# Topic: Communications (Test 2)

Topic: Communications
Q.1
In delta modulation, the slope overload distortion can be reduced by
A. decreasing the step size
B. decreasing the granular noise
C. decreasing the sampling rate
D. increasing the step size
Explaination / Solution:
No Explaination.

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Q.2
In a Direct Sequence CDMA system the chip rate is 1.2288 × 106 chips per second. If the processing gain is desired to be AT LEAST 100, the data rate
A.  must be less than or equal to 12.288 × 10bits per sec
B.  must be greater than 12.288 × 10bits per sec
C.  must be exactly equal to 12.288 × 10bits per sec
D.  can take any value less than 122.88 × 10bits per sec
Explaination / Solution:

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Q.3
A load of 50 Ω is connected in shunt in a 2-wire transmission line of Z0 = 50Ω as shown in the figure. The 2-port scattering parameter matrix (s-matrix) of the shunt element is

A.
B.
C.
D.
Explaination / Solution:

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Q.4
Two 4-array signal constellations are shown. It is given that φ1 and φ2 constitute an orthonormal basis for the two constellation. Assume that the four symbols in both the constellations are equiprobable. Let N0/2 denote the power spectral density of white Gaussian noise.
If these constellations are used for digital communications over an AWGN channel, then which of the following statements is true ?
A. Probability of symbol error for Constellation 1 is lower
B. Probability of symbol error for Constellation 1 is higher
C. Probability of symbol error is equal for both the constellations
D. The value of N will determine which of the constellations has a lower probability of symbol error
Explaination / Solution:
No Explaination.

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Q.5
Consider a system whose input x and output y are related by the equation where h(t) is shown in the graph.

Which of the following four properties are possessed by the system ?
BIBO : Bounded input gives a bounded output.
Causal : The system is causal,
LP : The system is low pass.
LTI : The system is linear and time-invariant.
A. Causal, LP
B. BIBO, LTI
C. BIBO, Causal, LTI
D. LP, LTI
Explaination / Solution:

Here h(t) ! 0 for t < 0. Thus system is non causal. Again any bounded input x(t) gives bounded output y(t). Thus it is BIBO stable.

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Q.6
A communication channel with AWGN operating at a signal to noise ration SNR >> 1 and bandwidth B has capacity C1. If the SNR is doubled keeping constant, the resulting capacity C2 is given by
A.
B.
C.
D.
Explaination / Solution:

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Q.7
The maximum value of ᶿ until which the approximation sin ᶿ=ᶿ holds to within 10% error is
A. 10º
B. 18º
C. 50º
D. 90º
Explaination / Solution:

Here, as we know

but for 10% error, we can check option (B) first

so, the error is more than 10%. Hence, for error less than 10%, ᶿ = 18º can have the approximation

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Q.8
Consider a Binary Symmetric Channel (BSC) with probability of error being p. To transmit a bit, say 1, we transmit a sequence of three 1s. The receiver will interpret the received sequence to represent 1 if at least two bits are 1. The probability that the transmitted bit will be received in error is
A.

p3 3p2(1 - p)

B. p3
C. (1 - p3)
D. pp2(1 - p)
Explaination / Solution:
No Explaination.

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Q.9
The signal  is
A. FM only
B. AM only
C. both AM and FM
D. neither AM nor FM
Explaination / Solution:
No Explaination.

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Q.10
In the following network, the switch is closed at t = 0- and the sampling starts from t = 0. The sampling frequency is 10 Hz.

The expression and the region of convergence of the z -transform of the sampled signal are
A.
B.
C.
D.