ration does not utilize the radio resources efficiently on the downlink and uplink RF carriers. The complementary grouping and scheduling of users would allow efficient use of downlink and uplink resources in an H-FDD oper- ation. The various duplex schemes are illustrated in Figure 9-14. 9.6 FRAME STRUCTURE As mentioned earlier, the mobile WiMAX systems are based on a subset of IEEE 802.16-2009 standard features [1]. Although the latter standard specifies a number of radio frame sizes, such as 2, 2.5, 4, 5, 8, 10, 12, and 20 ms, only the 5 ms radio frame is supported in the mobile WiMAX system profiles [6]. In order to satisfy the requirements for shorter airlink transmission latency, IEEE 802. 16m transparently modified the legacy frame structure. The modifications (as shown in Figure 9-18), are transparent to the legacy devices and are only visible to the new systems. The main idea behind the modifications was to shorten the transmission time intervals and to accelerate the HARQ re- transmissions. Therefore, the legacy radio frame was partitioned into a number of subframes. In order to improve the broadcast of system information in a predefined location and at deterministic intervals, the concept of superframe and superframe header were introduced. The modified frame structure further carries new synchronization and broadcast channels to overcome some of the limitations of the legacy system, such as problems with cell selection at low Signal-to-Interference + Noise Ratios (SINR) due to the poor design of the legacy preamble and the lack of robustness at low SINRs, resulting in poor performance at the cell edge and issues with cell selection/re-selection for cell-edge users. The new superframe concept would allow prescheduled transmission of time-critical system configuration information which further simplifies the operation of the system and reduces the system overhead and scanning latency. 9.6 Frame structure Frequency Division Duplex (FDD) Frequency Separation Time Division Duplex (TDD) Switching Gaps Half-