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We propose opportunistic transmission methods exploiting spatial holes in multi antenna systems with a cognitive multiple access or broadcast channel. The proposed methods determine a precoding matrix for each cognitive transmitter as well as a post processing matrix for each cognitive receiver. For both, the pre as well as the post processing step different strategies are given. With the help of these matrices the cognitive system can be described by an equivalent non cognitive multiple user system. This equivalent systems reveals the multiplexing gain of the considered cognitive scheme and allows for determining the achievable sum rate of the cognitive users by applying multi user waterfilling.