Spectrum-Efficient Coherent Photonic Terahertz Transmission System Based on Subcarrier Multiplexing With Offset Quadrature Amplitude Modulation

Abstract

To overcome the spectral efficiency (SE) limitations of conventional subcarrier multiplexing (SCM) caused by guard bands, a spectrum-efficient coherent photonic terahertz (THz) transmission system based on SCM with offset quadrature amplitude modulation (SCM-OQAM) is proposed and validated. By modeling intrinsic interference in spectrally compact configurations, we reveal that adjacent-channel interference exhibits an alternating real-imaginary structure. Leveraging this property, the proposed SCM-OQAM scheme achieves interference suppression in the real domain without requiring guard bandwidth. A coherent transmission architecture together with the corresponding digital signal processing (DSP) workflow is developed. Simulation results demonstrate that, under comparable performance conditions, the proposed system achieves approximately 15% improvement in SE compared with SCM-QAM. In addition, owing to the high tolerance of OQAM to truncation effects, the critical truncation length can be reduced from 7 T to 3 T, leading to a 57% reduction in the number of filter taps and a corresponding reduction in computational complexity. Furthermore, the proposed scheme is particularly well suited to short-reach photonic THz access and fronthaul scenarios, where spectrum resources are scarce and implementation efficiency is critical. These results highlight the potential of SCM-OQAM as an effective approach for jointly improving spectral utilization and DSP efficiency in future high-capacity THz communication systems.