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Multiple-input-multiple-output (MIMO) systems use multiple antennas in both transmitter and receiver ends for higher spectrum efficiency. The hardware implementation of MIMO detection becomes a challenging task as the computational complexity increases. This paper presents the architectures and implementations of two typical sphere decoding algorithms, including the Viterbo-Boutros (VB) algorithm and the Schnorr-Euchner (SE) algorithm. Hardware/software codesign technique is applied to partition the decoding algorithm on a single field-programmable gate array (FPGA) device. Three levels of parallelism are explored to improve the decoding rate: the concurrent execution of the channel matrix preprocessing on an embedded processor and the decoding functions on customized hardware modules, the parallel decoding of real/imaginary parts for complex constellation, and the concurrent execution of multiple steps during the closest lattice point search. The decoders for a 4times4 MIMO system with 16-QAM modulation are prototyped on a Xilinx XC2VP30 FPGA device with a MicroBlaze soft core processor. The hardware prototypes of the SE and VB algorithms show that they support up to 81.5 and 36.1 Mb/s data rates at 20 dB signal-to-noise ratio, which are about 22 and 97 times faster than their respective implementations in a digital signal processor.