# QoS-Aware Downlink Traffic Scheduling for Cellular Networks with Dual Connectivity
###### Abstract  
In a cellular network, how to preserve users' quality of service (QoS) demands is an important issue. To provide better data services, researchers and industry have discussed the deployment of small cells in cellular networks to support dual connectivity enhancement for user equipments (UEs). By such an enhancement, a base station can dispatch downlink data to its surrounding small cells, and UEs that are located in the overlapping areas of the base station and small cells can receive downlink data from both sides simultaneously. We observe that previous works do not jointly consider QoS requirements and system capabilities when making scheduling decisions. Therefore, in this work, we design a QoS traffic scheduling scheme for dual connectivity networks. The designed scheme contains two parts. First, we propose a data dispatching decision scheme for the base station to decide how much data should be dispatched to small cells. When making a dispatching decision, the proposed scheme aims to maximize throughput and ensure that data flows can be processed in time. Second, we design a radio resource scheduling method, which aims to reduce dropping ratios of high-priority QoS data flows, while avoiding wasting radio resources. In this work, we verify our design using simulation programs. The experimental results show that compared to the existing methods, the proposed scheme can effectively increase system throughput and decrease packet drop ratios. cellular networks; dual connectivity; small cell; scheduling; quality of service (QoS) 202220220220220220220220220220220220220220220220220220220222022202  
control sublayer (RLC), and packet data convergence protocol sublayer (PDCP). The MeNB and SeNB are connected through an X2 interface, which may be ADSL or fiber connections. By this architecture, the MeNB decides how to split downlink data for UEs that connect to both the MeNB and SeNB. More specifically, for a dual-connectivity UE, the MeNB's PDCP layer decides how much data will be dispatched to the MeNB's RLC and to the corresponding SeNB's RLC layer, respectively. As shown in Figure 1, the separated downlink data will then be aggregated by the UE's PDCP layer. It is not hard to see how the MeNB dispatches downlink data will affect network performance. For example, assume that a UE \(u\) has connected to both the MeNB and a SeNB, and the observed signal quality on the SeNB is poor. In this situation, if the MeNB dispatches more downlink data to the SeNB, the SeNB needs to spend more radio resources to process the UE \(u\)'s data, and thus, the system throughput will decrease. Some previous works (e.g., [4; 5]) discussed data dispatching mechanisms for the MeNB. However, we found that the previous schemes do not consider SeNBs' capabilities and statuses when making dispatching decisions, and thus, the packet drop ratio increases because data dispatches to SeNBs may not be processed in time. In a 3 GPP cellular network, the MAC layer of eNBs (i.e., the MeNB or SeNBs) assigns _radio resource blocks_ (_RBs_) to disseminate downlink data for UEs every _transmission time interval_ (_TTI_). Before assignment, the MAC layer collects channel quality indicator (CQI) values from UEs. Then, the MAC layer performs the adaptive modulation and coding algorithm (AMC) implemented by the network operator to determine the modulation and coding scheme (MCS) for UEs. Based on the MCS result, for a UE, the MAC layer can know the amount of data (in bits) that can be carried in an RB to the UE. Then, by referring to the remaining data amount of the UE, the MAC layer can determine how many RBs will be assigned to the UE in the next TTI. Obviously, RB assignment policies will affect network performance. In the literature, there are many QoS-aware MAC scheduling works (e.g., [6; 7; 8; 9]), but most of them simply favor data flows with higher priorities and do not consider the status of buffered packets; as a result, packet drop ratio may be increased, and thus, overall system throughput cannot be increased.