Timing and Phase Synchronization with Turbo Codes

The system performance analysis of turbo coded systems in wireless packet transmission applications usually assumes that the channel has been estimated and synchronization parameters are properly set. This paper discusses a realistic turbo coding system when the signal phase or timing offset have not been perfectly compensated. Its intention is to give an overview of the synchronization techniques using properties of turbo codes. For the constant timing offset case we propose an algorithm which uses turbo decoder characteristics to finetune the coarse timing estimate given by a typical ML device. The resulting performance is very close to the optimally synchronized system. To model the varying phase error we introduce a method separating the size and the sign of the offset. We show that using this method and increasing the state space of the decoders can improve the bit error probability without the need of increasing transmitted power. INTRODUCTION On of the most important factors determining the efficiency of a wireless system is the power required for successful transmission of data. The power needed to reliably transmit a signal can be reduced by using special coding techniques. One of the latest and the most prominent of such techniques is turbo coding with a performance which is almost able to reach the Shannon channel capacity limit [1]. The problem lies, however, in the practical applications of the turbo codes. Unfortunately, reducing the transmitted power makes it more difficult to estimate the channel and properly synchronize the phase and symbol timing of the incoming signal. Timing offset is relatively easy to model since its rate of change is much lower than the packet transmission rate. Thus timing offset can be assumed to be random but constant over the whole packet [3]. Phase offset may, however, change much faster and must be treated on a symbol basis with much more complicated modelling. In our paper we discuss models and algorithms aiming at improving performance of a turbo-coded system on the AWGN channel suffering from timing or phase offset. Note that, even though our discussion is based on the AWGN channel, the results can easily be extended to fading channels. THE SYSTEM MODEL Thesignal after the AWGN channel for turbo coding and QPSK usually has the form of