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2003 Course Advanced Communication Systems

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Total No. of Questions : 12] [Total No. of Pages : 5 [3764] - 233 B.E. (E & TC) ADVANCED COMMUNICATION SYSTEMS (404225) (2003 Course) P1477 Time : 3 Hours] [Max. Marks : 100 Instructions: 1) Answers to the two sections should be written in separate answer books. 2) 3) 4) 5) Neat diagrams must be drawn wherever necessary. Figures to the right indicate full marks. Assume suitable data, if necessary. All quetions are compulsary SECTION - I Q1) a) b) c) Q2) a) b) c) d) Q3) a) b) With the help of appropriate diagram describe the operational principle of WDM. [8] Briefly explain the working of Optoisolator. [4] Determine spectral bandwidth of optical fiber system, if usable spectral band = 105nM and operating center wavelength is 1550nM. [4] OR With the help of schematic diagram explain the working of 2*2 coupler, employing all glass optical fibre. Explain the purpose of tapered region in this coupler. [4] State the mathematical relation between input power, throughput power, coupled power and return loss for 2*2 coupler. State the relation of input with other outputs, considering coupling coefficients and axial distance. [4] With the help of neat diagram briefly explain Tap coupler. [2] Consider a commercially available 8*8 star coupler, made from 2*2 coupler, elements of 3 db attenuation. About 6% power is lost in each element. Find the excess loss, splitting ratio and total loss. [6] Explain how tunable light sources are useful in wavelength multiplexing. [6] Change of refractive index for a DFB laser is 0.75%. Determine the tuning rage (tune). If 60 channels are operating in this range, determine spectral width (SIGNAL) at 2.54Gbps. [6] P.T.O. c) Q4) a) Prepare rise time budget for an optical fiber link operating on Long Haul network with following data. i) Source rise time = 1ns. ii) Detector rise time = 1.1ns. iii) Intramodal fiber rise time = 0.1ns/km. iv) Intermodal fiber rise time = 0.001 ns v) Optical link length proposed: 1) First system = 10km. 2) Second system = 50km. [6] Estimate data rate using RZ format for both the systems. OR DFB laser transmitter is proposed to be used in 565 Mbps optical fiber trunk system operating at 1550 nM. Two types of receivers, one with Pi-N type and the other with APD type are planned. Using the data given in the Table -1 prepare optical link power budgets for systems using both the types of detectors. Determine if any excess margin is available. If yes, determine the extension of the link length. If No, Appropriately reduce the link lengths. Present the iterations and give final result graphically along with budget calculations. [18] Table - I SR.NO. 1 2 3 4 5 6 7 8 9 Q5) a) DESCRIPTION Transmitter output,dbm Receiver sensitivity,dbm Dispersion equalization penalty in dBs System safety margin dBs Link length in KMs Link loss in dB/km Attenuation per connectors dB Extinction ratio penalty Splice loss per splice per km TYPE OF RECEIVER WITH WITH P-i-N APD -3 -5 -36 -45 4 4 5 5 50 70 0.35 0.25 1 1 2 2 0.1 0.1 Explain in details any two of the following for orbital satellites: i) Communication Subsystems. ii) Power Subsystem. iii) Telemetry, Tracking and command systems. [8] b) A satellite is in circular orbit. The altitude of the satellite s orbit above the surface of earth is 1400km. [3764] - 233 -2- i) Q6) a) b) c) Q7) a) b) Q8) ) b) [3764] - Determine the centripetal and centrifugal accelerations. In Mts/sec2, acting on the satellite. ii) What is the velocity, in Mts/sec in, of the satellite in orbit? iii) Determine the orbital period, in hrs, min and seconds of the satellite in the orbit. [8] OR Explain with mathematical expressions how orbital height r is related with orbital period T [4] Explain briefly various look angles for satellite earth station. [4] A geostationary satellite working in Ku band is used to provide communication in United States of America. The Antennas on the satellite have bandwidth of 6 degrees in E-W direction and 3 degrees N-S direction. Separate Antennas are used for transmitting at 11 GHz and receiving at 14 GHz.Determine