Low complexity implementations of sphere decoding for MIMO detection

A new low complexity sphere decoding method for multiple-input multiple-output (MIMO) maximum-likelihood (ML) detection is proposed. One method that reduces the complexity of sphere decoding is the decoding order of MIMO sphere decoder using the soft-output signal of a suboptimum receiver as a reference. We refer to this method as ordered sphere decoder and we try to reduce its complexity. In order to do this, we use the reliability information of the transmitted vector to do channel ordering. This means that we make decisions on the elements of the transmitted vector starting from its most reliable element. To this end, we arrange the reliabilities in an increasing order. This ordering will define a permutation. The elements of the reference signal and also the columns of the channel matrix will be arranged according to this permutation. Then, we detect the permuted transmitted vector using ordered sphere decoder with the new permuted channel matrix and reference signal. In our proposed method, we start detecting the transmitted vector from its most reliable element and for each element, we start from the most probable transmitted symbol based on the information from the reference signal. This kind of ordering will help finding the candidate transmitted vectors quickly. Our method results in reducing the complexity of sphere decoder specially in low signal to noise ratios without compromising the performance of ML detection.