Joint ´dirty paper´ pre-coding and downlink beamforming

We consider the problem of joint interference balancing in a frequency flat vector broadcast channel with non-cooperating receivers and perfect channel information at the transmitter. We propose a transmission strategy, that combines vector Costa pre-coding and transmit beamforming. Our approach differs from related work (e.g. Caire and Shamai, 2001) in that we do not consider the sum rate as the design goal. Instead, it is required that the users fulfill individual target SINRs. If this problem is feasible for a given power constraint, excess transmission power can be minimized. By using an interesting duality between broadcast and multiple-access channels, an efficient algorithm is developed, which finds the optimal transmission powers and spatial pre-filters. For the 2-user scenario, we derive a closed-form solution, which illustrates the rate trade-off between two users under a given power constraint.