# Handover-Driven Interference Management for Co-Channel Deployment of Femto- and Macro-Cells
## 2 Motivation
### Limitations of Existing Techniques  
There is a rich body of research that has attempted to resolve the femtocell interference problem based on radio resource (power, frequency, and time) management. [PERSON] et al. [4] proposed a hybrid overlay and underlay spectrum access mechanism and solved a subchannel allocation problem as a coalition formation game among femtocell users. [PERSON] et al. [5] proposed an asynchronous coordination scheme of co-channel femtocells based on delayed signaling and asynchronous update behavior and derived the condition of configuration parameters that guarantees convergence to a global optimum. DACA [6] is an intra- and inter-cell resource coordination framework associated with control algorithms for full-duplex heterogeneous cellular networks. Recently, there have been an increasing number of proposals exploiting hybrid-access femtocells. [PERSON] et al. [7] analyzed the performance of hybrid access schemes in the perspective of MIMO array configurations. [PERSON] et al. [8] proposed a scheme to jointly optimize the ratio of dedicated resources for femtocells in open and hybrid-access femtocells. [PERSON] et al. [9] proposed a spectrum allocation method for hybrid-access femtocells where a macro BS allocates a portion of subchannels to femtocells with which each femtocell guarantees the throughput of served macrocell users. [PERSON] et al. [10] proposed a similar framework and designed an auction-based algorithm where each femtocell receives additional macro users as bidders while the macro BS acts as the auctioneer. [PERSON] et al. [11] developed a QoS-aware cell selection scheme that assigns mobile users to femtocells,incorporating the energy consumption of backhaul links. Game-theoretic incentive frameworks to motivate femtocells to adopt a hybrid access policy by pricing accessed macrocell users' resource usages were proposed in [12; 13]. However, most studies focused on intra-cell resource management combined with BS assignment, and none considered the optimality and convergence of dynamic handover management. Some research works focused on the cell selection problem (also known as the BS assignment problem) that is used to find optimal pairs of users and serving cells. The system energy efficiency of small cell networks was considered for cell selection in [14; 15]. There are numerous proposals to solve the problem in general cellular networks. [PERSON] and [PERSON] [16] proposed synchronous and asynchronous distributed algorithms that find the optimal power vector and BS assignment. [PERSON] et al. [17] developed a pricing-based BS assignment algorithm considering the congestion level of BSs. The use of iterative algorithms based on linear programming (LP) to solve the BS assignment problem was presented in [18]. [PERSON] and [PERSON] [19] considered a coordinated multipoint dynamic cell selection problem. However, they all focused on the optimality of resultant BS assignments only, and none of them considered the handover overhead. Some heuristic approaches that consider the handover overhead exist, but they do not guarantee the optimality [20]. _Range extension_, which expands the handover ranges of overlaid cells [21], was discussed in 3 GPP as a solution to address the interference problem. The use of cell ranges for distributing users is advantageous in that it can be implemented within a conventional cell-selection framework of existing cellular systems without modifying user devices or an air interface. To fully realize the benefit of the range extension, vendors need an algorithm to control the handover ranges. There has been a notable attempt at optimal range control for hybrid-access femtocells. ARCHoN [22] allocates radio resources (frequency, time, and power) and distributes/assigns users among overlapping cells by controlling hotzone cell ranges such that the overall DL and UL loads of a HetNet are minimized while per-user rate constraints are met.