Joint Multicast Beamforming and Admission Control

Wireless multicasting is quickly emerging as an important enabling technology for mass content distribution to mobile hand-held devices. With channel state information at the transmitter, it is possible to tailor transmissions by selective beamforming towards specific user groups, thereby harnessing the wireless 'broadcast advantage' and managing co-channel interference. Wireless (physical layer) multicasting options are currently being debated in the emerging UMTS-LTE standard. A key problem in this context is admission control: it is often infeasible or impractical to serve all subscribers from a single access point, due to mutual interference or power limitations. This paper considers the joint multicast beamforming and admission control problem, aiming to maximize the number of subscribers that can be served and minimize the power required to serve them. The joint problem is NP-hard, yet suitable reformulation reveals that it can be naturally relaxed to a semidefinite program (SDP), leading to an approximation by deflation over SDP. Experimental results using measured channels indicate that the proposed approximation approach is fast and efficient.