# Personal Picocell Scheme Using Adaptive Control CRE in Heterogeneous Mobile Networks +
## 2 Personal Picocell Scheme Using Adaptive Control CRE
### Adaptive Control CRE  
In general, cell selection is executed using downlink RSRP which is represented as Equation (1).  
\[j_{\text{u}}=\arg\max\,P_{\text{i,u}} \tag{1}\]  
where, i and u show the cell ID and user ID (UE ID), respectively. \(j_{\text{u}}\) is the donor cell ID that is chosen by UE\({}_{\text{u}}\), and \(P_{\text{i,u}}\) represents the RSRP at UE\({}_{\text{u}}\) obtained from the reference signal of cell-i. CRE is basically activated using RSRP-based method. According to CRE, UE makes an offset to the RSRP obtained from the reference signal of the picocell, which is given by Equations (2) and (3).  
\[j_{\text{u}}=\arg\max\,(P_{\text{i,u}}+\gamma_{\text{i}}) \tag{2}\]  
\[\gamma_{\text{i}}=\left\{\begin{array}{cc}\text{CSO};&\text{ if i is a picocell ID}\\ 0;&\text{ if i is a macrocell ID}\end{array}\right. \tag{3}\]  
where, \(\gamma_{\text{i}}\) represents the CSO which is applied to UE located within and/or close to cell-i. If CRE does not work, i.e., CSO = 0 dB, the picocell coverage remains unchanged. CRE is not applied for microcell; therefore, \(\gamma_{\text{i}}\) is fixed to 0.  
As mentioned in Section 2.2, it could be ideal for each UE to provide each CSO to realize the personal picocell scheme. However, it would be impossible to do that in real mobile networks because the number of UE is enormous. Therefore, we propose an adaptive control CRE that uses two different CSOs (CSO\({}_{\text{high}}\) and CSO\({}_{\text{low}}\)). In the proposed method, CSO is given by Equation (3).  
\[\gamma_{\text{i,u}}=\left\{\begin{array}{cc}\text{CSO}_{\text{high}};&\text{ if SINR}_{\text{u}}<\text{SINR}_{\text{th}}\\ \text{CSO}_{\text{low}};&\text{ otherwise}\end{array}\right. \tag{4}\]  
where \(\gamma_{\text{i,u}}\) indicates the CSO which is applied to UE\({}_{\text{u}}\) located near the picocell-i. SINR\({}_{\text{u}}\) is the downlink SINR measured at the UE\({}_{\text{u}}\) obtained from the reference signal of the donor microcell. SINR\({}_{\text{th}}\) is assumed to meet a fixed value of \(\alpha\) for cumulative distribution function (CDF), where the CDF is formed by the downlink SINR\({}_{\text{u}}\) measured at all UE obtained from the reference signal of macrocell.  
Figure 5 shows the principle of the adaptive control CRE and how to determine the two different CSOs. The macro-eNB first determines \(\alpha\), SINR\({}_{\text{th}}\) and two different CSOs (CSO\({}_{\text{high}}\) and CSO\({}_{\text{low}}\)) based on the results of the CDF. Then, the macro-eNB commands all UE within the macrocell in which CSO\({}_{\text{high}}\) is assigned to UE with a SINR lower than SINR\({}_{\text{th}}\), and CSO\({}_{\text{low}}\) is assigned to UE with a SINR greater  
Figure 4: Concept of personal picocell scheme in HetNet.  
than \(\text{SINR}_{\text{th}}\). Figure 5 also shows the difference in picocell coverage for UE\({}_{1}\) and UECSO\({}_{\text{high}}\) is assigned to UE\({}_{1}\) because \(\text{SINR}_{1}\) of UE\({}_{1}\) is lower than \(\text{SINR}_{\text{th}}\). Similarly, \(\text{CSO}_{\text{low}}\) is assigned to UE\({}_{2}\) because \(\text{SINR}_{2}\) of UE\({}_{2}\) is greater than \(\text{SINR}_{\text{th}}\). Consequently, the picocell coverage of UE\({}_{1}\) looks wider than that of UE\({}_{2}\). Figure 6 shows the algorithm of the adaptive control CRE. The fundamental concept for the offloading scenario is the almost same as that for the conventional CRE. The difference is that UE can choose two different CSOs (\(\text{CSO}_{\text{high}}\) and \(\text{CSO}_{\text{low}}\)) depending on their downlink received SINR performance.  
Figure 5: Adaptive control CRE using two different CSOs: \(\text{CSO}_{\text{high}}\) and \(\text{CSO}_{\text{low}}\).  
Figure 6: Algorithm of adaptive control CRE in HetNet.