 vendors are used typically. Also X2 interface between macro eNodeBs is open and allows to use different vendors in different geographical areas. The X2 interface between macro and small cell layer is standardized as well and makes it possible to have different cell layers from different vendors. The challenge still is that the eNodeB algorithms are not standardized and may require coordination between the different layers. Figure 8.23 illustrates the main interfaces. The interface between baseband and RF can use Open Base Station Architecture Initiative (OBSAI) or Common Public Radio Interface (CPRI) but those interfaces are generally not open and only work within one vendor. Core network RF units CPRI/ Macro baseband OBSAI Small cell layer Figure 8.23 Interfaces in HetNet deployments Small Cell Optimization Femto cells commonly use different vendor than the macro cells. Femto cell case is simpler for multivendor because the interference management is easier in the indoor deployments; femto cell transmission power is low; and the femto gateway hides the large number of small cells from the macro layer. Efficient operation of heterogeneous networks requires automation to control opex and achieve optimal network performances. Automation is provided by means of SON-based mechanisms. Although SON is largely specified for LTE the SON logic (algorithms) is typically vendor specific. SON is not a single feature but rather a family of several use cases; for those use cases whose logic is fully in the eNodeB, an open X2 assures multivendor capability. For those use cases whose logic is centralized, the multivendor tools at the network management level weds SON to other vendors' systems. The network management interfaces are not fully standardized and some part of integration and adaptation is required. 8.7 Summary This chapter showed that inter-site carrier aggregation and dual connectivity are beneficial for increasing user data rates in HetNet environment and improving the reliability of mobility. The results show that