  We consider two-way amplify and forward relaying, where multiple full-duplex
user pairs exchange information via a shared full-duplex massive multiple-input
multiple-output (MIMO) relay. We derive closed-form lower bound for the
spectral efficiency with zero-forcing processing at the relay, by using minimum
mean squared error channel estimation. The zero-forcing lower bound for the
system model considered herein, which is valid for arbitrary number of
antennas, is not yet derived in the massive MIMO relaying literature. We
numerically demonstrate the accuracy of the derived lower bound and the
performance improvement achieved using zero-forcing processing. We also
numerically demonstrate the spectral gains achieved by a full-duplex system
over a half-duplex one for various antenna regimes.
