1. A user equipment device, comprising:a processor circuit configured to:
establish a connection to a first base station operating in a Frequency Division Duplex (FDD) mode, the first base station designated as a primary serving cell (PCell); and
establish a connection to a second base station operating in a Time Division Duplex (TDD) mode, the second base station designated as a secondary serving cell (SCell);
a receiver circuit configured to receive subframe scheduling messages from the SCell; and
a transmitter circuit configured to transmit acknowledgment messages to the PCell, the acknowledgement messages corresponding to the subframe scheduling messages received on the SCell.