# QoS-Aware Downlink Traffic Scheduling for Cellular Networks with Dual Connectivity
## 3 Network Model and System Flow  
Figure 2 shows the network scenario. The network is composed of one MeNB and \(l\) SeNBs. These \(l\) SeNBs are modeled as the set \(S=\{s_{1},s_{2},\ldots,s_{l}\}\). The operating frequencies of the MeNB and \(l\) SeNBs are different, and thus, the wireless signals between the MeNB and a SeNB will not interfere with each other. The MeNB and \(l\) SeNBs are connected by X2 interfaces. The link capacities of these interfaces are \(R_{Mb}(s_{1}),R_{bh}(s_{2}),\ldots,R_{bh}(s_{l})\), respectively. In the network, there are \(m\) UEs, which are modeled by the set \(U=\{u_{1},u_{2},\ldots,u_{m}\}\). The set \(U\) can be divided further into \(U^{M}\) and \(U^{C}\) sets. A UE located in the \(U^{M}\) set indicates that the UE only connects to the MeNB. On the other hand, a UE located in the \(U^{C}\) set means that the UE connects both to the MeNB and a SeNB at the same time. For example, in Figure 2, there are two SeNBs, i.e., \(l=2\), and five UEs, i.e., \(m=5\). Based on the above model, the sets \(U^{M}=\{u_{2},u_{5}\}\) and \(U^{C}=\{u_{1},u_{3},u_{4}\}\).  
According to the 3 GPP specifications, each data flow will be carried by a radio bearer. A UE can initiate multiple radio bearers, i.e., a UE can have multiple data flows at the same time. However, to facilitate presentation, we simply assume that those \(m\) UEs in the network will initiate \(m\) radio bearers accordingly. These bearers are represented by a set \(B=\{b_{1},b_{2},..,b_{m}\}\), where the bearer \(b_{i}\) belongs to the UE \(u_{i}\). The set \(B\) can also be divided into \(B^{M}\) and \(B^{C}\) sets. For a bearer \(b_{i}\), its incoming data rate is \(r_{in}(b_{i})\). Furthermore, the bearer \(b_{i}\) will be assigned to a 3 GPP QoS class identifier (QCI) value [27]. The QCI information contains the bearer's priority level, delay constraint, and packet drop ratio requirements. We model the delay and packet drop ratio constraints of \(b_{i}\) as \(Q_{d}(b_{i})\) and \(Q_{i}(b_{i})\), respectively. When a packet of \(b_{i}\) arrives to the MeNB's PDCP layer at time \(t\)  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Reference** & **Consider QoS** & **Consider Fairness** & **Consider Packet Status** \\ \hline Ref. [8] & ✓ & & \\ \hline Ref. [9] & ✓ & & \\ \hline Ref. [25] & & ✓ & \\ \hline Ref. [21] & & & ✓ \\ \hline Ref. [23] & ✓ & ✓ & \\ \hline Ref. [20] & & & ✓ \\ \hline Ref. [24] & ✓ & & \\ \hline Ref. [19] & ✓ & & \\ \hline Ref. [26] & ✓ & ✓ & ✓ \\ \hline QED & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 2: The network scenario.  
this packet has to be sent to the corresponding UE no later than time \(t+Q_{d}(b_{i})\). Otherwise, this packet will be dropped. Besides, based on the QCI value, traffic can further be divided into the guaranteed bit rate (GBR) and non-guaranteed bit rate (non-GBR). The GBR traffics are delay sensitive, which are used to support real-time traffics (e.g., video stream and online game). On the other hand, the non-GBR traffics can tolerate more delay and the data size may be larger. According to the 3 GPP QCI settings, GBR traffic types will have higher priorities than non-GBR ones. Therefore, in this work, GBR traffics will be served earlier.  
Figure 3 indicates the proposed framework. As mentioned in Section 1, the MeNB's PDCP layer will receive UEs' downlink packets from the Internet. The MeNB buffers the received data and then makes dispatch decisions using the proposed QDD method (in Section 4). In our design, the MeNB periodically executes QDD every \(I_{t}=k\times TTI\) ms, where \(k\) is a predefined system parameter. By QDD, part of the received packets for these dual-connectivity UEs will be sent to the corresponding SeNBs. Furthermore, the downlink data will be sent to the MAC layer of the MeNB and SeNBs. Then, the MAC layer adopts the proposed QED method to allocate its radio resource blocks (RBs) to UEs underneath the corresponding MeNB or SeNB every TTI.