  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.
