CDMA offers an answer to the problem of capacity. The key to its high capacity is the use of noise as a carrier wave, as first suggested decades ago by Claude Shannon. Instead of dividing the spectrum or disconnecting time "cracks" that each user is assigned a different case noise carrier. While these waveforms are not rigorously orthogonal if they are about.
The practical application of this principle has always used digitally generated pseudo - noise, instead of the true thermal noise. The essential element is that retains the benefits, and simplifies the transmitters and receivers because they can be carried out using large portions of high density digital devices.
The greatest benefit of noise - as carriers is that the system sensitivity to interference is fundamentally altered. Traditional time and frequency slotted systems should be designed with reuse of proportion that satisfies the worst, but only a small fraction of users actually experience the worst.
The use of noise - as the bearers of all users occupying the same spectrum, makes the effective noise sum of all else - the signs of the user. The receiver correlates its input with the desired noise carrier, while reinforcing the sign to the ratio of noise to the discoverer. The improvement exceeds the noise added to provide an adequate SNR to the discoverer. The frequency reuse is universal, ie multiple users use each CDMA carrier frequency ...
The reuse model is now rainbow cells indicate that 1.25 MHz passband of integers is used by each user, and the same passband is reused in each cell. The capacity is determined by the balance between the required SNR for each user, and the gain spread spectrum processing. The figure of merit of a well - designed the digital receiver to indicate dimensions - to - noise ratio (SNR) The "noise" part of the SNR, in a spread spectrum, the system really is the sum of thermal noise and the other - user interference. The SNR required to achieve a particular error rate depending on several factors, such as forward error correction and multipath and fading environment.
For recipients typically used in commercial CDMA is typically about 3 dB to 9 dB. Energy is related by piece to mark power and proportion of data:
Noise + interference term is the power spectral density. If the spectrum of signs is roughly rectangular, with a bandwidth of W, then the noise + interference power spectral density that is where the first term represents the thermal noise level receptor (FN = figure receiver noise). Rewriting the SNR equation as regards the proportion of data and the spread - samples of bandwidth spectrum where the magic lies.
Interference in this equation is the sum of the signs of all users.
This equation is the key to understanding why CDMA was not explored for use in multiple access systems on Earth. It is also the key to innovation that led to commercial CDMA.
