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 eavesdropper, 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.