Certain aspects of the present disclosure provide techniques for determining resources for transmission of acknowledgement messages to a user equipment (UE). The techniques involve receiving a plurality of codewords from the UE via an uplink component carrier, and determining at least first and second index pairs identifying resources of a downlink channel to be used for transmitting acknowledgement messages for the received codewords. The first index pair is determined based on a first set of parameters, and the second index pair is determined based on the first set of parameters and a second set of one or more parameters. At least one of the parameters in the second set is a fixed value.