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This paper investigates downlink channel estimation in frequency-divisionduplex (FDD)-based massive multiple-input multiple-output (MIMO) systems. Toreduce the overhead of downlink channel estimation and uplink feedback in FDDsystems, cascaded precoding has been used in massive MIMO such that only alow-dimensional effective channel needs to be estimated and fed back. On theother hand, traditional channel estimations can hardly achieve the minimummean-square-error (MMSE) performance due to lack of the a priori knowledge ofthe channels. In this paper, we design and analyze a strategy for downlinkchannel estimation based on the parametric model in massive MIMO with cascadedprecoding. For a parametric model, channel frequency responses are expressedusing the path delays and the associated complex amplitudes. The path delays ofuplink channels are first estimated and quantized at the base station, then fedforward to the user equipment (UE) through a dedicated feedforward link. Inthis manner, the UE can obtain the a priori knowledge of the downlink channelin advance since it has been demonstrated that the downlink and the uplinkchannels can have identical path delays. Our analysis and simulation resultsshow that the proposed approach can achieve near-MMSE performance.

Design and Analysis of Downlink Channel Estimation Based on Parametric Model for Massive MIMO in FDD Systems