Time-division duplex (TDD) based massive MIMO systems rely on the reciprocityof the wireless propagation channels when calculating the downlink precodersbased on uplink pilots. However, the effective uplink and downlink channelsincorporating the analog radio front-ends of the base station (BS) and userequipments (UEs) exhibit non-reciprocity due to non-identical behavior of theindividual transmit and receive chains. When downlink precoder is not aware ofsuch channel non-reciprocity (NRC), system performance can be significantlydegraded due to NRC induced interference terms. In this work, we consider ageneral TDD-based massive MIMO system where frequency-response mismatches atboth the BS and UEs, as well as the mutual coupling mismatch at the BSlarge-array system all coexist and induce channel NRC. Based on theNRC-impaired signal models, we first propose a novel iterative estimationmethod for acquiring both the BS and UE side NRC matrices and then also proposea novel NRC-aware downlink precoder design which utilizes the obtainedestimates. Furthermore, an efficient pilot signaling scheme between the BS andUEs is introduced in order to facilitate executing the proposed estimationmethod and the NRC-aware precoding technique in practical systems.Comprehensive numerical results indicate substantially improved spectralefficiency performance when the proposed NRC estimation and NRC-aware precodingmethods are adopted, compared to the existing state-of-the-art methods.
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