| 单词 | CDMA (Code Division Multiple Access) |
| 释义 | CDMA (Code Division Multiple Access) 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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