A system receives a phase-modulated signal that carries client symbols and a sequence that includes first symbols on which a first tone is to be based and having a first power level, and second symbols on which a second tone is to be based and having a second power level; processes the sequence to generate the first tone and the second tone; determines a power difference based on the first power level and the second power level; retrieves condition information that identifies a threshold for determining whether a condition is associated with the signal; determines whether the power difference is greater than the threshold; and outputs an instruction, to adjust a parameter, used to transmit the signal, based on the determination that the condition is associated with the signal, where adjusting the parameter causes the power difference to decrease to a level that is less than the threshold.