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