techniques; in the simplest case, this means polarization multiplex. For LTE baseline, we can assume a spectral efficiency of roughly RLTE = 2 b/s/Hz, compared to a peak data rate on the relay link of about RR = 8 b/s/Hz. Therefore, by this simple calculation, a moderate relaying overhead of about 20% can be achieved because one fifth of the time is required for relaying of user data. As far as possible, one should minimize the additional backbone data traffic for organization of the cooperation. Different approaches may help to achieve this goal. First, because data traffic will increase with increasing size of a CA, the number of participating NBs in a CA should be as small as possible. A combination with other techniques like antenna tilting, IF coordination, and advanced IRC helps to achieve high performance gains in spite of this smaller size of the CAs. Additionally, © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes cooperation should be restricted to UEs with strong mutual interference and those that have large amounts of traffic to send. UEs with small data rates will result in a lot of overhead for organization of the cooperation while providing only result in performance gains. Here, voice over Internet protocol (VoIP) users should be mentioned specifically; they should be served conventionally without cooperation. Another helpful technique is advanced quantization schemes, which adapt channel quantization levels to the actual achievable cooperation gain. UEs with a modula- tion and coding scheme (MCS) support (e.g., 1 b/s) require less accurate precoding information than UEs with 6 b/s. In the following, the required data rates on the S1, X2 interface and the over- the-air data rates are estimated for different implementations of the previously described cooperation concepts. The S1 interface provides the data connection to the backbone network, while the X2 interface has been defined for inter-eNB con- nections, which are essential for classical inter-eNB cooperation. As it is def