Cdma meaning

(Code Division Multiple Access) A method for transmitting digital signals simultaneously over the same carrier frequency (channel). CDMA's basic principles were developed to secure radio signals at the beginning of World War II; however, the most widely known application of CDMA came much later as a cellular transmission method. In the U.S., Verizon and Sprint are CDMA carriers, and Qualcomm designs the chips for the air interface.CDMA and GSMFor years, CDMA and GSM have been the two primary cellular, but incompatible, 2G and 3G transmission systems. Old cellphones support either CDMA or GSM, while newer models handle both. Although all current phones support 4G LTE, if they do not have both CDMA and GSM built in, users cannot switch carriers and be guaranteed service. The problem is that in rural or highly congested areas, transmission may throttle down from LTE to 3G. See LTE, GSM and cellular generations.CDMA Was a Major AdvanceCDMA 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). See IS-95 and CDMA2000.Spread SpectrumUnlike GSM, which uses TDMA to divide the channel into time slots, CDMA's spread spectrum assigns a unique code to each conversation and uses the full bandwidth of the channel. An often-used analogy is the ability to detect one's own language in a room full of people speaking multiple languages.Each bit 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. See chip rate, BREW, FDMA, TDMA and spread spectrum.The following illustrations were created with the assistance of Klein Gilhousen, co-founder of Qualcomm and co-inventor of CDMA. They show the path of a single bit being transmitted and received.
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Short for code-division multiple access; a technology used in wireless communications for second-generation and third-generation wireless networks. It allows many signals to occupy a single transmission channel in order to optimize the available bandwidth, a process called multiplexing. CDMA is used in UHF (ultra-high- frequency) cellular telephone systems in the 800-MHz and 1.9-GHz bands. Wireless companies that use the CDMA technology include Sprint, PCS and its affiliates, Verizon Wireless, and Qwest Communi-cations International Inc.
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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.
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