Joint multiuser downlink beamforming and admission control in heterogeneous networks

This work studies the problem of joint multiuser downlink beamforming and admission control in multiple-input multiple-output (MIMO) heterogeneous networks. Considered is a network where a newly deployed femtocell base-station (FBS) has the coverage overlapped with that of an existing macrocell base-station (MBS). Our design objective is to serve as many femto-users (FUEs) as possible at their quality-of-service (QoS) requirements while maintaining the QoS requirements at the macro-users (MUEs). In the first part of this work, we consider the joint downlink beamforming and admission control problem as a joint optimization problem, which can be solved in a centralized manner with full coordination between the MBS and the FBS. In the second part, we propose a distributed algorithm in performing joint downlink beamforming and admission control at the femtocell with only limited MBS-FBS coordination. Specifically, after acquiring certain design parameters from the MBS, the FBS unilaterally determines its beamforming and admission control strategy while coordinating its induced interference to the macrocell. We then prove that the distributed algorithm will converge to a fixed-point where the QoS at the MUEs and admitted FUEs is guaranteed. Simulation results show that the distributed algorithm performs as well as the centralized one in terms of number of FUEs served with only a small penalty on the power usage at the MBS and the FBS.