A problem of suppression of intrachannel nonlinearities through the use of constrained coding is considered. Three different techniques are proposed and compared with respect to their efficiency, namely 1) constrained coding; 2) combined constrained and error control coding; and 3) deliberate error insertion. Significant Q-factor improvement up to 16 dB depending on code rate and number of spans is demonstrated. A combined constrained-iterative forward error correction (FEC) scheme can operate in the presence of strong intrachannel nonlinearities when even advanced FEC schemes would be overwhelmed with errors. It provides a coding gain of 12.1 dB at a bit error rate (BER) of 10/sup -9/. Deliberate error insertion is an efficient approach to balance the encoder complexity and the achievable coding gain.