An iterative decoding scheme with turbo code and iteratively demapped multi-dimensional QPSK serially concatenated

Turbo codes employ iterative decoding through the exchange of extrinsic information (EI) on systematic bits between two parallel concatenated recursive systematic convolutional (RSC) codes. The EI on parity bits can not be utilized during iterative decoding. In this paper, we propose a new type of iterative decoding scheme mapping parity bits of turbo codes to a multi-dimensional (MD) QPSK, which makes it possible to utilize the EI on parity bits by iterative demapping. We use extrinsic information transfer (EXIT) chart to predict the system´s convergence behavior and to help search good RSC codes and MD QPSK schemes. An analytical upper bound is also given to show the system´s asymptotic performance in medium to high E_b/N₀ region. We introduce two special schemes and do simulations on them. Particularly, the scheme with turbo code consisting of two 4-state (7, 6) codes and an eight dimensional QPSK outperforms Berrou turbo code with two 16-state RSC codes in water fall region and has similar error floor.