FDD Massive MIMO -- Antenna Duplex Pattern an-Reciprocity : A Missing Brick

TL;DR

This paper investigates the pattern reciprocity of antenna duplex patterns in FDD massive MIMO systems, revealing significant divergence in practical user handsets that impacts downlink channel state information accuracy.

Contribution

It demonstrates that real-world user devices exhibit duplex pattern divergence, challenging prior assumptions of pattern reciprocity in FDD massive MIMO systems.

Findings

Measured phones show duplex pattern divergence.

Mock-up phone exhibits depolarization and gain differences.

Pattern divergence significantly affects DL CSI assessment.

Abstract

Obtaining down link (DL) channel state information (CSI) at the base station (BS) is challenging for frequencydivision-duplex (FDD) massive MIMO (MM) systems. Considerable overhead is required for DL training and feedback. Instead studies often assume highly correlated average spatial signal signatures (i.e. directional clusters) between FDD duplex links. These assumptions, however, only represent use of antennas with the same radiation pattern over the duplex band, leading to illumination of the same cluster. In this paper, we investigate pattern reciprocity, over the duplex band, for practical user handsets. We first show how a population of measured contemporary phones exhibits noticeable duplex pattern divergence. We then show measured complex pattern duplex divergence of a mock-up phone, where depolarization comes on top of gain differences. Thus we reveal a significant but overlooked brick in DL CSI assessment for FDD MM operation. Namely, in order for the FDD MM performance studies to be realistic, practical user handsets have to be considered.