Channel Estimation Using In-Band Pilots for Cell-Free Massive MIMO

In this paper, a novel channel estimation technique using in-band pilots (IBPs) for uplink transmission is proposed for cell-free (CF) massive multiple-input multiple-output (MIMO) networks. In particular, compared to traditional superimposed pilot schemes in the time domain, the proposed IBP scheme is operated in the frequency domain, which is more effective and robust due to the removal of interference from the data frequen-cies at the pilot location. We first introduce a system model for the CF massive MIMO with the IBP design. Then, analytical expressions for the mean square error and the normalized mean square error (NMSE) are derived for performance evaluation. Analytical and simulated results reveal that the proposed IBP scheme for CF MIMO outperforms the conventional regular and superimposed pilots in terms of NMSE. Besides, other important insights include the IBP scheme works well in high transmit power regions, and this new channel estimation strategy is more suitable for the scenario with a large number of users since it is not that sensitive to the number of users.