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A new generalized sphere decoder (GSD), called a double-layer sphere decoder (DLSD), is proposed for under-determined MIMO systems with fewer receive antennas N than transmit antennas M. The proposed algorithm is significantly faster than those introduced in and . The basic idea is to partition the transmitted signal vector into two subvectors with M-N+1 and N-1 elements. After some simple transformations, we can use an outer layer sphere decoder (SD) to choose proper subvectors of length M-N+1 and then use an inner layer SD to decide the other subvector of length N-1, thus the whole transmitted signal vector is obtained. Simulation results show that DLSD has far less complexity than the existing GSDs.