6G NTN Waveforms: A Comparison of OTFS, AFDM and OCDM in LEO Satellite Channels

TL;DR

This work addresses the challenge of Doppler-induced degradation in 6G non-terrestrial networks by comparing three beyond-OFDM waveforms—OTFS, AFDM, and OCDM—over LEO satellite channels. A MATLAB-based BER framework is developed, applying MMSE-SD after equalization to ensure fair, identical evaluation across waveforms under four NTN TDL models per 3GPP TR 38.811, with realistic LEO parameters including Doppler up to $\pm 48\, \text{kHz}$ (S-band) and $\pm 480\, \text{kHz}$ (Ka-band). The results show OCDM is generally unusable due to an irreducible error floor, while AFDM and OTFS provide substantially better BER, with AFDM outperforming OTFS in high-SNR regimes for several TDLs and OTFS remaining competitive at lower SNR. The study offers practical guidance for waveform selection in 6G NTN and provides an open-source validation framework for researchers to reproduce and extend the results.$

Abstract

Sixth generation (6G) physical layer (PHY) is evolving beyond the legacy orthogonal frequency division multiplexing (OFDM)-based waveforms. In this paper, we compare the bit error rate (BER) performance of three beyond-OFDM waveforms, namely, orthogonal time-frequency-space (OTFS) modulation, affine frequency division multiplexing (AFDM), and orthogonal chirp division multiplexing (OCDM), which are particularly suitable for the highly mobile non-terrestrial network (NTN) vertical of 6G. In order to characterize the effect of mobility and Doppler shift in low Earth orbit (LEO) satellites, we performed BER comparisons over four different NTN tapped-delay-line (TDL) models, TDL-A, TDL-B, TDL-C, and TDL-D, as specified in the 3rd generation partnership project (3GPP) technical report TR 38.811. After channel equalization, a minimum mean squared error with successive detection (MMSE-SD) algorithm was used to enhance the BER performance. It was found that AFDM and OTFS consistently outperformed OCDM across all TDL models, while AFDM performed better than OTFS in TDL-B and TDL-C, in the high signal-to-noise ratio (SNR) regime. The complete simulation framework is made available as an open-source code for quick validation and further development.

Figures (4)

• Figure 1: 6G NTN architecture with a fast-moving LEO satellite and a highly mobile user equipment (UE). The gNodeB connects to the core via an NG interface, whereas it connects to the UE via a Uu interface through an NTN payload in a transparent manner.
• Figure 2: Transmitter and receiver schematic for waveform simulation. The modulator uses a discrete inverse transform, and the demodulator uses the corresponding discrete transform. OCDM $(c_1=c_2=1/(2N))$ and OFDM $(c_1=c_2=0)$ are special cases of AFDM. Channel estimator feeds a linear minimum mean square error equalizer.
• Figure 3: Comparison of different waveforms over NTN TDL-C channel with LMMSE and MMSE-SD: [Top] BER performance comparison, and [Bottom] Comparison of SNR requirement for achieving a target BER.
• Figure 4: BER performance comparison of AFDM, OCDM and OTFS with MMSE-SD over different NTN TDL channels.