Thorsten Clevorn

BER prediction using EXIT charts for BICM with iterative decoding

A method for the prediction of bit-error rates (BER) of bit-interleaved coded modulation systems with iterative decoding (BICM-ID) is presented, which is based solely upon the extrinsic information transfer (EXIT) chart and avoids extensive BER simulations. Comparisons show an accurate prediction, even for low BERs down to 10/sup -6/. Furthermore, an easy procedure to obtain the optimum rotation angle for a signal constellation set in conjunction with IQ Interleaving is described. Both methods can be applied to all types of channels.

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

PSK versus QAM for iterative decoding of bit-interleaved coded modulation

Bit-interleaved coded modulation with iterative decoding (BICM-ID) is a transmission scheme with excellent performance on fading channels. The mapping of the bit patterns to the elements of the signal constellation set (SCS) is the key design element of BICM-ID, especially regarding the asymptotic performance of the BER. We demonstrate that SCSes of PSK (phase shift keying) with optimum bit pattern assignment are significantly superior in the asymptotic error-floor region to the respective square QAM (quadrature amplitude modulation) SCSes. When modifying the PSK SCSes to non-regular SCSes, performance gains of more than 1 dB are realizable in the case of 4 bits-per-channel symbols. The achievable gains are theoretically determined and analyzed, and verified by simulations.

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.

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.

On redundant index assignments for iterative source-channel decoding

Iterative source-channel decoding (ISCD) exploits the residual redundancy of source codec parameters by using the Turbo principle. A powerful improvement for ISCD is the concept of redundant index assignments. In this letter we present new design and optimization guidelines for redundant index assignments. For block code based redundant index assignments, weak block codes such as a repetition code should be preferred. Simulation results show significant performance improvements.

Parameter SNR Optimized Index Assignments and Quantizers based on First Order A Priori Knowledge for Iterative Source-Channel Decoding

Two key design aspects regarding the source coding part of iterative source-channel decoding (ISCD) are the index assignment and the quanizer. While the conventional index assignments are not suited for ISCD the so far presented optimized index assignments do only consider zeroth order a priori knowledge or optimize the parameter SNR only indirectly. In this paper we present a new cost function which directly optimizes the parameter SNR incorporating the first order a priori knowledge. With the same cost function we can also optimize the code book of the quantizer. Simulation results show the excellent performance the new parameter SNR optimized index assignments and quantizers exhibit.

Iterative demodulation for DVB-S2

We propose the application of iterative demodulation at a DVB-S2 receiver. The demodulation is improved by feedback of extrinsic information from the decoder of the LDPC channel code. The presented simulation results show an enhancement of about 0.3 dB, which is notable taking the gap to the Shannon limit into account. The additionally required computational demand is negligible and the receiver only needs to be slightly modified. With the unmodified transmitter the proposed scheme is completely standard compliant

Optimized Mappings for Iteratively Decoded BICM on Rayleigh Channels with Interleaving

A cost function for the optimization of mappings for bit-interleaved coded modulation systems with iterative decoding (BICM-ID) for a Rayleigh channel with applied IQ interleaving is derived. For this channel model the optimum mapping depends on the channel quality and on the rotation of the signal constellation set. Simulation results reveal that mappings optimized with the new cost function yield a better performance than previously known mappings. Furthermore, a second, easy procedure to obtain the optimum rotation angle for a signal constellation set in conjunction with IQ interleaving is described