 The received signals on Ntr receive antennas, which are collected in the vector r, can be formulated as r=HWs+ n, (7.18) © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology where n is AWGN. The overall channel matrix H is an tx matrix and is composed of the channel matrices for each user, which are of size Nrx NBSNix. The following equation depicts this relationship: H=[H, ..,H . ,Hnms]1, j: user index. (7.19) Here, H is the transpose of the channel matrix for user j, which is an NNBNIX X Nrx matrix. The NNB eNBs need overall downlink channel state information H for the calculation of the precoding matrix W SO as to form multiple spatial beams, which enable independent and decoupled data streams for NUE users. The indi- vidual user j estimates its portion of the channel H and quantizes it by finding the best candidate from the predefined set of codes C The index of the chosen code ij is sent back to the eNB with the CU through a limited feedback channel. The eNB reconstructs the channel matrix H by looking up the codebook, which is shared by transmitters and receivers. This reconstructed channel matrix is used for the calculation of the precoding matrix W. Note that the notation of the general system is (NNB X N NMSXN. 7.4.1 Challenges, or "Is Cooperation Feasible?" In the preceding sections, many good reasons for cooperation have been given. Theoretically large capacity, as well as coverage gains in the order of several factors, has been predicted. Measurements in real-world outdoor scenarios have verified the potential cooperation gains by offline simulations based on the measured radio channels [28]. At the same time, simultaneous transmission to more than one single user on the same frequency-, time-, and, possibly, code-resource is challenging for a cellular mobile radio system because mutual time-varying interference may easily destroy any cooperation gain. The main issues in the context of cooperation and possible implementation strategies for LTE to overcome 