Non-Orthogonal Affine Frequency Division Multiplexing for Spectrally Efficient High-Mobility Communications

This paper proposes a novel non-orthogonal affine frequency division multiplexing {(nAFDM)} waveform for reliable high-mobility communications with enhanced spectral efficiency {(SE)}. The key idea is {to introduce} a bandwidth compression factor into the AFDM {modulator} to enable controllable subcarrier overlapping. We first {detail the proposed nAFDM transceiver} and derive the corresponding input-output {signal} relationship. Then, an efficient {nAFDM} signal generation method based on the inverse discrete Fourier transform (IDFT) is proposed, enabling practical implementation using existing inverse fast Fourier transform (IFFT) modules without additional hardware complexity. Next, to characterize the impact of non-orthogonal modulation, we derive a closed-form expression {of} inter-carrier interference (ICI), showing its dependence on the bandwidth compression factor. To mitigate the resulting interference, we propose a soft iterative detection algorithm and a low-complexity implementation approach that leverages the distribution characteristics of ICI. {Simulation results demonstrate that 1) in terms of bit error rate (BER), the proposed nAFDM can achieve near identical BER compared to conventional AFDM, while outperforms other waveform counterparts; 2) nAFDM is capable of striking higher SE compared to other existing waveforms; and 3) the proposed nAFDM achieves an attractive BER vs. SE trade-off, and the proposed soft ID scheme can attain a trade-off between BER and complexity.}