UL (6) UL (6) 6 OFDM Symbols= 0.583 ms 5 OFDM Symbols = 0.514 ms Switching Gaps Idle Time DL/UL (6) DL/UL (6) DL/UL (6) DL/UL (6) DL/UL (6) DL/UL (6) DL/UL (6) DL/UL (6) FDD Radio Frame = 5 ms FIGURE 9-16 The frame structure with CP= 1/8 Tu for FDD and TDD (DL/UL ratio 5:3) [2] An IEEE 802.1 16m frame for a CP= 1/16T has five type-1 subframes and three type-2 subframes for FDD; six type-1 subframes and two type-2 subframes for TDD for nominal channel bandwidths of 5, 10, and 20 MHz. The subframe preceding a DL to UL switching point is a type-1 subframe. Figure 9-17 illustrates an example of the TDD and FDD frame structure for 5, 10, and 20 MHz channel bandwidths with a CP 1/16T. With an OFDM symbol duration of 97.143 us and a CP length of 1/16T, the lengths of the type-1 and type-2 subframes are 0.583 ms and 0.680 ms, respectively. The TTG and RTG switching gaps are 82.853 us and 60 us, respectively. Other numerologies may result in different number of subframes per frame and symbols within the subframes. The Transmission Time Interval (TTI) is defined as the duration of the transmission of the physical layer encoded packets over the radio air interface, and is equal to an integer number of consecutive subframes. In other words, the TTI refers to the length of an independently decodable transmission on the airlink. A data burst occupies either one subframe (i.e., the default TTI or short TTI) or multiple consecutive subframes (i.e., the long TTI). The long TTI in FDD mode contains four subframes for both the DL and UL. The long TTI in TDD mode comprises all available DL/UL subframes depending on the direction of transmission. The legacy and IEEE 802.16m frames are offset by an integer number of subframes to accom- modate new features such as the IEEE 802.16m Advanced Preamble (preamble), Superframe Header (system configuration information), and control channels, as shown in Figure 9-18. The Frame Offset shown in Figure 9-18 is an offset between the start of the legacy frame and the start of the IEEE 802.16m frame