Abstract
In the user-centric, cell-free, massive multi-input, multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) system, a large number of deployed access points (APs) serve user equipment (UEs) simultaneously, using the same time–frequency resources, and the system is able to ensure fairness between each user; moreover, it is robust against fading caused by multi-path propagation. Existing studies assume that cell-free, massive MIMO is channel-hardened, the same as centralized massive MIMO, and these studies address power allocation and energy efficiency optimization based on the statistics information of each channel. In cell-free, massive MIMO systems, especially APs with only one antenna, the channel statistics information is not a complete substitute for the instantaneous channel state information (CSI) obtained via channel estimation. In this paper, we propose that energy efficiency is optimized by power allocation with instantaneous CSI in the user-centric, cell-free, massive MIMO-OFDM system, and we consider the effect of CSI exchanging between APs and the central processing unit. In addition, we design different resource block allocation schemes, so that user-centric, cell-free, massive MIMO-OFDM can support enhanced mobile broadband (eMBB) for high-speed communication and massive machine communication (mMTC) for massive device communication. The numerical results verify that the proposed energy efficiency optimization scheme, based on instantaneous CSI, outperforms the one with statistical information in both scenarios.
Highlights
Wireless network technology has developed rapidly within the last 30 years, and people’s lives have been profoundly changed by the development of wireless technology [1]
The cellfree massive MIMO-orthogonal frequency division multiplexing (OFDM) system divides N subcarriers into multiple resource block (RB), and allocates RBs according to different kinds of service requirements
In the enhanced mobile broadband (eMBB) scenario, the number of user equipments (UEs) is much smaller than the number of access points (APs); in the massive machine-type communication (mMTC) scenario, each UE occupies fewer subcarriers, while the number of UEs being served is much larger than the number of APs
Summary
Wireless network technology has developed rapidly within the last 30 years, and people’s lives have been profoundly changed by the development of wireless technology [1]. The author optimizes the power allocation for maximizing energy efficiency in a user-centric massive MIMO system operating at millimeter-wave frequency [20]. The channel hardening phenomenon of massive MIMO makes the small-scale fading disappear gradually with the increase in antenna array size [24,25] It is pointed out in the literature that channel hardening is conditional in the cell-free massive MIMO scheme [26]. We study the energy-efficient optimization problem of user-centric, cellfree massive MIMO-OFDM in frequency fading channels. Based on the phenomenon that the channel is less hardened in cell-free massive MIMO, we propose to use instantaneous CSI when using energy-efficient optimization. We propose downlink energy efficiency optimization to improve the energy efficiency of cell-free massive MIMO-OFDM systems by using instantaneous CSI. We compare the performance of energy-efficiency optimization with different channel information in cell-free massive MIMO-OFDM systems.
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