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We propose two low-complexity digital signal processing (DSP) techniques to improve the system performance of digital subcarrier multiplexing (DSCM) optical transmission system with a silicon photonics transmitter. We first analyze the impacts of various transmitter parameters on the system performance of the single carrier system versus the DSCM system. We show quantitatively that the DSCM system suffers from a high peak-to-average power ratio (PAPR) and nonlinear transfer functions that cannot be easily compensated for. Furthermore, using a high-driving-voltage silicon photonics modulator exacerbates this penalty at the transmitter. To combat the performance degradation caused by nonideal transmitters, we propose and demonstrate the functionality of an encoding scheme based on the fast Fourier transform (FFT) to decrease the PAPR of the transmitted DSCM signals. Then a simple and effective pre-mapping technique is proposed to compensate for the nonlinearity from the transmitter. After developing the theory of our proposed approach, both DSP blocks are verified with coherent optical transmission simulations and experiments. Using a 64 GBd 4-bit/s/Hz DSCM signal containing 8 subcarriers transmitted over 43.2 km of standard single-mode fiber (SSMF), the FFT encoding achieves a gain of 3.458 dB in terms of link loss, and the pre-mapping achieves a gain of 0.486 dB when compared to the raw DSCM system, at the HD-FEC bit error rate (BER) threshold of 3.8e-3. We also tested the performance of the system when the two techniques are combined. We found that this led to a power budget increase of 4.159 dB at the HD-FEC threshold. Since the total gain is more than the addition of the two gains from each DSP block, there is a gain enhancement effect between the two proposed algorithms that generates extra gain when implemented together. The proposed transmitter algorithms and the overall schematic is favorable to the implementation of DSCM systems when using silicon-photonics-modulator-based transmitters specifically and coherent transmitters generally.