(often called rank enhance- ment) because radio channels from multiple sites are less correlated than that from a single site, leading to SINRs beyond those of isolated radio cells [18]. Beam forming and diversity gains, in combination with rank enhancement and interference cancellation, are the reason for the huge per- formance gains, promised by cooperative antenna systems from theory [6] as well as in many SL simulations. The lower part of Figure 7.1 provides a differentiation of single-link MIMO, COOPA systems, and active interference management as seen by our group: MIMO. Conventional open- and closed-loop MIMO systems provide diver- sity (DIV), beam-forming (BF), and spatial multiplexing (SMUX) gain. COOPA. MU-MIMO such as SDMA (sometimes called virtual MIMO because, for example, several UEs with one AP each form a virtual antenna array) is a first step into the area of cooperation (see earlier discussion). The main focus of COOPA systems is on distributed MU-MIMO systems. For COOPA, several eNBs form so-called CAs of limited size with the goal to cancel interference within the CA completely-called zero forcing (ZF)- to a certain level-called minimum mean square error (MMSE)-based on algorithms like joint transmission or detection [23]. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Active interference management. Cooperation areas require large overhead in terms of CSI estimation and feedback of CSI from UEs to eNBs over uplink (UL) radio channels. Moreover, they generate significant backbone traffic for accurate and timely exchange of data samples and CSI information between the involved NBs. Therefore, the size of cooperation areas will have to be restricted to very few cells and be supplemented by other techniques. In the case of OFDMA, a coordination of radio resources in adjacent cells might avoid col- lision of worst-case users at the cell edge or antenna tilting reduces far off inter- ferers. Although antenna tilting is a well- known, proven, and of