Recent questions and answers in Communications

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GATE ECE 2020 | Question: 8
The components in the circuit shown below are ideal. If the op-amp is in positive feedback and the input voltage $V_{i}$ is a sine wave of amplitude $1$ $\text{V}$, the output voltage $V_{o}$ is a non-inverted sine wave of $2$ $\text{V}$ amplitude. an inverted sine ... . a square wave of $5$ $\text{V}$ amplitude. a constant of either $+5$ $\text{V}$ or $-5$ $\text{V}$.

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2020 | Question: 12
A digital communication system transmits a block of $N$ bits. The probability of error in decoding a bit is $\alpha$. The error event of each bit is independent of the error events of the other bits. The received block is declared erroneous if at least one of its bits is decoded ... $\alpha ^{N}$ $1-\alpha ^{N}$ $1-\left ( 1-\alpha \right )^{N}$

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2020 | Question: 22
A binary random variable $X$ takes the value $+2$ or $-2$. The probability $P(X=+2)=\alpha $. The value of $\alpha$ (rounded off to one decimal place), for which the entropy of $X$ is maximum, is __________.

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2020 | Question: 42
For the modulated signal $x\left ( t \right )=m\left ( t \right )\cos\left ( 2\pi f_{c}t \right )$, the message signal $m\left ( t \right )=4\cos\left ( 1000\pi t \right )$ and the carrier frequency $f_{c}$ is $1\: MHz$. The signal $x(t)$ is passed ... $\cos\left ( 920\pi t \right )$ $\cos\left ( 1000\pi t \right )$ $\cos\left ( 540\pi t \right )$

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2020 | Question: 47
$S_{PM}(t)$ and $S_{FM}(t)$ as defined below, are the phase modulated and the frequency modlated waveforms, respectively, corresponding to the message signal $m(t)$ ... $S_{PM}(t)$ and $S_{FM}(t)$ are same, then the value of the ratio $\dfrac{K_{p}}{K_{f}}$ is ________ seconds.

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2020 | Question: 48
In a digital communication system, a symbol $S$ randomly chosen from the set $\left \{ s_{1},s_{2},s_{3},s_{4} \right \}$ is transmitted. It is given that $s_{1}=-3,s_{2}=-1,s_{3}=+1$ and $s_{4}=+2$. The received ... decoding when the transmitted symbol $S=s_{i}$. The index $i$ for which the conditional symbol error probability $P_{i}$ is the highest is ___________.

go_editor asked in Communications Feb 13, 2020

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GATE ECE 2019 | Question: 7
A linear Hamming code is used to map $4-bit$ message to $7-bit$ codewords. The encoder mapping is linear. If the message $0001$ is mapped to the codeword $0000111$, and the message $0011$ is mapped to the codeword $1100110$, then the message $0010$ is mapped to $0010011$ $1100001$ $1111000$ $1111111$

Arjun asked in Communications Feb 12, 2019

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GATE ECE 2019 | Question: 34
A single bit, equally likely to be $0$ and $1$, is to be sent across an additive white Gaussian noise (AWGN) channel with power spectral density $N_{0}/2.$ Binary signaling with $0 \mapsto p(t),$ and $1 \mapsto q(t),$ is used for the transmission, along with ... $E$ would we obtain the $\textbf{same}$ bit-error probability $P_{b}$? $0$ $1$ $2$ $3$

Arjun asked in Communications Feb 12, 2019

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GATE ECE 2019 | Question: 45
Let a random process $Y(t)$ be described as $Y(t)=h(t) \ast X(t)+Z(t),$ where $X(t)$ is a white noise process with power spectral density $S_{x}(f)=5$W/Hz. The filter $h(t)$ ... power spectral density as shown in the figure. The power in $Y(t),$ in watts, is equal to _________ $W$ (rounded off to two decimal places).

