说明：  New deployments are needed to address the future needs of 4G networks like high data rate support for clustered users. As a result, there is a shift in paradigm for MIMO increasing the number of antennas at a single cell site to distributing multiple antennas over a cell area. Furthermore, such distributed antennas may be in different power levels, and also in different antenna configurations. These add an interesting aspect and require scalable designs as well as interference management as part of MIMO improvements. Geographically separated antennas implemented as remote radio heads can be controlled by a centrally located baseband unit using high-speed standardized fiber interfaces. Such low-latency fiber interfaces ensure good synchronization across antennas and make the transmission distributed antennas as a natural extension of MIMO.