d in more detail. 7.5 Conclusions The motivation for cooperation, theoretical analysis, and simulation results have been presented in this chapter, giving a clear view of the high potential of coop- erative antenna systems that promise performance gains in the order of several hundred percent or even more. At the same time, practical and robust implementa- tions for COOPA systems are challenging, although feasible if properly designed. Specifically, active interference management that combines different techniques like intra-eNB cooperation with antenna tilting, IRC, and interference coordination seems to be promising because the sizes of so-called cooperation areas can be kept small and practical. Different implementation strategies have been presented and some important issues for the core element of any cooperation area have been addressed for the example of intersector cooperation. The newly proposed COOPA HARQ concept allows reducing feedback overhead and, at the same time, solves the critical issue of feedback delay. In the meantime, many research groups and companies are con- fident that cooperative antenna systems are realizable in real systems. Therefore, © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology further discussions will not be about the question of whether cooperation should be standardized but, rather, about what type of cooperation should be standardized. Due to the multitude of implementation options, many interesting discussions can be expected in this area. References 1. Matthias, R. 2008. Future network infrastructure for a world connected. NGNM conference at CeBIT, Hannover, Germany, March 6, 2008. 2. ITU homepage: http://www.itu.int/net/home/index.aspx Schulz, E., and J. Kahtava. 2007. Nokia Siemens networks and Nokia's view on IMT-advanced minimum requirements. FuTURE Forum: Seminar on the require- ment and development of IMT-advanced technology and system, Beijing, September 25, 2007. 4. Hideaki Takagi, H., and B. H. Walke. 2008. Spectrum