for the small cells all the time but only on those predefined windows. The small cell transmission activity can be kept low to minimize the power consumption. The transmission activity of the discovery signal can be very low, for example, with 100 ms period or more. 8.6 Multivendor Macro Cell and Small Cells The mobile networks 10 years ago commonly had different radio vendors in 2G and in 3G networks. Another typical multivendor case is home femto cells that use different vendor than macro cells. But things are changing and there are multiple reasons why the multivendor networks are becoming more difficult lately; see Figure 8.22: LTE carrier aggregation combines all LTE bands together. The carrier aggregation from Release 10 works only within one vendor in practice. LTE Small Cell Optimization LTE2600 Carrier aggregation of all LTE bands HSPA2100 + LTE2100 Multiband HSPA LTE1800 + GSM1800 Single RF unit for all technologies within HSPA900 + one band GSM900 CoMP in HetNet LTE800 Inter-site carrier aggregation in HetNet Figure 8.22 Interconnections between radio technologies and cell layers Multiradio RF implementation allows to use single RF unit for all technologies within one band: HSPA900 and GSM900 can share the RF unit; LTE1800 and GSM1800 can share the RF unit; and LTE2100 and HSPA2100 can share the RF unit. Also dual-band RF units are available combining for example 800 and 900 MHz bands. Tighter interworking between macro cells and small cells with CoMP and inter-site carrier aggregation. CoMP works only within one vendor while the inter-site carrier aggregation can also be implemented between vendors over open X2 interface. The target is to optimize the complex implementation and improve the system performance, which leads to higher interworking between frequencies, technologies and cell layers. Such tighter interworking can make the multivendor case challenging even if the standardized and open interfaces enable multivendor rollouts. S1 interface between radio and core open and different radio and core