ssible, while performance degradations compared to a constant radio channel are relatively small, even for the middle of the frame. Note that interpolation accuracy will be smallest at the center of the frame, if correct precoding is assumed for the beginning and end of the frame. Figures 7.24 and 7.25 contain simulation results highlighting achievable SIR with and without the COOPA HARQ concept. The relative overall Tx power of AW compared to W has been analyzed with respect to At and mobile speed as well as feedback delay. For moderate scenar- ios, values of AW will be small and in the order of -20 to -10 dB because the radio channel typically evolves slowly. For this reason, we can retransmit Tx(t3) = Hdelayd X SCd, where SCd is a proper scaling factor, SO that, at the receiver, = 3 kmh, w/o interpolation, 140 symbols delay. ampl/phase = 5/7 bits V = 3 kmh Delay = 70 symbols a 700 us V = 3 kmh Delay = 126 symbols a 700 us Figure 7.24 CDF of achieved SIR for COOPA HARQ concept for UEs 1 and 2. Left: without interpolation, FB delay = 10 ms; middle: with interpolation for worst-case FB delay of 5 ms; right: with interpolation for FB delay = 9 ms. © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes 50% CDF 5% CDF 50% CDF w/o 5% CDF w/o -10.0 # of Subframes a 1 ms Figure 7.25 Achievable SIR for COOPA HARQ with ( ) and without (w/o) inter- polation for different FB delays at 50 and 5% CDF values. the signal can be scaled down by 1/SCd. Alternatively, Tx(t3) is modified to Tx(t3) = AW X H delay d + W X Hdelayd, increasing the combining gain at the receiver. COOPA HARQ can be easily combined with partial IRC at the UEs for sup- pression of residual IF, thus partly reducing the SDF at the UE side to other cell interference, but at the same time stabilizes the overall system further. COOPA HARQ leads to a robust system design but, as usual for HARQ systems, at the cost of an additional retransmission, thus increasing the required Tx resources. Optimum usage of these resources is currently being studie