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Cell-free (CF) massive multiple-input multiple-output (MIMO) systems showgreat potentials in low-mobility scenarios, due to cell boundary disappearanceand strong macro diversity. However, the great Doppler frequency offset (DFO)leads to serious inter-carrier interference in orthogonal frequency divisionmultiplexing (OFDM) technology, which makes it difficult to providehigh-quality transmissions for both high-speed train (HST) operation controlsystems and passengers. In this paper, we focus on the performance of CFmassive MIMO-OFDM systems with both fully centralized and local minimum meansquare error (MMSE) combining in HST communications. Considering the localmaximum ratio (MR) combining, the large-scale fading decoding (LSFD)cooperation and the practical effect of DFO on system performance, exactclosed-form expressions for uplink spectral efficiency (SE) expressions arederived. We observe that cooperative MMSE combining achieves better SEperformance than uncooperative MR combining. In addition, HST communicationswith small cell and cellular massive MIMO-OFDM systems are compared in terms ofSE. Numerical results reveal that the CF massive MIMO-OFDM system achieves alarger and more uniform SE than the other systems. Finally, the train antennacentric (TA-centric) CF massive MIMO-OFDM system is designed for practicalimplementation in HST communications, and three power control schemes areadopted to optimize the propagation of TAs for reducing the impact of the DFO.

Cell-Free Massive MIMO-OFDM for High-Speed Train Communications