A combinatorial precoding scheme of cell-free massive MIMO with channel aging

Here, we investigate the precoding schemes for the downlink data transmission of a time-division duplex cell-free massive multiple-input multiple-output (MIMO) system with channel aging, which arises from the user mobility. Closed-form spectral efficiency (SE) expressions of the downlink with the normalized conjugate beamforming (NCB), and the full-pilot zero-forcing (FZF), are derived, which are used for the analytical system performance evaluation. Then, a novel combinatorial precoding scheme with enhanced system SE performance, which adopts either NCB or FZF according to each user channel aging condition, is proposed. Moreover, a pilot allocation strategy is proposed to alleviate the extra interference brought by the combinatorial precoding scheme. Also, a statistical channel cooperative power control is employed to further improve the performance for all the above precoding schemes. Numerical results show that the proposed precoding scheme can substantially improve the average downlink SE.