Affine frequency division multiplexing (AFDM) is an emerging multicarrier waveform that offers a potential solution for achieving reliable communications over time-varying channels. This paper proposes two maximum-likelihood (ML) estimators of symbol time offset and carrier frequency offset for AFDM systems. One is called joint ML estimator, which evaluates the arrival time and carrier frequency offset by comparing the correlations of samples. Moreover, we propose the other so-called stepwise ML estimator to reduce the complexity. Both proposed estimators exploit the redundant information contained within the chirp-periodic prefix inherent in AFDM symbols, thus dispensing with any additional pilots. To further mitigate the intercarrier interference resulting from the residual frequency offset, we design a mirror-mapping-based scheme for AFDM systems. Numerical results verify the effectiveness of the proposed time and carrier frequency offset estimation criteria and the mirror-mapping-based modulation for AFDM systems.

翻译：仿射频分复用（AFDM）是一种新兴的多载波波形技术，为在时变信道上实现可靠通信提供了潜在解决方案。本文针对AFDM系统提出了两种符号时间偏移与载波频率偏移的最大似然（ML）估计器。第一种称为联合ML估计器，通过比较样本的相关性来评估到达时间和载波频率偏移。此外，本文还提出了另一种称为逐步ML估计器的低复杂度方案。两种估计器均利用AFDM符号中啁啾周期前缀固有的冗余信息，从而无需额外导频。为进一步抑制残余频率偏移导致的子载波间干扰，本文设计了一种基于镜像映射的AFDM系统方案。数值结果验证了所提出的时间与载波频率偏移估计准则及基于镜像映射的调制方案在AFDM系统中的有效性。