# Green Small Cell Operation of Ultra-Dense Networks Using Device Assistance +
## 2 System Model  
### Assumptions  
We assume that small cells can be deployed by users or an MNO; thus, the deployment can be unplanned, and small cells are randomly deployed, resulting in a dense and possibly redundant deployment. It is plausible that if the APs are placed in a home, they can use the Internet as the backhaul to communicate with the macro base station. We assume that APs are deployed in the coverage of a macro BS. For example, in NTTDOCMO's phantom cell concept [19], the control plane and user plane are separated in different frequency bands in which high capacity and good mobility can be supported. We assume that the control plane and the user plane are provided by the macro BS and small APs, respectively. We introduce an entity located in the macro BS called the small cell controller (SCC). From [2], the AP on/off operation can be classified into two cases: the core network (CN) controlling and UE controlling. By taking advantage of CN and UE controlling, we propose a device-assisted framework shown in Figure 1. In the device-assisted framework, UEs measure beacon signals that APs broadcast and send feedback to the SCC. Then, the SCC determines the set of turned-on APs by exploiting the feedback. To enhance energy efficiency, turning off APs is an effective solution when APs are densely deployed. While saving energy, however, it is important to maintain the acceptable level of quality of service, which is mainly determined by the data rate. Interestingly, turning off APs can also increase the data rate when APs are densely deployed because interference among APs can be mitigated by properly selecting the set of turned-off APs. In Section 6, we show that a higher or similar average data rate is achieved by using our algorithms compared to the baseline. We assume that the set of APs \(\mathcal{J}\) is located in the macro base station coverage, and the set of UE \(\mathcal{I}\) is in the coverage provided by APs, where \(j\in\mathcal{J}\) is the index of AP and \(i\in\mathcal{I}\) is the index of UE. Figure 1: System model of the green small cell operation assisted by devices. **Definition 1** (AP on/off state indication vector). : _When \(x_{j}\) stands for the turned-on or off state of AP \(j\in\mathcal{J}\), the AP on/off state indication vector \(\mathbf{x}=\{x_{1},\cdots,x_{j},\cdots,x_{l}\}\) is defined by:_  
\[x_{j}= \begin{cases}1,&\text{if AP $j$ is turned on,}\\ 0,&\text{otherwise}\end{cases}  \tag{1}\]  
where \(j\) is the number of APs in the set \(\mathcal{J}\). Note that the AP state indication vector shows a possible state of each AP. Once the SCC determines the set of turned-on APs, the vector indicates whether each AP is turned on or not. Then, we define the power consumption of the APs. While the power consumption of the APs can be affected by the utilization or load levels of APs, the power consumption of APs can be approximated as a constant value since we consider small-sized APs that consume much smaller power compared to the conventional macro BSs. Moreover, considering that the power consumption of small cell BSs is mostly from the fixed power consumption and, thus, transmission power consumption is marginal, the total power consumption can be assumed to be constant; for example, according to the data sheet of a Wi-Fi AP that has a similar hardware size of a small cell AP, the Cisco Aironet 1140 Series [20] consumes 12.95 W, while its transmission power varies from 0.78 mW to 100 mW, which is less than 1% of the total power consumption. Thus, the approximately constant power consumption can be seen as a plausible assumption. Therefore, if an AP is turned on, it consumes the operational power \(\overline{P_{j,OP}}\) that includes the transmission power \(\overline{P_{j,Tx}}\) dissipated into the air. Thus, the operational power vector of AP \(j\in\mathcal{J}\) is \(\mathbf{P_{Op}}(\mathbf{x})=\{P_{1,OP}(x_{1}),\cdots,P_{j,OP}(x_{j}),\cdots,P_ {j,OP}(x_{j})\}\) where:  
\[P_{j,OP}(x_{j})= \begin{cases}\overline{P_{j,OP}},&\text{ if }x_{j}=1,\\ 0,&\text{otherwise},\end{cases}