  This paper introduces a practical precoding method for the downlink of Filter
Bank Multicarrier-based (FBMC-based) massive multiple-input multiple-output
(MIMO) systems. The proposed method comprises a two-stage precoder, consisting
of a fractionally spaced prefilter (FSP) per subcarrier to equalize the channel
across each subcarrier band. This is followed by a conventional precoder that
concentrates the signals of different users at their spatial locations,
ensuring each user receives only the intended information. In practical
scenarios, a perfect channel reciprocity may not hold due to radio chain
mismatches in the uplink and downlink. Moreover, the channel state information
(CSI) may not be perfectly known at the base station. To address these issues,
we theoretically analyze the performance of the proposed precoder in presence
of imperfect CSI and channel reciprocity calibration errors. Our investigation
covers both co-located (cell-based) and cell-free massive MIMO cases. In the
cell-free massive MIMO setup, we propose an access point selection method based
on the received SINRs of different users in the uplink. Finally, we conduct
numerical evaluations to assess the performance of the proposed precoder. Our
results demonstrate the excellent performance of the proposed precoder when
compared with the orthogonal frequency division multiplexing (OFDM) method as a
benchmark.
