radio resources by the switching gaps. The frame structure from the point of view of an H-FDD mobile station is similar to the TDD frame structure, with the exception that the DL and UL transmissions occur in two separate frequency bands. Transmission gaps are required between the DL and UL subframes to allow switching of transmit/ receive circuitry (i.e., a duplexer-free operation) and accommodation of maximum permissible RTT in each deployment scenario (cell size, RF carrier frequency, etc.). For nominal channel bandwidths of 5, 10, and 20 MHz, an IEEE 802. 16m frame for CP= 1/8T has eight type-1 subframes for FDD, and seven type-1 subframes and one type-3 subframe for TDD. Figure 9-16 shows example TDD and FDD frame structures for a 5, 10, and 20 MHz channel bandwidth with a CP = 1/8T Assuming OFDM symbol duration Tu = 102.857 us and CP = 1/8T, the length of type-1 and type-3 subframes are 0.617 ms and 0.514 ms, respectively. The Transmit/Receive Transition Gap (TTG) and Receive/Transmit Transition Gap (RTG) are 105.714 us and 60 us, respectively. Other numerologies may result in a different number of subframes per frame and of symbols within the subframes. In FDD mode, the structure of a frame is identical for the DL and UL in each specific frame. The length of the TTG gap is selected greater than the maximum round trip time in the cell plus the switching time of the RF circuitry from transmission to reception, i.e., tGap > 2rmaxlc + tswitching where rmax denotes the maximum cell radius, C denotes the speed of light, and tswitching represents the switching time of RF circuitry. Assuming a typical value of 30 us for tswitching and the value of switching gap given above, a maximum cell size of 11.4 km can be supported. Larger values of switching gaps should be used for support of larger cell sizes. The IEEE 802. 16m OFDMA parameters further include CP 1/4T for support of large cell sizes and frequency bands of less than 1 GHz. 9.6 Frame structure TDD Radio Frame = 5 ms DL (6) DL (6) DL (6) DL (6) DL (5) UL (6) 