Arjun asked in Communications Feb 12, 2019

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GATE ECE 2019 | Question: 46
A voice signal $m(t)$ is in the frequency range $5\:kHz$ to $15\:kHz$. The signal is amplitude-modulated to generated an AM signal $f(t)=A\left(1+m(t)\right)\cos 2\pi f_{c}t,$ where $f_{c}=600\: kHz.$ ... for the encoding. The rate, in Megabits per second (rounded off to $2$ decimal places), of the resulting stream of coded bits is ________ Mbps.

Arjun asked in Communications Feb 12, 2019

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11

GATE ECE 2019 | Question: 55
In the circuit shown, $V_{1}=0$ and $V_{2}=V_{dd}.$ The other relevant parameters are mentioned in the figure. Ignoring the effect of channel length modulation and the body effect, the value of $I_{out}$ is _________ $mA$ (rounded off to $1$ decimal place).

Arjun asked in Communications Feb 12, 2019

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12

GATE ECE 2016 Set 3 | Question: 23
A binary baseband digital communication system employs the signal $p(t) = \begin{cases}\large\frac{1}{\sqrt T_s},&0\leq t\leq T_s \\ \: 0, & \text{otherwise} \end{cases}$ for transmission of bits. The graphical representation of the matched filter output $y(t)$ for this signal will be

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 2 | Question: 22
A speech signal is sampled at $8$ $kHz$ and encoded into $PCM$ format using $8$ bits/sample. The $PCM$ data is transmitted through a baseband channel via $4$-level $PAM$. The minimum bandwidth (in $kHz$) required for transmission is _________

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 2 | Question: 48
An information source generates a binary sequence $\left \{ \alpha _{n} \right \}$. $\alpha _{n}$ can take one of the two possible values $-1$ and $+1$ with equal probability and are statistically independent and identically distributed. This sequence is precoded ... there is a null at $f=\frac{1}{3T}$ in the power spectral density of $X(t)$, then $k$ is ________

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 2 | Question: 49
An ideal band-pass channel $500 \: Hz –2000 \:Hz$ is deployed for communication. A modem is designed to transmit bits at the rate of $4800$ bits/s using $16-QAM$. The roll-off factor of a pulse with a raised cosine spectrum that utilizes the entire frequency band is __________

Milicevic3306 asked in Communications Mar 28, 2018

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16

GATE ECE 2016 Set 2 | Question: 50
Consider a random process $X\left ( t \right )=3V(t)-8,$ where $V(t)$ is a zero mean stationary random process with autocorrelation $R_{v}\left ( \tau \right )=4e^{-5\mid \tau \mid}.$ The power in $X(t)$ is _________

Milicevic3306 asked in Communications Mar 28, 2018

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17

GATE ECE 2016 Set 2 | Question: 51
A binary communication system makes use of the symbols zero and one . There are channel errors. Consider the following events: ... in bits that you obtain when you learn which symbol has been received (while you know that a zero has been transmitted) is _________

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 1 | Question: 21
Consider binary data transmission at a rate of $56$ kbps using baseband binary pulse amplitude modulation (PAM) that is designed to have a raised-cosine spectrum. The transmission bandwidth (in kHz) required for a roll-off factor of $0.25$ is ________

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 1 | Question: 24
Concentric spherical shells of radii $2$m, $4$m, and $8$ m carry uniform surface charge densities of $20 \: nC/m^2$, $ – 4 \: nC/m^2$ and $\rho_s$ respectively. The value of $\rho_s$ ($nC/m^2$) required to ensure that the electric flux density $\overrightarrow D$ = $\overrightarrow 0$ at radius $10$ m is ________

Milicevic3306 asked in Communications Mar 28, 2018

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GATE ECE 2016 Set 1 | Question: 49
A digital communication system uses a repetition code for channel encoding/decoding. During transmission, each bit is repeated three times ($0$ is transmitted as $000$, and $1$ is transmitted as $111$). It is assumed that the source puts ... a $1$. Assuming a binary symmetric channel with crossover probability $p=0.1$, the average probability of error is _________

