Reza Shakeri Hamidreza Khakzad Abbas Taherpour Tamer Khattab Mazen Omar Hasna Hybrid underlay/overlay cognitive radio system with hierarchical modulation in the presence of channel estimation error. 967-972 2014 GLOBECOM https://doi.org/10.1109/GLOCOM.2014.7036934 conf/globecom/2014 db/conf/globecom/globecom2014.html#ShakerKTKH14