74161-mt----spread Spectrum Techniques

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Code No: 74161/MT M.Tech. – I Semester Supplementary Examinations, September, 2008 SPREAD SPECTRUM TECHNIQUES (Electronics & Communications) Time: 3hours

Max. Marks:60 Answer any FIVE questions All questions carry equal marks ---

1.a) b) 2.a) b) 3.a)

b)

4.a)

b)

Explain briefly the model of spread spectrum digital communication system, with a neat block diagram. Define the processing gain and the Jamming margin parameters in connection with the spread spectrum. State and explain some applications of DS spread spectrum signals. Distinguish between the processing gains of DS and FH spread spectrum. Mention two basic criteria to be followed by the transmitted signal of a communications system so that it may be considered a spread spectrum system. Mention various types of spread spectrum system. If a direct sequence spread spectrum signal is interfered with a sinusoidal signal of normalized power PJ at carrier frequency fo, 1 then prove the error probability Pe = erfc Ps PJ ( f c f b ) . 2 The symbols have their usual significance. A spread spectrum system has the following parameters. Message bit rate fb=3Kbps, PN sequence chip rate fc = 3.072 × 106. If the average probability of error is not to exceed 10-5, find the Jamming margin. With diagrams, explain the operation of direct sequence spread spectrum (DS/SS) transmitter and receiver system.

5.a) State and prove the properties of maximal-length sequences. b) Construct a Galois field having 32 elements and list all elements in polynomial format together with its multiplicative inverse element. The polynomial 1+D+D5 is primitive. Contd…2

Code No: 74161/MT 6.a) b) 7.a) b) 8.

::2::

Discuss briefly partial auto correlation properties of m-sequences. Explain briefly the principle of operation of a typical gold code generator. Discuss briefly synchronization of spread spectrum systems with a neat block diagram. Discuss briefly the operation of noncoherent tracking loop. Write short notes on: (a) Time-hopping (TH) spread spectrum system. (b) Delay – Locked Loop (DLL) for PN code tracking. ******