In a wireless communication system, downlink and uplink transmissions of information (control signaling or data) can be according to either a frequency division duplex (FDD) mode or a time division duplex (TDD) mode. In the FDD mode, uplink and downlink transmissions are separated in the frequency domain, by transmitting uplink data using a first carrier frequency, and transmitting downlink data using a second carrier frequency. In the TDD mode, on the other hand, both uplink and downlink transmissions occur on the same carrier frequency; however, uplink and downlink transmissions are separated in the time domain, by sending uplink and downlink transmissions in different time periods.
In some wireless communications systems, different uplink-downlink configurations may be defined. A particular uplink-downlink configuration can specify that, within a frame, a first subset of subframes in the frame is used for uplink transmissions, and a second subset of subframes in the frame is used for downlink transmissions. Different uplink-downlink configurations can employ different numbers of uplink and downlink subframes.