ne compares a (1,1) single-input, single-output (SISO) system without coop- eration with a cooperative (4,4) MIMO system. Thus, in spite of the tremendous © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology technical challenges described in this chapter, it will be important to define and standardize the most suitable type of cooperation in one of the next LTE releases. In early 2008, a study item was launched at 3GPP [12] searching for use- ful enhancements of LTE release 8, which might end in a new LTE release R9. In parallel, there are strong global activities for the definition of the successor of IMT-2000, the so-called IMT-advanced systems [2], where it is likely that some form of cooperation will be included. Accordingly, research on cooperative antenna systems has become one of the hottest topics; it will form the input of ongoing stan- dardization activities and has led already to proposals for the LTE evolution [14]. In IEEE 802.1 [15]-the relaying task group defining mobile multihop relay (MMR) solutions for WiMAX (worldwide interoperability for microwave access) systems-cooperative relaying has been adopted as one type of relaying. Although theoretical and simulation results are very promising, at the same time it is clear that full cooperation in a cellular radio system is not feasible because this would require accurate channel knowledge from all eNBs to all UEs, processing of very large channel matrices at a central processing unit (CPU), and the exchange of tremendous amounts of data between all eNBs involved in a cooperative network with extremely short delays (in the order of a few milliseconds). In LTE release 8, the final decision about DL MU-MIMO has been shifted from LTE release 8 to future releases due to a limited time schedule. MU-MIMO is tightly related to cooperative systems, and it is widely anticipated as a precursor of cooperation. Note that relevant LTE specifications for open- and closed-loop MIMO can be found in reference 16. Very simp