Channel Reciprocity Calibration in TDD Hybrid Beamforming Massive MIMO Systems

A hybrid analog-digital (AD) beamforming structure is a very attractive solution to build low-cost massive multiple-input multiple-output systems. Typically, these systems use a set of fixed beams for transmission and reception to avoid the need to obtain channel state information at transmitter (CSIT) for each antenna element individually. However, such a method cannot fully exploit the potential of hybrid AD beamforming systems. Alternatively, CSIT can be estimated by assuming a model for the propagation channel, whereas this model is only validated in millimeter-wave band; thanks to its poor scattering nature. In this paper, we focus on time division duplex systems with hybrid beamforming structure and propose a reciprocity calibration scheme that allows to acquire full CSIT. Different from existing CSIT acquisition methods, our approach does not require any assumption on the channel model and can estimate full CSIT.