A feedback radio resource in a license-exempt frequency band maps from a data radio resource to at least one frequency sub-band which the data resource excludes. The feedback resource is also spaced in time from the data resource by a predetermined interval. Feedback (ACK/NACK) for data received in the data resource is then sent in the feedback resource. In various embodiments the sub-band is one or more edges of the data resource channel, or of a license-exempt component carrier. The predetermined interval may be a function of how much of the data resource is occupied by data. The data transmitting device delays sending its data by a time offset from the end of a previous transmission on that channel, or if it does not know the end time it delays until the predetermined interval plus the length of the feedback resource have lapsed. Various synchronization aspects are also disclosed.