J. Qiu, D. Grace, G. Ding, J. Yao and Q. Wu, Blockchain-Based Secure Spectrum Trading for Unmanned-Aerial-Vehicle-Assisted Cellular Networks: An Operator’s Perspective, in IEEE Internet of Things Journal, vol. 7, no. 1, pp. 451-466, Jan. 2020, doi: 10.1109/JIOT.2019.2944213.