We developed two types of practical maximum-likelihood detectors (MLD) for multiple-input multiple-output (MIMO) systems, using a field programmable gate array (FPGA) device. For implementations, we introduced two simplified metrics called a Manhattan metric and a correlation metric. Using the Manhattan metric, the detector needs no multiplication operations, at the cost of a slight performance degradation within 1 dB. Using the correlation metric, the MIMO-MLD can significantly reduce the complexity in both multiplications and additions without any performance degradation. This paper demonstrates the bit-error-rate performance of these MLD prototypes at a 1 Gbps-order real-time processing speed, through the use of an all-digital baseband 44 MIMO testbed integrated on the same FPGA chip.

Multiple-input multiple-output (MIMO) wireless systems can realize large spectral efficiency and high performance communication links. For wideband transmissions, the combination of MIMO systems with orthogonal frequency division multiplexing (OFDM) has recently attracted a lot of attention and it is well known as MIMO-OFDM. In MIMO-OFDM systems, the overlapped signals over each subcarrier need to be separated by the receiver. For fading coefficients based receivers, the performance of the receiver depends largely on the accuracy of the estimated channel. Especially, when the channel varies with time, accurate channel tracking is needed. Conventional optimized channel tracking has large computational complexity because large matrix inverse computation is required. In order to reduce the complexity, a simplified channel tracking scheme assuming PSK modulation has been considered. However, such a simplified scheme is found to suffer from large performance degradation when applied to multi-level QAM modulation. In this paper, we derive a new simplified but improved channel tracking scheme for MIMO-OFDM systems that can be applied to both PSK and multi-level QAM modulation. The performance and the complexity of the proposed scheme are evaluated with comparisons to conventional schemes.