Reference Signals Design for Hybrid Analog and Digital Beamforming

There are many issues regarding the implementation of digital beamforming (BF) structures in massive MIMO: calibration, complexity, and cost. In a practical massive MIMO deployment, hybrid digital and analog BF structures with active antennas can be an alternative choice. In this letter, an
$N$
(the number of transceivers) by
$M$
(the number of active antennas per transceiver) hybrid BF structure is investigated, where the analog BF (ABF) is performed per transceiver and digital BF (DBF) is performed across
$N$
transceivers. A hybrid BF scheme is proposed, where the same ABF is applied to each transceiver, on top of which a DBF is designed to maximize the gain in a certain direction around the ABFs main beam direction. The proposed hybrid BF design establishes an inherent connection between the DBF and the beam direction of hybrid BF. Therefore, beam domain reference signals (RSs) can be conveniently implemented via applying properly designed DBF on subcarriers within the coherent bandwidth. Simulation results are given to show the validity of the proposed scheme.