Method and apparatus for determining HARQ timing in communication systems

A method for transmitting, at a user equipment, a Hybrid Automatic Repeat Request (HARQ) feedback to a base station in a communication system supporting Carrier Aggregation (CA) of a primary cell and at least one secondary cell, wherein the primary cell and the secondary cell support either Frequency Divisional Duplex (FDD) or Time Divisional Duplex (TDD), the method comprising:

• receiving a downlink physical channel through one of the primary and the secondary cells;
• transmitting the HARQ feedback corresponding to the downlink physical channel of the primary cell at a first timing predetermined for the primary cell;
• and
• transmitting the HARQ feedback corresponding to the downlink physical channel of the secondary cell at a second timing,
• wherein the second timing is determined according to one or more of duplex modes of the primary cell and the at least one secondary cell, and a predefined rule,
• wherein the duplex modes are selected from TDD and FDD, and the predefined rule includes that the HARQ feedback is transmitted on a component carrier of the primary cell,
• wherein if the user equipment is not configured to monitor a control channel in the primary cell for scheduling the secondary cell, and when the primary cell is configured as TDD and the secondary cell is configured as FDD, the second timing is identical with a fourth timing, wherein the fourth timing is determined as below:
• for a first subframe which is downlink subframe in both the primary cell and the secondary cell, the second timing of transmitting the HARQ feedback corresponding to the downlink physical channel of the first subframe in the secondary cell is identical with the first timing;
• and
• for a second subframe which is uplink subframe in the primary cell and is downlink subframe in the secondary cell, the second timing of transmitting the HARQ feedback corresponding to the downlink physical channel of the second subframe in the secondary cell is identical with the first timing of transmitting the HARQ feedback corresponding to the downlink physical channel of a downlink subframe of the primary cell which is closest to the second subframe.