oversampled discrete multitone, additive channel), which is merely of theoretical interest. In any case, consecutive zeros, in the output of the OFDM modulator, do not correspond to real subcarriers. Pilot tones are transmitted for synchronization or channel-estimation purposes. These pilot tones are generally scattered among the information ones. Our approach is to use these pilot tones as syndromes, in order to correct impulsive noise. We show that the correction capacity is conditioned by the position of these pilot tones in the transmitted sequence. A protection subsystem based on hypotheses tests is introduced after the decoding operation in order to detect malfunctions of this decoder. The efficiency of this technique is corroborated with simulations in the slightly modified Hiperlan2 context. Other extensions are then provided in order to increase the practical usefulness of the method.

We introduce a computationally efficient FFT-based algorithm for the design of OFDM/OQAM pulse shaping filters of arbitrary length and with arbitrary overlapping factors. The basic idea of our approach is to start from an arbitrary initial (nonorthogonal) filter and to perform an orthogonalization which yields an ISI- and ICI-free pulse shaping filter. The orthogonalization is performed in the discrete Zak transform domain and can be implemented using the FFT. We design filters for OFDM/OQAM systems with up to 1024 channels and overlapping factor 32. Finally, we present simulation examples assessing the performance of the designed pulse shaping filters in the presence of Doppler spread.

Zero padding (ZP) of multicarrier transmissions has been proposed as an appealing alternative to the traditional cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM) to ensure symbol recovery regardless of the channel zero locations. In this paper, both systems are studied to delineate their relative merits in wireless systems where channel knowledge is not available at the transmitter. Two novel equalizers are developed for ZP-OFDM to tradeoff performance with implementation complexity. Both CP-OFDM and ZP-OFDM are then compared in terms of transmitter nonlinearities and required power backoff. Next, both systems are tested in terms of channel estimation and tracking capabilities. Simulations tailored to the realistic context of the standard for wireless local area network HIPERLAN/2 illustrate the pertinent tradeoffs.

We extend the turbo demodulation procedure proposed by Li and Ritcey (see IEEE Journal on Selected Areas in Communications, vol.17, no.4, p.715-24, 1999) and by Magniez, Muquet, Duhamel, Buzenac and de Courville (see Proc. Int. Symp. on Turbo Codes and Related Topics, Brest, France, p.169-72, 2000) to OFDM systems with cyclic prefix (CP-OFDM) or zero-padding (ZP-OFDM). The extensions account for the noise color introduced by linear equalization. Resorting to realistic simulations, we also show that the turbo demodulation procedure used with set partitioned labeling can significantly outperform non-iterative decoding with Gray labeling. We also show that the proposed turbo demodulation scheme increases the performance gap between ZP and CP-OFDM relative to non-iterative decoding because it amplifies the performance gain brought by the guaranteed symbol recovery of ZP-OFDM.

This paper proposes a new blind channel estimation method for orthogonal frequency division multiplexing (OFDM) systems. The algorithm makes use of the redundancy introduced by the cyclic prefix to identify the channel based on a subspace approach. Thus, the proposed method does not require any modification of the transmitter and applies to most existing OFDM systems. Semi-blind procedures taking advantage of training data are also proposed. These can be training symbols or pilot tones, the latter being used for solving the intrinsic indetermination of blind channel estimation. Identifiability results are provided, showing that in the (theoretical) situation where channel zeros are located on subcarriers, the algorithm does not ensure uniqueness of the channel estimation, unless the full noise subspace is considered. Simulations comparing the proposed method with a decision-directed channel estimator finally illustrates the performance of the proposed algorithm.

We introduce a subspace-based algorithm for the blind identification of time-dispersive channels in pulse shaping OFDM systems based on offset quadrature amplitude modulation (OFDM/OQAM). Our approach exploits cyclostationarity induced by the use of overlapping pulse shaping filters and uses second-order statistics only. The proposed method is similar in spirit to modulation-induced cyclostationarity. Our algorithm assumes symbol-rate sampling in the receiver and does not require knowledge of the channel order. We provide simulation results demonstrating the performance of the new method.

We describe herein a method providing access to both enantiomers of 3-acetylthio-2-benzylpropionic acid via enzymatic desymmetrization of 2-benzyl-1,3-propanediol. These compounds are respectively the starting materials for the synthesis of ecadotril, and dexecadotril, which are powerful inhibitors of NEP (EC 3.4.24.11) and have been developed as therapeutic agents.

Aldol 5b, easily obtained from dihydrocarvone 6b, reacts with organolithium and Grignard reagents leading to enantiomerically enriched alcohols 8 (ee up to 35%). Thus, aldol 5b is a synthetic equivalent of benzaldehyde with a masked prochiral face.

The reaction of a silyl enol ether bearing a nitrogen atom with a hemiacetal vinylog in the presence of a catalytic amount of boron trifluoride etherate led to the corresponding ketoaldehyde. The cyclisation of the ketoaldehyde into azaspiroenone and nitrogen deprotection was performed in basic medium. The N-benzylation of this enone gave the known key intermediate of total synthesis of (±)-perhydrohistrionicotoxin.

