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We present a soft-output multiple-input multiple-output (MIMO) detection algorithm that achieves near max-log optimal error rate performance with low- and fixed-computational complexity. The proposed smart ordering and candidate adding (SOCA) algorithm combines a smart-ordered QR decomposition with smart candidate adding and a parallel layer-by-layer search of the detection tree. In contrast to prior algorithms that use smart candidate adding, the proposed algorithm has fixed computational complexity, and it never visits a node more than once. Results indicate that the SOCA algorithm has an attractive performance-complexity profile for both fast and slow fading 4 Ã 4 and 8 Ã 8 MIMO channels with quadrature amplitude modulation (QAM) inputs.