precoder is given as: (10-57) The above code is composed of two SFBC codes which are transmitted on antenna ports 0, 2, and 1, 3, respectively. The reason for distributing a single SFBC code in such an interlaced manner on every other antenna port instead of consecutive antenna ports is related to the fact that the first two cell- specific antenna ports have a higher reference signal density than the last two; and hence, provide better estimation of the channel. The above transmit-diversity scheme can be used for all downlink channels other than PHICH. For the latter, four different ACK/NACK bits are multiplexed using orthogonal codes with a spreading factor of four over a group of four sub-carriers, and the resulting group is repeated three times in the frequency-domain to achieve frequency-diversity gain. To maintain the orthogonality between different codes in each repetition of four sub-carriers, antenna switching is not applied within each repetition. Instead, the set of antennas changes across different repetitions. When there are multiple PHICHs transmitted, using type 1 or type 2 alternatively for different PHICHs would be helpful to keep uniform power distribution over the eNB transmit antennas. Precoding for spatial multiplexing using antenna ports with cell-specific reference signals is only used in combination with layer mapping for spatial multiplexing. Spatial multiplexing supports two or four antenna ports and the set of antenna ports used is p = 0,1 or p = 0,1,2,3, respectively with p denoting the antenna port number. The precoding for spatial multiplexing without CDD is defined as: = (W(i) (10-58) where W(i) is the Np X N precoding matrix, Np is the number of antenna ports, N denotes the number of layers, i = 0, 1, ...,M's symb - 1, and MP symb = For spatial multiplexing, the values of W(i) are selected among the precoder elements in the codebook configured in the eNB and the UE. The eNB can further confine the precoder selection in the UE to a subset of the elements in the codebook using 10.9 Mu