1xRTT, often abbreviated as 1x, is a designation referring to an operating mode of CDMA2000, a wireless technology used in CDMA cellular networks. The 1x in 1xRTT stands for one times the number of 1.25MHz channels, while RTT stands for radio transmission technology. CDMA2000 1xRTT is also called 3G1X, IMT-CDMA Multi-Carrier, or IS-2000.

CDMA stands for code division multiple access. It is a mobile digital radio technology that transmits streams of bits. Unlike older cellular technology, all radios on the network can be active at the same time. CDMA was developed by a commercial company, Qualcomm. CDMA is the root technology of CDMA 1xRTT.

CDMA2000 is a family of wireless standards that use CDMA technology to transmit voice, data, and signal information between cellular phones and cell sites. 1xRTT is the version of CDMA2000 that uses a pair of 1.25MHz radio channels. 1xRTT supports peak data speeds of up to 144 kilobits per second (Kbps). Since it offers high speed data services in addition to voice capability, CDMA2000 1xRTT is technically part of the third generation (3G) of cellular systems. However, because it is slower than other 3G systems, 1xRTT is sometimes classified as “second and a half” generation, an unofficial term for technologies that are seen as falling somewhere between the second and third generation of wireless cellular systems.

1xRTT is seen as an improvement over previous CDMA technologies for a number of reasons. It provides up to double the voice capacity of second-generation cellular systems. 1xRTT is so much more efficient because it uses a pilot signal - a frequency transmitted for control, equalization, and reference purposes - as well as more channels between fixed cell stations and mobile users. CDMA2000 1xRTT also has media and link access protocols, allowing for greater use of data services than 2G systems.

1xRTT is not the only operating standard for CDMA2000. There is also CDMA2000 1x Evolution-Data Optimized, or 1xEV-DO. It is also referred to as EVDO.

EVDO is an improvement over 1xRTT because it has a higher data rate capability. Wireless carriers use it for streaming data and video services, which would not be possible on 1xRTT. Evolution-Data/Voice (EVDV) transfers data at similar speeds, but development has stalled since most wireless carriers began adopting EVDO.

In the United States, Verizon Wireless, Sprint PCS, Alltel, and U.S. Cellular all use 1xRTT. 1xRTT is also used by Bell Mobility and TELUS Mobility in Canada, and by Iusacell and Unefon in Mexico.