s; - beamforming: similar to spatial expansion, each vector of constellation points from all the space-time streams is multiplied by a matrix of steering vectors to produce the input to the transmit chains. i) Inverse discrete Fourier transform (IDFT) converts a block of constellation points to a time domain block. j) CSD (Cyclic Shift Diversity) insertion is where the insertion of the cyclic shifts prevents unintentional beamforming. CSD insertion may occur before or after the IDFT. k) GI insertion prepends to the symbol a circular extension of itself. 1) Windowing optionally smooths the edges of each symbol to increase spectral decay. 802.11n Interface Figure 4.7 shows the blocks used to generate the HT-SIG field of the PPDU data unit in HT_MF mode. GI Insertion Windowing GI Insertion Windowing GI Insertion Windowing Single spatial stream GI Insertion Windowing Transmission streams Figure 4.7. Transmission chain - Diagram 1 These blocks are also used to generate the NON_HT part of the PPDU data unit in HT_MF mode. The BCC encoder and the interleaver are not used when generating the L-STF and L-LTF fields. Figure 4.8 shows the blocks used to generate the DATA field for HT_MF and HT_GF modes. A subset of these blocks consisting of the constellation mapper and the CSD blocks, as well as the blocks on the right, including the spatial mapping block, is also used to generate the HT-STF, HT-GF-STF and HT-LTF fields. The HT-GF-SIG field is generated using the blocks shown in Figure 4.7, augmented by additional CSD blocks and spatial mapping. 78 Wi-Fi Integration to the 4G Mobile Network Constellation GI Insertion Interleaver Mapper Windowing Constellation GI Insertion Interleaver Mapper Windowing Constellation GI Insertion Interleaver Mapper Windowing Constellation Interleaver GI Insertion Mapper Windowing Spatial streams Space-time streams Transmission streams Figure 4.8. Transmission chain - Diagram 2 4.3.2. Frequency plan The 802.11n interface operates in the N-NII (Unlicensed-National Information Infrastructure) ba