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We consider the spectrum sharing of multiple cellular operators with independent infrastructures where their cells are overlapping in the same geographical area. Operators, which share their spectrum, need to deal with co-channel interference. We propose to use transmit-beamforming to reduce the inteference in the downlink transmission because beamforming requires minimum message passing between the operators; only the Channel State Information (CSI) of the involved mobile terminals. Due to the fact there are multiple decision-makers, interference rejection is only possible if the mobile terminals possess multiple antennas. Since we push the complexity to the base stations, the base stations with multiple antennas have to optimize the co-channel interference while trying to achieve a satisfactory level of data-rates. As multiple operators try to do the same, this will result in a conflict between the base stations. The selection of the beamforming weights and transmit-power levels are critical. We propose to combine the advantages of Signal-to-Leakage-plus-Noise Ratio (SLNR)-based beamforming with a power-allocation using a Stackelberg Game with a leader-follower relationship. Simulations show the performance gain of our approach as compared to other approaches which are more selfish in selecting the transmission strategies.