Downlink Spectral Efficiency of Cell-Free Massive MIMO Systems With Multi-Antenna Users

This paper studies a cell-free massive multiple-input multiple-output (MIMO) system where its access points (APs) and users are equipped with multiple antennas. Two transmission protocols are considered. In the first transmission protocol, there are no downlink pilots, while in the second transmissi...

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Bibliographic Details
Published in:IEEE transactions on communications 2020-08, Vol.68 (8), p.4803-4815
Main Authors: Mai, Trang C., Ngo, Hien Quoc, Duong, Trung Q.
Format: Article
Language:English
Subjects:
Antennas
Cell-free massive MIMO
Channel estimation
Closed form solutions
Codes
Downlink
Downlinking
Efficiency
Exact solutions
Massive MIMO
MIMO (control systems)
MMSE-SIC
Power control
Protocol
Protocols
Spectra
spectral efficiency
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Summary:This paper studies a cell-free massive multiple-input multiple-output (MIMO) system where its access points (APs) and users are equipped with multiple antennas. Two transmission protocols are considered. In the first transmission protocol, there are no downlink pilots, while in the second transmission protocol, downlink pilots are proposed in order to improve the system performance. In both transmission protocols, the users use the minimum mean-squared error-based successive interference cancellation (MMSE-SIC) scheme to detect the desired signals. For the analysis, we first derive a general spectral efficiency formula with arbitrary side information at the users. Then analytical expressions for the spectral efficiency of different transmission protocols are derived. To improve the spectral efficiency (SE) of the system, max-min fairness power control (PC) is applied for the first protocol by using the closed-form expression of its SE. Due to the computation complexity of deriving the closed-form performance expression of SE for the second protocol, we apply the optimal power coefficients of the first protocol to the second protocol. Numerical results show that two protocols combining with multi-antenna users are prerequisites to achieve the sub-optimal SE regardless of the number of user in the system.
ISSN:0090-6778
1558-0857
DOI:10.1109/TCOMM.2020.2990951