In paper 'To Harvest and Jam: A Paradigm of Self-Sustaining Friendly Jammers for Secure AF Relaying', the paper mentions that another related paper studies collaborative use of relays to form effective beams jamming the eavesdropper, i.e., secure collaborative relay beamforming, for relay-wiretap channels with single eavesdropper. You've also read that paper. Provide the full name of the paper.