Milicevic3306 asked in Communications Mar 28, 2018

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21

GATE ECE 2015 Set 3 | Question: 22
The modulation scheme commonly used for transmission from GSM mobile terminals is $4$-QAM $16$-PSK Walsh-Hadamard orthogonal codes Gaussian Minimum Shift Keying (GMSK)

Milicevic3306 asked in Communications Mar 28, 2018

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22

GATE ECE 2015 Set 3 | Question: 36
A three bit pseudo random number generator is shown. Initially the value of output $Y\equiv Y_{2} Y_{1} Y_{0}$ is set to $111.$ The value of output $Y$ after three clock cycles is $000$ $001$ $010$ $100$

Milicevic3306 asked in Communications Mar 28, 2018

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23

GATE ECE 2015 Set 2 | Question: 51
Consider a binary, digital communication system which uses pulses $g(t)$ and $-g(t)$ for transmitting bits over an AWGN channel. If the receiver uses a matched filter, which one of the following pulses will give the minimum probability of bit error?

Milicevic3306 asked in Communications Mar 28, 2018

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24

GATE ECE 2015 Set 1 | Question: 50
The transmitted signal in a GSM system is of $200$ kHz bandwidth and $8$ users share a common bandwidth using TDMA. If at a given time $12$ users are talking in a cell, the total bandwidth of the signal received by the base station of the cell will be at least (in kHz) ___________.

Milicevic3306 asked in Communications Mar 28, 2018

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25

GATE ECE 2014 Set 4 | Question: 23
In a double side-band (DSB) full carrier AM transmission system, if the modulation index is doubled, then the ratio of total sideband power to the carrier power increases by a factor of __________.

Milicevic3306 asked in Communications Mar 26, 2018

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26

GATE ECE 2014 Set 4 | Question: 51
An M-level PSK modulation scheme is used to transmit independent binary digits over a band-pass channel with bandwidth $100$ kHz. The bit rate is $200$ kbps and the system characteristic is a raised-cosine spectrum with $100 \%$ excess bandwidth. The minimum value of M is __________

Milicevic3306 asked in Communications Mar 26, 2018

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27

GATE ECE 2014 Set 3 | Question: 19
A modulated signal is $y(t)= m(t)\cos(40000\pi t)$, where the baseband signal $m(t)$ has frequency components less than $5$ KHz only. The minimum required rate (in KHz) at which $y(t)$ should be sampled to recover $m(t)$ is _______.

Milicevic3306 asked in Communications Mar 26, 2018

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28

GATE ECE 2014 Set 3 | Question: 23
Consider an $FM$ signal $f(t)= \cos\left [ 2\pi f_{c} t+\beta _{1}\sin2\pi f_{1}t+\beta _{2}\sin2\pi f_{2}t\right ].$ The maximum deviation of the instantaneous frequency from the carrier frequency $f_{c}$ is $\beta _{1}f_{1}+\beta _{2}f_{2}$ $\beta _{1}f_{2}+\beta _{2}f_{1}$ $\beta _{1}+\beta _{2}$ $f _{1}+f _{2}$

Milicevic3306 asked in Communications Mar 26, 2018

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29

GATE ECE 2014 Set 3 | Question: 49
Let $X(t)$ be a wide sense stationary $(WSS)$ random process with power spectral density $S_{X}(f).$ If $Y(t)$ is the process defined as $Y(t)= X(2t-1)$, the power spectral density $S_{Y}( f )$ ... $S_{Y}( f )= \frac{1}{2}S_{X}\left ( \frac{f}{2} \right )e^{-j2\pi f }$

Milicevic3306 asked in Communications Mar 26, 2018

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30

GATE ECE 2014 Set 3 | Question: 50
A real band-limited random process $X(t)$ ... -pass filter of unity gain with centre frequency of $8$ $kHz$ and band-width of $2$ kHz. The output power (in Watts) is ___________.

