On the Capacity of Fading Channels with Peak and Average Power Constraints at Low SNR

The capacity of fading channels under peak and average power constraints in the low-SNR regime is investigated. We show that the capacity scales essentially as ${C \approx A \ \text{SNR} \int_{1- \frac{1}{A}}^1 F^{-1}\left(t\right)dt}$, where $A$ is the peak to average power ratio (PAPR), and $F(\cdot)$ is the cumulative distribution function of the fading channel. We also prove that an On-Off power scheme is sufficient to asymptotically achieve the capacity. Furthermore, by considering the variable PAPR scenario, we generalize the scalability of the capacity and derive the asymptotic expression for the capacity in the low-SNR regime.