ng points would require use of idle symbols to accommodate the switching gaps. In the case of TDD operation with the basic frame structure, the last symbol in the subframe immediately preceding a downlink-to-uplink/uplink-to-downlink switching point may be reserved for guard time and consequently not transmitted. An example frame partitioning is shown in Figure 9-18. 9.8 SUBCHANNELIZATION AND PERMUTATION The IEEE 802.16-2009 standard defines a permutation zone as a number of contiguous OFDM symbols in the DL or the UL that use the same permutation formula. The DL or UL portions of the radio frame may contain more than one permutation zone. One of the drawbacks of the sub- channelization schemes specified in the IEEE 802.16-2009 standard is that the distributed and localized allocations cannot be located in the same permutation zone. Furthermore, in the legacy standard, a slot requires both a time and a subchannel dimension for completeness, and is the minimum possible data allocation unit. The definition of an OFDMA slot depends on the OFDM symbol structure, which varies for UL and DL, for Full Usage of Subchannels (FUSC) and Partial Usage of Subchannels (PUSC), and for the distributed sub-carrier permutations and the adjacent sub-carrier permutations. As shown in Figure 9-20, a data region in the legacy standard is defined as a two-dimensional resource unit consisting of a group of contiguous subchannels, in a group of contiguous OFDM symbols. A two-dimensional allocation may be visualized as a rectangle such 4 X 3, 6 X 1 etc. Therefore, to address a particular rectangle in the DL or UL MAP, two parameters are required, i.e., time and frequency offsets (or alternatively symbol offset and subchannel offset). In order to reduce the overhead of signaling allocations in two-dimensional subchannelization schemes,as well as to allow both localized and distributed allocations in the same time-zone, IEEE 802.16 introduced new one-dimensional subchannelization schemes and their associated permu- tation methods to achieve