A suboptimum iterative decoder for space-time trellis codes - Communications, 2004 IEEE International Conference on

The main problem of space-time trellis codes is constituted by their complexity, which grows exponentially with the number of transmit antennas. To avoid this shortcoming, one can think to a suboptimum decoder in which a simplified metric, together with a preliminary filtering step, is used. In this paper, we develop this idea and give some possible choices for the filter design. These different choices are compared with one another by means of analytical tools and simulations. In two independent papers, Telatar [6] and Foschini [3] have shown that, for high signal-to-noise ratio, the capacity of a multi-input multi-output (MIMO) fading channel with t transmit and r receive antennas grows linearly with min (r, t). Since then, space-time codes (STCs), i.e., the coding schemes for MIMO channels, have received considerable attention. In the family of STCs, block STCs (STBCs), like the Alamouti code, have been until now more popular in applications, because of their low decoding complexity. Instead, trellis STCs (STTCs) have a decoding complexity that is exponential with the number of transmit antennas and the constellation size. In order to reduce the complexity of STTC decoding, Biglieri et al. [2] have designed suboptimum decoders, which make use of a linear or nonlinear interface, conceptually similar to the multiuser detectors that are designed for multipleaccess channels to suppress multiuser interference. In this context, the interface is useful to suppress inter-antenna interference (IAI) and for this reason, throughout the paper, we will refer to it as the antenna separator (AS). By using a simple AS, the decoding complexity of an STTC grows only linearly with the number of transmit antennas. In [2], it is also introduced an iterative Turbo-like scheme in which, through several iterations, a soft-input soft-output AS (SISOAS) exchanges information with a SISO trellis decoder. In this paper, we face with the design of a linear SISO-AS to be inserted in such an iterative space-time decoder. While the SISO-AS described in [2] is a fixed MMSE filter, we will introduce other kinds of SISO-AS, with different complexity, that outperform the original design. The improvement in performance will be demonstrated both analytically and by simulations. The structure of the paper is the following. In Sect. I, the system is described. In Sect. II, the new designs for the SISOAS are described and compared. In Sect. III, simulation results are shown. Finally, in Sect. IV, we draw some conclusions. Space-Time Trellis Encoder π1