n for a mobile speed of 30 km/h, 40-symbol = 2.9 ms feedback delay, and 4/6 bit for A and PHI. The high performance gain without delay is almost completely lost for the delayed feedback case. (For a further reduction of feedback, see [61].) © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes 7.4.5 COOPA HARQ The high sensitivity to feedback delay calls for some specific solutions to come to a robust system concept and additionally keep overall overhead small. For this pur- pose, the COOPA HARQ concept has been developed. It integrates the conven- tional LTE HARQ concept with optimized CB-based precoding. The main goal of HARQ is to adapt optimally to varying link conditions. Thus, in a first transmis- sion, only low redundancy is transmitted, but in cases in which the receiver can- not decode a data packet, a nonacknowledge (NACK) message is fed back to the transmitter. After reception of a NACK message, the transmitter sends additional redundancy for the first data packet. In contrast to ARQ schemes, in the case of HARQ, the received data from the first transmission are stored and will be com- bined suitably with the retransmitted receive signal [36]. For COOPA HARQ, the concept is extended to precoding. Instead of addi- tional redundancy for the data packet, the retransmission of the data packet is done with an optimized precoding matrix WQ,2. Optimization can be done with respect to higher quantization levels A and PHI or with respect to a time-corrected channel matrix Hdelay In the first case, the first PMI feedback of each UE is, for example, only 3 bit, to conform to LTE release 8. As can be seen from Figure 7.22, only a few UEs will succeed if higher CIR (such as >10 dB) is requested; therefore, it will be necessary to send a modified NACK message, COOPA NACK. In addition to the NACK information, COOPA NACK contains some further PMI information, SO the overall PMI information is, for example, 5 bit, long. With this additional PMI information, according to Figure 7.22 (bottom), a much hig