# Handover-Driven Interference Management for Co-Channel Deployment of Femto- and Macro-Cells
## 1 Introduction  
Recently, there has been a growing interest in femtocell technology, which promises cost-effective enhancement of indoor network coverage and capacity to meet increasing demands for bandwidth-hungry data services within home or enterprise environments [1]. A femtocell is a small indoor area covered by a low-power base station (BS), referred to as a _femto BS_ in this paper. A significant capacity improvement is expected to be achieved with femtocells via their enhanced spatial reuse of spectrum resources. In addition, femtocells can offload macrocells and improve indoor coverage at low capital and operational costs. Femto BSs are installed on the subscriber's premise without the operator's lengthy planning and are typically connected to an operator's core network via public Internet connections, such as DSL and cable modems. A key challenge faced by femtocell technology is how to protect macrocell user services from femtocell interference when macro- and femtocells are allocated the same frequency band for economic reasons, called _co-channel deployment_, while exploiting spatial reuse of channel resources within femtocells. Co-channel deployment of macro- and femtocells is expected to be commonplace due to the high cost of the licensed spectrum. Under this co-channel deployment, transmissions within femtocells may cause harmful interference to user services within macrocells, and vice versa, as reported bymany researchers [2]. The resultant performance degradation has become a major obstacle to market penetration of the femtocell technology, since the deteriorated service to existing users will increase the churn rate. There have been many proposals to resolve the femtocell interference problem based mostly on radio resource coordination between macro- and femtocells, but their achievable capacity is limited. To overcome the capacity limit of existing approaches by adding a degree-of-freedom (DoF) when solving the interference problem, a new solution architecture called _HandOver-driven Femtocell Interference Management_ (HO-FIM) is proposed. HO-FIM mitigates the interference problem and offloads the overall heterogeneous networks by exploiting handovers between macrocells and hybrid-access femtocells based on the users' measurement reports. A new challenge associated with this approach is how to minimize the number of triggered handovers and suppress ping-pong handovers while reaching a desired system condition, since handovers accompany inevitable interruptions of ongoing user services being handed over. This paper proposes to meet this challenge by triggering handovers only when absolutely needed and, once triggered, to find an optimal handover sequence to reach a target system condition with minimal handovers. HO-FIM accompanies a greedy per-user control algorithm which has a fine control granularity and thus enables convergence to an optimal solution. HO-FIM is composed of the following three interacting controllers:  
* _Master controller_: Makes decisions on whether to activate/deactivate the handover controller based on the current status of radio resource usage;
* _Handover controller_: Determines a sequence of handovers to be made to reach a target system condition with minimal handovers while avoiding ping-pong handovers;
* _Radio resource controller_: Handles intra-cell scheduling of frequency, time and power resources to meet users' target service levels. The approach of HO-FIM is also applicable to the general concept of _heterogeneous networks_ (HetNets) that represents cellular deployments with heterogeneous types of cells of different sizes and overlapping coverage areas including femto and small (hotzone) cells. The evaluation results show that HO-FIM improves the performance of both macro- and femtocell users under various environmental settings.