SASTRA University 2010 B.Tech Electronics and Communications Engineering Digital communication - Question Paper

Digital communication
B. Tech Degree exams
1. What is aliasing effect? Suggest methods to eliminate it.
2. describe absolute Bandwidth.
3. Write the properties of real valued power signal interms of its auto correlation.
4. A signal x(t) =3 sin (500 t) is sampled and quantized using 10 bit PCM. obtain SNR and step size.
5. obtain the avg. normalized power for x(t) = A cos2pf0tusing time averaging.
6. For the input, 10110 provide the unipolar RZ representation.
7. Mention the ideal solution for zero ISI.
8. find the output of precoder and output of duobinary coder for the input binary data 1011101.
9. Show that SNR of the method filter is 2E/NO.
10. What is the purpose of intergrate and dump receiver?
11. Draw the FSD and PSK waveform for the input binary sepuence 1001011.
12. Differentiate coherent and non-coherent detection example.
13. QPSK and BPSK have identical bit fault probability. Justify it.
14. 4 signals S1(t),S2(t) and S4 (t) are of equal energy 'E' and are described for finite time interval 0<=t<=T as s1(t)=(e/t)?½ f1(t) s2(t)= (e/t)?½ f2(t);s3(t) =s1(T);s4(t)=s2(t). Also f1 (t) and f2(t)
Ar orthonormal functions described for the interval 0 <=t<= T.
presume the signals are transmitter=d with equal probability over AWGN channel . Determine (a) the dimension of the observation space (b) signal constellation
15. List any 2 advantages of QPSK over BPSK.
16 provide the frame structure of FA-TDM.
17. Compare fixed and demand-assignment multiple access.
18. provide the diagrammatic representation of S- ALOHA operation.
19. elaborate the steps involved in multiple access info flow?
20. List the advantages of CDMA over other MA techniques.

Part-B
21. (A) describe uniform and non-uniform quantization. (3)
(B) Derive the expression for signal to quantization noise ratio for a PCM system that the input to the PCM system that employs linear quantization technique. presume that the input to the PCM system Is a sinusoidal signal.

(c) A signal g(t) consists of 2 frequency components f1=3.9 kHz and f2 = 4.1 kHz in such a Relationship hey cancel out every other when g(t) is sampled at the instants t=0,T,2T… where T=125 µs. The signal is described b g(t)= cos [2pf1t+Acos[2pf2t+f].
obtain the values of A and f of the 2nd frequency component.
(OR)
22. (A) Derive the transfer function of an ideal LPF.
(B) discuss in detail about sources of corruption.
23. (A) discuss ISI, and prove its occurrence mathematically.
(B)The info in analog waveform with maximum frequency fm =3 kHz is to be transmitted over an M-ary PAM system, where the number of pulse level is m=16. The quantization distortion is specified not to exceed ±1% of the peak-peak analog signal
(I)What is the minimum number of bits/sample word that should be used in digitizing the analog wave form?
(II)What is the minimum needed sampling rate and what is the resulting bit transmission rate?
(III) What is the PAM pulse sampling rate?
(IV) If the transmission bandwidth equals 12 kHz, Determine bandwidth of this system.

(OR)

24. (A) Derive the probability fault for matched filter and explain when a matched filter becomes an integrator.
(B) In a binary transmission, 1 of the messages is represented by a rectangular pulse x (1). The other message is transmitted by the absence of the pulse. compute SNR at t=T. Assuming white noise with PSD NO/2. Also sketch the IR of the matched filter and output of the matched filter.
25. (A) compute the probability of fault for an ASK signal.
(B) A binary sequence 101101 is transmitted over a communication channel using DPSK transmitter. The channel introduces a phase reversal of 180°.
(I)Sketch the transmitted DPSK waveform assuming an initial bit of '1'.
(II) Assuming the channel is noise free, show that the DSPK detector is the receiver which produces the original binary sequence, despite the 180° phase reversal which produces the original binary sequence, despite the 180° phase reversal in the channel.

(OR)

26. Generate and detect a QPSK signal. find its probability of fault.
27. Compare FDMA and TDMA by considering
(I)Bit rate equivalence
(II) Message delays
(OR)
28. (A) with suitable diagram, discuss CDMA.
(B) discuss in detail about CDMA frequency hopping modulation process.

Earning:   Approval pending.