the dimensions and Gains of transmitting and receiving [8] antennas in N-S and E-W directions. Write short notes on any two of following. i) Preemphasis and de-emphasis in FM systems. ii) Psophometric weighting in communication system. iii) Energy dispersal in FM/FDM satellite communication systems. [10] A television transmissions in NTSC format uses C Band transponder of 36 MHz bandwidth. The maximum video frequency is 4.2 MHz. The system uses frequency modulation with standard signal processing techniques. At the receiving earth station clear Air C/N ratio possible is 15dBs. Determine video S/N ratio achievable at the earth station output. If downlink attenuation doubles due to worst atmospheric condition, estimate the new video S/N ratio, under such adverse conditions.Assume preemphasis improvement factor of 9.0dB and subjective video improvement factor of 8.0dB in the calculations. [8] OR With help of neat diagrams briefly explain generation and coherent detection for QPSK modulation realized as a combination of two BPSK streams. [5] With the help of neat diagram briefly explain Raised Cosine Filtering (RCF) based signal shaping techniques used in digital satellite communication systems. Illustrate the trade-off between usable bit rate [5] and rolloff factor for RCF signal shaping. 233 -3- c) Adigital satellite system Ku band uses QPSK modulation with raised cosine signal shaping with roll off factor 0.25. The computer data at 8.0 Mbps is transferred through this link. Satellite link provides Eb/No of 10.16 dBs. Determine the following:i) Symbol transmission rate, occupied bandwidth and C/N ratio in dBs for the digital satellite link. ii) Effective Bit error rate that the system can provide. assume erfc (4.07 ) = 0.001*10-6 [8] Q9) a) Explain the meaning of Noise temperature of receiver. With the help of overall noise model of the receiver, derive the expression for effective system noise temperature Ts for the receiving earth station. [6] b) A satellite in geostationary orbit is at a distance of 39000km from earth station. The required flux density at the satellite to saturate the transponder at frequency of 14.3 GHz is stipulated as -90.0 dBw/mts2. The transmitting earth station provides gain of 52.0 dBs. Determine EIRP of earth station in dBW, minimum output power of earth station transmitter in watts to saturate the transponder. Also determine the diameter for parabolic antenna for the earth station, assume antenna aperture efficiency 60%. [10] OR Q10) ) Explain the significance for any two of the following: i) G/T ratio for the earth station. ii) Antenna noise temperature for the earth station antenna. iii) C/N ratio in satellite communications. [6] b) A geostationary satellite carries a transponder with a 20 watt transmitter at 4.0GHz. The output back off in the satellite is set at 3.0dB. Transmitting Antenna of the satellite provides gain of 1000. Determine the following of the earth station terminal, located at the center of the coverage zone, at a distance 38500km. i) The flux density in dB W/mts2, at the earth station. ii) The power received in dBW by antenna with a gain of 8000. iii) EIRP of satellite in dBW. [10] Q11) ) What does the acronym VSAT stand for and explain how does it justify the underlying technology? What is the typical range, in meters, of the aperture diameter for a VSAT operating with Ku band satellite? [6] b) Explain what MESH and STAR architectures are in a VAST network. Give two advantages and disadvantages of MESH and STAR architectures. [6] c) Explain with neat diagram the TDMA frame structure. [4] [3764] - 233 -4- OR Q12)a) Describe CDMA system and discuss its advantages over other multiple access systems. [8] b) A36 MHz C-Band transponder has output spectrum for downlink in the range 3705-3742MHz. It carries two unmodulated carriers at 3718 and 3730 MHz with equal magnitude at the input to HPA. i) Show that intermodulation frequency will be within transponder bandwidth. ii) If carriers are modulated and modulated signal has a bandwidth of 8 MHz around exact centre frequency, find the intermodulation products. [8] ***** [3764] - 233 -5-

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