lti-antenna techniques in 3GPP LTE x0 (i) yo (i) XN1-1 (i) -1 (i) N1 Layers Np Antennas FIGURE 10-50 Open-loop spatial multiplexing with Nt antennas and N transmission layers [101] codebook subset restrictions. For large-delay CDD, precoding for spatial multiplexing is defined by (see Figure 10-50): y(0) (i) x(0)(i) = W(i)D(i)U (10-59) (Np-1)(i) (N,-1) where D(i) and U are N1 X N1 matrices supporting cyclic delay diversity, as shown in Table 10-34, for a different number of layers. For two antenna ports, the precoder is selected according to W(i) = C1 where C1 denotes the precoding matrix corresponding to precoder index 0 in Table 10-35. For four antenna ports, the UE may assume that the eNB cyclically assigns different precoders to different vectors (x(0)(i),x(1) (i), x(N1-1) (i))2 on the physical downlink shared channel. A different precoder is used every N1 vectors, where N1 denotes the number of transmission layers in the case of spatial multiplexing. In particular, the precoder is selected according to W(i) = Ck, where k is the precoder index given by k = (|i/N/|mod4) + 1€ {1,2,3,4} and C1, C2, C3, C4, denote precoder matrices. When two antenna ports are configured, the number of codewords is equal to the transmission rank and codeword n is mapped to layer n. For transmission on two antenna ports (p = 0,1), the precoding matrix W(i) is selected from Table 10-35 or a subset of it. The open-loop spatial multiplexing may be used when reliable PMI feedback is not available at the eNB, for example, when the UE moves fast or when the feedback overhead on uplink is too high. The open-loop spatial multiplexing with N layers and Np antenna ports (Np > N1) is illustrated in Figure 10-50. The feedback consists of the RI and the CQI in open-loop spatial multi- plexing. In contrast to closed-loop spatial multiplexing, the eNB only determines the transmission rank, and a fixed set of precoding matrices are applied cyclically across all the scheduled sub-carriers in the frequency-domain. It is noted that in the open-loop s