Synergistic Gain for OTFS/AFDM Multi-Satellite Transmission System

The thriving of satellite communication (SatCom), particularly the expansion of constellation size, offers significant opportunities for cooperative multi-satellite transmission (MST). MST leverages the diversity of fading channels arising from the spatial separation of satellites through novel waveform schemes, such as delay-Doppler (DD) domain-based orthogonal time frequency space (OTFS) and chirp domain-based affine Fourier division multiplexing (AFDM). This paper demonstrates that MST can achieve substantial synergistic gain by combining signal-to-noise ratio (SNR) gain and diversity gain with proper waveform design. We prove that OTFS/AFDM avoids the loss of diversity distinguishability compared to current orthogonal frequency division multiplexing (OFDM) and single-carrier (SC) systems, thereby maximizing the synergistic gain of MST. Furthermore, our simulation results indicate that, although the maximum likelihood (ML) receiver can theoretically achieve the SNR gain of MST under weak small-scale fading channels, the MMSE equalizer fails to do so. Overall, the results suggest that MST is more suitable for severe fading channels and highlight a challenge for future receiver designs to achieve SNR gain under weak fading channels.