GATE EC|| COMMUNICATION SYSTEM || PYQs (2000 - 2025)

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Question 1

Q.14 A digital communication system transmits through a noiseless bandlimited channel [−𝑊 𝑊]. The received signal z(t) at the output of the receiving filter is given by z(t) = \sum_{n} b[n] \, x(t - nT) where b[n] are symbols x(t) is the overall system response to a single symbol. The received signal is sampled at 𝑡 = 𝑚𝑇. The Fourier transform of x(t) is X (f) must satisfy for zero intersymbol interference at the receiver is______.

  • \text{(B)} \quad \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = {T}


  • [Tex]\text{(B)} \quad \sum_{m=-\infty}^{\infty} X\left(f + \frac{m}{T}\right) = \frac{1}{T}[/Tex]


  • [Tex]\text{(B)} \quad \sum_{m=-\infty}^{\infty} X\left(f + {m}{T}\right) = {T}[/Tex]

  • [Tex]\text{(B)} \quad \sum_{m=-\infty}^{\infty} X\left(f + {m}{T}\right) = \frac{1}{T}[/Tex]

Question 2

In a digital communication system, the overall pulse shape p(t) at the receiver before the sampler has the Fourier transform P(f). If the symbols are transmitted at the rate of 2000 symbols per second, for which of the following cases is the inter symbol interference zero?

  • ss


  • ss


  • ss


  • ss


Question 3

Which of the following protocols provides a connection-oriented communication system with guaranteed delivery?

  • UDP

  • IP

  • TCP

  • ARP

Question 4

A bandlimited signal is sampled at the Nyquist rate. The signal can be recovered by passing the samples through

( GATE 2001|| EC || MCQ ||2 MARK)

  • an RC filter.

  • an envelope detector.

  • a PLL.

  • an ideal low-pass filter with the appropriate bandwidth.

Question 5

Consider a system shown in Figure. Let X(f) and Y(f) denote the Fourier transforms of x(t) and y(t) respectively. The ideal HPF has the cut-off frequency 10 kHz.

Annotation-2025-09-21-220930

(PYQ || 2004 MCQ|| 1 MARK)

  • 1 kHz and 24 kHz

  • 2 kHz and 24 kHz

  • 1 kHz and 14 kHz

  • 2 kHz and 14 kHz

Question 6

A low-pass filter having a frequency response H(jω) =A(ω) e jφ(ω) does not produce any phase distortion if

(PYQ || 2006 MCQ|| 1 MARK)

  • A(ω) = Cω 2 , ϕ(ω) = kω 3

  • A(ω) = Cω 2 , ϕ(ω) = kω

  • A(ω) = Cω, ϕ(ω) = kω 2

  • A(ω) = C, ϕ(ω) = kω –1

Question 7

In the figure, M(f) is the Fourier transform of the message signal m(t) where A = 100 Hz and B = 40 Hz. Given v(t) = cos(2f c t) and w(t) = cos (2(f c + A) t), where fc > A. The cut-off frequencies of both the filters are f c .

Screenshot-2025-09-30-002514

The bandwidth of the signal at the output of the modulator (in Hz) is _______.

(PYQ || 2006 NAT|| 2 MARK)

  • 60

Question 8

The amplitude modulated waveform s(t) = A c [1 + Ka m(t)] cos ωct is fed to an ideal envelope detector. The maximum magnitude of Ka m(t) is greater than 1. Which of the following could be the detector output?

(PYQ || 2000 MCQ|| 1 MARK)

  • Ac m(t)

  • A c 2 [1 + K a m(t)] 2

  • [A c |1 + k a m(t))|]

  • A c |1 + K a m(t)| 2

Question 9

A message m(t) band-limited to the frequency f m has a power of P m . The power of the output signal in given figure is-

Screenshot-2025-09-30-003320

(PYQ || 2000 MCQ|| 2 MARK

  • Pmcosθ/2

  • Pm/4

  • Pm sin2θ/4

  • Pmcos2θ/4

Question 10

A DSB-SC signal is to be generated with a carrier frequency f c = 1 MHz using a nonlinear device with the input-output characteristic v 0 = a 0 v i + a 1 v i 3 where a 0 and a 1 are constants. The output of the nonlinear device can be filtered by an appropriate band-pass filter. Let v i = A c i cos(2πf c i t) + m(t) where m(t) is the message signal. Then, the value of f c i (in MHz) is

(PYQ || 2003 MCQ|| 1 MARK)

  • 1.0

  • 0.333

  • 0.5

  • 3.0

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