A discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) scheme is attractive for its effective peak-to-average power ratio (PAPR) reduction. This paper presents another important issue about DFT-S OFDM, which is noise spread induced by the extra inverse discrete Fourier transform for DFT-S OFDM data recovery at the receiver. It would reduce the optical signal-to-noise ratio (OSNR) and spread the noise over the whole DFT-S OFDM subband, make DFT-S OFDM being more sensitive to noise distribution, and weaken the improvement induced by PAPR reduction. The principle of DFT-S OFDM generation and theoretical analysis of the noise spread in DFT-S OFDM signal processing at the receiver is presented. 100 Gb/s multiband DFT-S OFDM simulation system and 2.5 Gb/s experiment have been demonstrated to prove that the DFT-S OFDM scheme has worse performance than the conventional OFDM scheme under the condition of low OSNR and bad channel estimation; hence, it is more sensitive to the OSNR in transmission channel and needs more accurate channel estimation.