A downlink data transmission is performed in a downlink subframe. One or more resources are determined. The one or more resources are to be used for transmission or reception of error information for the received transmission in an uplink subframe based at least on an association set between one or more downlink or special subframes and an uplink subframe. The determined association set depends on a downlink reference configuration. The association set for each uplink subframe comprises a grouping of the downlink or special subframes into at least two groups. An association set is defined only for the subframes which are uplink subframes in a corresponding downlink reference configuration. The error information is transmitted or received using the determined one or more resources in the uplink subframe. Methods, apparatus, programs, and program products are disclosed.