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In telecommunications, the term channel noise level has the following meanings: The ratio of the noise in the communication channel at any point in a transmission system to an arbitrary level chosen as a reference. [a] [b] The noise power spectral density in the frequency range of interest. The average noise power in the frequency range of ...
Telecommunication systems strive to increase the ratio of signal level to noise level in order to effectively transfer data. Noise in telecommunication systems is a product of both internal and external sources to the system. Noise is a random process, characterized by stochastic properties such as its variance, distribution, and spectral density.
Moreover, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often ...
Modern digital telephone systems have less trouble in the voice frequency range as only the local line to the subscriber now remains in analog format, but DSL circuits operating in the MHz range on those same wires may suffer severe attenuation distortion, which is dealt with by automatic equalization or by abandoning the worst frequencies.
/ must be used with care on interference-limited channels since additive white noise (with constant noise density ) is assumed, and interference is not always noise-like. In spread spectrum systems (e.g., CDMA), the interference is sufficiently noise-like that it can be represented as and added to the thermal noise to produce the overall ratio
where is the overall noise factor of the subsequent stages. According to the equation, the overall noise factor, , is dominated by the noise factor of the LNA, , if the gain is sufficiently high. The resultant Noise Figure expressed in dB is:
The theorem establishes Shannon's channel capacity for such a communication link, a bound on the maximum amount of error-free information per time unit that can be transmitted with a specified bandwidth in the presence of the noise interference, assuming that the signal power is bounded, and that the Gaussian noise process is characterized by a ...
English: pdf Version of english wikibook on Communication Systems This file was created with MediaWiki to LaTeX . The LaTeX source code is attached to the PDF file (see imprint).