Spectrally efficient SCM-QPSK optical system: design and performance evaluation over long-haul fiber

The evolution toward 6G networks demands unprecedented data rates, spectral efficiency, and energy efficiency, motivating the integration of wireless and optical communication technologies to exploit the complementary advantages of both domains. Recently, subcarrier multiplexing (SCM) has received great attention because it allows sending multiple data streams over a single optical carrier. However, SCM is susceptible to impairments such as chromatic dispersion, nonlinearities, and accumulation of amplified spontaneous emission (ASE) noise in the amplifiers that adversely affect the performance of the SCM optical link. To address the need for spectrally efficient optical transmission, we numerically investigate a four-subcarrier system using quadrature phase shift keying (QPSK) over a 400-km multi-span optical fiber link. The four subcarriers are symmetrically positioned at ±5.5 and ±16.5 GHz and shaped using root-raised cosine filtering. The transmission link is modeled using the symmetric split-step Fourier method, including chromatic dispersion, Kerr nonlinearity, fiber attenuation, and ASE noise introduced after each 80-km erbium-doped fiber amplifier span. The individual subcarriers are recovered independently and evaluated using bit error rate (BER), error vector magnitude, signal-to-noise ratio, constellation diagrams, and eye diagrams. The results provide a numerical characterization of the impairment-induced degradation experienced by the individual SCM subcarriers after long-haul propagation. At the selected 400-km and 0-dBm operating point, substantial degradation is observed across all four subcarriers, with comparable performance among the symmetric subcarriers. A separate Additive White Gaussian Noise validation confirms the correct implementation of the QPSK modulation, demodulation, and BER calculation. The study provides a baseline for understanding subcarrier-level impairment behavior. However, nonlinear impairment mitigation is outside the scope of the present work.