# Communication Engineering (Test 8)

## Tancet Anna University : Ece Electronics And Communication Engineering

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Communication Engineering
| Signals and Systems | | Communications | | Networks, Signals and Systems |
Q.1
In a GSM system, 8 channels can co-exist in 200 kHz bandwidth using TDMA. A GSM based cellular operator is allocated 5 MHz bandwidth. Assuming a frequency reuse factor of 1/5, i.e. a five-cell repeat pattern, the maximum number of simultaneous channels that can exist in one cell is
A. 200
B. 40
C. 25
D. 5
Explaination / Solution:

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Q.2
For a Hertz dipole antenna, the half power beam width (HPBW) in the E-plane is
A. 3600
B. 1800
C. 900
D. 450
Explaination / Solution:

The beam-width of Hertizian dipole is 1800 and its half power beam-width is 900

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Q.3
Noise with double-sided power spectral density on K over all frequencies is passed through a RC low pass filter with 3 dB cut-off frequency of fc. The noise power at the filter output is
A. K
B. Kfc
C. kπfc
D.

Explaination / Solution:

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Q.4
Suppose that the modulating signal is  and the carrier signal is , which one of the following is a conventional AM signal without over-modulation
A. x(t) = AC m(t) cos (2𝜋fC t)
B. x(t) = AC [1 + m(t)] cos (2𝜋fC t)
C. x(t) = AC cos (2𝜋fC t) +AC /4 m(t) cos (2𝜋fC t)
D. x(t) Acos (2𝜋fC tcos (2𝜋fm t)+Asin (2𝜋fm tsin (2𝜋fC t)
Explaination / Solution:
No Explaination.

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Q.5
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 inter symbol interference zero?
A.
B.
C.
D.
Explaination / Solution:

For ISI free pulse,
If P(t) is having spectrum P(f)
Then  = constant

Rs = 2KSpa

Thin condition is met by pulse given in option B.

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Q.6
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.7
The Column-I lists the attributes and the Column-II lists the modulation systems. Match the attribute to the modulation system that best meets it Column-I P. Power efficient transmission of signals Q. Most bandwidth efficient transmission of voice signals R. Simplest receiver structure S. Bandwidth efficient transmission of signals with Significant dc component Column-II 1. Conventional AM 2. FM 3. VSB 4. SSB-SC
A. P-4;Q-2;R-1;S-3
B. P-2;Q-4;R-1;S-3
C. P-3;Q-2;R-1;S-4
D. P-2;Q-4;R-3;S-1
Explaination / Solution:

In FM the amplitude is constant and power is efficient transmitted. No variation in power. There is most bandwidth efficient transmission in SSB- SC. because we transmit only one side band. Simple Diode in Non linear region ( Square law ) is used in conventional AM that is simplest receiver structure. In VSB dc. component exists.

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Q.8
An antenna pointing in a certain direction has a noise temperature of 50K. The ambient temperature is 290 K. The antenna is connected to a pre-amplifier that has a noise figure of 2 dB and an available gain of 40 dB over an effective bandwidth of 12 MHz. The effective input noise temperature Te for the amplifier and the noise power Pao at the output of the preamplifier, respectively, are
A. Te = 169.36K and Pao = 3.73 × 10-10W
B. Te = 1.36K and Pao = 3.73 × 10-10W
C. Te = 182.5K and Pao = 3.83 × 10-10W
D. Te = 160.62K and Pao = 4.6 × 10-10W
Explaination / Solution:
No Explaination.

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Q.9
The response h(t) of a linear time invariant system to an impulse δ(t) , under initially relaxed condition is  The response of this system for a unit step input u(t) is
A.
B.
C.
D.
Explaination / Solution:

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Q.10
The state variable formulation of a system is given as

The response y(t) to the unit step input is
A.
B.
C.
D.