Hong Wan Rong-Rong Chen Yingbin Liang Optimality of beamforming in MIMO multi-access channels via virtual representation. 2573-2577 2008 ISIT https://doi.org/10.1109/ISIT.2008.4595456 conf/isit/2008 db/conf/isit/isit2008.html#WanCL08