Affine frequency division multiplexing (AFDM), tailored as a novel multicarrier technique utilizing chirp signals for high-mobility communications, exhibits marked advantages compared to traditional orthogonal frequency division multiplexing (OFDM). AFDM is based on the discrete affine Fourier transform (DAFT) with two modifiable parameters of the chirp signals, termed as the pre-chirp parameter and post-chirp parameter, respectively. These parameters can be fine-tuned to avoid overlapping channel paths with different delays or Doppler shifts, leading to performance enhancement especially for doubly dispersive channel. In this paper, we propose a novel AFDM structure with the pre-chirp index modulation (PIM) philosophy (AFDM-PIM), which can embed additional information bits into the pre-chirp parameter design for both spectral and energy efficiency enhancement. Specifically, we first demonstrate that the application of distinct pre-chirp parameters to various subcarriers in the AFDM modulation process maintains the orthogonality among these subcarriers. Then, different pre-chirp parameters are flexibly assigned to each AFDM subcarrier according to the incoming bits. By such arrangement, aside from classical phase/amplitude modulation, extra binary bits can be implicitly conveyed by the indices of selected pre-chirping parameters realizations without additional energy consumption. At the receiver, both a maximum likelihood (ML) detector and a reduced-complexity ML-minimum mean square error (ML-MMSE) detector are employed to recover the information bits. It has been shown via simulations that the proposed AFDM-PIM exhibits superior bit error rate (BER) performance compared to classical AFDM, OFDM and IM-aided OFDM algorithms.

翻译：仿射频分复用（AFDM）作为一种面向高移动通信的新型多载波技术，利用啁啾信号相较于传统正交频分复用（OFDM）展现出显著优势。AFDM基于离散仿射傅里叶变换（DAFT），其中包含两个可调啁啾信号参数：预啁啾参数和后啁啾参数。通过精细调整这些参数，可避免不同时延或多普勒频移的信道路径重叠，从而提升性能，尤其在双弥散信道中效果显著。本文提出一种基于预啁啾索引调制（PIM）的新型AFDM结构（AFDM-PIM），通过将额外信息比特嵌入预啁啾参数设计，实现频谱效率与能量效率的双重提升。具体而言，我们首先证明在AFDM调制过程中对不同子载波应用差异化预啁啾参数可保持子载波间的正交性；随后根据输入比特灵活为每个AFDM子载波分配不同预啁啾参数。由此，除传统相位/幅度调制外，额外二进制比特可通过所选预啁啾参数实现的索引隐式传输，且无需额外能量消耗。在接收端，采用最大似然（ML）检测器与低复杂度ML-最小均方误差（ML-MMSE）检测器恢复信息比特。仿真结果表明，与经典AFDM、OFDM及IM辅助OFDM算法相比，所提AFDM-PIM具有更优的误码率（BER）性能。