  A dual-layer multiple-input multiple-output (MIMO) system with multi-level
modulation is considered. A computationally efficient soft-input soft-output
receiver based on the exact max-log maximum a posteriori (max-log-MAP)
principle is presented in the context of iterative detection and decoding. We
show that the computational complexity of our exact max-log-MAP solution grows
linearly with the constellation size and is, furthermore, less than that of the
best known methods of Turbo-LORD that only provide approximate solutions. Using
decoder feedback to change the decision thresholds of the constellation
symbols, we show that the exhaustive search operation boils down to a simple
slicing operation. For various simulation parameters, we verify that our
solution performs identically to the brute-force exhaustive search max-log MAP
solution.
