tion for the proposed scheme over PRS reference signals are given in Figure 7.19. Feedback overhead. Closely related to optimum channel estimation is the challenge to feed back CSI from all UEs to all eNBs involved in the CA SO that the eNBs can do proper precoding. For TDD systems, one can partly rely on channel reciprocity, but for FDD systems one has to assume basically uncorrelated DL and UL radio channels. UL feedback in cellular systems is very expensive because it costs valuable UE power, thus reducing battery lifetime. At the same time, UEs have smaller Tx power and will have only limited UL capacity, in particular at the cell edge. Codebook-based precoding as defined in LTE release 8 is already a big step in reducing feedback; at the same time, more feedback will be required for high-performance precoding solutions. Many ideas exist for reducing feed- back, such as the subspace approach [23], hierarchical codebooks, tracking CBs [34], or other compression techniques. Later we will propose a further technique for robust and self-adapting feedback overhead in the combina- tion with advanced hybrid automatic repeat request (HARQ; in the follow- ing called COOPA HARQ) techniques. Feedback delay between measurement of the radio channel and availability at the CU is even more challenging. Based on the LTE RS and frame struc- ture, it will take several 1-ms subframes to estimate the radio channel, pre- pare the UL message, decode the message at the eNB, and react by updating the precoder at the CU. The timing of these actions adds up easily to 10 ms and might be even higher if backbone delay is also involved. During this time duration, the radio channels might have changed significantly, even in © 2009 by Taylor & Francis Group, LLC Cooperative Transmission Schemes the case of relatively low mobile speed. COOPA HARQ also addresses this issue with the goal for a robust transmission mode. An organizational issue is the coordination of different scheduling units, espe- cially in cases of distributed cooperation. T