hus, one unit has to be defined as CU, which will control other scheduler units. This has to be done in a self-organizing manner as being investigated in 3GPP as self-optimizing networks (SONs). 7.4.2 Cooperation Area of Minimum Size as LTE Enhancement In the following, the simplest type of CA is analyzed-namely a(2x1,2x1) CA consisting of two different eNBs or RNs serving two UEs simultaneously, where each radio station has only one single antenna port. This can be seen as the core of any CA but at the same time allows analysis of most of the relevant issues addressed previously. Figure 7.12 (bottom) is a schematic representation of this basic CA for the application of intersector cooperation. It is assumed that a set of active UEs feed back their optimum PMIs and the CU of the CA selects two suitable UEs out of the set of active UE for cooperation on one, several, or all RBs. Suitable criteria for user group- ing are similar path loss (PL), low channel correlation, and large mutual interference. Similar PL minimizes power rise for ZF or MMSE and low channel correlation helps to minimize the feedback overhead or maximizes performance for a given feedback overhead; large mutual interference guarantees high performance gains due to coop- eration. In the following, only the two selected UEs will be considered further. The CA is organized for the selected two UEs based on LTE release 8 according to the following parameters: downlink/uplink, full duplex 2.53/2.68 GHz antennas eNB/UE 2 TRx/2 Rx sampling rate 30.72 MHz used bandwidth 20 MHz symbol period downlink/uplink long (short) block 71.4/70.1 (37.5) ns cyclic prefix downlink/uplink 4.6875/4.13 ns total number of subcarriers 2.048 number of used subcarriers/RB up to 1.200/100 transmission scheme downlink OFDMA modulation in downlink 4, 16, 64 QAM radio frame 10 ms subframe or TTI symbols/subframe (DL) resource block size 12 SCs in one subframe © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Full Channel Matrix H Re