Similarly, both the network tiers are co-channel deployment.
For the co-channel deployment of the network, both the network tiers share the same set of available sub-channels (denoted by C).
In universal frequency reuse scheme, the existing spectrum can be aggressively and effectively reused by all of the coexisting network tiers.
While MM carries critical importance for HetNets to minimize HOFs and PPs, mobility load balancing is also crucial to achieve load balancing among different network tiers.
In fact, research on multi-tier HetNets where communications among different network tiers (e.g., macro, micro and small-cell tiers) share the same microwave spectrum has been one of the most active research topics over the past years [57].
Such networks are typically referred as heterogeneous networks (HetNets), and their deployment entails a number of challenges in terms of capacity, coverage, mobility management (MM), and mobility load balancing across multiple network tiers [2].
This will lead to higher spatial spectrum utilization and network usage capacity at the expense of an increased possibility of interference among network tiers and of a reduced quality of service (QoS).
This heterogeneous network architecture also enables efficient traffic offloading among different network tiers and RATs to better support the growing mobile traffic with enhanced QoS, communication rates, and energy efficiency [51 56].
Moreover, the work [64] proposes enhanced inter-cell interference coordination (eICIC) techniques for the two-tier HetNet where the optimal cell range expansion bias and almost blank subframe rate are obtained to achieve the fair resource sharing and interference management between two network tiers.
Principally, there are two types of interferences that exist in the two-tier femtocell network: cross-tier and co-tier [3].
