Abstract In this paper we present the design and a numerical proof-of-concept of an RSOA-based colorless multilevel transmitter, suitable for use in optical network units at user premises. Simple electrical predistortion of the M-QAM modulation signal theoretically allows the transmitter to be operated at up to 4 GBd symbol rate with 16-QAM modulation accounting for the transmission channel impairments and noise at the detection stage, if forward error correction is implemented. We also numerically confirm that 4-QAM operation is possible without the modulation signal predistortion, as long as the constellation is derotated after detection, which can be easily done using digital feed-forward algorithm.