Francesca Vatta

Nonsystematic turbo codes

In this paper, we introduce the concept of nonsystematic turbo codes and compare them with classical systematic turbo codes. Nonsystematic turbo codes can achieve lower error floors than systematic turbo codes because of their superior effective free distance properties. Moreover, they can achieve comparable performance in the waterfall region if the nonsystematic constituent encoder has a low-weight feedforward inverse. A uniform interleaver analysis is used to show that rate R=1/3 turbo codes using nonsystematic constituent encoders have larger effective free distances than when systematic constituent encoders are used. Also, mutual information-based transfer characteristics and extrinsic information transfer charts are used to show that rate R=1/3 turbo codes with nonsystematic constituent encoders having low-weight feedforward inverses achieve convergence thresholds comparable to those achieved with systematic constituent encoders. Catastrophic encoders, which do not possess a feedforward inverse, are shown to be capable of achieving low convergence thresholds by doping the code with a small fraction of systematic bits. Finally, we give tables of good nonsystematic turbo codes and present simulation results comparing the performance of systematic and nonsystematic turbo codes.

Design and performance analysis of a new class of rate compatible serially concatenated convolutional codes

In this paper, a novel class of serially concatenated convolutional codes (SCCCs) is addressed. In contrast to standard SCCCs, where high rates are obtained by puncturing the outer code, the heavy puncturing is moved to the inner code, which can be punctured beyond the unitary rate. We derive analytical upper bounds on the error probability of this code structure by considering an equivalent code construction consisting of the parallel concatenation of two codes, and address suitable design guidelines for code optimization. It is shown that the optimal puncturing of the inner code depends on the outer code, i.e., it is interleaver dependent. This dependence cannot be tracked by the analysis for standard SCCCs, which fails in predicting code performance. Based on the considerations arising from the bounds analysis, we construct a family of rate-compatible SCCCs with a high level of flexibility and a good performance over a wide range of code rates, using simple constituent codes. The error rate performance of the proposed codes is found to be better than that of standard SCCCs, especially for high rates, and comparable to the performance of more complex turbo codes.

Hybrid concatenated codes with asymptotically good distance growth

Turbo Codes and multiple parallel concatenated codes (MPCCs) yield performance very close to the Shannon limit. However, they are not asymptotically good, in the sense of having the minimum distance grow linearly with the length of the code. At the other extreme, multiple serially concatenated codes (MSCCs), for example very simple repeat-accumulate-accumulate codes, have proven to be asymptotically good, but they suffer from a convergence threshold far from capacity. In this paper, we investigate hybrid concatenated coding structures consisting of an outer MPCC with very simple memory-1 component encoders serially concatenated with an inner accumulator. We show that such structures exhibit linear distance growth with block length and that they have better thresholds than MSCCs. The results indicate a fundamental tradeoff between minimum distance growth and convergence threshold in turbo-like codes.

Transfer function bounds on turbo codes performance in the Rician fading channel

As a powerful coding technique, turbo codes are a prime candidate for improving the reliability of communication over wireless channels, where fading is the main impairment. However, to date, only limited attention has been given to the performance of turbo codes on fading channels, and the influence of multipath fading has been fairly accurately investigated as far as the Rayleigh fading distribution is concerned only. Therefore, it is the purpose of this work to conduct a study of turbo codes performance considering the more general Rice fading case: the study is performed in conjunction with both the fully-interleaved channel and the correlated Rice slow-fading channel.

Video quality estimation in wireless IP networks: Algorithms and applications

This article proposes three methods to estimate the distortion deriving from packet losses in wireless video communication. The proposed methods take into account the short-term properties of the encoded video sequences. A suitable set of functions is adopted to model the distortion envelope resulting from multiple losses. The estimated performance is compared with the actual distortion, evaluated by decoding the received sequence with a properly designed decoder. Numerical results confirm the accuracy of the proposed models in approximating the actual Mean Square Error (MSE) for a wide range of loss rates. Some applications of the proposed algorithms are presented.

