Uplink control signaling for joint fdd nad tdd carrier aggregation

A user equipment device, comprising:

• a processor circuit configured to:
• establish a connection to a first base station operating in a Time Division Duplex (TDD) mode, the first base station designated as a primary serving cell (PCell);
• and
• establish a connection to a second base station operating in a Frequency Division Duplex (FDD) mode, the second base station designated as a secondary serving cell (SCell);
• a receiver circuit configured to receive Physical Downlink Shared Channel (PDSCH) messages in a downlink subframe from the SCell;
• and
• a transmitter circuit configured to transmit acknowledgment messages corresponding to the PDSCH messages to the PCell in a selected uplink subframe, wherein the selected uplink subframe is the first valid uplink subframe following the downlink subframe.

A method for operating a mobile device, comprising:

• establishing a connection to a first base station designated as a primary serving cell (PCell);
• establishing a connection to a second base station designated as a secondary serving cell (SCell), wherein the SCell uses a different operating mode than the PCell;
• receiving subframe scheduling messages from the SCell in a downlink subframe;
• and
• selecting an uplink subframe to send a scheduling message acknowledgement based upon the operating mode of the PCell.