Understanding CDMA January 2, 2010
Posted by admin in : Internet Tips , trackbackIn a previous post I had discussed What is CDMA and GSM? Ok, as promised I will explain about the CDMA is more comprehensive in terms of science communication.
CDMA or Code Division Multiple Access is a multiple access technique based on spectral spread communication system, where each user is given a specific code that will distinguish one user to another user. This system was originally developed in the military because of reliability in the fight against high noise, the nature of anti jamming, and high data confidentiality.
Scattered Spectral System Definition
By definition, the spectral spread communication system is a modulation technique where the sender of the signal occupies a bandwidth much greater than the minimum spectrum required to deliver the information. This concept is based on the theory for C.E Shannon channel capacity, namely:
C = W log 2 (1 + S / N)
Where: C = capacity of the transmission channel (bps)
W = transmission bandwidth (Hz)
N = noise power (Watt)
S = signal power (Watt)
From the above theory shows that to deliver greater information on the channel noise to be taken in two ways:
- With the conventional way, where a small W and S/N large.
- How to spread spectrum, where the big W and S/N small.
On the Spectral system spread plated on the information signal frequency band much wider than the information bandwidth. The spread is done by an independent spreader function of signal information in the form of pseudorandom signal which have similar spectral characteristics of noise, is called pseudorandom noise (PN code).
There are several modulation techniques can be used to generate spread spectrum signals such as Direct Sequence Spread Spectrum (DS-SS) in which the information carrier signal is multiplied directly with the signal of high-speed spreaders, Frequency Hopping Spred Spectrum (FH-SS) where the carrier frequency signal information changes according to the lines of code will be given and constant during a certain period, called T (chip period). Time Hopping Spread Spectrum (TH-SS) in which the information carrier signal is not sent in a continuous but sent in the form of short duration bursts that depend on the signal burst pengkodeannya, and hybrid modulation which is a combination of two or more modulation techniques on which aims to combine the advantages each technique. Modulation technique most widely used today, including CDMA2000 1x system, is the Direct Sequence Spread Spectrrum (DS-SS) because of its realization is simpler than other modulation techniques.
In DS-SS, demodulate the signal carriers in directly by the coded data which is a row of data that has been encoded with a high-speed line of code generated by a Pseudo Random Generator (PRG) and a pseudo random characteristics that are predictable and periodic. Scattered signal is then modulated by using BPSK modulation, QPSK, or MSK. In the CDMA2000 1x system used QPSK modulation technique.
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