Optimal power allocation assisted with relay in open access femtocell network with the registered users protection

Femtocell with the open access mode benefits the macrocell users (MUE) which locate closely to the femtocell base station (FBS). However, how to enhance these users´ performance with the femtocell registered users quality of service (QoS) protection becomes a major problem. Deploying relay outside the femtocell networks is an economical and effective option to reduce the cross-tier interference while improving the network capacity and indoor coverage in the cellular networks. In this paper, an optimal power allocation scheme considering the protection of the femtocell registered users is proposed in the open access femtocell network assisted with a relay node, which operates under a novel amplify-and-forward (AF) relaying protocol, for the downlink. The power allocation problem is formulated with the objective of maximizing the sum-transmission-rate of the neighboring macrocell users (NMUEs) which perform the handovers from the macrocell to the femtocell due to the degraded channel QoS of the macrocell. This optimization is subject to the constraints of the required transmission rates of the femtocell registered users (FUEs), the thresholds of the cross-tier interferences to the users kept in the macrocell, and the total transmit powers of the FBS and the relay node, etc. We solve the optimization problem via the dual decomposition method. Simulation results have shown that our proposed scheme can protect the QoS required by the FUEs. It also illustrates the better performance of the proposed optimization methodology in terms of the sum-transmission-rate of the NMUEs than the traditional femtocell method which has no presence of the relay node.