In this study, a simple and high performance precoding scheme for downlink multiuser MIMO (DL MU-MIMO) systems is investigated. Sorted QR decomposition algorithm is adapted for zero-forcing (ZF) Tomlinson-Harashima precoding (THP) with user-stream permutation in order to select a permutation pattern which maximizes a sum-rate of the DL MU-MIMO system. In addition, allocating unequal number of bits to each user-stream improves the DL performance of the ZF-THP in the MU-MIMO scenario. Numerical results show that the proposed ZF-THP scheme outperforms a conventional MMSE-THP scheme.
Abstract:In this report, we propose a new MMSE channel estimation algorithm for OFDM systems. It is well-known that time domain maximum likelihood estimators (MSEs) show highly accurate impulse response estimation by using the time domain long preamble of the OFDM frame. On the other hand, the impulse response estimation based on the minimum mean square error (MMSE) criterion achieves superior channel estimation in low SNR conditions; however, it requires prior statistical information such as delay profiles of channels. In addition, the computational complexity becomes large because of the inverse matrix calculation. In order to overcome these issues, we propose to estimate the power delay profile of the channel by using the MLE, and to suppress the increase of the computational complexity for the matrix inverse calculation by applying the conjugate gradient method without degradation of the PER performance.
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