The acquisition of channel state information is crucial in millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems. However, the previous studies for mmWave channel estimation only focus on the conventional static channel model without considering the Doppler shifts in a time-varying scenario. Since the variations of angles are much shorter than that of path gains, the mmWave time-varying channel has block-sparse and low-rank characteristics. In this paper, we show that the block sparsity, along with the low-rank structure, can be utilized to extract the Doppler shifts and other channel parameters. Specially, to effectively exploit the block-sparse and low-rank structures, a twostage method is proposed for mmWave time-varying channel estimation. In the first stage, we formulate a block-sparse signal recovery problem for AoAs/AoDs estimation, and we develop a block orthogonal matching pursuit (BOMP) algorithm to estimate the AoAs/AoDs. In the second stage, we formulate a low-rank tensor due to the low-rank structure of time-varying channels, and based on the results of the first stage, a CANDECOMP/PARAFAC (CP) decomposition-based algorithm is proposed to estimate the Doppler shifts and path gains. In addition, in order to compare with conventional tensor decompositionbased algorithms, two tensor decomposition-based time-varying channel estimation algorithms are proposed. Simulation results demonstrate that the proposed channel estimation algorithm outperforms the conventional compressed sensing-based algorithms and the tensor decomposition-based algorithms, and the proposed algorithm remains close to the Cramér-Rao Lower Bound (CRLB) even in the low SNR region with the priori knowledge of AoAs/AoDs. INDEX TERMS Time-varying channel estimation, block-sparse, low-rank, compressed sensing, tensor decomposition.
Based on the framework of the Cooperative Principle, the qualitative research method was adopted to analyze the compliance and violation of the Cooperation Principle between doctors and patients in the outpatient departments. The research object of this thesis is the dialogue between doctors and patients in outpatient departments of Chinese hospitals. The corpus of this study was collected by the author in four outpatient departments of a Grade A hospital in Shenzhen in 2019. In this thesis, 20 cases of doctor-patient dialogues were randomly selected from 83 cases of recorded corpus according to proportion of each department, and the following conclusions were drawn: 1) When answering the doctor's question, most patients violate the Maxim of Quantity. 2) Many patients often violate the Maxim of Relevance. 3) Both doctors and patients often comply with the Maxim of Manner. 4) Most doctors often comply with the Maxim of Quality and violate the Maxim of Relevance. This thesis purposes the existing problems in Chinese outpatient conversation, and provides guidance for the doctor-patient communication in the future.
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