Date Added: Nov 2012
This paper studies a novel underlay MIMO Cognitive Radio (CR) network where the instantaneous or statistical Channel State Information (CSI) of the interfering channels to the Primary Receivers (PRs) is completely unknown to the CR. The authors first show that low-rank CR interference is preferable for improving the throughput of the PRs compared with spreading less power over more transmit dimensions. Based on this observation, they then propose a rank minimization CR transmission strategy assuming a minimum information rate must be guaranteed on the CR main channel. They propose a simple solution referred to as Frugal WaterFilling (FWF) that uses the least amount of power required to achieve the rate constraint with a minimum-rank transmit covariance matrix.