An NTN Uplink Radio Interface Based on Constant-Envelope Multicarrier Modulations and NOMA

The evolution of the radio interface design in 6G NonTerrestrial Networks (NTNs) is moving toward the adoption of new waveforms and non-standard multiple access methodologies. Recent studies, conducted within the framework of the Next Generation EU RESTART ITA-NTN project, have demonstrated that a modular SDR-based PHY-layer design can be profitably leveraged to provide a wide range of programmable waveforms. Among these, constant envelope (CE) multicarrier modulations (CE-OFDM and CE-SCOFDM) may play a key role in future NTN transmissions due to their immunity to nonlinear distortion and their enhanced diversity against frequency-selective multipath propagation. CE-MC waveforms are implemented by imposing a non-linear phase modulation on a normalized real-valued multicarrier signal. Such techniques enable orthogonal multiple access (OMA) in the downlink. On the contrary, OMA is not allowed in the uplink because the multiple carriers attributed to a user are nonlinearly “packed” by the phase modulator into a single carrier RF signal, which cannot be slotted into the numerical Discrete Fourier Transform (DFT) domain, as happens for OFDMA and SC-FDMA. To leverage the advantages of CE-MC modulations in the NTN uplink, we propose using Power-Domain Non-Orthogonal Multiple Access (PD-NOMA) as a multi-user transmission scheme with Serial-Interference Cancellation (SIC) at the receiver side. As our investigation is preliminary, a basic two-user configuration has been considered. An NTN uplink transmission scenario, based on small LEO satellites operating in the S-band, has been simulated. A first series of simulation results discusses the impact of various degrees of freedom in CEMC waveforms on NOMA performance, while keeping the power allocation ratio as a variable. Building on these results, a subsequent series of simulations concerns a realistic NOMA scenario, where the power gap between two users located at the borders of the satellite footprint is evaluated based on the propagation conditions. The achieved simulation results provide a preliminary, however reliable, evaluation of the proposed multi-user CE-MC-based NOMA radio interface, also offering some hints for the future implementation of the proposed transmission system in more complex multi-user NTN uplink scenarios.