A nonracemizing route to a set of chiral 3-aminopyrrolidines, based on 4-hydroxy-(l)-proline, is described. The induction potential of the lithium amides derived from these diamines has then been investigated in the asymmetric addition of alkyllithium compounds onto various aldehydes. Enantiomeric excesses up to 76% have been obtained in the case of the condensation of n-butyllithium onto o-tolualdehyde under standard experimental conditions (THF, −78 °C). Interestingly, the presence of a second asymmetric center, such as an α-methylbenzyl group, on the lateral 3-amino group gives access, according to its configuration, to one or the other of the 1-o-tolylpentan-1-ol enantiomers.

The potassium dienolate of prenal obtained by treatment of the corresponding dienoxysilane with tBuOK was reacted with enaldehydes. In all cases a γ-specific reaction occurs. According to the reaction conditions a γ; 1,2 or a γ; 1,4 coupled product was selectively obtained with enals. The γ; 1,2 reaction provided an efficient prenylation procedure. A short two-step synthesis of retinal is described.

The use of chiral alkoxides obtained from ephedrines for enantioselective proton abstraction is described in full. These reagents allow the practical obtention of axially dissymmetric 1,3-dioxanes via highly enantioselective dehydrohalogenation reactions. The conditions for a catalytic use of these chiral alkoxides are defined.

A new image coding technique based on an L/sup /spl infin//-norm criterion and exploiting statistical properties of the reconstruction error is investigated. The original image is preprocessed, quantized, encoded, and reconstructed within a given confidence interval. Two important classes of preprocessing, namely linear prediction and iterated filterbanks, are used. The approach is also shown to be compatible with previous techniques. The approach allows a great flexibility in that it can perform lossless coding as well as a controlled lossy one: specifications are typically that only p% the reconstructed pixels are different from the original ones.

This paper presents conventional and emerging applications of orthogonal synthesis/analysis transform configurations (transmultiplexer) in communications. It emphasizes that orthogonality is the underlying concept in the design of many communication systems. It is shown that orthogonal filter banks (subband transforms) with proper time-frequency features can play a more important role in the design of new systems. The general concepts of filter bank theory are tied together with the application-specific requirements of several different communication systems. Therefore, this paper is an attempt to increase the visibility of emerging communication applications of orthogonal filter banks and to generate more research activity in the signal processing community on these topics.

L'etude du transfert de chiralite, dans la reaction de condensation du n-butyllithium sur les aldehydes aromatiques en presence d'amidures de lithium derives d'une serie de 3-aminopyrrolidines optiquement actives, est abordee via la determination de la structure du complexe forme entre amidure et n-BuLi par RMN haut champ 1 H, 13 C et 6 Li dans des conditions proches de celles de la reaction.

Designing filter banks for source coding purposes classically relies on perfect reconstruction filters. However, several studies have shown recently that taking the quantization noise into account in the design could yield noticeable reduction of the reconstruction distortion. In particular, a joint optimization of the synthesis filters and quantizers with respect to the output mean squared error (MSE) was proposed, and the resulting scheme was called an minimum MSE (MMSE) filter bank. However, this approach was dedicated to parallel uniform filter banks, which require a very large computational complexity. This paper is concerned with the possibility of providing MMSE filters under the constraint that the filter bank is either of a modulated or a tree-structured kind and thus of low implementation cost. Various approaches for the optimization of the synthesis bank are presented, taking into account the structural constraint. Depending on the number of parameters to be optimized, we obtain various tradeoffs between decoder complexity, transmitted bit rate, and reconstruction distortion.

The structure of two chiral N,N‘-disubstituted-3-aminopyrrolidine lithium amides 3 and 4 in solution in THF-d8 has been studied at low temperatures by high-field 1H, 13C, and 6Li NMR spectroscopy. Despite their structural analogy, these two compounds adopt very different conformations in solution: while the amide 3 pyrrolidinic ring undergoes only minor changes with respect to its amino precursor 1, amide 4 presents a norbornyl-like bridged structure around 6Li+. When excess Bu6Li is added to both amide solutions, 1:1 amide−Bu6Li complexes arise, and their structures appear, this time, very similar and organized around a parallepipedic N−Li2−C core, the two lithium cations bridging the amide and the alkyl chain. The 4−Bu6Li complex appears very tight, with two distinct signals corresponding to each of the diastereotopic α-protons of butyllithium, by contrast with the looser 3−Bu6Li complex. From all these data, we propose a model to interpret the results obtained using these chiral amides in the asymmetr...

New N,N′-disubstituted-3-aminopyrrolidines lithium amides have been used as chiral auxiliaries in the asymmetric condensation of n-butyllithium onto aromatic aldehydes, leading to e.e.s up to 73%.