Synchronization in Turbo Coded Systems

Sammanfattning: Turbo codes and their unusual properties have attracted significant attention of the information theory community. There already  exist many articles and papers devoted to the theory of these codes. This thesis, however, focuses on practical implementation issues of turbo codes, with synchronization problems as the main topic.

Turbo codes are usually discussed under assumptions of perfect channel estimation and synchronization. It simplifies the analysis but does not
show their true performance in realistic conditions. This thesis investigates, among others, the effects of non-perfect timing and phase offset recovery on bit error rate. It is shown that non-perfect synchronization can severely impair turbo coding gains.

The typical ML synchronization algorithms may not be a good solution for turbo code systems, mostly due to the low signal-to-noise region in which such codes operate. This work postulates using the soft bit output of the BCJR algorithm as the source of better statistics for synchronization algorithms. Two algorithms using soft bits for timing recovery are presented and shown to outperform the classical ML NDA method. The phase offset compensation can also be significantly improved  when using soft bits.

The salient advantage of using the BCJR algorithm for synchronization purposes lies, however, in its ability to detect non-linear phenomena of cycle-slip and M-fold phase ambiguity. This eliminates the necessity of transmitting additional pilot symbols, which increases the available bandwidth.