  We consider a downlink multicast and unicast superposition transmission in
multi-layer Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division
Multiple Access (OFDMA) systems when only the statistical channel state
information is available at the transmitter (CSIT). Multiple users can be
scheduled by using the time/frequency resources in OFDMA, while for each
scheduled user MIMO spatial multiplexing is used to transmit multiple
information layers, i.e., single user (SU)-MIMO. The users only need to
feedback to the base-station the rank-indicator and the long-term average
channel signal-to-noise ratio to indicate a suitable number of transmission
layers, a suitable modulation and coding scheme and allow the base-station to
perform user scheduling. This approach is especially relevant for the delivery
of common (e.g., popular live event) and independent (e.g., user personalized)
content to a high number of users in deployments in the lower frequency bands
operating in Frequency-Division-Duplex (FDD) mode, e.g., sub-1 GHz. We show
that the optimal resource allocation that maximizes the ergodic sum-rate
involves greedy user selection per OFDM subchannel and superposition
transmission of one multicast signal across all subchannels and single unicast
signal per subchannel. Degree-of-freedom (DoF) analysis shows that while the
lack of instantaneous CSI limits DoF of unicast messages to the minimum number
of transmit antennas and receiver antennas, the multicast message obtains full
DoF that increases linearly with the number of users. We present resource
allocation algorithms consisting of user selection and power allocation between
multicast and unicast signals in each OFDM subchannel. System level simulations
in 5G rural macro-cell scenarios show overall network throughput gains in
realistic network environments by superposition transmission of multicast and
unicast signals.
