Marc Adrat

Iterative source-channel decoder using extrinsic information from softbit-source decoding

In digital mobile communications, efficient compression algorithms are needed to encode speech or audio signals. As the determined source parameters are highly sensitive to transmission errors, robust source and channel decoding schemes are required. This contribution deals with an iterative source-channel decoding approach where a simple channel decoder and a softbit-source decoder are concatenated. We mainly focus on softbit-source decoding which can be considered as an error concealment technique. This technique utilizes residual redundancy remaining after source coding. We derive a new formula that shows how the residual redundancy transforms into extrinsic information utilizable for iterative decoding. The derived formula opens several starting points for optimizations, e.g. it helps to find a robust index assignment. Furthermore, it allows the conclusion that softbit-source decoding is the limiting factor if applied to iterative decoding processes. Therefore, no significant gain will be obtainable by more than two iterations. This will be demonstrated by simulation.

Iterative source-channel decoding: improved system design using EXIT charts

The error robustness of digital communication systems using source and channel coding can be improved by iterative source-channel decoding (ISCD). The turbo-like evaluation of natural residual source redundancy and of artificial channel coding redundancy makes step-wise quality gains possible by several iterations. The maximum number of profitable iterations is predictable by an EXIT chart analysis. In this contribution, we exploit the EXIT chart representation to improve the error correcting/concealing capabilities of ISCD schemes. We propose new design guidelines to select appropriate bit mappings and to design the channel coding component. A parametric source coding scheme with some residual redundancy is assumed. Applying both innovations, the new EXIT-optimized index assignment as well as the appropriately designed recursive nonsystematic convolutional (RNSC) code, allows to outperform known approaches to ISCD by far in the most relevant channel conditions.

Convergence behavior of iterative source-channel decoding

Whenever digital communication is subject to transmission errors, the utilization of reliability information at the receiver is beneficial. Soft-channel decoding techniques improve the error correcting capabilities and in addition, softbit source decoding performs error concealment. While the reliability gain of channel decoding is based on artificial diversities explicitly introduced at the transmitter, softbit source decoding exploits implicit redundancies remaining in the bitstream after state-of-the-art source encoding. In an iterative source-channel decoding scheme the reliability gains of both are exchanged iteratively in order to enhance the common error resistance. However, first investigations have shown that the number of profitable iterations is limited to small values. In this paper, the convergence behavior is analyzed using a modern tool called EXIT charts. EXIT charts permit us to predict the convergence behavior by studying the individual components. As a novelty, EXIT characteristics are specified for softbit source decoding. The convergence analysis is confirmed by simulation.

Error concealment by near optimum MMSE-estimation of source codec parameters

In digital communications source coding is indispensable to achieve a high bandwidth efficiency in applications where bandwidth is a limited resource. Usually these source coding algorithms determine speech or audio parameters which are highly sensitive to transmission errors. This paper deals with an error concealment technique that benefits from residual redundancy remaining after source coding. Due to delay and complexity constraints a certain amount of inter-frame as well as intra-frame correlation between source codec parameters remains which might be exploited to enhance the subjective audio quality. In this contribution we propose an algorithm which is able to utilize both types of redundancy if the source parameters exhibit a two-dimensional Markov property. Actually, this approach is nearly optimal in the minimum mean square error (MMSE) sense but much less complex compared to the optimal estimator.

Iterative Source-Channel Decoding Using Short Block Codes

Iterative source-channel decoding (ISCD) exploits the residual redundancy of the source, e.g., codec parameters, for quality improvements. In contrast to the well-known convolutional coded ISCD systems, we propose in this paper an ISCD scheme which features a superior performance but is based solely on two short block codes. The block codes serve as highly redundant index assignment and rate-1 inner channel code respectively. The improved capabilities are confirmed by EXIT charts. Finally, the effects of imperfect knowledge of the receiver on the residual redundancy are analyzed. With the proposed flexible index assignment no feedback from the receiver to the transmitter is required and competitive results are obtained even for slightly erroneously estimated residual redundancy

Efficient and portable SDR waveform development: The Nucleus concept

Future wireless communication systems should be flexible to support different waveforms (WFs) and be cognitive to sense the environment and tune themselves. This has lead to tremendous interest in software defined radios (SDRs). Constraints like throughput, latency and low energy demand high implementation efficiency. The tradeoff of going for a highly efficient WF implementation is the increase of porting effort to a new HW platform. In this paper, we propose a novel concept for WF development, the Nucleus concept, that exploits the common structure in various wireless signal processing algorithms and provides a way for efficient and portable implementation. Tool assisted WF mapping and exploration is done efficiently by propagating the implementation and interface properties of Nuclei. The Nucleus concept aims at providing software flexibility with high level programmability, but at the same time limiting HW flexibility to maximize area and energy efficiency.

Turbo decodulation: iterative combined demodulation and source-channel decoding

We propose the combination of iterative demodulation and iterative source-channel decoding as a multiple turbo process. The receiver structures of bit-interleaved coded modulation with iterative decoding (BICM-ID), iterative source-channel decoding (ISCD), and iterative source coded modulation (ISCM) are merged to one novel turbo system, in which in two iterative loops reliability information is exchanged between the three single components, demodulator, channel decoder and (softbit) source decoder. Simulations show quality improvements compared to the different previously known systems, which use iterative processing only for two components of the receiver.

The EXIT-characteristic of softbit-source decoders

We outline a new technique to compute the EXIT-characteristic of softbit-source decoders analytically without extensive histogram measurements. Based on the analytic considerations it is straightforward to derive a compact determination rule for the maximum value of attainable extrinsic information. We also show that the area under the EXIT-characteristic grows almost logarithmically with the prediction gain which is utilizable due to the residual redundancy in the source data.

2nd Order Cyclostationarity of OFDM Signals: Impact of Pilot Tones and Cyclic Prefix

This paper deals with 2nd order cyclostationarity of orthogonal frequency division multiplex (OFDM) signals. A new generalized formula for the spectral correlation density (SCD) function is derived. Compared to related work in the literature, our derivation is not restricted to the case that all sub-carriers of an OFDM signal carry statistically independent data. The reason for that is that correlated data in terms of pilot tones are typically introduced on different carriers for channel estimation and synchronization purposes. The new formula allows us to analyze the impact of such pilot tones on the SCD. In addition, it gives extra information about the impact of the cyclic prefix.

EXIT Chart Based System Design for Iterative Source-Channel Decoding with Fixed-Length Codes

Audio-visual source encoders for digital wireless communications extract parameter sets on a frame-by-frame basis. Due to delay and complexity constraints these parameters exhibit some residual redundancy which manifests itself in non-uniform parameter distributions and intra- as well as inter-frame correlation. This residual redundancy can be exploited by iterative source-channel decoding (ISCD) to improve the robustness against impairments from the channel. In the design process of ISCD systems the well known EXIT charts play a key role. However, in case of inter-frame parameter correlation, the classic EXIT charts do not provide reliable bounds for predicting the convergence behavior of ISCD. We explain the reasons for the so-called overshooting effect and propose a novel extension to the EXIT chart computation which provide significantly better bounds for the decoding trajectories. Four advanced ISCD system configurations are proposed and investigated using the benefits of the improved EXIT chart based system design. These configurations include regular and irregular redundant index assignments. In addition, we incorporate unequal error protection in the optimization of irregular index assignments. We show how to realize a versatile multi-mode ISCD scheme which operates close to the theoretical limit.