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Canonical Massive MIMO uses time division duplex (TDD) to exploit channelreciprocity within the coherence time, avoiding feedback of channel stateinformation (CSI), as is required for precoding at the base station. We extendthe idea of exploiting reciprocity to the coherence bandwidth, allocatingsubcarriers of a multicarrier (OFDM)-based system in an interlaced fashion toup- and downlink, respectively, referred to as (OFDM-)subcarrier interlaced FDD(IFDD). Exploiting this "two-dimensional" channel reciprocity within both timeand frequency coherence does not require any CSI feedback and, even more so,offers faster-than-TDD channel tracking. To address imperfections of actualhardware, 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. Aswill be shown, such hardware challenges like self-interference or frequencyoffsets are not critical for massive MIMO operation, but can be neglected asthe number of antennas grows large. Finally, we quantify how IFDD allows totrack the channel variations much faster than TDD over a wide range of commonlyused pilot symbol rates.

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