On the Robustness of AFDM and OTFS Against Passive Eavesdroppers

We investigate the robustness of affine frequency division multiplexing (AFDM) and orthogonal time frequency space (OTFS) waveforms against passive eavesdroppers performing bruteforce 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 O(√N) for OTFS and O(N²) for AFDM, where N is the number of subcarriers, indicating that AFDM has superior resilience over OTFS. Simulation results demonstrate that AFDM is capable of preventing reliable demodulation at the eavesdropper side with a bit error rate (BER) of 0.5, while OTFS allows partial signal recovery under equivalent conditions.