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

TL;DR

This paper investigates cell-free massive MIMO-OFDM systems for high-speed train communications, addressing Doppler effects and proposing a practical train antenna-centric design with power control to enhance spectral efficiency.

Contribution

It derives closed-form spectral efficiency expressions considering Doppler effects and compares cooperative and uncooperative combining methods in CF massive MIMO-OFDM for HST.

Findings

Cooperative MMSE combining outperforms MR combining in spectral efficiency.

CF massive MIMO-OFDM achieves larger, more uniform SE than small cell and cellular systems.

Train antenna-centric design with power control improves system performance under Doppler effects.

Abstract

Cell-free (CF) massive multiple-input multiple-output (MIMO) systems show great potentials in low-mobility scenarios, due to cell boundary disappearance and strong macro diversity. However, the great Doppler frequency offset (DFO) leads to serious inter-carrier interference in orthogonal frequency division multiplexing (OFDM) technology, which makes it difficult to provide high-quality transmissions for both high-speed train (HST) operation control systems and passengers. In this paper, we focus on the performance of CF massive MIMO-OFDM systems with both fully centralized and local minimum mean square error (MMSE) combining in HST communications. Considering the local maximum ratio (MR) combining, the large-scale fading decoding (LSFD) cooperation and the practical effect of DFO on system performance, exact closed-form expressions for uplink spectral efficiency (SE) expressions are derived. We observe that cooperative MMSE combining achieves better SE performance than uncooperative MR combining. In addition, HST communications with small cell and cellular massive MIMO-OFDM systems are compared in terms of SE. Numerical results reveal that the CF massive MIMO-OFDM system achieves a larger and more uniform SE than the other systems. Finally, the train antenna centric (TA-centric) CF massive MIMO-OFDM system is designed for practical implementation in HST communications, and three power control schemes are adopted to optimize the propagation of TAs for reducing the impact of the DFO.