In a mobile wireless channel, the small-scale multipath fading induces temporal channel fluctuations in the form of peaks and deep fades. The channel capacity degradation with fading severity in the high signal-to-noise ratio (SNR) regime is well known in the wireless communication literature: the probability of deep fades increases significantly with higher fading severity resulting in poor performance. In this paper, we focus on double-fading pinhole channels under perfect CSIT to show a very counter-intuitive result that - higher fading severity enables higher ergodic capacity at sufficiently low SNR. The underlying reason is that at low SNRs, ergodic capacity depends crucially on the probability distribution of channel peaks (simply tail distribution); for the pinhole channel, the tail distribution improves with increased fading severity. This allows a transmitter operating at low SNR to exploit channel peaks more efficiently resulting in a net improvement in achievable spectral efficiency. We derive a new key result quantifying the above dependence for the double-Nakagami-$m$ fading pinhole channel - that is, the ergodic capacity ${C} \propto (m_T m_R)^{-1}$ at low SNR, where $m_T m_R$ is the product of fading (severity) parameters of the two independent Nakagami-$m$ fadings involved.

翻译：在移动无线信道中，小尺度多径衰落会引起信道在时间上的波动，表现为峰值和深度衰落。高信噪比条件下，衰落严重性导致的信道容量退化现象在无线通信文献中已广为人知：随着衰落严重性增加，深度衰落的概率显著上升，导致性能恶化。本文聚焦于完美信道状态信息（CSIT）下的双重衰落针孔信道，展示了一个反直觉的结果——在足够低的信噪比下，更高的衰落严重性反而能实现更高的遍历容量。其根本原因在于，低信噪比条件下，遍历容量主要取决于信道峰值的概率分布（即尾部分布）；对于针孔信道，随着衰落严重性增加，尾部分布会得到改善。这使得低信噪比下的发射机能够更高效地利用信道峰值，从而在可达频谱效率上实现净提升。我们推导出一个关键的新结果，量化上述依赖关系对双Nakagami-$m$衰落针孔信道的影响：在低信噪比下，遍历容量${C} \propto (m_T m_R)^{-1}$，其中$m_T m_R$为两个独立Nakagami-$m$衰落（严重性）参数的乘积。