# Personal Picocell Scheme Using Adaptive Control CRE in Heterogeneous Mobile Networks +
## 2 Personal Picocell Scheme Using Adaptive Control CRE  
### HetNets and CRE  
Figure 1 shows a typical HetNet combined with a three-sector macrocell served by a macro-eNB and picocell for each macro-sector served by each pico-eNB. Picocells are primarily added to increase the system capacity in hot spots with considerable UE, including indoor areas. By offloading traffic from the macrocell, HetNet can improve the system performance, i.e., provide increased user throughput.  
From a carrier frequency perspective, two types of HetNets have been discussed, as shown in Figure 2. The first is single-band HetNet in which the carrier frequency of the picocell is the same as that of the macrocell, as shown in Figure 2a. The second is multi-band HetNet in which the carrier frequencies between the macrocell and picocell are different, as shown in Figure 2b. The single-band HetNet causes interference between the macrocell and picocell, as well as interference between picocells. However, multi-band HetNet causes interference only between picocells. In this paper, a HetNet with time-domain enhanced inter-cell interference coordination (eICIC) to eliminate the interference between the macrocell and picocell is not considered [19].  
In HetNets, it is important for UE to determine which access is better to communicate with the macro- or pico-eNBs, which is referred to as cell selection. UE is normally connected to a cell with a stronger downlink RSRP or with a higher signal-to-interference plus noise ratio (SINR). Therefore, the connection ratio of the macro-eNB is greater than that of the pico-eNB because the transmission power of the macro-eNB is larger than that of the pico-eNB. CRE technique is very effective at increasing the connection ratio of the pico-eNB through the use of a positive CSO for the downlink RSRP from the pico-eNB, as shown in Figure 3, although the downlink transmission power of  
Figure 1: HetNet combined with macrocell served by macro-eNB and picocell served by pico-eNB [20].  
Figure 2: Two types of HetNets. (**a**) Single-band HetNet; (**b**) multi-band HetNet.  
the pico-eNB is fixed (i.e., unchanged for UE). In this case, the picocell coverage appears to be wider as an \"extended picocell\" for the UE in the CRE zone, as shown in Figure 3. Consequently, the connection point of the UE in the CRE zone should be changed from macro-eNB to pico-eNB when CRE is activated. Like this, CRE technique is effectively used in HetNets to enhance the system capacity as well as cell-edge user throughput. However, CRE degrades the downlink RSRP of the UE that is forcibly served by the pico-eNB located in the CRE zone because the downlink RSRP from the macro-eNB is in reality higher than that from the pico-eNB. According to the CRE drawback, the user throughput of the UE that is served by the pico-eNB in the CRE zone may worsen.  
Interference scenario for pico-eNB installed at outdoor is different from that for indoor pico-eNB. Therefore, the usefulness of CRE is scenario dependent. For example, the CRE may not be useful for indoor pico-eNB, because buildings wall provide isolation between the microcell and picocell.  
From the uplink performance perspective, CRE would increase the uplink received power and/or SINR at pico-eNB when the connection point is changed to the pico-eNB from macro-eNB by the CRE. It is well-known that uplink cell border is different from the downlink border in HetNets. In general, the uplink border is closer to macro-eNB compared with the downlink border. Therefore, the received SINR at pico-eNB for UE within CRE zone can be improved, because the path-loss to the pico-eNB is less than that to the macro-eNB.