. 5.6.3 Femto Cell Architecture Femto cells are connected to operator's network by using the broadband line at home or in the office. The number of femto cells can be very high: there are operators where the number of 3G femto cells is 10-20 times more than the number of macro base stations. The network architecture and scalability need to be considered with such a high number of small cells. The two architecture options are shown in Figure 5.10. One option is to use femto gateway between femto cells and packet core network elements Mobility Management Entity (MME) and Serving Gateway (S-GW). The target of femto gateway is to hide the large number of femto cells from the core network. The femto gateway is always used with 3G femto cells and it can also be used with LTE femto cells. Another option is to connect femto cells directly to the packet core in the same way as marco cells. It is also possible to use femto gateway only for the control plane towards MME. We consider the pros and cons of the different architecture options below. With femto gateway Femto Femto gateway Without femto gateway Femto Figure 5.10 Femto architecture options Small Cell Deployment Options Femto cell architecture with femto gateway: - Low scalability requirements to MME and S-GW. The packet core network does not connect to each individual femto cell but only to the femto gateway. - Switching off and on femto cells will not cause signalling load to MME. If there is a large scale power failure in the housing area and the power comes back, then large number of femto cells will register simultaneously to the network. - Less mobility signalling towards EPC. If there are multiple femto cells in the enterprise premises and UE is moving between those cells, the mobility is managed by the femto gateway and does not impact the packet core. - Femtos do not need to support S1-Flex since the support is in the gateway. S1-Flex is used for supporting multiple core networks. - Paging optimization can be done in femto gateway. - Femto gateway can prote