Performances of Weighted Cyclic Prefix OFDM with Low-Complexity Equalization

International audience—In this paper, we justify low-complexity equalization techniques for weighted cyclic prefix (WCP)-OFDM. This modulation technique refers to filter bank based multicarrier (FBMC) transmission system provided with short filters. It allows the use of non-rectangular waveforms in order to mitigate interference caused by time-frequency selective channels while preserving an efficient implementation. Index Terms—Time-varying multipath channels, filter bank based multicarrier modulations, equalization, efficient realization

Analyse du système linéaire optimal pour les communications multiporteuses au-delà de la cadence de Nyquist

Les communications au delà de la cadence de Nyquist permettent une augmentation de l'efficacité spectrale en contre-partie d'une complexité plus élevée. Concernant les communications multiporteuses, les travaux menés jusque là se sont principalement focalisés sur l'étude des systèmes non-linéaires exploitant des techniques de codage et/ou d'égalisation, sans considération ou optimisation particulière de la partie linéaire du système. Dans cet article, nous analysons le compor-tement du système linéaire multiporteuse optimal lorsqu'il est utilisé seul ou avec des structures de réception non-linéaires (décodage itératif et égalisation à retour de décision). Nous nous intéressons également aux limites des hypothèses com-munément utilisées lors de l'implémentation de ces systèmes non-linéaires. L'utilisation du système linéaire optimal permet une expression analytique de la probabilité d'erreur qui peut être utilisée pour prédire les performances et aider la conception de systèmes codés. Ce travail met aussi en avant le bon compromis performances/complexité offert par l'égaliseur à retour de décision dans le contexte des communications au-delà de la cadence de Nyquist

Analysis of the optimal linear system for multicarrier FTN communications

National audienceFaster-than-Nyquist signalization allows for a better spectral efficiency at the expense of an increased complexity. Regarding multicarrier communications, previous work mainly relied on the study of non-linear systems exploiting coding and/or equalization techniques, with no particular optimization regarding the linear part of the system. In this paper, we analyse the behavior of the optimal linear multicarrier system when used with non-linear receiving structures (iterative decoding and direct feedback equalization), or in a standalone fashion. We also investigate the limits of the assumptions commonly made for the implementation of such non-linear systems. The use of this optimal linear system allows for a closed-form expression of the bit-error probability which can be used to predict the performances and help the design of coded systems. Our work also highlights the great performance/complexity trade-off offered by decision feedback equalization in a faster-than-Nyquist context.Les communications au delà de la cadence de Nyquist permettent une augmentation de l'efficacité spectrale en contre-partie d'une complexité plus élevée. Concernant les communications multiporteuses, les travaux menés jusque là se sont principalement focalisés sur l'étude des systèmes non-linéaires exploitant des techniques de codage et/ou d'égalisation, sans considération ou optimisation particulière de la partie linéaire du système. Dans cet article, nous analysons le compor-tement du système linéaire multiporteuse optimal lorsqu'il est utilisé seul ou avec des structures de réception non-linéaires (décodage itératif et égalisation à retour de décision). Nous nous intéressons également aux limites des hypothèses com-munément utilisées lors de l'implémentation de ces systèmes non-linéaires. L'utilisation du système linéaire optimal permet une expression analytique de la probabilité d'erreur qui peut être utilisée pour prédire les performances et aider la conception de systèmes codés. Ce travail met aussi en avant le bon compromis performances/complexité offert par l'égaliseur à retour de décision dans le contexte des communications au-delà de la cadence de Nyquist

FTN multicarrier transmission based on tight Gabor frames

A multicarrier signal can be synthesized thanks to a symbol sequence and a Gabor family (i.e., a regularly time-frequency shifted version of a generator pulse). In this article, we consider the case where the signaling density is increased such that inter-pulse interference is unavoidable.Over an additive white Gaussian noise channel, we show that the signal-to-interference-plus-noise ratio is maximized when the transmitter and the receiver use the same tight Gabor frame. What is more, we give practical efficient realization schemes and show how to build tight frames based on usual generators. Theoretical and simulated bit-error-probability are given for a non-coded system using quadrature amplitude modulations. Such a characterization is then used to predict the convergence of a coded system using low-density parity-check codes. We also study the robustness of such a system to errors on the received bits in an interference cancellation context

