Downlink interference management in femtocell networks - a comprehensive study and survey

Femtocell has gained considerable attention to provide indoor ubiquitous mobile broadband coverage at higher data rate along with excellent voice service. It is a low power, short range, user installed Home Base Station for connection of personal equipments such as laptops, notepads and mobile phones provided priority for voice service. Operating at licensed spectrum, femtocells are overlaid on the existing macrocell network, forms a two-tier heterogeneous network. Unplanned ad-hoc manner deployment and restricted access mode of femtocells create much stronger and highly variable interference in the two-tier heterogeneous macro-femtocellular networks. The downlink is more critical in terms of femto-macro interference, because it depends on the proximity of victim UE from base station. If the victim is in much close to the aggressor base station, the QoS of victim gets much affected and there is degradation in overall system capacity and greater outage probability. Emerging interactive and content uploading services stress the need for proper femtocell interference management in such two-tier heterogeneous network. This paper presents a comprehensive study on downlink interference scenarios under co-channel deployment and detailed survey on downlink interference management strategies in such networks. It is concluded that intelligent subchannel allocation using cognitive radio concept and transmit beamforming along codebook restriction, macrocell beam subset selection, ICBM, G-ACCS and partial co-channel deployment schemes play significant role on interference management in OFDMA femtocell networks whereas, in CDMA femtocells distributed adaptive power control, joint power control with admission control and IA code assignment strategy can be used for interference management. Issues such as signaling and channel sensing overhead, computational complexity, potential error in the estimation of RB usage and time complexity in channel selection and power allocation are yet to be solved and indicate rich and clear research prospects in interference management of femtocell networks.