## 3 Proposed Scheme
### Proposed FFR-3 Scheme  
In this paper, we propose a resource allocation scheme called fractional frequency reuse 3 (FFR-3). The reason for naming the proposed scheme FFR-3 is that interference mitigation and resource reuse methods are designed by dividing each cell and radio frequency into three areas. In the semi-static scheme, some resources are fixedly allocated for particular links and some resources are reused across links. In the proposed FFR-3, fixed orthogonal resources are allocated to cellular links in the same cell to avoid mutual interference. On the other hand, to minimize throughput loss, D2D links reuse resources allocated to cellular links to the extent that interference is minimized. The detailed procedure of the proposed scheme is as follows. Figures 3 and 4 show the cell structure and frequency partitioning in FFR-3, respectively. As shown in Figure 3, a cell is divided into three sectors named \(A\), \(B\), and \(C\). Figure 2: The case of interference in D2D WMSN. Each sector is again divided into two zones, and each zone is identified by a number after the sector name such as \(A1\) and \(A2\). As a result, each cell is divided into six zones consisting of \(A1\), \(A2\), \(B1\), \(B2\), \(C1\) and C2. In FFR-3, uplink resources are divided into frequencies for UEs and for sensors. The frequency for sensors is divided into six sections as shown in Figure 4. The proposed FFR-3 allocates orthogonal resources to cellular links in the same cell. As seen in Figure 5, orthogonal frequencies consisting of \(a1\), \(a2\), \(b1\), \(b2\), \(c1\) and \(c2\) are allocated to cellular links of a zone from \(A1\) to \(C2\), exclusively. Accordingly, there is no intra-cell interference for cellular links. Moreover, the inter-cell interference is also avoided in FFR-3 by designing cell structure so that zones of the same name are not contiguous. To avoid the interference between cellular and D2D links and increase the cell throughput, the D2D links in each zone reuse the frequencies used by the cellular links in the neighboring zones. For example, as seen in Figure 6, the D2D links in \(A1\) zone use \(b1\) and \(c1\) frequencies that are used by cellular links in \(B1\) and C1 zones, respectively. In the same way, the D2D links in \(A2\) zone uses frequencies \(b2\) and \(c2\) used by the cellular links in the neighboring zones. According to the cell structure of FFR-3, the D2D links of each zone allocate orthogonal resources to the D2D links of the neighboring zone. As a result, in FFR-3, each zone has one frequency section for cellular links and two frequency sections for D2D links. Note that frequency reuse factor (FRF) for the cellular link is one because one zone, which is \(1/6\) of the entire cell area, uses \(1/6\) of the entire frequency domain. FRF for the D2D link is two because one zone, which is \(1/6\) of the entire cell area, uses \(2/6\) of the entire frequency domain. Figure 4: Frequency partitioning in FFR-3. Figure 3: The cell structure in FFR-3. In this section, we compare the interference effects of the proposed FFR-3 with the LTE D2D system, which has a similar interference environment to the sensor deployed system. The resource allocation in the comparative LTE D2D system assumes the semi-static scheme in [48], which is orthogonal resource allocation to uplink and D2D link. Figures 7 and 8 show the interferers affecting the target cellular link in semi-static and FFR-3, respectively. As shown in Figure 7, there is no intra-cell interference for the cellular link in the existing semi-static scheme. This is because orthogonal resources are allocated between D2D and cellular links in the same cell and time domain interference control is applied between cellular links. On the other hand, in the proposed FFR-3, there is intra-cell interference from the D2D link to the cellular link. This is because the D2D link reuses the cellular link resources of the adjacent zones to minimize the throughput reduction in FFR-3.