 relaying of backhaul traffic, which means that neighboring eNBs are served by one central serving eNB NBs with fiber backbone connection, while the served eNBs NBR have only a small data pipe to the backbone network. Thus, NBRs have relay functionality but "look" to UEs like conventional eNBs. This way, the advantage of reduced backbone infrastructure is combined with the benefit of IF cancellation within a CA, formed by one NBs and one or two NBRS. All proposals are different versions of a cooperative antenna system (in the fol- lowing, called the COOPA system). Depending on side conditions and scenarios, one or another cooperation scheme out of solutions (a)-(e) might be selected. For those MNOs having easy access to fast low-latency fiber connections-as might be the case in South Korea or China-DAS or conventional inter-eNB cooperation is very promising. MNOs with low data rate digital subscriber line (DSL) connec- tions per eNB will prefer CR, thus overcoming backbone limitations and improv- ing overall system performance in one step. In Figure 7.10, the potential gains of different system configurations are com- pared with respect to achievable spectral efficiencies, depending on the degree of cooperation-thus giving a rough idea about the potential of cooperative antenna systems. Different 2 x 2 as well as 4 X 4 MIMO system concepts with varying degrees of cooperation and with linear and NL precoding and different limitations for the backbone network are given. It is interesting that, for 2 X 2 MIMO and CAs with advanced interference management with spectral efficiencies of about 7 b/s, performance similar to that for full cooperation can be achieved. 7.3.1 Cooperation and the Backbone Network One important issue in the context of CAs, which might be surprising at first glance, is the typically increased data traffic over the backbone network. One might assume © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology that backbone traffic is of lower relevance compar