le MU-MIMO schemes had been evaluated for release 8, where only one single PMI (precoding matrix indicator) for the full frequency band is fed back per UE to the eNB, resulting in a simpler implementation with very small feedback overhead as well as in very small perfor- mance gains due to inaccurate adaptation to frequency-selective radio channels. In contrast, results under the assumption of accurate channel knowledge have led to significantly higher MU-MIMO performance gains [17]. In this way, for the first time, it has been demonstrated that the spectral efficiency scales linearly with the overall number of antennas in the interference-limited network also. Under practi- cal system considerations, a compromise has to be found between full network- wide cooperation and LTE release 8 SU-MIMO precoding based on restricted size LTE codebooks, with the same precoding matrix applied to the full bandwidth. Throughout this chapter, different COOPA concepts will be introduced, such as intra- and inter-eNB cooperation, distributed antenna systems (DASs) based on remote radio heads (RRHs), or as self-organizing networks [18-20], addressing different aspects of the previously mentioned issues. Cooperative relaying seems to be especially promising because it combines multihop coverage gains with MIMO capacity gains. As a further application of cooperative relaying, the integration of backbone functionality into the air interface will be analyzed. Rising backhaul traffic is one of the main challenges for MNOs for the introduction of LTE with its increased data traffic compared to GSM and even high-speed downlink packet © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes access (HSDPA). For that purpose, adjacent NBs are directly interconnected over the air in the same licensed spectrum and served in this way from a central eNB. The central eNB is the only one that has a fixed broadband backhaul connection, reducing the overall infrastructure effort. The combination of cooperative antenna systems with dif