duced Matrix HR = HR of Size 2 X 2 after IRC IF Suppression by R Block Diagonal Channel Block Diagonal Matrix WH Matrix WHR After ZF Figure 7.20 Reduced size X 2 H = HR channel matrix for analysis of CAs. In this list, two Tx and two Rx antenna ports per eNB and UE are stated, and the overall simulated system is of type (2 X 1, 2 X 1). The background is the overall system concept in mind, where the SDF of the two UE AEs is used for IRC by the receive matrix R of size 2 x 2 for I=1, 2, as illustrated in Figure 7.20. The IRC will form the UE beam former SO that one or several of the strongest interferers from other eNBs can be suppressed. If we use the SDF in terms of additional Rx antennas for IRC of intracell interference, this SDF will be missing for intercell interference cancellation. In mobile systems, the IRC of the strongest other cell interference has been proven to be quite effective because the UE can adapt if dif- ferent eNBs become the strongest interferers [18]. As a result, the UEs feed back the CSI for the composite radio channel HR of size 2 X 2 instead of 2 x 4 or 4 X 4; thus, SDF is saved and feedback is reduced as well. The reduced model can be simply written as r=HWd+n, where n is AWGN and H, W are of size 2 x 2. In the following, the CA will be organized without taking AWGN into account (i.e., the purely interference-limited case will be analyzed). Thus, the comparison of different implementations will be independent of the cell size or Tx power. Codebook-based precoding is assumed, but for different types of codebooks. © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes 7.4.3 Channel Estimation Generally, a 2 x 2 CA for intersector cooperation can be implemented by two dif- ferent strategies: One might "hide" the intersector cooperation as a conventional MU-MIMO transmission from a single sector. Each sector sends two different RS grids for AP 1, 3 and AP 2, 4, respectively. This allows the UEs to estimate their radio channels on orthogonal RS grids without knowing that the