On the Robustness of AFDM and OTFS Against Passive Eavesdroppers

TL;DR

This paper analyzes the robustness of AFDM and OTFS waveforms against passive eavesdroppers, showing AFDM's superior resilience due to higher brute-force complexity, with BER simulations confirming its near-undecodability.

Contribution

The paper provides an analytical comparison of AFDM and OTFS against brute-force eavesdropping, revealing AFDM's higher complexity and robustness.

Findings

AFDM has a brute-force complexity of O(N^2), higher than OTFS's O(√N).

BER simulations show AFDM remains nearly undecodable for eavesdroppers.

OTFS allows partial signal recovery under brute-force attack.

Abstract

We investigate the robustness of affine frequency division multiplexing (AFDM) and orthogonal time frequency space (OTFS) waveforms against passive eavesdroppers performing brute-force demodulation to intercepted signals, under the assumption that eavesdroppers have no knowledge of chirp parameters (in AFDM) or the delay-Doppler grid configuration (in OTFS), such that they must search exhaustively over possible demodulation matrices. Analytical results show that the brute-force complexity scales as $\mathcal{O}(\sqrt{N})$ for OTFS and $\mathcal{O}(N^2)$ for AFDM, where $N$ is the number of subcarriers, indicating that AFDM has superior resilience over OTFS. Bit error rate (BER) simulations confirm the analysis by showing that, with AFDM, the signal remains nearly undecodable at the eavesdropper, while OTFS allows partial signal recovery under equivalent conditions.