BITS PILANI- K.K. BIRLA GOA-CAMPUS
Semester II, 2016-17
Course: EEE F 431, MOBILE TELECOMMUNICATION NETWORKS
Compree, Regular [Closed book]
Date: 15/05/2017
Wt., 40%, MM: 120
Duration 3 Hrs
Instructions:1. Solve (showing all steps) all the Questions in Main Answer Sheet and write only the final
answers in the space provided in A4 Answer sheet. 2. Overwritten answers will not be Re-evaluated. 3.
Assume/Use suitable data/constants where-ever required and mention the assumptions clearly.
Q1. LTE downlink is based on OFDM. Consider the normal subcarrier spacing Δf= 15 kHz, and assume the
transmission bandwidth is 20 MHz, corresponding to FFT size 2048. LTE defines two types of cyclic prefix
(CP): the normal CP with 144 samples and the extended CP with 512 samples. Different CPs are needed for
different application scenarios.
[12]
(a) What is the maximum possible symbol rate on each subcarrier?
(b) What is the OFDM symbol duration for LTE?
(c) What is the sampling time is Ts for OFDM symbols?
(d) Why do we need CP in OFDM?
(e) What is the power loss with normal CP and extended CP, respectively?
(f) If the delay spread in the operating environment is 7µs, which type of CP should we use?
Q2. Consider a 4-branch diversity system where each branch received an independent Rayleigh fading signal.
Given that the instantaneous received power over the 4-branches respectively are -92 dBm, -91 dBm, -94 dBm,
and -95 dBm. Assuming that interference is negligible and background noise is measured at -99 dBm.
[12]
(a) What is the SNR at the receiver detector if selection combining is used
(b) What is the SNR at the receiver detector if maximal ratio combining is used
(c) What is the SNR at the receiver detector if equal gain combining is used
(d) How many branches are needed to provide an outage probability of 4% using selection combining given that
the average received power on all branches in dBm is -92 dBm.
Q3. A cellular system has a total RF bandwidth of 25 MHz and simplex channel spacing of 60 kHz. The
system is required to cover a total area of 7200 km2.
[12]
(a) Calculate the number of traffic channels/cell, if the cluster size is 18.
(b) Find the reuse distance for part (a), if the area of each cell is 16 km2.
(c) Determine the total number of cells needed to cover entire area.
(d) How many calls can be simultaneously processed by each cell if 16 users can share each channel?
Q4. Assume that extensive measurements have been done in an area and a multipath channel model is
constructed whose power-delay profile consists of 4 paths arriving 0.1µs apart. First path arrives and assumed
to have unit power (0dB), and, subsequent paths have 3dB reduction in power as compared to the previous one.
Assuming 90% correlation is required for flat fading. Draw the power delay profile and find the largest
transmitted signal bandwidth that would go through this channel without experiencing frequency selective
fading?
[12]
Q5. Suppose that, by law, a service operator is not allowed to radiate more than 30 watts of power. Using the
two-ray model, what antenna height is required for a service radius of 1 km? 10 km? Assume that the receiver
sensitivity is -100 dBm and unity gain for transmitter and receiver antenna.
[12]
Q6. (1). Describe all the physical circumstances that relate to a stationary transmitter and a moving receiver
such that the Doppler shift at the receiver is equal to (a) 0 Hz; (b) fdmax; (c) -fdmax; and (d) fdmax /2, where fdmax is
the maximum Doppler shift.
[8]
(ii). What would be the maximum Doppler shift when operating in the cellular band (800MHz)? What range of
symbol rates (symbols/second) can go through this channel experiencing slow fading?
[4]
Q7. Consider the filter h(t) shown below in Fig Q7, (a) Determine the signal pulse g(t) to which this filter is
matched and sketch it as a function of time. (b) If the signal pulse g(t) obtained in part (a) is input to the filter
h(t), plot the output as a function of time. (c) What is the peak value of the matched filter output?
[12]
Fig. Q7
Fig. Q9
Q8. Assume a 4-cell reuse pattern. A total of 96 channels are available for each cluster. In a specific cluster the
traffic load is not uniformly distributed over the cells. To cater for the different requirements, the available
channels are allocated as follows: {42, 30, 18, 6} for the cells in the cluster.
[12]
(a) Given a blocking probability of 2%, what traffic loads can each cell accommodate?
(b) Due to interference problems, 120 degrees sectorisation is introduced into the system. What traffic loads can
each cell now accommodate?
(c) Assume that we can categorize the traffic patterns into 3 classes. Short message of duration 20 sec/call,
normal calls of duration 4 min/call and teenage gossiping of duration 1.5 hours/call. Let there be 5 normal calls
and 10 short message for each teenager calling. How many calls per hour can be accommodated on average in
the cluster when sectorisation is not used?
Q9. A mobile communication system operates at 910 MHz carrier frequency with Pt = 12 W, Gt = 10 dB, Gr = 5
dB and L =2. The diffraction region is shown in Fig Q9. (Note: 1 m =3.28 ft, 1 mile = 1.61 Km)
[12]
(a) Compute the received power for the knife edge geometry shown.
(b) Compute free space received power without the obstruction.
(c) What is path loss due to diffraction.
Q10. (1). Assume a binary bit stream is to be modulated on an RF carrier. If the base band bit stream has a data
rate of 1 Megabit per sec, then : (a) What is the first-zero crossing bandwidth of the RF spectrum if simple
rectangular pulses are used, assuming BPSK is used? (b) What is the bandwidth of the RF spectrum if Raised
cosine pubes with roll-off factor 0.25 are used, assuming BPSK is used? (c) If GMSK modulation is to be
generated and a 3 dB bandwidth of 500 kHz is used for the Gaussian low pass filter, what is the proper choice
for the frequency deviation Δf .
[6]
(2). The GMSK modulation is used in the GSM system with a channel bandwidth of 200 kHz and a data rate of
270.8 kbps. Calculate (a) the frequency shift between binary 1 and binary 0, (b) the transmitted frequencies if
the carrier frequency is 900 MHz, and (c) the bandwidth efficiency in bps/Hz.
[3]
(3). The BPSK modulation is used in a channel that adds white noise with single-sided PSD N0 = 10-10 W/HZ.
Calculate the amplitude A of the carrier signal to give Pe = 10-6 for a data rate of 100 kbps.
[3]
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