**4.5 TUSC permutation**

The Tile Usage of Sub-channels is a downlink subcarrier permutation mode that is identical to the uplink PUSC. If closed loop advanced antenna systems (AAS) are to be used with the PUSC mode, explicit feedback of the channel state information (CSI) from the MS to the BS would be required even in the case of TDD, since the UL and DL allocations are not symmetric, and channel reciprocity cannot be used. TUSC allows for a DL allocation that is symmetric to the UL PUSC, thus taking advantage of UL and DL allocation symmetry and eliminating the requirement for explicit CSI feedback in the case of closed-loop AAS for TDD systems. Refer to TTD frame structure shown in Fig. 10.
