these topics include: Backbone. Some of the cooperation concepts have tough requirements regarding overall data traffic as well as delay constraints, which have to be fulfilled from the supporting backbone network, as analyzed in detail earlier. With respect to a viable business case of cooperation, this might be a big issue. From a technical point of view, fiber connections might provide any data rate and, in the case of transparent data lines or tunnels between the NBs, very small latencies will also be possible. Tight synchronization in time and frequency is one of the first issues that must be solved. Time synchronization for OFDMA systems has to ensure that receive signals from two different radio stations are received well within the guard interval (GI) for LTE of an OFDM symbol of length 4.68 us; otherwise, intersymbol interference will occur. Analysis of measurements in the area of Berlin, Germany, has shown that typical delay spreads (DSs) of the combined radio channel for two cooperating UEs is about 1.5 times © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes Inter-eNB Cooperation 1.3 km = 4 us delay +3 us 0.2 km = .7 us delay +3 us 0.3 km = 1 us delay 1 km = 3 us delay At = .7 -3 us = -2.3 us At = 1-4 us us Corrected: At = .7 -(4 -6.3 us !! Corrected: At = (1 + 3) -4 us = 0 us Figure 7.13 Timing advance for inter-eNB cooperation. that of the DS to a single UE []. This is an encouraging result because dif- ferent time delays for different eNB-UE links cannot be compensated by simple timing advance mechanisms known for single links, as is clear from Figure 7.13. Frequency synchronization errors cause OFDMA intercarrier interference or, if seen from a time domain perspective, different phase rotations for each transmitting radio station. For LTE, a quite narrow SC spacing of only 15 kHz has been chosen, so, without appropriate countermeasures, frequency stability of local oscillators (LOs) has to be extremely high. Keep in mind that for an RF frequency of 2.6 GHz, even a very challeng