Abstract
In centralized massive multiple-input multiple-output (MIMO) systems, the channel hardening phenomenon can occur, in which the channel behaves as almost fully deterministic as the number of antennas increases. Nevertheless, in a cell-free massive MIMO system, the channel is less deterministic. In this paper, we propose using instantaneous channel state information (CSI) instead of statistical CSI to obtain the power control coefficient in cell-free massive MIMO. Access points (APs) and user equipment (UE) have sufficient time to obtain instantaneous CSI in a slowly time-varying channel environment. We derive the achievable downlink rate under instantaneous CSI for frequency division duplex (FDD) cell-free massive MIMO systems and apply the results to the power control coefficients. For FDD systems, quantized channel coefficients are proposed to reduce feedback overhead. The simulation results show that the spectral efficiency performance when using instantaneous CSI is approximately three times higher than that achieved using statistical CSI.
Highlights
Massive multiple-input multiple-output (MIMO) with InstantaneousThe global annual wireless data traffic continues to grow rapidly
This paper proposes that in a slowly time-varying channel environment, both the central processing unit (CPU) and user equipment (UE) have sufficient time to obtain instantaneous channel state information (CSI) to use for power control
We exploit the fact that the channel hardening phenomenon is not evident in cell-free massive MIMO compared with centralized massive MIMO
Summary
The global annual wireless data traffic continues to grow rapidly. Devices are growing at a compound annual growth rate (CAGR) of approximately 10%, approaching 30 billion devices by 2023 [1]. Many works use statistical CSI for cell-free massive MIMO power control [8,13,14,15] The advantage of this approach is that large-scale fading tends to change slowly, so statistical information can reduce the amount of calculation. If cell-free massive MIMO is deployed on a campus, in a conference venue or in other walkingoriented scenarios, where users move slowly and the channel coherence time is long [19], the feedback of downlink CSI is lower, and there is sufficient time for estimation and feedback. The channels between users are orthogonal; the channel hardening phenomenon is not as evident as that in centralized massive MIMO [16] Based on this fact, we propose to use accurate instantaneous CSI instead of statistical CSI for downlink transmission and power control. Entropy 2021, 23, 1552 of a complex matrix, respectively. ◦ stands for the Hadamard product, and vec(.) indicates vectorization of a matrix
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