Cross-layer Resource Allocation for the Downlink of SDMA Systems

In this paper, we study the resource allocation problem for the downlink of spatial-division multiple-access (SDMA) systems. We develop a general architecture to determine the optimal subset of users and their transmission rates. The constraints posed by the physical layer (in terms of spatial orthogonality) and by the link layer (in terms of QoS requirements) are jointly taken into consideration. As an implementation case, we propose efficient algorithms for systems that employ zeroforcing beamforming. Extensive simulations and analysis are performed to demonstrate the effectiveness of our proposal.