  In-phase/quadrature-phase (I/Q) imbalance is one of the most important
hardware impairments in communication systems. It arises in the analogue parts
of direct conversion radio frequency (RF) transceivers and can cause severe
performance losses. In this paper, I/Q imbalance (IQI) aware widely-linear (WL)
channel estimation and data detection schemes for uplink multi-cell massive
multiple-input multiple-output (MIMO) systems are proposed. The resulting
receiver is a WL extension of the minimum mean square error (MMSE) receiver and
jointly mitigates multi-user interference and IQI by processing the real and
the imaginary parts of the received signal separately. The IQI arising at both
the base station (BS) and the user terminals (UTs) is then taken into account.
The considered channel state information (CSI) acquisition model includes the
effects of both estimation errors and pilot contamination, which is caused by
the reuse of the same training sequences in neighboring cells. We apply results
from random matrix theory to derive analytical expressions for the achievable
sum rates of the proposed IQI aware and conventional IQI unaware receivers. Our
simulation results show that the performance of the proposed IQI aware WLMMSE
receiver in a system with IQI is close to that of the MMSE receiver in an ideal
system without IQI. Moreover, our results for the sum rate of the IQI unaware
MMSE receiver reveal that the performance loss due to IQI can be large and, if
left unattended, does not vanish for large numbers of BS antennas.
