ements for the X2 interface between cooperating eNBs [20]. The main idea is to do the same processing at each eNB in parallel at so-called mirror CUs. If each mirror CU selects its precoding matrix W for the same PMIs of all involved UEs and performs precoding for all data on the same RBs, the result on the air interface will be the same as for conventional inter-eNB cooperation. To enable distributed processing at mirror CUs, all UEs broadcast their PMIs to all involved eNBs. For precoding all involved eNBs, NNB need all data d1 to d of all supported UEs, NUE which can be easily fulfilled over the back- bone network by IP multicast protocols [27]. Additionally, the medium access control (MAC) decisions at the eNBs have to be exchanged over the air on eNB-UE-eNB links. This is one of the challenges because over- the-air transmission is naturally less robust than a conventional X2 inter- eNB connection. At the same time, self-organizing networks can react © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes very quickly, if properly implemented; by applying semistatic scheduling decisions as well as a suitable combination with data exchange over the X2-interface, robust implementations are possible. In addition to moder- ately higher data traffic on the S1-interface, fast data exchange in the order of a few milliseconds is the main advantage of DSOs over conventional inter-eNB cooperation because it allows faster adaptation to varying radio channel conditions. Cooperative relaying (CR), in its most advanced form, not only provides diversity gains but also forms a CA including the AEs of one sector and one or more RNs. Similar to DSOs, the cooperation is organized without direct involvement of the backbone network, which is replaced by in-band relay- ing. The CU is located at the central eNB (requiring fast exchange of CSI and PMI over the relay links) or is replaced by two mirror CUs at the RNs, similarly to DSOs. A very interesting application of CR is the combination of cooperation with in-band