A transmitter (12) obtains data packets at a higher layer (16A) of a protocol stack (16), and duplicates each data packet at the higher layer (16A) to obtain duplicates of each data packet. The duplicates are transmitted on different respective channels from the higher layer (16A) to a lower layer (16B) of the protocol stack (16). The transmitter (12) generates different transport blocks to include data from the different respective channels, such that the duplicates of each data packet are conveyed by different transport blocks. For each transport block, the transmitter (12) transmits or does not transmit at least a part of the transport block within at least a part of a different respective transmission opportunity on a link (26) to a receiver (14) using unlicensed frequency spectrum (28), depending on whether or not a channel sensing procedure indicates the unlicensed frequency spectrum (28) is clear during that at least part of the transmission opportunity.