A 3G cellular radio system based on earlier CDMA versions TIA/EIA IS-95a and IS-95b, CDMA2000 has been standardized by the EIA/TIA as IS-856 and approved by the ITU-R as part of the IMT-2000 family. The initial version, known as CDMA2000 1xRTT offers 2.5G capabilities within a single standard 1.25 MHz channel, effectively doubling the voice capacity of the predecessor 2G cdmaOne systems and offering theoretical data speeds up to 153 kbps through the use of quadrature phase shift keying (QPSK) modulation. An enhanced 3G version known as 1xEV-DO is a high data rate (HDR) version that employs 16-QPSK modulation in support of a peak data rate of 2.4 Mbps on the downlink and 153 kbps on the uplink. 3x, also known as IS-2000-A, is an enhancement that uses three cdmaOne carriers for total bandwidth of 3.75 MHz. See also 16-QPSK, 1XEV-DO, 1xRTT, 2G, 2.5G, 3G, 3x, bandwidth, carrier, CDMA, cdmaOne, cellular radio, downlink, IMT2000, ITU-R, modulation, QPSK, and uplink.