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Many practical downlink multi-user multiple-input multiple-output (MIMO) linear precoding methods have been proposed recently. The block diagonalisation (BD) method has more advantages when each user is equipped with multiple receive antennas. However, BD has poor performance at the low and medium SNR regime with no consideration of noise. Hence, the regularised block diagonalisation (RBD) method has been proposed, which also has a high computational complexity for using singular value decomposition to suppress multi-user interference and noise. Proposed is a novel method of Cholesky factorisation for the RBD method, which can reduce computational complexity effectively without performance loss. Analysis and simulation results show the effectiveness of the proposed method.Introduction: Multiple-input multiple-output (MIMO) systems have attracted much attention owing to large spectral efficiencies. Recently, it has been shown that the sum-capacity region is achieved by dirty paper coding (DPC). Motivated by DPC, several nonlinear precoding schemes have been developed but they require high complexity. On the other hand, linear precoding schemes have been proposed as practical approaches with low complexity. The most intuitive scheme for single antenna receivers is to use the inversion of the channel as the precoding matrix [1]. An extension for receivers with multiple antennas is the block diagonalisation (BD) of the channels [2, 3], which attempts to completely eliminate multi-user interference (MUI) without any consideration of noise and has a constraint on the number of transmit and receive antennas, otherwise the nullspace may not exist. Although the regularised block diagonalisation (RBD) method [4] overcomes the drawbacks of BD, it still has high computational complexity when using singular value decomposition (SVD) to suppress MUI and noise. In this Letter, we propose a novel method of Cholesky factorisation for the RBD method, which is a simple way to get the precoding matrix with lower computational complexity and is equivalent to the original RBD. Theoretical analysis and simulation results show there is no performance degradation.

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