User Group Selection Method in Multiuser MIMO-OFDM Transmission with Adaptive PAPR Reduction Using Null Space in MIMO Channel

In this paper, we investigate the user group selection method in multiuser multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) transmission when the adaptive peak-to-average power ratio (PAPR) reduction using the null space in a MIMO channel is applied. The user group to be spatially multiplexed is optimally selected by evaluating the throughput after the PAPR reduction process for all combinations of user candidates. However, the computational complexity of the optimal user group selection becomes prohibitive as the number of candidate users increases when performing the actual PAPR reduction process. To establish a computationally efficient user group selection method, we first discuss the relationship between the beamforming (BF) matrix and the achievable performance of the PAPR reduction using the null space in a MIMO channel. Based on the discussion, we propose a method that pre-selects user groups based on the BF matrix. By limiting the full assessment of the throughput evaluation after the PAPR reduction process to only the pre-selected user groups, the computational complexity of the proposed method is reduced. Computer simulation results show that the proposed method suppresses the loss in the PAPR-vs.-throughput performance while the computational complexity is significantly reduced.