Lan Zhang Ying-Chang Liang Yan Xin Joint Beamforming and Power Allocation for Multiple Access Channels in Cognitive Radio Networks. 38-51 2008 26 IEEE Journal on Selected Areas in Communications 1 http://dx.doi.org/10.1109/JSAC.2008.080105 db/journals/jsac/jsac26.html#ZhangLX08