ferent other tech- niques, such as antenna tilting, interference rejection combining (IRC), or interfer- ence (IF) coordination, is very promising because it allows restricting cooperation to a few cells in the order of three to five. The small size of these interference-free zones-denoted as CAs-limits the processing as well as the feedback effort to these zones. The combination with the previously mentioned techniques guarantees maximum benefit in spite of this reduced effort. By damping the general interfer- ence floor stemming from far-off eNBs, canceling of a limited number of residual strongest interferers within the CAs boosts overall performance, thereby justify- ing the relative high overhead for organizing the CAs. Interference between CAs should be handled and controlled by other means, such as resource coordination. This leaves the task to organize a CA comprising only two transmitting and two receiving radio stations in the simplest case. For example, two NBs at one site may cooperate to overcome intersector interference between two UEs. The possible implementation of this fundamental building block of any cooperative system within the framework of LTE and its implications will be the main topic of this chapter. The chapter is organized into five sections. Section 7.2 provides a basic analysis of cooperative antenna systems, Section 7.3 classifies the basic types of cooperation systems, Section 7.4 is concerned with implementation issues, and Section 7.5 concludes the chapter. Note that basic concepts of LTE, such as resource blocks, codebook-based closed-loop MIMO precoding with unitary precoding matrices, and single-stream and dual-stream transmission will not be explained in detail in this chapter. The interested reader is referred to the previous chapters. Basic Analysis of Cooperative Antenna Systems This section will first give main rationales for cooperative antenna systems. A theo- retical analysis will be given in the second part based on interference functions. 7.2.1 The Benefits of Coopera