Subcarrier-Interlaced FDD for Faster-than-TDD Channel Tracking in Massive MIMO Systems

TL;DR

This paper introduces subcarrier-interlaced FDD (IFDD), a novel multicarrier scheme that exploits channel reciprocity in both time and frequency, enabling faster-than-TDD channel tracking in massive MIMO systems without CSI feedback.

Contribution

The paper proposes and validates a new interlaced FDD scheme that leverages two-dimensional channel reciprocity for rapid channel tracking in massive MIMO, bypassing the need for CSI feedback.

Findings

IFDD enables faster channel tracking than TDD across various pilot rates.

Hardware imperfections like self-interference are manageable and diminish with more antennas.

Experimental measurements confirm the practical viability of IFDD in real-world scenarios.

Abstract

Canonical Massive MIMO uses time division duplex (TDD) to exploit channel reciprocity within the coherence time, avoiding feedback of channel state information (CSI), as is required for precoding at the base station. We extend the idea of exploiting reciprocity to the coherence bandwidth, allocating subcarriers of a multicarrier (OFDM)-based system in an interlaced fashion to up- and downlink, respectively, referred to as (OFDM-)subcarrier interlaced FDD (IFDD). Exploiting this "two-dimensional" channel reciprocity within both time and frequency coherence does not require any CSI feedback and, even more so, offers faster-than-TDD channel tracking. To address imperfections of actual hardware, we conducted measurements, verifying the practical viability of IFDD. As it turns out, the scheme incurs similar transmit/receive isolation issues (self-interference) as well-known from the full-duplex (FDup) literature. As will be shown, such hardware challenges like self-interference or frequency offsets are not critical for massive MIMO operation, but can be neglected as the number of antennas grows large. Finally, we quantify how IFDD allows to track the channel variations much faster than TDD over a wide range of commonly used pilot symbol rates.