Abstract: Delay alignment modulation (DAM) is recently proposed as an effective technique to address the inter-symbol interference (ISI) issue, which circumvents the conventional channel equalization and multi-carrier transmission. Moreover, wireless communications are vulnerable to malicious eavesdropping and attacks due to their inherent open and broadcast nature. In particular, DAM not only eliminates the ISI at the desired receiver but may also introduce ISI to other locations, and thus is quite promising for secure communications. This paper considers the near-field secure wireless communication with DAM. To gain useful insights, it is first shown that when the antenna number of Alice is much larger than the number of multipaths for Bob and Eve, the delay compensation and low-complexity path-based maximal-ratio transmission (MRT) beamforming achieve a communication free of ISI and information leakage, owing to the asymptotically orthogonal property brought by the near-field nonuniform spherical wave (NUSW). The secrecy rate performance of path-based zero-forcing (ZF) beamforming toward ISI-free communication is then evaluated. Furthermore, the path-based optimized DAM beamforming scheme is proposed to maximize the secrecy rate, by considering the general case in the presence of some tolerable ISI. As a comparison, the benchmarking scheme of the artificial noise (AN) based orthogonal frequency-division multiplexing (OFDM) is considered. Simulation results show that DAM achieves a higher secrecy rate and lower peak-to-average-power ratio (PAPR) than the AN-based OFDM.

Key words: Near-field; Nonuniform spherical wave; Delay alignment modulation (DAM); Inter-symbol interference (ISI) free communication; Physical layer security; Secrecy rate

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