Subject Code:EC1629 Subject :Digital Communication

-----------------------------------------------------------------------EC 1629 Digital Communication (VI SEMESTER)

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UNIT 1 PULSE MODULATIONPART - A1. State sampling theorem. A band limited signal of finite energy, which has no frequency components higher than W hertz, is completely described by specifying the values of the signal at instants of time separated by 1/2W seconds and, A band limited signal of finite energy, which has no frequency components higher than W hertz, may be completely recovered from the knowledge of its samples taken at the rate of 2W samples per second.2. What is aliasing or fold over?Aliasing effect takes place when sampling frequency is less than nyquist rate. Under such condition, the spectrum of the sampled signal overlaps with itself. Hence higher frequencies take the form of lower frequencies. This interference of the frequency components is call ed aliasing effect.3. Define Nyquist rateLet the signal be band limited to W Hz. Then Nyquist rate is given as,Nyquist rate = 2W samples/secAliasing will not take place if sampling rate is greater than Nyquist rate.4. What is meant by PCM?Pulse code modulation (PCM) is a method of signal coding in which the message signal is sampled; the amplitude of each sample is rounded off to the nearest one of a finite set of discrete levels and encoded so that both time and amplitude are represented in discrete form. This allows the message to be transmitted by means of a digital waveform.5. What is meant by quantization?While converting the signal value from analog to digital, quantization is performed. The analog value is assigned to the nearest digital level. This is called quantization. The quantized value is then converted to equivalent binary value. The quantization levels are fixed depending upon the number of bits. 6. What is meant by prediction error?The difference between the actual sample of the process at the time of interest and the predictor output is called a prediction error.7. Define delta modulation.Delta modulation transmits only one bit per sample. That is the present sample value is compared with the previous sample value and the indication, whether the amplitude is increased or decreased is sent.8. Define adaptive delta modulation.The performance of a delta modulator can be improved significantly by making the step size of the modulator assume a time- varying form. In particular, during a steep segment of the input signal the step size is increased. Conversely,when the input signal is varying slowly, the step is reduced , In this way, the step size is adapting to the level of the signal. The resulting method is called adaptive delta modulation (ADM).9. Name the types of uniform quantizer?1. Mid tread type quantizer.2. Mid riser type quantizer.3. Biased type quantizer.10. Define mid tread quantizer.Origin of the signal lies in the middle of a tread of the staircase.

11. Define mid-riser quantizer?Origin of the signal lies in the middle of a riser of the staircase12. Define quantization error?Because of quantization, inherent errors are introduced in the signal. This error is called quantization error.13. What you mean by non-uniform quantization?Step size is not uniform. Non-uniform quantizer is characterized by a stepsize that increases as the separation from the origin of the transfer characteristics is increased. Non-uniform quantization is otherwise called as robust quantization14. What do you mean by companding? Define compander.The signal is compressed at the transmitter and expanded at the receiver. This is called as companding. The combination of a compressor and expander is called a compander.15. What is PAM?PAM is the pulse amplitude modulation. In pulse amplitude modulation, the amplitude of a carrier consisting of a periodic train of rectangular pulses is varied in proportion to sample values of a message signal.16. How the messages can be recovered from PAM.The message can be recovered from PAM by passing the PAM signal through reconstruction filter. The reconstructing filter integrates the amplitude of PAM pulses. Amplitude smoothing of the reconstructed signal is done to remove amplitude discontinuities due to pulses.17. Write an expression for bandwidth of binary PCM with N messages each with a maximum frequency of fm Hz.If v number of bits are used to code each input sample, theb bandwidth of PCM is given as,BT >= N v fmHere v fm is the bandwidth required by one message.18. What is meant by adaptive delta modulation?In adaptive delta modulation, the step size is adjusted as per the slope of the input signal. Step size is made high if slope of the input signal is high. This avoids slope distortion.19. What is the advantage of delta modulation over pulse modulation schemes?Delta modulation encodes one bit per sample. Hence signaling rate is reduced in DM.20. What is the advantage of delta modulation over PCM? Delta modulation transmits only one bit for one sample. Thus the signaling rate and transmission channel bandwidth is quite small for delta modulation. The transmitter and receiver implementation is very much simple for delta modulation. There is no analog to digital converter involved in delta modulation.21. What are the different types of sampling techniques? Ideal sampling or Instantaneous or Impulse sampling Natural Sampling Flat Top or Rectangular pulse sampling.22. Define TDMIn PAM, PDM, PWM the pulse is present for short duration and form most of the the time between the two pulse can be occupied by pulses from other channels. This is called Time Division Multiplexing.23. What is the disadvantage of delta modulation?The delta modulation has two drawbacks Slope overloads distortion or Startup errorThe rate of rise of input signal x(t) is so high that the staircase signal cannot approximate it, the step size becomes too small for staircase signal u(t) to follow the steep segment of x(t). Thus there is a large error between the staircase approximated signal and the original input signal x(t). This error is called slope overload distortion. To reduce this error, the step size should be increased when slope of the signal x(t) is high. Granular noise or Hunting noiseGranular noise occurs when the step size is too large compared to small variations in the input signal. That is for very small variations in the input signal, the staircase signal is changed by large amount because of large step size.The error between the input and approximated signal is called granular noise.

