Downlink transmission scheme 319 time, half-duplex operation has an impact on the data rates that can be provided to/from a single mobile terminal. It should be noted that full/half-duplex capa- bility may be frequency-band dependent such that a terminal may support only half-duplex operation in certain frequency bands while being capable of full- duplex operation in the remaining supported bands. In case of TDD operation (lower part of Figure 16.2), there is only a single carrier frequency and uplink and downlink transmissions are always separated in time also on a cell basis. As the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An essential aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink nor uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions as elaborated upon in Chapter 19. For LTE, this guard time is provided by special subframes (subframe 1 and, in some cases, subframe 6), which are split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission.