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2003 Course Advanced Communication Systems

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Total No. of Questions : 12] [Total No. of Pages :5 [3864] - 273 B.E. (E & TC) ADVANCED COMMUNICATION SYSTEMS (404225) (2003 Course) P1154 Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answers to the two sections should be written in separate books. 2) 3) 4) 5) Neat diagrams must be drawn wherever necessary. Figures to the right indicate full marks. Assume suitable data, if necessary. Use of logarithmic tables, slide rule, electronic pocket calculator is allowed. SECTION - I Q1) a) b) c) With the help of appropriate schematic diagram describe operational principle of WDM. [8] Explain in brief WDM standards. [4] Find spectral bandwidth of optical fiber system when usable spectral [4] band, =100 nm and centre wavelength is 1420 nm. OR Q2) a) With help of block schematic explain basic star coupler for splitting or combining optical powers. [6] b) With help of appropriate mathematical expressions, explain in brief following terms. i) ii) iii) iv) Splitting ratio. Excess loss. Insertion loss. Return. [6] P.T.O. c) For a 2 x 2 coupler, input power level is 300 microwatts. An output power at port 1 is, 100 microwatts, at port 2 is 95 microwatts and at port 3 is 5 nanowatts. Find: i) Coupling ratio (in %). ii) Excess loss. iii) Insertion loss. iv) Return loss. [4] Q3) a) b) Explain optical link power budget. [4] A Long Haul optical fiber link is operating at 512 Mbps, with bit error rate 10-9. A 1550 nm Laser Diode Transmitter launches optical power of + 3dBm. Two receivers with following sensitivity figures are available for installation on this link. i) RecA ; -32 dBm. ii) RecB; -51 dBm Connectors at receiver and transmitters have a loss of 1 dB per connector. Splice loss is estimated to be 0.1 dB per splice per km. The optical link available has loss coefficient of 0.35 dB/km. A jumper cable, if desired to be used, is available in 1 mtr link length and has a loss coefficient of 0.0065 dB per mtr. However connectors need to be connected to this jumper. The system margin allowed is 5 dB and extinction ratio penalty is 2 dB. Find out optimum link length for both receivers. Prepare a power budget & represent it graphically. [14] OR Q4) Write short notes on: a) b) Q5) a) b) Optical power meter. OTDR. [6] [12] Explain what the terms centrifugal and centripetal mean with regard to a satellite in orbit around the earth. Support your answer with the help of suitable diagram & mathematical equations. [4] What are the differences, between a geosynchronous satellite and a geostationary satellite orbit? What is the period of a geostationary satellite? What is the name given to this orbital period? What is the velocity in km/ sec of a geostationary satellite in its orbit? [4] [3864] - 273 -2- c) Satellite is in a 322 km high circular orbit. Determine: i) The orbital angular velocity in radians per second. ii) The orbital period in minutes. iii) The orbital velocity in meters per second. [8] OR Q6) a) b) Describe in brief five major subsystems required on a satellite. [8] A geostationary satellite provides service to a region which can be covered by the beam of an antenna on the satellite with a beam width of 1.8o. The satellite carries transponders for Ku band and Ka band, with separate antennas for transmit and receive. For center frequencies of 14.0/11.5 GHz and 30.0/20.0 GHz. Determine the diameters of the four antennas on the satellite. [8] SECTION - II Q7) a) b) Q8) a) b) Define and explain the following terms. [8] i) Pre - emphasis and De - emphasis. ii) Sampling and Quantizing. A standard NTSC signal has a base band video bandwidth of 4.2 MHz and is transmitted over the satellite link in an RF bandwidth of 30 MHz using Frequency Modulation and standard Pre - emphasis and De emphasis. At the receiving earth station the C/N ratio in clear sky conditions is 15 dB. Calculate the base band Signal to Noise ratio for the video signal. Assume a de - emphasis improvement of 9 dB and a subjective improvement factor of 8 dB in the base band signal to noise [8] ratio. OR Define and explain the following terms. [8] i) Bit and Symbol Error rate. ii) Compression and Expansion. A Single Channel Per Carrier - Frequency Modulation satellite link has an RF channel bandwidth of 36 kHz and a base band maximum frequency of 4 kHz. De - emphasis provides a subjective improvement in base band S/N ratio of 8 dB. Calculate the base band S/N ratio for the voice channel for a receiver C/N ratio of 15 dB. If the FM demodulator has an FM threshold at 8 dB, what is the link margin for this system? [8] [3864] - 273 -3- Q9) a) b) c) Q10) Derive step by step, the power received by an earth station Pr, from a satellite transmitter in terms of Pt - power transmitted, Gr, Gt - Gain of transmitting & receiving antenna, respectively, losses associated with transmitting & receiving antenna & attenuation in atmosphere. [8] Explain the terms Noise temperature & Noise figure. State the relation between them. [6] What is the significance of G/T ratio and explain how does it affect C/N ratio for satellite communication system. [4] OR A C -band earth station has an antenna with a transmit gain of 54 dB. The transmitter output power is set to 100 W at a frequency of 6.100 GHz. The signal is received by a satellite at a distance of 37,500 km by an antenna with a gain of 26 dB. The signal is then routed to a transponder with a noise temperature of 500 K, a bandwidth of 36 MHz, and a gain of 110 dB. signal is then retransmitted towards ground terminals with transmitting antenna with gain of 24.0 dB at frequency of 3.85 GHz. Calculate the following. i) The path loss at 6.1 GHz. ii) The power received at the satellite, in dBW, iii) The noise power at the transponder input, in dBW, iv) The C/N ratio, in dB, at the input of the transponder. v) The carrier power, in dBW and in watts, at the transponder output. vi) The power flux density received at ground terminals in dBW/mt2. [18] Q11)a) Compare TDMA & CDMA techniques used in satellite communication system. [8] b) Three identical earth stations access 54 MHz bandwidth transponder using FDMA. The saturated output of the transponder is 60 w and transponder is operated with 3 db output backoff. The bandwidth of earth station signals are . Station A 25 MHz Station B 15 MHz Station C 10 MHz Determine power level at the output of the transponder in dBw for each earth station signal. (Assume the transponder is operating in its linear region) [8] [3864] - 273 -4- Q12)a) b) c) OR Draw and explain various network architectures for a VSAT network. [4] Draw & explain block diagram of DBS - TV receiver. [6] A VSAT system operating at 14.02 GHz / 11.72 GHz is at a range of 38,500 Km. It uses BPSK, with transmitted bit rate of 256 Kbps. System noise temperature is 600dBK at transponder and Boltzman s constant K = 1.38 * 10-23 J/K (-228.6 dBw/K/Hz). Calculate. i) Noise power at the transponder input in dBm units. ii) Free space uplink & downlink path loss. [6] ***** [3864] - 273 -5-

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