Communication Systems : Analog communication: Noise & Bandwidth

By Abhinav Gupta|Updated : July 7th, 2021

The article contains fundamental notes on "Analog communication: Noise & Bandwidth"  topic of "Communication Systems in  Electrical Engineering" subject. Also useful for the preparation of various upcoming exams like GATE Electrical Engineering(EE)/ IES/ BARC/ ISRO/ SSC-JE /State Engineering Services exams and other important upcoming competitive exams.

Noise is ever present and effect the performance of virtually every system. The presence of noise reduce the performance of the Analog and digital communication systems. This article deals with how noise affects different Analog modulation techniques. After studying this article candidates should be familiar with the following:

  • Various performance measures of communication systems.
  • SNR calculations for DSB-SC, SSB-SC, Conventional AM, FM (threshold effect, threshold extension, pre-emphasis and de-emphasis) and PM.
  • The figure of merit of All the above systems.
  • Comparisons of all analog modulation systems.
  • Bandwidth efficiency, power efficiency, ease of implementation.

2. Important Factors Related to Noise

  • The presence of noise degrades the performance of the Analog and digital communication systems.
  • The extent to which the noise affects the performance of communication system is measured by the output signal-to-noise power ratio or the probability of error.
  • The SNR is used to measure the performance of the Analog communication systems, whereas the probability of error is used as a performance measure of digital communication systems.
  • figure of merit = γ = SNRo/SNRi
  • The loss or mutilation of the message at low pre-detection SNR is called as the threshold effect. The threshold occurs when SNRi is about 10dB or less.
  • Output SNR:
    • So = output signal power
    • Si = input signal power
    • fM = baseband signal frequency range
    • The input noise is white with spectral density = η/2
  • Figure of Merit:

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(S/N)0 depends mainly on modulation scheme and receiver characteristics.

  • Figure of Merit of a DSB system:

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        Where m2(t) = power in the message signal = P

         For a single tone modulation:

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     Input to the PM is DSB signal + Narrow band noise = Ac m(t) cos 2πfct + ni(t) cos 2πfct – nq(t) sin 2πfct

      Output of the PM = Ac m(t) cos2 2πfct + ni(t) cos2 2πfct – nq(t) cos 2πfct sin 2πfct

      Output of the LPF =

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 N0 = (1/4) [In phase component noise power]

        = (1/4) [2WN0] = (N0W)/2

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  • Figure of Merit of a SSB system:

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     Input to the PM = SSB + Narrow band Noise  = [Ac m(t) cos 2πfct + Ac (t) sin 2πfct]. ½ + ni(t) cos 2πfct – nq(t) sin 2πfct

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  • Noise in AM

    s(t)AM = Ac cos 2πfct + Ac Ka m(t) cos 2πfct

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  • Noise in FM

    s(t) = Ac cos [2πfct + 2πKf ]

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  • The noise power spectral density at the output of the demodulator in PM is flat within the message bandwidth whereas for FM the noise power spectrum has a parabolic shape.
  • The modulator filter which emphasizes high frequencies is called the pre-emphasis filter(HPF) and the demodulator filter which is the inverse of the modulator filter is called the de-emphasis filter(LPF)

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