We describe a co-channel interference rejection scheme that is suitable for QR-decomposition maximum likelihood detection (MLD) in multiple-input and multiple output (MIMO) systems. A pre-whitening matrix for interference rejection is decomposed into a triangular matrix and its Hermitian matrix by using a complex Gaxpy version of the Cholesky algorithm. The decomposed triangular matrix is used as a spatial pre-filter to whiten co-channel interference. Simulation results demonstrate that the proposed scheme can suppress co-channel interference streams at the cost of receive diversity order and achieves better transmission performance than QR-decomposition MLD itself in MIMO channels with co-channel interference.

A rate control scheme is described for zero-forcing beamforming (ZFBF) multiuser multiple-input and multiple-output (MU-MIMO) systems with a QR-decomposition maximum likelihood detector (MLD) at the receiver. For selected users, a modulation-and-coding set is selected for each substream by estimating the per-substream post-MLD signal-to-interference-plus-noise ratio. Iterative modified QR-decomposition MLD is employed at the receiver to achieve the throughput expected from the transmitter. The simulation results demonstrated that the proposed rate-control scheme achieved the target packet error rate while increasing the throughout for ZFBF-MU-MIMO systems as the number of user candidates increases.

A precoding scheme is described for multiple-input and multiple-output orthogonal frequency-division multiplexing systems with a QR-decomposition maximum likelihood detector (MLD) incorporated with a parallel interference canceller (PIC) at a receiver. Transmit antenna ranking based on received substream signal power or per-substream minimum Euclidean distances is fed back to a transmitter. Based on the ranking information, precoding matrices are determined as permutation matrices such that specific packets are transmitted from transmit antennas with higher channel quality over the whole subcarriers. The simulation results demonstrated that precoding effectively utilizes PIC by reducing the possibility that all substreams are incorrectly decoded and thus improves the transmission performance of a QR-decomposition MLD with PIC.