  In this work, we develop a new iterative turbo receiver for LDPC-coded
multi-antenna systems based on semidefinite relaxation (SDR). For a classical
turbo receiver, forward error correction (FEC) code is only used at decoder.
Nonetheless, by taking advantage of FEC code in the detection stage, our
proposed SDR detector can output extrinsic information with much improved
reliability to the decoder. We also propose a simplified SDR turbo receiver
that solves only one SDR problem per codeword instead of solving multiple SDR
problems in the iterative turbo processing. This scheme significantly reduces
the time complexity of SDR turbo receiver, while the error performance remains
similar as before. In fact, our simplified scheme is generic in the sense that
it is applicable to any list-based iterative receivers.
