  We consider a multipair massive multiple-input multiple-output (MIMO) two-way
relaying system, where multiple pairs of single-antenna devices exchange data
with the help of a relay employing a large number of antennas $N$. The relay
consists of low-cost components that suffer from hardware impairments. A
large-scale approximation of the spectral efficiency (SE) with maximum ratio
(MR) processing is derived in closed form, and the approximation is tight as $N
\to \infty$. It is revealed that for a fixed hardware quality, the impact of
the hardware impairments vanishes asymptotically when $N$ grows large.
Moreover, the impact of the impairments may even vanish when the hardware
quality is gradually decreased with $N$, if a scaling law is satisfied.
Finally, numerical results validate that multipair massive MIMO two-way
relaying systems are robust to hardware impairments at the relay.
