  Multicasting, where a base station (BS) wishes to convey the same message to
several user equipments (UEs), represents a common yet highly challenging
wireless scenario. In fact, guaranteeing decodability by the whole UE
population proves to be a major performance bottleneck since the UEs in poor
channel conditions ultimately determine the achievable rate. To overcome this
issue, two-phase cooperative multicasting schemes, which use conventional
multicasting in a first phase and leverage device-to-device (D2D)
communications in a second phase to effectively spread the message, have been
extensively studied. However, most works are limited either to the simple case
of single-antenna BS or to a specific channel state information at the
transmitter (CSIT) setup. This paper proposes a general two-phase framework
that is applicable to the cases of perfect, statistical, and topological CSIT
in the presence of multiple antennas at the BS. The proposed method exploits
the precoding capabilities at the BS, which enable targeting specific UEs that
can effectively serve as D2D relays towards the remaining UEs, and maximize the
multicast rate under some outage constraint. Numerical results show that our
schemes bring substantial gains over traditional single-phase multicasting and
overcome the worst-UE bottleneck behavior in all the considered CSIT
configurations.
