# Resource Allocation of Multiple Base Stations for Throughput Enhancement in UAV Relay Networks
## 6 Numerical Results  
In this section, we present the simulation results to demonstrate that multi-BSs-URN achieves a higher throughput of \(U_{m}\) without an outage of \(U_{o}\) over mBS-URN or sBS-URN. In addition, it is demonstrated that an optimal time allocation \(\lambda_{opt}\) extends the operation range of UAV compared to an equal time allocation (i.e., \(\lambda=0\), denoted as \(\lambda_{0}\) in Figures 3 and 4) and achieves a larger throughput gain.  
The following are assumed for simulations. \(U_{m}\) and \(U_{o}\) are located inside and outside the mBS coverage, respectively; therefore, \(U_{o}\) cannot have a direct link from mBS. sBS is in the middle between \(U_{m}\) and \(U_{o}\), and the distance between sBS and \(U_{o}\) is large, so that there is no direct link between them. UAV moves from \(U_{m}\) to \(U_{o}\), and a relative distance \(\Delta\) is defined as \(\Delta=\frac{d_{U_{m},UAV}}{d_{U_{m},U_{o}}}\). A large \(\Delta\) indicates that the UAV stays close to \(U_{o}\), while a small \(\Delta\) means that the UAV stays close to \(U_{m}\). An equal power allocation at mBS is assumed in the first time slot, so that \(P^{1 st}_{mBS,U_{m}}=P^{1 st}_{mBS,ULAV}=15\) [dBm], \(P_{M}=30\) [dBm], \(P_{S}=P_{ULAV}=25\) [dBm], path loss \(=3\), the height of UAV \(=70\) [m], \(\eta=0.1\) [bps/Hz], \(\epsilon=0.1\), and \(\tau=0.01\).  
Figure 3 demonstrates the superior performance of multi-BSs-URN over mBS-URN for both \(\lambda_{opt}\) and \(\lambda_{0}\). The multi-BSs-URN achieves a higher throughput within an entire operation range of UAV without an outage of \(U_{o}\). For comparison, two mBS-URNs are considered; mBS-URN with or without interference from sBS (i.e., mBS-URN w/ \(I_{sBS}\) and mBS-URN w/o \(I_{sBS}\), respectively, in Figure 3). In mBS-URN w/ \(I_{sBS}\), an outage of \(U_{o}\) always occurs for both \(\lambda_{opt}\) and \(\lambda_{0}\), thereby resulting in the throughput degradation of \(U_{m}\) within the operation range of UAV. Even in mBS-URN w/o \(I_{sBS}\), an outage of \(U_{o}\) occurs when \(0.4\leq\Delta\leq 0.9\) for \(\lambda_{opt}\), and \(\Delta\geq 0.25\) for \(\lambda_{0}\). This is because a time allocation for two time slots is not enough for reliable relay transmission to \(U_{o}\). When \(\Delta\geq 0.9\) for \(\lambda_{opt}\), \(U_{m}\) achieves high throughput because UAV remains very close to \(U_{o}\) and a small amount of time is needed to set up an access link. Even though mBS-URN suffers from an outage without interference from sBS, it can extend the valid operation range of the UAV from \(0\leq\Delta\leq 0.25\) to \(0\leq\Delta\leq 0.4\) and \(\Delta\geq 0.9\) with an optimal \(\lambda_{opt}\). Figure 4 compares the throughput of multi-BSs-URN with that of sBS-URN. Without a time allocation, sBS-URN suffers from an outage of \(U_{o}\) for an entire operation range of the UAV. For an optimal \(\lambda_{opt}\), the throughput of \(U_{m}\) is very low when \(0\leq\Delta\leq 0.5\) due to the severe interference from mBS. However, it increases as \(\Delta\) increases because the UAV receives less interference when it approaches \(U_{o}\).  
Figure 4: Throughput comparison of multi-BSs-URN and sBS-URN.  
Figure 3: Throughput comparison of multi-BSs-URN and mBS-URN.  
Simulation results in Figures 3 and 4 demonstrate that simultaneous transmissions from mBS and sBS can achieve higher throughput over a single transmission from mBS or sBS without an outage of \(U_{o}\), thereby guaranteeing a reliable relay link to \(U_{o}\). Furthermore, the optimal time allocation can provide additional throughput gain and extend the UAV operation range. Therefore, simultaneous transmissions from multiple BSs in the URN with optimal time allocations are essential for the throughput maximization of URN and stable relay transmission to \(U_{o}\).