(Code Division Multiple Access) A method for transmitting real-time signals over a shared portion of the spectrum. The foremost application of CDMA is the digital cellular phone technology that operates in the 800MHz band and 1.9GHz . Unlike GSM and TDMA, which divides the spectrum into different time slots CDMA uses a spread spectrum technique to assign a code to each conversation. After the speech codec converts voice to digital, CDMA spreads the voice stream over the full 1.25MHz bandwidth of the CDMA channel coding each stream separately so it can be decoded at the receiving end. The rate of the spreading signal is known as the “chip rate,” as each bit in the spreading signal is called a “chip” voice conversations use the full bandwidth at the same time. One bit from each conversation is multiplied into 128 coded bits by the spreading techniques, giving the receiving side an enormous amount of data it can average just to determine the value of one bit.
CDMA phones are noted for their excellent call quality and low current draw CDMA is less costly to implement, requiring fewer cell sites than the GSM and TDMA digital cell phone systems and providing three to five times the calling capacity. It provides more than 10 times the capacity of the analog cell phone system (AMPS). CDMA is also expected to become the third-generation (3G) technology for GSM.
CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and TDMA methods, CDMA’s wide spreading signal makes it difficult to detect and jam.
One of the most important concepts to any cellular telephone system is that of “multiple access”, meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporary allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how channels are allocated to the many users of the system.