Milicevic3306 asked in Communications Mar 26, 2018

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31

GATE ECE 2014 Set 3 | Question: 51
In a $PCM$ system, the signal $m(t)=\left \{\sin(100\pi t)+\cos(100\pi t) \right \}V$ is sampled at the Nyquist rate. The samples are processed by a uniform quantizer with step size $0.75$ $V$. The minimum data rate of the $PCM$ system in bits per second is ______.

Milicevic3306 asked in Communications Mar 26, 2018

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32

GATE ECE 2014 Set 2 | Question: 23
Consider sinusoidal modulation in an AM system. Assuming no overmodulation, the modulation index ($\mu$) when the maximum and minimum values of the envelope, respectively, are $3$ V and $1$ V, is ___________.

Milicevic3306 asked in Communications Mar 26, 2018

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33

GATE ECE 2014 Set 2 | Question: 40
In the circuit shown, choose the correct timing diagram of the output (y) from the given waveforms $W1$, $W2$, $W3$ and $W4$. $W1$ $W2$ $W3$ $W4$

Milicevic3306 asked in Communications Mar 26, 2018

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34

GATE ECE 2014 Set 2 | Question: 44
The input-output relationship of a causal stable LTI system is given as $y[n] = \alpha \: y[n-1] + \beta \: x[n]$. If the impulse response $h[n]$ of this system satisfies the condition $\sum_{n=0}^{\infty}h[n]= 2$, the relationship between $\alpha$ and $\beta$ is $\alpha = 1-\beta /2$ $\alpha = 1+\beta /2$ $\alpha = 2\beta$ $\alpha = -2\beta$

Milicevic3306 asked in Communications Mar 26, 2018

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35

GATE ECE 2014 Set 2 | Question: 50
Coherent orthogonal binary FSK modulation is used to transmit two equiprobable symbol waveforms $s_{1}( t )= \alpha \cos 2\pi f_1 t$, and $s_2(t) = \alpha \cos 2 \pi f_2 t$ where $\alpha = 4$ ... error probability for a data rate of $500$ kbps is $Q(2)$ $Q(2\sqrt{2})$ $Q(4)$ $Q(4\sqrt{2})$

Milicevic3306 asked in Communications Mar 26, 2018

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36

GATE ECE 2014 Set 2 | Question: 51
The power spectral density of a real stationary random process $X(t)$ is given by $ S_X (f) = \begin{cases} \frac{1}{W}, & \mid f \mid \leq W \\ 0, & \mid f \mid > W \end{cases}$ The value of the expectation $E \left [ \pi X(t)X\left ( t-\frac{1}{4W} \right ) \right ]$ is __________.

Milicevic3306 asked in Communications Mar 26, 2018

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GATE ECE 2014 Set 1 | Question: 22
In a code-division multiple access $(\text{CDMA})$ system with $N = 8$ chips, the maximum number of users who can be assigned mutually orthogonal signature sequences is ________.

Milicevic3306 asked in Communications Mar 26, 2018

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GATE ECE 2014 Set 1 | Question: 51
Let $Q(\sqrt{\gamma})$ be the BER of a BPSK system over an AWGN channel with two-sided noise power spectral density $N_{0}/2.$ The parameter $\gamma$ is a function of bit energy and noise power spectral density. A system with two independent and identical ... both the channels. If the BER of this system is $Q( b \sqrt{\gamma})$ then the value of $b$ is _____.

Milicevic3306 asked in Communications Mar 26, 2018

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GATE ECE 2014 Set 1 | Question: 52
A fair coin is tossed repeatedly until a ‘Head’ appears for the first time. Let $L$ be the number of tosses to get this first ‘Head’. The entropy $H(L)$ in bits is _____.

Milicevic3306 asked in Communications Mar 26, 2018

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40

GATE ECE 2013 | Question: 49
Bits $1$ and $0$ are transmitted with equal probability. At the receiver, the pdf of the respective received signals for both bits are as shown below. The optimum threshold to achieve minimum bit error rate $(BER)$ is $\frac{1}{2} \\$ $\frac{4}{5} \\$ $1\\$ $\frac{3}{2} $

Milicevic3306 asked in Communications Mar 26, 2018

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