Download GTU (Gujarat Technological University Ahmedabad) B.Tech/BE (Bachelor of Technology/ Bachelor of Engineering) 2020 Summer 8th Sem 2171004 Wireless Communication Previous Question Paper

Enrolment No.___________

**GUJARAT TECHNOLOGICAL UNIVERSITY**

**BE - SEMESTER? VIII EXAMINATION ? SUMMER 2020**

**Subject Code: 2171004**

**Date:27/10/2020**

**Subject Name: WIRELESS COMMUNICATION**

**Time: 10:30 AM TO 01:00 PM**

**Total Marks: 70**

**Instructions:**

**1. Attempt all questions.**

**2. Make suitable assumptions wherever necessary.**

**3. Figures to the right indicate full marks.**

**MARKS**

**Q.1 (a)**Give full forms of the following acronyms: WLAN, VoIP,

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AMPS.

**(b)**Assume a 1 Amp-hour battery is used on a cellular telephone.

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Also, Assume that the cellular telephone draws 35 mA in idle

mode and 250 mA during a call. Estimate the battery life for

the following cases:

(i) If the user leaves the phone on continually and has one 3-

minute call every day.

(ii) If the user leaves the phone on continually and has one 3-

minute call every 6 hours.

(iii) If the user leaves the phone on continually and has one 3-

minute call every hour.

(iv) Calculate the maximum talk time available on the phone

in this example.

**(c)**Draw a neat sketch of GSM architecture and state the functions

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of the following: BSC, MSC, VLR, HLR, AUC.

**Q.2 (a)**How the cell coverage radius would be affected by the system

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loading for each of the following technologies: AMPS-FDMA,

IS-95 CDMA and IS-54 TDMA.

**(b)**State the reason(s) for the IS-95 CDMA system to use rate ?

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convolution encoding in the forward channel and rate 1/3

convolution encoding in the reverse channel.

**(c)**Briefly discuss security threats in wireless network and suggest

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possible ways of protection.

**OR**

**(c)**Explain the hidden-node problem and exposed-node problem

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in context of mobile ad-hoc networks.

**Q.3 (a)**A Wi-Fi system operates at 1 Mbps. Calculate the data transfer

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time required for a 20 KB file.

**(b)**Compute the rms delay spread for the power delay profile in

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FIG.1.

If BPSK modulation is used, what is the maximum bit rate that

can be sent through the channel without needing an equalizer?

1

P() =1

0 ?s 1 ?s 2 ?s

FIG.1

**(c)**A spectrum of 30 MHz is allocated to a wireless FDD cellular

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system which uses two 25 kHz simplex channels to provide

full duplex voice and control channels. Compute the number

of channels available per cell if a system uses (i) 4-cell reuse

(ii) 7-cell reuse and (iii) 12-cell reuse. If 1 MHz of the allocated

spectrum is dedicated to control channels, determine an

equitable distribution of control channels and voice channels

in each cell for each of the three systems.

**OR**

**Q.3 (a)**A Wi-Fi access point transmits +20 dBm power. The receiver

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sensitivity of a wireless device in the network is specified as -

85 dBm. Determine the maximum allowable path loss,

ignoring other power losses.

**(b)**In a digital cellular system, if fc = 900 MHz and the mobile

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velocity is 60 km/hr, calculate the received carrier frequency if

the mobile is (i) Directed towards the transmitter (ii) going

away from the transmitter and (iii) moving in a direction which

is perpendicular to the direction of the arrival of the transmitted

signal.

**(c)**For given path loss exponent (A) n=4 and (B) n=3, find the

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frequency reuse factor and the cluster size that should be used

for maximum capacity. The signal to interference ratio (S/I) is

required to be at least 15 dB for satisfactory system

performance. There are six co-channel cells in the first tier and

all of them are at the same distance from the mobile. Use

suitable approximations.

**Q.4 (a)**Calculate the change in received signal power in a mobile

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environment with path loss exponent n=4 at two different

points such that the distance of the second point is two times

the distance of the first point. Express your answer in dB.

**(b)**Explain salient features of CDMA.

**04**

**(c)**Consider two different cellular systems that share the

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following characteristics. The frequency bands are 825-845

MHz for uplink and 870-890 MHz for the downlink. A duplex

circuit consists of one 30 kHz channel in each direction. The

systems are distinguished by the reuse factor, which are 4 and

19 respectively. For these systems, (i) Find the maximum

number of simultaneous conversations that can be supported

by a single cell in each system. (ii) Suppose that in each

systems the cluster of cells (4, 19) is duplicated 16 times, find

the number of simultaneous conversations that can be

supported by each system. Assume 8 TDMA slots/frame for

each system.

**OR**

**Q.4 (a)**Calculate the change in received signal power in a free-space

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propagation environment at two different points such that the

distance of the second point is ten times the distance of the first

point. Express your answer in dB.

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**(b)**Discuss the frame efficiency of TDMA with apt equations.

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**(c)**Prove that for a regular hexagonal geometry, the frequency

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reuse ratio and cluster size are related by the relationship Q =

(3N)1/2 , where N= i2 +j2 +ij.

**Q.5 (a)**Explain briefly how a RAKE receiver improves the received

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signal strength.

**(b)**In the following cases, tell whether the two-ray model could

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be applied, and explain why or why not:

Case 1: ht = 35 m, hr = 3 m, d = 250 m.

Case 2: ht = 30 m, hr = 1.5 m, d = 450 m.

**(c)**In a cellular radio operating at 800 MHz, the transmitter and

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the receiver are separated by 500m from each other. The knife-

edge diffraction object between them has a height of 30 m. The

diffraction object is 100 m from transmitter. Find:

(i)

The excess path length

(ii)

The phase difference corresponding to the excess

path length.

(iii)

The Fresnel-Kirchhoff diffraction parameter.

(iv)

The radius of the second Fresnel zone.

**OR**

**Q.5 (a)**List types of diversity techniques and explain any one of them

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in 3-4 sentences.

**(b)**Define following terms:

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(i) Dwell time (ii) Soft handoff (iii) mobile assisted

handoff (iv) Transceiver

**(c)**If a baseband binary message with a bit rate of 100 kbps is

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modulated by an RF carrier using BPSK. Calculate the

following:

(i) Find the range of values required for rms delay spread of

the channel such that the received signal is a flat-fading signal.

(ii) If the modulation carrier frequency is 5.8 GHz, what is the

coherence time of the channel for vehicle speed of 50 km/h.

(iii) For your answer in part (ii), is the channel "fast" or "slow"

fading?

(iv) Given your answer in (ii), how many bits are sent while

the channel appears static?

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This post was last modified on 04 March 2021