A multiplexing technique used in radio networks, CDMA is rooted in spread spectrum (SS) technology developed in the 1940s. Spread spectrum is a wideband radio transmission technology that spreads of the transmitted signal over a spectrum of radio frequencies that is much wider than that required to support the native narrowband transmission.Thereby, multiple transmissions can simultaneously use the entire system wideband, rather than just individual time slots or frequency channels. CDMA employs a variant known as frequency hopping spread spectrum (FHSS), which transmits short bursts of data over a range of frequency channels within the wideband carrier. Each transmission is assigned a 10-bit pseudorandom binary code sequence, which comprises a series of ones and zeros in a seemingly random pattern known to both the transmitter and receiver.The original code sequence is mathematically self-correlated to yield a code that stands out from all others, at least on average.The paired transmitters and receivers recognize their assigned and correlated code sequences, which look to all others as pseudorandom noise (PN). FHSS phase-modulates the carrier wave with a continuous string of PN code symbols, or chips. So, the chip rate is much faster than the bit rate. Thereby, the noise signal occurs with much greater frequency than the original data signal and spreads the signal energy over a much wider band. The transmitter and receiver hop from one frequency to another in a carefully choreographed hop sequence under the control of the centralized base station transceiver. Each transmission dwells on a particular frequency for a very short period of time (no more than 400 milliseconds for FCC-controlled applications), which may be less than the time interval required to transmit a single data packet, or symbol, or even a single bit. So, the chip rate can be faster than the bit rate. A large number of other transmissions also may share the same range of frequencies simultaneously, with each using a different hop sequence.The potential remains, however, for the overlapping of packets.The receiving device can distinguish each packet in a packet stream by reading the various codes prepended to the packet data transmissions, and treating competing signals as noise. See also bandwidth, carrier, channel, chip, chip rate, DSSS, FHSS, frequency, hop sequence, modulation, narrowband, PN, signal, SS, symbol, transceiver, and wideband.
(Code Division Multiple Access) A method for transmitting multiple digital signals simultaneously over the same carrier frequency (the same channel). Although used in various radio communications systems, the most widely known application of CDMA is for cellphones. As of 2011, there were more than 500 million CDMA cellular users worldwide, with more than half in Asia. Verizon and Sprint are CDMA carriers in the U.S., while TELUS uses CDMA in Canada. Qualcomm designs the chips for the CDMA air interface. See IS-95 and CDMA2000. CDMA provides up to 10 times the calling capacity of earlier analog networks (AMPS) and up to five times the capacity of GSM systems. CDMA is also the basis for the WCDMA and HSPA 3G technologies used by GSM carriers (see WCDMA and HSPA). Spread Spectrum Unlike GSM and earlier digital systems, both of which use TDMA to divide the channel into time slots, CDMA's spread spectrum overlaps every transmission on the same carrier frequency by assigning a unique code to each conversation. Each voice conversation uses the full bandwidth simultaneously, and the often-used analogy is the ability to detect one's own language in a room full of people speaking other languages. After the speech codec converts the caller's voice to digital, CDMA spreads the digital stream over the full 1.25 MHz bandwidth of the channel with a unique coding pattern. The rate of the spreading signal is known as the "chip rate," as each bit in the spreading signal is called a "chip," with no relation at all to an integrated circuit chip. Each bit of a conversation is multiplied into 128 coded bits, giving the receiving circuit an enormous amount of data it can average just to determine the value of a single bit when decoding the signal. More Secure CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and TDMA, CDMA's wide spreading signal makes it difficult to detect and jam. For more information, contact the CDMA Development Group (CDG) at www.cdg.org. See BREW, cellular generations, IS-95, CDMA2000, WCDMA, GSM, FDMA, TDMA, CDPD, CDG and spread spectrum.