A system to communicate digital data symbols with higher than QPSK modulation comprises a transmitter and receiver. The transmitter comprises, a modulator and circuitry to split and encode the data into a first block of more significant bits and a second block of less significant bits for modulating by. The receiver receives digital data bits by iterative determination of soft estimates of bits followed by a hard decision as to what bit was intended, and comprises a first processor to provide first soft estimates of bits of the received signal, and a second processor to decode the first soft estimates and to provide second soft estimates of the bits. The receiver also comprises a first combiner to provide adapted first soft estimates to the second processor, the adapted first soft estimates of each bit being dependent upon the respective first soft estimate and a respective previous first soft estimate, and a second combiner to provide third soft estimates back to the first processor for subsequent further decoding, the third soft estimates of each bit being dependent upon the respective second soft estimate and a respective previous second soft estimate.