Efficient filter-bank pilot structures for multi-user massive MIMO systems

This work proposes a novel pilot structure (PS) to improve channel estimation (CE) in massive multiple-input multiple-output filter bank multi-carrier (mMIMO-FBMC) communications using offset quadrature amplitude modulation (OQAM). Accurate CE is essential to fully benefit from the corresponding spectral efficiency (SE) advantages and robustness against dispersive channels. Unlike orthogonal frequency division multiplexing (OFDM), FBMC/OQAM shows only real-field orthogonality. Therefore, corresponding signals suffer from intrinsic interference, motivating the use of alternative CE techniques. Furthermore, typical CE methods for mMIMO-FBMC systems require large training overheads, particularly for a large number of users or for flat-fading channel conditions over each subcarrier band. To address these issues, this paper proposes a PS that reduces the training overhead by interleaving user pilots in frequency while using conventional OFDM CE techniques, particularly the least squares (LS) method in the context of single-user (SU) and multi-user (MU) scenarios. Analytical expressions for the mean square error (MSE) and the Cramer-Rao lower bound (CRLB) are derived for the estimation technique considering the proposed PSs. Furthermore, performed simulations over the 5G QuaDRiga channel show that the proposed PS improves SE while delivering optimal performance in both preamble-based MU-mMIMO and SU systems.