  This paper proposes an energy-efficient scheme for multicell multiple-input,
multiple-output (MIMO) simultaneous transmit and reflect (STAR) reconfigurable
intelligent surfaces (RIS)-assisted broadcast channels by employing rate
splitting (RS) and improper Gaussian signaling (IGS). Regular RISs can only
reflect signals. Thus, a regular RIS can assist only when the transmitter and
receiver are in the reflection space of the RIS. However, a STAR-RIS can
simultaneously transmit and reflect, thus providing a 360-degrees coverage. In
this paper, we assume that transceivers may suffer from I/Q imbalance (IQI). To
compensate for IQI, we employ IGS. Moreover, we employ RS to manage intracell
interference. We show that RIS can significantly improve the energy efficiency
(EE) of the system when RIS components are carefully optimized. Additionally,
we show that STAR-RIS can significantly outperform a regular RIS when the
regular RIS cannot cover all the users. We also show that RS can highly
increase the EE comparing to treating interference as noise.
