nabled, the system may define and utilize addi- tional component carriers to improve the user experience and to meet QoS requirements for various services, or to provide services through additional component carriers which might have been configured or optimized for those services. Figures 11-4 and 11-5 depict the frame structure used for single-carrier and multi-carrier operation. A number of narrowband component carriers may be aggregated to support virtual wideband operation. Each component carrier may include synchroni- zation and broadcast channels. A multi-carrier-enabled MS can utilize radio resources across multiple component carriers using a common MAC. Depending on the mobile station's capabilities, such utilization may include aggregation or switching of traffic across multiple component carriers controlled by a single MAC instance. As mentioned earlier, the component carriers utilized in multi-carrier operation may be contiguous or non-contiguous. If component carriers are in the same frequency band and Legacy Radio Frame New Radio Frame Frame Offset Legacy Zones New Zones Used Guard Sub-carriers Un-used Guard Sub-carriers FIGURE 11-4 An example illustration of carrier aggregation when supporting legacy systems [2] 11.2 Sub-carrier alignment and use of guard sub-carriers Superframe = 20 ms Single Multicarrier Carrier Terminals Terminals FIGURE 11-5 An example of a multi-carrier frame structure [2] physically adjacent, and the frequency separation of two adjacent carriers is an integer multiple of OFDM sub-carrier spacing, no guard sub-carriers are necessary between adjacent component carriers (see Figure 11-4). When component carriers are in a non-contiguous spectrum, the number of uplink subframes is not necessarily the same for all the carriers in the TDD mode. The same frame structure is used for each component carrier in a multi-carrier operation. Each component carrier is required to carry a superframe header which may include part of a super- frame header. Figure 11-5 illustrates an example of f