Two‐Dimensional Iterative Processing for DAB Receivers Based on Trellis‐Decomposition

Wim J Van Houtum,Frans M J Willems

doi:10.1155/2012/394809

Abstract

We investigate iterative trellis decoding techniques for DAB, with the objective of gaining from processing 2D‐blocks in an OFDM scheme, that is, blocks based on the time and frequency dimension, and from trellis decomposition. Trellis‐decomposition methods allow us to estimate the unknown channel phase since this phase relates to the sub‐trellises. We will determine a‐posteriori sub‐trellis probabilities, and use these probabilities for weighting the a‐posteriori symbol probabilities resulting from all the sub‐trellises. Alternatively we can determine a dominant sub‐trellis and use the a‐posteriori symbol probabilities corresponding to this dominant sub‐trellis. This dominant sub‐trellis approach results in a significant complexity reduction. We will investigate both iterative and non‐iterative methods. The advantage of non‐iterative methods is that their forwardbackward procedures are extremely simple; however, also their gain of 0.7 dB, relative to two‐symbol differential detection (2SDD) at a BER of 10−4, is modest. Iterative procedures lead to the significantly larger gain of 3.7 dB at a BER of 10−4 for five iterations, where a part of this gain comes from 2D processing. Simulations of our iterative approach applied to the TU‐6 (COST207) channel show that we get an improvement of 2.4 dB at a Doppler frequency of 10 Hz.

Highlights

Digital audio broadcasting (DAB) systems, DAB+ systems, and terrestrial-digital multimedia broadcasting (T-DMB) systems use orthogonal frequency division multiplexing (OFDM), for which every OFDMsubcarrier is modulated by π/4-Differentially EncodedQuaternary PSK (DE-QPSK) [1]
Focussing on 2D blocks was motivated by the fact that the channel coherence-time is typically limited to a small number of OFDM symbols, and since DAB transmissions use time-multiplexing of services, which limits the number of OFDM symbols in a codeword
We have investigated decoding procedures for DAB-like systems, focussing on trellis decoding and iterative techniques, with a special focus on obtaining an advantage from considering 2D blocks and trellis decomposition

Summary

Introduction

We will propose methods based on iteratively demodulating and decoding blocks of received symbols in a DAB-transmission stream. The SOVA was used for differential detection as well as for decoding of the convolutional code They used in the coherent setting an estimate of the phase based on a block of three by three received symbols, which are adjacent in time and frequency direction. The channel-phase and gain might be assumed to be fixed for a number of adjacent subcarriers and consecutive symbols This is the assumption on which we base our investigations. OFDM reception can be regarded as parallel matchedfiltering corresponding to B complex orthogonal waveforms, one for each subcarrier This results in a channel model, holding for a 2D block of symbols, that is, given by rm,n = |h|e jφsm,n + nm,n,.

Detection and Decoding

Performance for TU-6 Channel Model

Conclusions