multiple Access by distribution in code or AMRC in summary ( Code Division Multiple Access (CDMA)) is a system of coding used in Mobile telephony particularly with the the United States, in space, military telecommunications in particular, and in the navigation systems by satellites like the GPS, Glonass or Galileo.
Origins
It is a Hollywood actress, Hedy Lamarr, helped of George Antheil, which is at the origin of this system. Hedy Lamarr and George Antheil even deposited a patent in 1941. They proposed their system so that the radio operator guidance of torpedes cannot be intercepted by the enemy. However, the American army waited until the patent expires to exploit it.
Techniques
For mobile telephony, three techniques are possible to make pass several channels on same the carrying Fréquence: the temporal Multiplexing (AMRT, in English TDMA ), multiplexing of frequency (AMRF, in English FDMA ) and multiplexing by code (AMRC, in English CDMA ).- the standard GSM does not resort to the AMRC, but to the AMRT and the AMRF.
The current standard is CDMA-2000.
Standards
CDMA-2000 (Code Division Multiple Access) is an evolution of the CDMAOne system used in the United States of America. It allows flows going theoretically up to 2 MBits/sStandard CDMA-2000 is based on interface radio CDMA developed originally by the army Américaine.
CDMA-2000 is developed by the 3GPP2 (3rd Generation Partnership Project #2) which gathers the TIA in America, ARIB, CWTS and TTA in Pacific Asia and the international forum TTC.
ETSI European as well as American T1P1 do not form part and concentrate of it only on UMTS.
The CDMA-2000 is an American standard TIA under reference IS-2000.
The CDMA is complex. Developed in the Years 1980 for the satellite communications, it consists in “spreading out the spectrum” by means of a code allocated with each communication. The receiver uses this same code to demodulate the signal that it receives and to extract the useful information. The code itself does not transport any information. The operation requires important capacities of calculation, therefore more expensive components for the terminals general public. On the other hand, the operators have recourse to the CDMA for the connections by satellite of their fixed network.
Spreading out of spectrum
There exist two manners of spreading out the spectrum:
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direct Sequence: each bit of information is replaced by a series of bits, which we will call code. Let us imagine a flow R of 10 kbits/s requiring a band-width of 10 Khz. By replacing each bit by his code (let us say 10 bits per code), one multiplies the flow transmitted by 10, which gives 100 kbit/s. the fact of transmitting 10 times more quickly thus widens the spectrum transmitted in a report/ratio 10. Concretely, the flow of the transmissions is increased, but the flow of useful information is unchanged after decoding.
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Escape (Jump) from frequency ( Frequency Hopping , FH ): one uses NR frequencies for a communication. The choice of the frequencies is done according to a preset model in advance (in order to allow the receiver to recover the communication). It is said that the FH is slow if one changes frequency after the sending of several symbols, or rapid if one changes frequencies during the sending of a symbol.
The W-CDMA
Whereas the CDMA and CDMA-2000 were systems of distribution in circuits mode, the introduction of the package mode and the widening of the spectrum led to Wideband-CDMA (W-CDMA) physical layer of the standard UMTS.
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