GUJARAT TECHNOLOGICAL UNIVERSITY
BE- SEMESTER-VII (NEW) EXAMINATION - WINTER 2020
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Subject Code:2171004 Date:25/01/2021Subject Name:Wireless Communication
Time:10:30 AM TO 12:30 PM Total Marks: 56
Instructions:
1. Attempt any FOUR questions out of EIGHT questions.
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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, VolIP, 03
AMPS.
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(b) Assume a 1 Amp-hour battery is used on a cellular telephone. 04
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:
(1) If the user leaves the phone on continually and has one 3-
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minute call every day.(i1) If the user leaves the phone on continually and has one 3-
minute call every 6 hours.
(ii1) If the user leaves the phone on continually and has one 3-
minute call every hour.
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(iv) Calculate the maximum talk time available on the phonein this example.
(¢) Draw aneat sketch of GSM architecture.and state the functions 07
of the following: BSC, MSC, VLR; HLR, AUC.
Q.2 (a) How the cell coverage radius.would be affected by the system 03
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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 %2 04
convolution encoding in the forward channel and rate 1/3
convolution encoding in the reverse channel.
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(c) Briefly discuss security threats in wireless network and suggest 07
possible ways of protection.
Q.3 (a) A Wi-Fisystem operates at | Mbps. Calculate the data transfer 03
time required for a 20 KB file.
(b) Compute the rms delay spread for the power delay profile in 04
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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?
O us 1pus 2 s
FIG.1
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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
(i1) 7-cell reuse and (iii) 12-cell reuse. If 1 MHz of the allocated
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spectrum is dedicated to control channels, determine anequitable distribution of control channels and voice channels
in each cell for each of the three systems.
Q.4 (a) A Wi-Fiaccess point transmits +20 dBm power. The receiver 03
sensitivity of a wireless device in the network is specified as -
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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 04
velocity is 60 km/hr, calculate the received carrier frequency if
the mobile is (i) Directed towards the transmitter (ii) going
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away from the transmitter and (iii) moving in a direction whichis perpendicular to the direction of the arrival of the transmitted
signal.
(¢) For given path loss exponent (A) n=4 and (B) n=3, find the 07
frequency reuse factor and the cluster size that should be used
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for maximum capacity. The signal to interference ratio (S/I) isrequired 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.
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Q.5 (a) Calculate the change in received signal power in a mobile 03
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
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(¢) Consider two different cellular systems that share the 07
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
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19 respectively. For these systems, (i) Find the maximumnumber 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
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supported by each system. Assume 8 TDMA slots/frame foreach system.
Q.6 (a) Calculate the change in received signal power in a free-space 03
propagation environment at two different points such that the
distance of the second point is ten times the distance of the first
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point. Express your answer in dB.(b) Discuss the frame efficiency of TDMA with apt equations. 04
(¢) Prove that for a regular hexagonal geometry, the frequency 07
reuse ratio and cluster size are related by the relationship Q =
(B3N)"2, where N= i +j2 +ij.
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Q.7 (a) Explain briefly how a RAKE receiver improves the received 03
signal strength.
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be applied, and explain why or why not:
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Case 1: hy=35m, h=3m,d =250 m.Case 2: hy=30m, hy=1.5m, d =450 m.
(¢) In a cellular radio operating at 800 MHz, the transmitter and 07
the receiver are separated by 500m from each other. The knife-
edge diffraction object between them has a height of 30 m. The
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diffraction object is 100 m from transmitter. Find:(1) The excess path length
(i1) The phase difference corresponding to the excess
path length.
(ii1)) The Fresnel-Kirchhoff diffraction parameter.
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(iv) The radius of the second Fresnel zone. Q.8 (a) Listtypes of diversity techniques and explain any one of them 03
in 3-4 sentences.
(b) Define following terms: 04
(1) Dwell time (i1) Soft handoff (ii1) mobile assisted
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handoff (iv) Transceiver (¢) If a baseband binary message with a bit rate of 100 kbps is 07
modulated by an RF carrier using BPSK. Calculate the
following:
(1) Find the range of values required for rms delay spread of
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the channel such that the received signal is a flat-fading signal.(i1) If the modulation carrier frequency is 5.8 GHz, what is the
coherence time of the channel for vehicle speed of 50 km/h.
(i11) For your answer in part (i1), is the channel “fast” or “slow”
fading?
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(iv) Given your answer in (i), how many bits are sent whilethe channel appears static?
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