Nonsystematic turbo codes: Design and bounds on effective free distance

In this paper, we address the design of turbo codes employing nonsystematic constituent encoders. Nonsystematic encoders have superior effective free distances compared to equivalent systematic encoders of the same complexity, and hence they are capable of achieving lower error floors. We derive bounds on the effective free distance of turbo codes employing nonsystematic constituent encoders and compare their distance and convergence properties with systematic turbo codes.