Wei-dang Lu Xuanli Wu Qingzhong Li Naitong Zhang Secondary Spectrum Access Based on Cooperative OFDM Relaying. 1-5 2012 VTC Spring https://doi.org/10.1109/VETECS.2012.6240018 conf/vtc/2012s db/conf/vtc/vtc2012s.html#LuWLZ12