  This paper proposes schemes to improve the spectral efficiency of a
multiple-input multiple-output (MIMO) broadcast channel (BC) with I/Q imbalance
(IQI) at transceivers by employing a combination of improper Gaussian signaling
(IGS), non-orthogonal multiple access (NOMA) and simultaneously transmit and
reflect (STAR) reconfigurable intelligent surface (RIS). When there exists IQI,
the output RF signal is a widely linear transformation of the input signal,
which may make the output signal improper. To compensate for IQI, we employ
IGS, thus generating a transmit improper signal. We show that IGS alongside
with NOMA can highly increase the minimum rate of the users. Moreover, we
propose schemes for different operational modes of STAR-RIS and show that
STAR-RIS can significantly improve the system performance. Additionally, we
show that IQI can highly degrade the performance especially if it is overlooked
in the design.
