In this paper, a Hermitian symmetry based orthogonal frequency division multiplexing (HS-OFDM) signal transmitter is proposed. It can be used in a digitized radio-over-fiber (DRoF) system to reduce the system complexity by transmitting a real digital sequence. In this HS-OFDM transmitter, differential pulse code modulation (DPCM) schemes are used instead of the traditional pulse code modulation (PCM) to quantify the bandpass sampled signals, which can improve the signal-to-quantization noise ratio (SQNR) by 4~7 dB. Moreover, a high-order pulse amplitude modulation (PAM) is utilized to increase the spectral efficiency of the DRoF system. Simulation results show that the digital code with PAM- 16 can improve the spectral efficiency by 3 times with the same error vector magnitude (EVM) performance.