14-08-2013, 04:22 PM
EFFECT OF NOISE ON ANALOG COMMUNICATION SYSTEMS
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Effect of noise on baseband systems
Baseband system serves as a basis for comparison of various modulation systems.
There exists no modulator-receiver consists only of a LPF with bandwidth W.
The noise power at the output of the receiver is therefore
Threshold effect in Angle Modulation
In non-linear modulation systems, generally the signal and noise processes at the output of the demodulator are completely mixed in a single process by a complicated non-linear functional.
At high SNRs, they can be approximated as additive form.
At lower SNRs, signal and noise are inter-mingled so that we can’t recognize one from the other.
So, no meaningful SNR can be defined. In such cases, the signal is not distinguishable from noise and a mutilation or threshold effect is present.
There exists a specific SNR at the i/p of the demodulator, known as threshold SNR, beyond which signal mutilation occurs.
Existence of threshold effect places an upper limit on the trade-off between bandwidth and power in a FM system.
In another way
High level of noise is in the high freq. components of the message in FM. It is desirable to attenuate the high freq. components of the demodulated signal. This causes reduction in noise level, but also attenuate high-freq. components of the message signal. To compensate for the attenuation of higher components of message signal we can amplify these components at the transmitter before modulation. Therefore, at the transmitter we need a HPF and at the receiver we need a LPF.