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Sixth Semester B.E. Degree Examination, May/June 2010 Digital Communication Note: Answer any FIVEfull questions, selecting at least TWO questionsfrom each part. PART-A a. With a block diagram, explain the basic signal processing operations involved in a digital communication system. (07 Marks) b. Explain 'flat-top' sampling, using waveforms and equations. (07 Marks) c. The signal x(t) = 2cos400nt + 6cos640nt is ideally sampled at fs = 500 Hz. If the sampled signal is passed through an ideal low pass filter with cut off frequency of 400 Hz: i) Determine the spectrum of the sampled signal and sketch. ii) What frequency components will appear in the filter output? (06 Marks) b. Show that the signal to quantization noise power ratio of a uniform quantizer is [SNRl m = 1.8+ 6n and n = number of bits/sample. (06 Marks) c. What is the necessity of non uniform quantization? Explain compounding. (06 Marks) d. A telephone signal band limited to 4 KHz is to be transmitted by PCM. The signal to quantization noise power ratio is to be at least 40 db. Find the number of levels into which the signal has to be encoded. Also find the transmission band width. (04 Marks) a. With neat diagrams, explain the operation of DPCM. (07 Marks) b. A DM system is tested with a 10KHz sinusoidal signal with 1 V peak to peak at the i/p. It is sampled at 10 times the Nyquist rate. What is the step size required to prevent slope overload? (04 Marks) c. Explain T1 - carrier system. (05 Marks) d. For the binary bit sequence 10110100, draw the waveforms using: (04 Marks) i) Unipolar NRZ ii) Unipolar RZ iii) Polar NRZ iv) Bipolar NRZ. a. Describe Nyquist's criteria for distortionless baseband transmission. (06 Marks) b. Explain the need for a precoder in a duobinary signaling. For i/p binary data 1011101, obtain the o/p precoder and o/p of duobinary coder. Explain how data can be detected at the receiver. (08 Marks) c. What is equalization? Explain adaptive equalization for data transmission. (06 Marks) 5 a. Explain the working of: i) Coherent BFSK transmitter and ii) QPSK transmitter. (10 Marks) b. The bit stream 1011100011 is to be transmitted using DPSK technique. Determine the encoded sequence and transmitted phase sequence. Also write the block diagram of the modulator and demodulator for the same and explain. (10 Marks) 10f2

6 a. A binary data is transmitted using ASK over a AWON channel at a rate of 2.4 Mbps. The carrier amplitude at the receiver is 1 ~V. Noise spectral density NJ2 is 10-15 Watt/Hz. Find the average probability of error if the detection is coherent. Take Erfc(5)::: 3xl 0-6. (06 Marks) b. With a diagram, explain the model of digital communication system. (08 Marks) c. Explain geometric interpretation of signals. (06-Marks) 7 a. Explain the detection of known signals in noise. (10 Marks) b. Write a note on minimum mean square error estimate. (04 Marks) c. A polar NRZ waveform is to be received by a matched filter. Binary 1 is represented by a rectangular positive pulse and binary 0 is represented by a rectarlgular negative pulse. Find out the impulse response of the matched filter and sketch it. (06 Marks) a. Mention the applications of spread spectrum system. sequence spread spectrum system. b. Explain the frequency hopped spread spectrum system. c. A slow FHIMFSK system has the following parameters: The number of bits / MFSK symbol = 04 The number of MFSK symbol/hop = 05 Calculate the processing gain of the system in decibels. Explain the principle of direct (08 Marks) (08 Marks)

Sixth Semester B.E. Degree Examination, May/June 2010 Digital Communications Note: 1. Answer any FIVE full questions. 2. Assume suitable data for numerical problems. 1 a. What are the two primary channel characteristics? Explain different types of digital communication channels. (07 Marks) b. What is quadrature sampling? Explain the generation and reconstruction of quadrature sampled bandpass signals. (08 Marks) c. Find the Nyquist rate for the analog signal given by x (t) x (t) = 3 Cos (50 nt) + 10 Sin (300 nt) - Cos (100 nt).. (05 Marks) 2 a. What is flat-top sampling? Derive the expression for the spectrum of flat-top sampled signal. (08 Marks) b. A flat-top sampling system samples a signal of maximum 1Hz with 2.5 Hz sampling frequency. The duration of the pulse is 0.2 seconds. Calculate the amplitude distortion due to aperture effect at highest signal frequency. Also find out the equalization characteristic. (08 Marks) c. Explain the concept oftdm. (04 Marks) 3 a. Explain the blocks of basic elements ofpcm system. (08 Marks) ----------------------- b. Show that for!j. = A, the!j. -law and A -law have the same companding gain. (06 Marks) c. A modulating signal is given by x (t) = A sin (Pt). Find the maximum value of A for no slope over load distortion. Assume step size 8 = 2 mv and a sampling rate of f s = 10kHz. (06 Marks) 4 a. Explain T 1 carrier multiplexing system. Also show that how the higher carrier multiplexing takes place for high bit rates. (06 Marks) b. Derive the expression for NRZ polar waveform PSD (Power Spectral Density). (08 Marks) c. The binary data stream 0010110 is applied to the input of a duo binary system. Construct the duo binary coder output and corresponding receiver output. Assume that there is 'a pre coder at the input. (06 Marks) 5 a. Explain the operation of binary phase shift keying (BPSK) transmitter and coherent BPSK receiver. (06 Marks) b. Obtain the signal space diagram for binary frequency shift keying (BFSK) and hence prove that the average probability of symbol error is given by Pe = ~ erfc~ Eb. (08 Marks) 2 2No c. Input binary sequence to QPSK modulator is given by {bi} = {O 1 0 0 1 0 1 1 0 1 1 O}. Sketch the transmitted time. lof2 phase of the carrier as a function of (06 Marks)

6 a. Explain the two stage Gram-Schmidt orthogonalization procedure to find orthonormal basis functions. (10 Marks) b. Derive the equation for maximum likelihood to find maximum likelihood estimation. rolvfilrks) 7 a. Explain the properties of maximum length sequences. (06 Marks) b. Explain the operation of direct sequence spread coherent PSK transmitter and receiver. (08 Marks) c. In direct sequence spread spectrum modulation scheme, a 14 - stage feedback linear shift register is used to generate the PN code sequence. Find the period of the code sequence and processing gain. (06 Marks) 8 a. Explain the idealized model of baseband spread spectrum with the illustration of waveforms in the transmitter for a data sequence of 0011101. (10 Marks) b. With a neat block diagram, explain the working of frequency hopped transmitter and receiver combination for M - ary frequency shift keying. (10 Marks)