Outage Performance and IRA Code Design for MIMO Block Fading With Unitary Scrambling

We consider coded multiple-input-multiple-output (MIMO) systems in block-fading channels employing iterative receivers, where the MIMO channels are scrambled by unitary matrices. We show that the unitary scrambling improves the outage performance with discrete input when the outer code rate is high, i.e., R
_o
> 0.5. We also demonstrate that the performance improvement from scrambling can be achieved by a small number of random unitary matrices. Moreover, with unitary scrambling, the signal model can be simplified, and the performance of the MIMO demodulator can be characterized by the singular values of the channel matrix. We then consider the design of irregular repeat-accumulate (IRA) codes for unitary scrambled MIMO block-fading channels. We propose a code design method by optimizing the IRA codes for extrinsic information transfer (EXIT) average with a given effective mutual information. We also propose a pessimistic code design for some system settings to improve the frame error rate performance.