A digital communication technique, implementable in a digital communication system, includes performing a training phase in which the transmitter sends one or more transmissions of pulse amplitude modulation (PAM) signals wherein the PAM signals are modulated using one or more data-to-PAM-symbol mapping schemes, performing a feedback phase in which the transmitter receives a feedback message from a receiver, the feedback message indicative of a change to be made to data-to-PAM-symbol modulation schemes, and implementing a data transmission phase in which data is transmitted using a current PAM symbol mapping scheme based on the feedback message, wherein at least some symbols of the current PAM symbol modulation scheme are unequally spaced with respect to each other.