Guodong Zhao Jun Ma Ye Li Tao Wu Young Hoon Kwon Anthony C. K. Soong Chenyang Yang Spatial Spectrum Holes for Cognitive Radio with Directional Transmission. 3034-3038 2008 GLOBECOM http://dx.doi.org/10.1109/GLOCOM.2008.ECP.582 conf/globecom/2008 db/conf/globecom/globecom2008.html#ZhaoMLWKSY08