A 3G cellular radio standard under development by the Chinese Academy of Telecommunications Technology (CATT), an agency of the People's Republic of China (PRC), and adopted by the 3rd Generation Partnership Project (3GPP) as UTRA TDD 1.28Mcps Option (UMTS Terrestrial Radio Access Time Division Duplex 1.28 Mega chips per second Option).TD-SCDMA is based on code division multiple access (CDMA), but uses time division multiple access (TDMA), as well, with the uplink signal synchronized by the base station (BS). Like European versions of W-CDMA, TD-SCDMA uses time-division duplex (TDD) to support bidirectional communications by derived uplink and downlink time slots within the same frequency band. TD-SCDMA specifications also include a frequency division duplex (FDD) mode. The modulation scheme on the uplink is quaternary phase shift keying (QPSK) and on the downlink either binary phase shift keying (BPSK) or QPSK.The synchronization of the signal at the base station improves the orthogonality of the coded transmissions, which serves to reduce interference between users and, therefore, allow increased capacity.TD-SCDMA runs in the 2000 MHz (2 GHz) band. Each 5 MHz carrier supports 120 voice channels in TDD mode (one carrier for both uplink and downlink transmission) and 250 channels in FDD mode (one uplink carrier and one downlink carrier).Asynchronous data rates range from 1.2 kbps to 2 Mbps, symmetrical. See also 3GPP, BPSK, carrier, CDMA, channel, chip, chip rate, downlink, FDD, frequency band, interference, orthogonal, QPSK, symmetric, synchronize, TDD, TDMA, time slot, uplink, and W-CDMA.