rame structure to support multi-carrier operation. The preamble and superframe headers are replaced with OFDM symbols carrying user data and control in the uplink direction in FDD mode. Figure 11-4 shows an example of an IEEE 802.16m frame structure supporting legacy systems. A number of narrowband component carriers supporting an IEEE 802. 16m system can be aggregated to CHAPTER 11 Multi-Carrier Operation enable wideband operation of mobile stations. One or more component carriers can be designated as the legacy carrier(s), depending on the number of legacy terminals in the network. A different number of usable guard sub-carriers can be allocated on both sides of the component carrier. For uplink transmissions, either time-division or frequency-division multiplexing of the new and legacy mobile stations may be supported over the mixed-mode component carrier. If two adjacent component carriers include the IEEE 802. 16m and the legacy zones (e.g., component carrier 0 in Figure 11-4), the frame offset and uplink multiplexing scheme (i.e., TDM or FDM) applied to these component carriers must be the same. The information about the frame configuration of other component carriers is provided through the S-SFH SP1 or via the AAI_MC-ADV and AAI_NBR-ADV messages. If two adjacent component carriers contain the IEEE 802.16m and legacy zones, the guard sub-carriers in the uplink are not used in order to allow use of the legacy UL-PUSC subchannelization scheme. Each component carrier may utilize sub-carriers at its band edges as additional data sub-carriers. The guard sub-carriers are grouped to form an integer number of physical resource units. The PRU structure used for the guard resource is the same as the structure of the regular physical resource units described in Chapter 9. Contiguous resource units may be constructed from physical resource units, including those formed by guard sub-carriers. The guard sub-carriers are not used for the transmission of control channels. As shown in Figure 11-6, the guard sub-carriers be