West Bengal Institute of Technology (WBIT) 2005 B.Tech Electronics and Tele-Communication Engineering digital communication - Question Paper

Code: A-21 Subject: DIGITAL COMMUNICATIONS
Time: three Hours Max. Marks: 100


NOTE: There are 11 ques. in all.



ques. one is compulsory and carries 16 marks. ans to Q. 1. must be written in the space given for it in the ans book supplied and nowhere else.

ans any 3 ques. every from Part I and Part II. every of these ques. carries 14 marks.

Any needed data not explicitly given, may be suitably presumed and said.





Q.1 select the accurate or best option in the following: (2x8)



a. In QAM scheme _________________ of the carrier signal is / are varied.


(A) both frequency and amplitude. (B) both amplitude and phase.

(C) both phase and frequency. (D) only phase.


b. In an FH/MFSK system, the PN sequence makes the carrier hop over a number of frequencies in a pseudo-random manner, with the outcome the spectrum of the transmitted signal is spread in a



(A) random manner.

(B) sequential manner.

(C) non sequential manner.

(D) in synchronization with PN-sequence.



c. When the performance of a digital transmission system is tested with an eye-diagram, excessive deformation of pulses is indicated by a/an


(A) increased opening of the eye. (B) wide open eye.

(C) closing down of the eye. (D) blinking of the eye.



d. A concatenated code uses 2 levels of coding, an inner code and an outer code to achieve the desired fault performance. The outer code is used for


(A) reducing channel fault.

(B) reducing noise generated in modulator and demodulator.

(C) reducing probability of fault to a specified level.

(D) controlling the fault burst.



e. For M-ary PSK signals if the number of levels M increases, the bandwidth efficiency



(A) reduces.

(B) remains constant.

(C) reaches an optimum value and then reduces.

(D) increases.



f. The Viterbi decoding algorithm performs



(A) sequential decoding. (B) maximum likelihood decoding.

(C) feedback decoding. (D) threshold decoding.



g. Companding is a special technique used to achieve


(A) low signal to noise ratio. (B) non uniform sampling.

(C) differential encoding. (D) uniform sampling.



h. Block codes in which the message bits are transmitted in unaltered form are called

Earning:   Approval pending.