Resource allocation and transmission optimization for MIMO cognitive femtocells

This paper considers resource allocation and transmission optimization for femtocell networks, where the orthogonal frequency division multiple access (OFDMA) strategy is used to cope with the multi-user communication. In an attempt to mitigate cross-tier interference and optimize multi-user network performance, a two-layered beamforming scheme is proposed, facilitated by the multiple-input-multiple-output (MIMO) transmission. Two different operation models are discussed, namely open and closed access. It is shown that the beamforming design problem for closed access turns out to be convex and can be efficiently solved, whereas a non-convex optimization has to be treated when adopting the open access. For the latter non-convex problem, we propose an iterative algorithm, which has ensured convergence and closed-form solutions for subproblems of each alternating step. Numerical results validate the efficiency of the proposed approach.