Analysis of a FTN Multicarrier System: Interference Mitigation Based on Tight Gabor Frames

Cognitive radio applications require flexible waveforms to overcome several challenges such as opportunistic spectrum allocation and white spaces utilization. In this context, multicarrier modulations generalizing traditional cyclic-prefix orthogonal frequency-division multiplexing are particularly justified to fit time-frequency characteristics of the channel while improving spectral efficiency.In our theoretical framework, a multicarrier signal is described as a Gabor family the coefficients of which are the symbols to be transmitted and the generators are the time-frequency shifted pulse shapes to be used. In this article, we consider the case where non-rectangular pulse shapes are used with a signaling density increased such that inter-pulse interference is unavoidable. Such an interference is minimized when the Gabor family used is a tight frame. We show that, in this case, interference can be approximated as an additive Gaussian noise. This allows us to compute theoretical and simulated bit-error-probability for a non-coded system using a quadrature phase-shift keying constellation. Such a characterization is then used in order to predict the convergence of a coded system using low-density parity check codes. We also study the robustness of such a system to errors on the received bits in an interference cancellation context

Improving Spectral Efficiency While Reducing PAPR Using Faster-Than-Nyquist Multicarrier Signaling

Multicarrier modulations are widely used in mobile radio applications due to their adaptability to the time-frequency characteristics of the channel, thus enabling low-complexity equalization. However, their intrinsically high peak-to-average power ratio (PAPR) is a major drawback with regard to implementation issues (e.g., power amplification efficiency, regulatory constraints...). In this paper, we confirm that the PAPR can be decreased as the signaling density (i.e., spectral efficiency at fixed constellation size) increases, even in the case where symbols cannot be perfectly reconstructed using a linear system. In such a two-dimensional generalization of faster-than-Nyquist (FTN) systems, PAPR distribution models from the literature are confirmed by simulation results. Furthermore, for a fixed number of subcarriers, we show that a sufficient condition to yield the optimal PAPR distribution at the output of a critically sampled transmitter is to specify pulse shapes as tight frames. Finally, simulation are performed in the more realistic case of an oversampled transmitted signal

On the study of faster-than-Nyquist multicarrier signaling based on frame theory

Multicarrier transmissions are classically based on undercomplete or exact Weyl-Heisenberg Riesz (biorthogonal or orthogonal) bases implemented thanks to oversampled filter-banks. This can be seen as a transmission below the Nyquist rate. However, when overcomplete Weyl-Heisenberg frames are used, we obtain a “faster-than-Nyquist” (FTN) system and it is theoretically impossible to recover exactly transmitted symbols using a linear receiver. Various studies have shown the interest of this high density signaling scheme as well as practical implementations based on trellis and/or iterative decoding. Nevertheless, there is still a lack of theoretical justifications with regard to pulse design in the FTN case. In this paper, we consider a linear transceiver operating over an additive white Gaussian noise channel. Using the frame theory and simulation results, we show that the mean squared error (MSE) is minimized when tight frames are used

On zero-forcing equalization for short-filtered multicarrier faster-than-Nyquist signaling

Within the context of faster-than-Nyquist signaling, a low-complexity multicarrier system based on short-length filters and zero-forcing turbo equalization is introduced. Short-length filters allow a reduced-size block processing while zero-forcing equalization allows a linear reduced-complexity implementation. Furthermore, rectangular and out-of-band energy minimization pulse shaping demonstrates competitive performance results over an additive white Gaussian noise channel while keeping a lower computational cost than other multicarrier faster-than-Nyquist systems

ZF OFDM Receiver for Underwater Communications

International audienceIn this paper, we propose a new scheme of transmission and reception of OFDM ( Orthogonal Frequency Division Multiplexing) signals for underwater wireless communications. The transmitter makes use of a kind of superimposed pilot signals, which are used by the receiver for estimating both the channel parameters (paths gain and delay) and the Doppler. Then, the estimated parameters are used for estimating the informative symbols in the zero forcing (ZF) sense. A new estimation scheme based on the resolution of two harmonic retrieval problems is proposed. Unlike standard methods, data resampling and estimation of residual carrier frequency offset are avoided. The efficiency of the proposed scheme is evaluated by means of simulations