On Layer Ordering Techniques for Near-optimal MIMO Detectors

Recently a soft-output multiple-input multiple-output (MIMO) detector has been proposed, namely LORD (layered orthogonal lattice detector), able to achieve maximum-likelihood (ML) performance (in the max-log sense) for two transmit antennas, and near-optimal for greater than two, in MIMO-OFDM bit interleaved coded modulation (BICM) systems. This paper presents a near-optimal layer ordering technique applicable to the hard-output version of the algorithm, essential to achieve near-ML performance up to high-dimensional MIMO systems. The Euclidean distance metrics are minimized over a constant number of transmit sequences, a property desirable to maintain constant throughput and not guaranteed by the most known sphere decoder. Differently from previous art works, attention is also paid to the optimization of the implementation complexity. Finally, it is also shown how the above mentioned ordering technique should be modified for the soft-output version of the algorithm, significantly improving the performance of MIMO-OFDM BICM systems. Simulations in the realistic context of next generation WLANs (802.11n) confirm this statement.