uld be used to achieve a robust sys- tem concept. The main advantage here is the small delay and relatively simple implementation. Sophisticated cooperative relaying solutions are very promising and com- bine in-band backhaul functionality, support of active interference management, and very fast adaptation to varying channel conditions. The values for WL are rough estimates if all optimization potential is exploited and are subject to further research. Basic Cooperation Area It is important to analyze cooperative antenna systems from a very high level, as has been done in the preceding, to understand the impact on system level; however, at the same time, one has to look from the bottom up to understand all implications for a real-world realization. Starting from analysis of a single CA (see Figure 7.12), © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes PMI-1 PMI-2 PMIUE1 Data UE 1 PMIUEx Data UE 2 PMIUEx =HWd+n=Htx+n PMIUE2 Figure 7.12 Basic cooperation area consisting of two eNBs and two UEs with one AE each. this section will end with a likely LTE-conform implementation or give some indi- cations about required enhancements of LTE R8. The general closed-loop or precoded MU-MIMO system forming a CA consists of a group of NNB eNBs and NUE UEs having N and Nrx antennas, respectively. As defined for LTE R8, codebook-based precoding is assumed. The data symbol block, d = [d1, with Ntr = NMsN is precoded by an Ntt X N tr matrix W with Ntt = NNBNW in a case in which the number of data streams for each user Nd (nd < N N is N xx. Here, the first NYx data symbols are intended for the first user, the next NYx symbols for the second user, and SO on. When denoting iNBliUE as the eNB/UE index and illi, as the transmit/receive antenna index, respectively, we can denote hij-where i = NIXLIVE - 1) + i,j= the channel coefficient between the i,th receive antenna of the inth UE and the ith transmit antenna of the ingth eNB. The channel coefficients can be expressed as the NN channel matrix H with [H],==hij =