Analysis and Design of Tuned Turbo Codes

It has been widely observed that there exists a fundamental tradeoff between the minimum (Hamming) distance properties and the iterative decoding convergence behavior of turbo- like codes. While capacity-achieving code ensembles typically are asymptotically bad in the sense that their minimum distance does not grow linearly with block length, and they therefore exhibit an error floor at moderate-to-high signal-to-noise ratios, asymptotically good codes usually converge further away from channel capacity. In this paper, we introduce the concept of tuned turbo codes, a family of asymptotically good hybrid concatenated code ensembles, where asymptotic minimum distance growth rates, convergence thresholds, and code rates can be tradedoff using two tuning parameters: λ and μ. By decreasing λ, the asymptotic minimum distance growth rate is reduced in exchange for improved iterative decoding convergence behavior, while increasing λ raises the asymptotic minimum distance growth rate at the expense of worse convergence behavior, and thus, the code performance can be tuned to fit the desired application. By decreasing μ, a similar tuning behavior can be achieved for higher rate code ensembles.

Tuned turbo codes

As implied by previous studies, there exists a fundamental trade-off between the minimum distance and the iterative decoding convergence behavior of a turbo code. While capacity achieving code ensembles typically are asymptotically bad in the sense that their minimum distance does not grow linearly with block length and they therefore exhibit an error floor at medium to high signal to noise ratios, asymptotically good codes usually converge further away from channel capacity. In this paper we present so-called tuned turbo codes, a family of asymptotically good hybrid concatenated code ensembles, where minimum distance growths and convergence thresholds can be traded-off using a tuning parameter lambda. By decreasing lambda, the asymptotic minimum distance growth rate coefficient is reduced for the sake of improved iterative decoding convergence behavior, and thus the code performance can be tuned to fit the desired application.

Rate-compatible punctured serial concatenated convolutional codes

We propose and compare some good rate-compatible serial concatenated convolutional code (SCCC) families. To obtain rate-compatible SCCCs, the puncturing is limited to inner coded bits. However, and this is the novelty proposed, we do not limit the puncturing to inner parity bits only, but we extend it also to inner systematic bits, thus obtaining higher rate SCCCs (i.e., beyond the outer code rate). The two main applications of this technique are its use in hybrid ARQ/FEC schemes and to achieve unequal error protection (UEP) of an information sequence.

Robust, efficient and balanced (REB) rate-compatible puncturing schemes, for hybrid ARQ algorithms using turbo codes

In this paper, we propose and evaluate compatible puncturing schemes for turbo codes for Forward Error Correction / Automatic Repeat Request (FEC/ARQ) algorithms, in which the data transmission is supposed to go on progressively. The aim is 1) to obtain a better performance w.r.t. hybrid ARQ schemes already proposed in the literature when the user has to cope with some random packet erasures due, e.g., to deep fading conditions on wireless links, or to congestion on wired networks, i.e., when the number of lost data packets (erasures on an erasure channel) is hard to predict; 2) to obtain a performance comparable to one of the most efficient hybrid ARQ schemes proposed in the literature when all the packets are available to the final user, even if seriously corrupted, as on a pure wireless link. To obtain an unbiased comparison of the different schemes, an analytical method based on the sphere-packing bound is employed. To assess the accuracy of the bound, simulation results are also presented.

Performance of hybrid ARQ schemes for the LEO satellite channel

This paper investigates the performance of hybrid ARQ schemes for the mobile Low Earth Orbit (LEO) satellite channel. The proposed schemes adapt their characteristics to the time-varying behavior of the channel. To obtain an unbiased comparison of different ARQ schemes an approximated analytical method is adopted. More precisely, the sphere-packing bound is used to estimate the best possible performance of a hybrid ARQ scheme. To assess the accuracy of the bound, simulation results, based on the use of turbo codes, are also presented.