How to obtain turbo gains in coherent and non-coherent orthogonal transmit diversity

— We consider turbo detection for coherent and non-coherent orthogonal transmit diversity. The turbo iterations are done between a convolutional code and the diversity combiner. We show that due to the orthogonal structure of the space-time block codes, extrinsic information which enables turbo gains can only be obtained for special symbol mappings. Such optimized symbol mappings are given for 8-PSK, 16-PSK and 16-QAM. Furthermore , we present a simple non-coherent soft-output detector for differential space-time block codes. Extrinsic information transfer (EXIT) charts are used as a powerful tool for analysis and performance prediction of the turbo detector. We show that a turbo detector benefits from spatial diversity in terms of earlier convergence. For quasistatic fading, we introduce the ¢ ¡-EXIT chart. I. INTRODUCTION AND SYSTEM MODEL Two hot topics have dominated research activities in wireless transmission technology in recent years: Turbo detection and space-time processing for wireless systems with multiple transmit and receive antennas. Turbo detection [1], [2] refers to iterative detection where extrinsic information is exchanged between different detection stages such as error correcting de-coders, equalizers, multiuser detectors or demodulators. The use of multiple antennas at both ends of the wireless link was motivated by results from information theory which promise large capacity gains in fading environments [3]. Among the variety of multiple antenna transmission schemes such as space-time codes [4] or spatial multiplexing (e.g. BLAST [5]), we focus on orthogonal space-time block codes as proposed for two transmit antennas in [6] and generalized to more transmit antennas in [7], [8] since they provide a powerful, simple, and flexible transmit diversity scheme which is also fairly robust to channel imperfections such as spatial correlation or rank deficiency. Space-time block codes require channel estimation at the receiver. Channel estimation is more difficult for multiple transmit antenna systems than for single transmit antenna systems since more channel coefficients have to be estimated and the power of the pilot symbols has to be distributed over several antennas. Therefore, differential space-time block codes as proposed in [9], [10] or [11], [12] which can be detected without channel estimation at the receiver are an attractive alternative to coherent space-time block codes. In this paper,we focus on the approach in [9] where the transmit symbols constitute orthogonal designs. Naturally, the application of the turbo principle to multiple antenna techniques has gained a lot of attraction. As far as space-time codes are concerned, most papers have …