Abstract
In this paper, optimum soft-output (SO) multiple-input multiple-output (MIMO) sphere detectors (SDs) are studied. Noting that ordering the channel matrix columns plays an important role in reducing the tree-search complexity of a SD, we propose an optimized layer-ordering scheme based on the minimum cumulative residual criterion. The proposed scheme is studied in the context of a 4 × 4 MIMO system, and a low-complexity dataflow architecture is proposed. The implementation employs a permutation-robust QR decomposition (PR-QRD) scheme, based on the modified Gram-Schmidt orthogonalization procedure. Simulations demonstrate that using the proposed scheme, the node count of a SO MIMO SD is reduced by one order of magnitude, while the QRD overhead is reduced by more than 25% in computations and 36% in time, without incurring any performance degradation.