Disclosed is a method including communicating, by a mobile device, with a base station via first and second component carriers having different frequency bands and time division duplexing (TDD) configurations. The method may include receiving one or more downlink transmissions via the second component carrier. The method may include selecting a hybrid automatic repeat request (HARQ) timing sequence based on the TDD configurations of the first and second component carriers. The method may include transmitting one or more positive acknowledgment and/or negative acknowledgement (ACK/NACK) signals, associated with the one or more downlink transmissions, according to the selected HARQ timing sequence. Other embodiments may be described and claimed.