CDMA (code-division multiple access) refers to several protocols used in 2G, 3G and 4G cellular communications
(it is sometimes referred to as direct-sequence spread spectrum). CDMA technology is a form of multiplexing that allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultrahigh-frequency (UHF) cellular communication systems across many frequency bands.
CDMA uses analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code) so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this makes cloning difficult and enhances security and privacy.
The mobile cellular families:
CDMA technology is used by almost all mobile operators in the U.S. (AT&T, Sprint, Verizon and T-Mobile) and around the world. We often confuse CDMA2000 (a family of standards supported by Verizon and Sprint) with CDMA (the physical layer multiplexing scheme).
It is important for mobile managers to understand that there are several major mobile cellular "families" using the CDMA technology:
GSM family of standards: This includes GPRS, EDGE, UMTS, HSPA, HSPA+ and LTE. The GSM family of standards began to use CDMA (spread spectrum) starting with UMTS (a 3G technology). Why? Because it is a highly efficient mechanism to multiplex many user conversations over the same wireless communications channel. In the U.S., AT&T and T-Mobile support the GSM family of standards and therefore use CDMA.
CDMA2000 family of standards: This includes 1xRTT, EV-DO Rev 0, EV-DO Rev A and EV-DO Rev B (now called Ultra Mobile Broadband -- UMB). The CDMA2000 family of standards is deployed by Verizon Wireless and Sprint in the U.S. and uses CDMA as the underlying multiplexing scheme. We often confuse CDMA2000 with CDMA.
Mobile WiMAX family of standards: This is the new guy on the block. Clearwire is deploying mobile broadband services using the WIMAX technology (IEEE 802.16e) throughout the U.S. and Europe. Clearwire is owned by Sprint (51%), Google, Intel, Time Warner, Comcast and Bright House Networks.
It is important for mobile managers to consider for the long term that the CDMA2000 family is a "dead end." Verizon is abandoning CDMA2000 and moving to the GSM family for its 4G network (Long Term Evolution). Sprint is also abandoning CDMA2000 and moving to WiMAX for its 4G rollout.
The future battle in cellular communications will be between WiMAX and Long Term Evolution (a GSM technology). The underlying technology is less of an issue because WiMAX and LTE are very similar. They both use Orthogonal Frequency Division Multiple Access (OFDMA) for their physical layer. They both use Multiple-Input, Multiple-Output (MIMO) and both are completely packet-based. This last point is huge: All voice communication will use voice over IP.
Understanding CDMA technology will help you gain a deeper insight into a widely used multiplexing scheme. More important, however, is the selection of the cellular solution (GSM, CDMA2000, or WiMAX) you will use.
About the co-author: Paul DeBeasi is a senior analyst at the Burton Group and has more than 25 years of experience in the networking industry