her SIR can be achieved and decoding probability is significantly increased. This helps to reduce overall feedback because a high number of feedback bits is generated only in those cases where high precoding accuracy is required also. In case of significant feedback delay At between the time t1 of measuring H(t1) and transmitting the data packet over the radio channel at time t2 = t1 + At, the radio channel will have evolved into the radio channel Hdelay(t2). Thus, precoding with W()Hdelay(t2) will go wrong, as can be seen from the preceding simulation results. For COOPA HARQ (see schematic representation in Figure 7.23), it is important that the UE can measure Hdelay(t2), which has been the radio channel for the first transmission and can therefore adapt its PMI feedback accordingly-eithe by calculating the difference of AW = W(t2) - W(t) itself or by feeding back W(t2) SO that the NB can calculate the difference. At the retransmission, the CU trans- mits Tx(t3) = AW X Hdelay (d is the same data vector as for the first transmission). The UEs combine both receive signals from t1 and t2 to rsum = r(t1) + r(t2) = (W(t) Hdelay(t2) + AW X Hdelay) d = W(t)HHdelay(t)). Note that the equation is true if Hdelay and Hdelay(t2) are equal; otherwise, there will be only partial correction. This technique is nonetheless very impor- tant because it allows correcting precoding errors after transmissions have already taken place. Moreover, it allows for interpolation between both time instances, © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Data d Data d Broadcast Precode with Precode with W(PMI) W(PMI) Hdelay(t1) Hdelay(t2) CSI Estimation UE1,2 CSI Estimation CSI Estimation Calculate PMI Calculate PMI Calculate PMI, Decode Decode + rx2 Hdelay(t1) Wd+n1 rx2 = Hdelay(t2) AW delay (W+AW)d + n12 Figure 7.23 Schematic representation of COOPA HARQ concept. and, accordingly, evaluations have revealed that even very simple linear interpola- tion over a full frame length of 10 ms is po