Abstract: In a frequency division multiplexed (FDM) telecommunications system, data is encoded on carriers stepped in frequency within each of a sequence of symbols. The carriers may be arranged in groups between which there are frequency bands in which interference may be expected. The symbols are of predetermined length and are transmitted at a predetermined frequency, and synchronisation to the symbols is needed at a receiver in order to decode the received symbols. During a synchronisation time, a sequence of synchronisation symbols carrying predetermined data is transmitted. The receiver performs unsynchronised Fast Fourier Transforms (FFTs) on the received symbols, starting successive FFTs at about the predetermined symbol frequency. The output of each FFT is evaluated by removing the predetermined data coding from the detected carriers and summing the detected phase differences between the carriers.