 is called the fronthaul, as shown in Fig. 1.23. In order to reduce the power consumption across network and to reduce inter-cell interference, the C-RAN architecture allows shutting off idle RRHs. This flexibility provides network adap- tation based on traffic profiles that vary temporally and/or geographically that further reduces the interference to neighboring cells in order to optimize overall system perfor- mance. From a hardware usage perspective, scheduling baseband resources on-demand to perform communication process for multiple RATs while taking advantage of network vir- tualization techniques can improve operational efficiency and reduce energy consumption. CoMP, dual connectivity, virtual multiple input and multiple output (MIMO), and coordi- nated beamforming are effective standard approaches to improve network capacity specified by 3GPP [47]. To support CoMP schemes with joint transmission/processing, the network configures the UEs to measure channel-state information and to periodically report the mea- surements to a set of collaborating nodes, which results in a large amount of signaling over- head. The logical interface between various RATs in a centralized BBU model would enable signaling exchange among participating nodes and node selection for collaborative transmission and reception. Since most of the current and the future deployment scenarios are in the form of ultradense heterogeneous networks, multi-RAT coexistence will be a key issue. 3GPP LTE was designed to support inter-RAT handover for GSM, UMTS, and interworking with wireless local area network. As shown in Fig. 1.24, the architecture for interworking between LTE and different RATs requires connection of their core networks for higher layer signaling. This multi-RAT architecture might be feasible but the latency of an inter-RAT handover may be prohibitive for many applications and services. In order to improve the user experience, simplified net- work architectures have been studied for lower latency based on C-RAN concept. If the 