A transmitter has a transmit chain that is configured to format transmit data into a frame structure that comprises signal blocks interspersed with training sequences for assisting a receive operation. The transmit chain is configured to insert two training sequences between a signal block and its following signal block in the frame structure. The transmitter also comprises a sequence generator configured to form the two training sequences, and in particular to form them such that a first of the training sequences is a negative of the second. Having one training sequence being the negative of the other helps to avoid the Direct Current (DC) subcarrier and provides good properties for channel estimation.