A Scalable Framework for CSI Feedback in FDD Massive MIMO via DL Path Aligning

TL;DR

This paper presents a scalable framework for efficient downlink CSI feedback in FDD massive MIMO systems by exploiting limited reciprocity of long-term channel parameters, reducing feedback overhead significantly.

Contribution

It introduces a novel DL pilot design and feedback scheme leveraging reciprocity of long-term parameters, enabling reduced-dimensionality feedback in practical wideband FDD massive MIMO systems.

Findings

Users can feed back one scalar per DL path for CSI recovery

The framework reduces feedback overhead while maintaining accurate CSI

Numerical results validate the effectiveness of the proposed method

Abstract

Unlike the time-division duplexing (TDD) systems, the downlink (DL) and uplink (UL) channels are not reciprocal anymore in the case of frequency-division duplexing (FDD). However, some long-term parameters, e.g. the time delays and angles of arrival (AoAs) of the channel paths, still enjoy reciprocity. In this paper, by efficiently exploiting the aforementioned limited reciprocity, we address the DL channel state information (CSI) feedback in a practical wideband massive multiple-input multiple-output (MIMO) system operating in the FDD mode. With orthogonal frequency-division multiplexing (OFDM) waveform and assuming frequency-selective fading channels, we propose a scalable framework for the DL pilots design, DL CSI acquisition, and the corresponding CSI feedback in the UL. In particular, the base station (BS) can transmit the FFT-based pilots with the carefully-selected phase shifts. Then the user can rely on the so-called time-domain aggregate channel (TAC) to derive the feedback of reduced imensionality according to either its own knowledge about the statistics of the DL channels or the instruction from the serving BS. We demonstrate that each user can just feed back one scalar number per DL channel path for the BS to recover the DL CSIs. Comprehensive numerical results further corroborate our designs.