# Intelligent Interference Management in UAV-Based HetNets +
## 3 System Model
### SE with 3 GPP Release-10/11 ICIC Techniques  
In a HetNet, the MBSs transmit at higher powers and have higher ranges compared to the lower power UABSs. Nevertheless, the UABSs can extend their coverage and associate a larger number of UEs by using the cell range expansion (CRE) technique defined in 3 GPP Release-8. The CRE of a UABS is defined as the factor by which UEs are biased to associate with that UABS. For example, in Figure 2, UABS 1 uses a CRE of 15 dB to force UE 4 to associate with itself. The use of CRE, however, results in increased interference to those UEs in the extended cell regions. This interference from MBSs to UEs near the edge of range-extended UABS cells can be mitigated using time-domain-based ICIC techniques defined in 3 GPP Release-10/11. These techniques require the MBS to transmit with reduced power during specific subframes on the physical downlink shared channel (PDSCH). Radio subframes with reduced power are termed coordinated subframes (CSF), and those with full power are termed uncoordinated subframes (USF). We denote this power reduction factor by \(\alpha\) where \(0\leq\alpha\leq 1\). We note that \(\alpha=1\) implies no ICIC, while eICIC techniques use \(\alpha=0\), and FeICIC techniques allow \(\alpha\) to vary between 0 and 1. We use \(\beta\) to denote the USF duty cycle, and hence, the CSF duty cycle is given by \((1-\beta)\). Figure 3 shows, for the scenario depicted in Figure 2, how MBS 1 and MBS 2 use power reduction factors, \(\alpha_{1}\) and \(\alpha_{2}\) respectively, to reduce interference to UE 3. We note that \(\alpha_{1}<\alpha_{2}\), as MBS 2, being farther away from UE 3, can transmit at a higher power without degrading the performance of UE 3. Individual MBSs or UABSs can schedule their UEs in USF or CSF based on the scheduling thresholds \(\rho\) and \(\rho^{\prime}\), respectively. Then, a UE may be served either by an MOI or UOI and by the CSF or USF resources of the MOI/UOI, resulting in four different association categories. Let \(\Gamma\) denote the signal to interference ratio (SIR) at the MOI-USF, \(\Gamma^{\prime}\) denotes the SIR at the UOI-USF, and \(\tau\) denotes the CRE that positively biases the UABS SIR to expand its coverage. Then, the four different resource categories where a UE may be scheduled can be summarized as follows. If \(\Gamma>\tau\Gamma^{\prime}\), we associate the UE with the MOI; otherwise, we schedule it with the UOI. The intuition behind this condition is straightforward: associate the UE with the nearest base station that gives the best SIR, taking into account the CRE. On the other hand, if \(\Gamma>\rho\) or \(\Gamma^{\prime}\leq\rho^{\prime}\), we schedule the UE in CSF, and otherwise in USF. This condition is based on the following intuition: scheduling a UE in the CSF of an MOI degrades that UE's SIR, whereas scheduling a UE in the CSF of a UOI improves that UE's SIR. Thus, the \"stronger\" UEs that have sufficiently high SIRs and are close to the MBS should be scheduled in the CSFs of that MBS, as these \"stronger\" UEs can take the performance hit. Similarly, the \"weaker\" UEs that have low SIRs and are close to the cell edge of a UABS should be scheduled in the CSF of that UABS, as they need to be protected as a priority.