CDMA stands for Code Division Multiple Access. It is a wireless communication technology that allows multiple people to use a single radio channel at the same time with little interference and very high security. Conventional communication systems transmit and receive on one constant frequency; CDMA "spreads" a radio signal over a large frequency range, using a unique code to identify each transmitter/receiver pair. This allows many users to communicate on the same channel while making each transmission largely immune to natural interference, eavesdropping, and jamming.
Most security and interference issues in radio frequency transmissions can be addressed by using CDMA technology. Because each sender/receiver pair has its own pseudo-random number (PN) code that is used to spread the signal across the frequency spectrum, multiple pairs can use the same channel without hearing each other. To receivers that don't know the code, the signals look like noise rather than data and is ignored. On top of this, the actual signals themselves are encrypted, so anyone trying to eavesdrop would need to know both the encryption key and the PN code, which makes it extremely difficult to do.
By using a spread spectrum, a jammer is less likely to be able to block the entire signal. That wide bandwidth also means that narrow band interference and multipath fading are unlikely to cause significant problems because they don't affect the entire spectrum. Since a very large number of people can use the same channel, switching from one transmission tower to another is usually not a problem because there is less concern that the new tower might not have the capacity to handle the signal and drop it.
Although CDMA has a very high capacity, more users sharing a channel means they will generate more noise. This can come across as static in the transmission, and each individual receiver will have to use more power to get its signal out. At some point, the noise will become too loud, and decrease the range of the transmission tower.
Other Methods of Transmission
Radio communication systems use a spectrum of differing frequencies to carry signals. Early systems required each individual sender and receiver pair to occupy a single frequency to avoid interference, a system known as Frequency Division Multiple Access (FDMA). Although effective in reducing the likelihood that transmissions between different pairs would interfere with each other, it offered little security as any third party capable of tuning to the same frequency could listen to or jam the transmissions. Another of the outdated multiple use techniques is time division multiple access (TDMA) which requires pairs to communicate on the frequency band at specific times so that there was less interference. Again, although this method is effective at eliminating cross-talk, it is not secure.
The differences between these three multiple access disciplines can be illustrated by imagining a group of people in a small room. If everyone tried to have a conversation at the same time, the results would be chaotic. To best make sense of the conversations, each pair of people could speak a different language (CDMA), allowing them to speak privately and ignore everyone else because their voices would just be noise. The alternatives might be pitching their voices at different frequencies (FDMA) or taking turns speaking (TDMA). There would, however, still be no privacy since everyone could understand each other, and delays in getting messages across.
Uses of CDMA
CDMA was originally of primary interest to the military, which needed to be able to send radio messages that were difficult to block or listen in on. Its use has expanded greatly, however, and different types have come to form the basis for a wide range of wireless communication, mobile phone, and data transfer technologies. In the US, the name CDMA is most closely associated with one of the main types of cell phone service.
When referring to cell phones, CDMA is more accurately called CDMA2000 or cdmaOne. This technology is most common in the US, although it is used in parts of Asia and elsewhere around the world. It is often contrasted with Global System for Mobile Communications (GSM) technology, a competing cell phone standard that was originally based on TDMA and FDMA methods. With third generation (3G) mobile phone standards, however, both use technology based on code-division multiplexing. CDMA phones are perhaps best known for storing all data in the handset rather than in removable SIM cards like GSM.