MIMO-OCDM-NOMA: Enhancing Spectral Efficiency with Improved Tolerance to Doppler and Interference in High-Mobility Environments

This paper presents the integration of Orthogonal Chirp Division Multiplexing (OCDM) and Non-Orthogonal Multiple Access (NOMA) within a Multiple-Input Multiple-Output (MIMO) framework, referred to as MIMO-OCDM-NOMA, to address the limitations of conventional multicarrier systems in high-mobility and interference-prone environments. The proposed MIMO-OCDM-NOMA framework enhances spectral efficiency (SE) by leveraging the power domain multiplexing of NOMA. The use of chirp signals in OCDM improves bit error rate (BER) performance, owing to their tolerance to noise, multipath fading, and interference. A closed-form average BER bound, a diversity-order analysis, and a carrier frequency offset (CFO) characterization are derived for the integrated framework and supported through simulations. The analysis of imperfect successive interference cancellation (SIC) reveals that the chirp-based modulation in OCDM reduces residual interference, thereby enhancing the system's overall tolerance to channel impairments. Simulation results demonstrate that the proposed MIMO-OCDM-NOMA system mitigates channel interference and maintains higher SE under doubly-dispersive and high-mobility conditions, even with imperfect SIC.