The performance of Radio Frequency (RF) fingerprinting techniques is negatively impacted when the training data is not temporally close to the testing data. This can limit the practical implementation of physical-layer authentication solutions. To circumvent this problem, current solutions involve collecting training and testing datasets at close time intervals -- this being detrimental to the real-life deployment of any physical-layer authentication solution. We refer to this issue as the Day-After-Tomorrow (DAT) effect, being widely attributed to the temporal variability of the wireless channel, which masks the physical-layer features of the transmitter, thus impairing the fingerprinting process. In this work, we investigate the DAT effect shedding light on its root causes. Our results refute previous knowledge by demonstrating that the DAT effect is not solely caused by the variability of the wireless channel. Instead, we prove that it is also due to the power-cycling of the radios, i.e., the turning off and on of the radios between the collection of training and testing data. We show that state-of-the-art RF fingerprinting solutions double their performance when the devices under test are not power-cycled, i.e., the accuracy increases from about 0.5 to about 1 in a controlled scenario. Finally, we propose a new technique to mitigate the DAT effect in real-world scenarios. Our experimental results show a significant improvement in accuracy, from approximately 0.45 to 0.85. Additionally, our solution reduces the variance of the results, making the overall performance more reliable.

翻译：射频（RF）指纹识别技术的性能在训练数据与测试数据时间间隔较大时会受到负面影响，这限制了物理层认证解决方案的实际应用。为解决该问题，现有方案通常要求训练与测试数据集在相近时间间隔内采集——但这种做法不利于物理层认证方案的实际部署。我们将此问题称为"后天效应"（Day-After-Tomorrow, DAT），其普遍归因于无线信道的时变性掩盖了发射器的物理层特征，从而影响指纹识别过程。本研究深入探究DAT效应的根本成因，实验结果推翻了既有认知，证明DAT效应并非完全由无线信道时变性导致。我们证实，设备开关机操作（即训练与测试数据采集期间无线电的开启与关闭）同样是重要影响因素。研究表明，若被测设备不经历开关机操作，现有最优RF指纹识别方案的性能可提升一倍：受控场景下准确率从约0.5提升至约1.0。最后，我们提出一种缓解实际场景中DAT效应的新技术，实验结果显示准确率从约0.45显著提升至0.85，同时降低了结果方差，使整体性能更具可靠性。