How many users should be turned on in a multi-antenna broadcast channel?

Wei Dai Youjian Liu Brian Rider Wen Gao How many users should be turned on in a multi-antenna broadcast channel? 1526-1535 2008 26 IEEE Journal on Selected Areas in Communications 8 http://dx.doi.org/10.1109/JSAC.2008.081018 db/journals/jsac/jsac26.html#DaiLRG08