In this paper we implement a turbo based Overlay Cognitive radio Z-channel with Trellis shaping, where the primary and cognitive users transmits and receives concurrently, utilizing the same spectrum. We ensure that bit error rate (BER) performance of the Primary does not degrade even in the presence of cognitive user. We derive bit error rate BER curves for both users, and show significant improvement in the performance of primary even in the presence of cognitive user by implementing turbo decoding instead of Viterbi decoder. Simulation results show that turbo decoder provides a coding gain of 9.6dB for Primary and 8.7dB for cognitive user with out trellis shaping and 11.2dB total gain with Trellis shaping for cognitive user, where as conventional Viterbi decoder provides 1.8dB coding gain only.