24. How to reduce slope overload noise and granular noise in DM?The large step size is required to accommodate wide dynamic range of the input signal to reduce slope overload distortion and small step size are required to reduce granular noise. Adaptive delta modulation is the modification to overcome these errors.

25. What are the advantages of adaptive delta modulation? The signal to noise ratio is better than ordinary delta modulation because of the reduction in slope overload distortion and granular noise. Because of the variable step size, the dynamic range of ADM is wide. Utilization of bandwidth is better than delta modulation.26. Compare PCM and DPCM.ParameterPCMDPCM1. Number of bitsIt can use 4, 8, or 16 bits/sampleBits can be more than One but are less than PCM2. BWHighest BW is requiredBW required is lower than PCM3. FeedbackThere is no feedback in Tx and RxFeedback exists.4. ApplicationAudio and video telephonySpeech and video

PART B

1. Draw the block diagram of PCM transmitter and receiver and explain functions of each.

2. Explain delta modulation system with block diagram and discuss the noise in delta modulation.

3. Explain Adaptive delta modulation system with block diagram and discuss the advantage of adaptive delta modulation.

4. Draw the block diagram of differential PCM transmitter and receiver and explain functions of each.

5. With neat diagram explain Time Division Multiplexer.

6. Draw and explain the different methods of pulse modulation.(16)

UNIT 2 BASEBAND PULSE TRANSMISSION

PART A

1. Define data signaling Rate.Data signaling rate is defined as the rate measured in terms bits per second(b/s) at which data are transmitted.Data signaling rate Rb=I/TbWhere Tb=bit duration.2. Define modulation rate.It is defined as the rate at which signal level is changed depending on the nature of the format used to represent the digital data. It is measured in Bauds or symbols per second.3. State NRZ unipolar formatIn this format binary 0 is represent by no pulse and binary 1 is Represented by the positive pulse.4. State NRZ polar format.Binary 1 is represented by a positive pulse and binary 0 is represented by a Negative pulse.5. State NRZ bipolar format.Binary 0 is represented by no pulse and binary one is represented by the alternative positive and negative pulse.6. State Manchester format.Binary 0 first half bit duration negative pulse and the second half bit duration positive pulse.Binary 1 first half bit duration positive pulse and the second half Bit duration negative pulse.

7. What is an eye pattern?Eye Pattern is used to study the effect of intersymbol interference.8. What is the width of the eye?(L 1) It defines the time interval over which the received waveform can be sampled without error from inter symbol interference.9. What is sensitivity of an eye?(L 1)The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.10. What is margin over noise?(L 1)The height of the eye opening at a specified sampling time defines the margin over noise.11. What is Inter symbol interference?(L 1)The transmitted signal will undergo dispersion and gets broadened during its transmission through the channel. So they happen to collide or overlap with the adjacent symbols in the transmission. This overlapping is called Inter Symbol Interference.12. How eye pattern is obtained?(L 1)The eye pattern is obtained by applying the received wave to the vertical deflection plates of an oscilloscope and to apply a saw tooth wave at the transmitted symbol rate to the horizontal deflection plate.

13. Draw the waveform for NRZ Unipolar waveform for binary data 01101001

14. Draw the waveform for Polar NRZ signaling for binary data 01101001

15. Draw the waveform for Unipolar RZ signaling for binary data 01101001

16. Draw the waveform for Bipolar RZ signaling for binary data 01101001

17. Draw the waveform for Split-phase or Manchester code for binary data 01101001

PART B

1. Discuss the principles of correlative coding.2.Describe the data communications modems and explain where they are used in data communications circuits(16)

3.Explain Inter symbol Interference (ISI). How it is avoided?4.What is eye pattern? Explain the measurement of different parameters using eye pattern. Explain the various data formats and compare them.5. Explain the following

channel voice coder linear predictive coder

UNIT 3 PASSBAND PULSE TRANSMISSION

PART A

1. Define QAM.Quadrature amplitude modulation is a form of digital modulation where the digital information is contained in both the amplitude and phase of the transmittedcarrier.

2. What is the error probability of a binary FSK System?Error probability of binary FSK is given as,Pe = 1 erfc 0.6E 2 N0Where E = PTb is energy of one bitand N0 is the psd of white noise. 2

3. Compare binary PSK with QPSK.

4. Differentiate coherent and non coherent methods.Coherent (synchronous) detection: In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.

Non coherent (envelope) detection: This type of detection does not need receiver carrier to be phase locked with transmitter carrier. The advantage of such a system is that the system becomes simple, but the drawback is that error probability increases. The different digital modulation techniques are used for specific application areas. The choice is made such that the transmitted power and channel bandwidth are best exploited.

5. What are antipodal signals?In BPSK, the two symbols are transmitted with the help of following signals,Symbol 1 s1 (t) = 2P cos (2 f0 t)Symbol 0 s2 (t) = 2P cos (2 f0 t + )Here observe that above two signals differ only in a relative phase shift of 180. Such signals are called antipodal signals.

6. Which digital modulation technique gives better error probability?Binary PSK gives reduced error probability compared to ASK and FSK. It is given as,Pe = 1 erfc E 2 N0

7. What are the advantages of QPSK as compared to BPSK?1. For the same bit error rate, the bandwidth required by QPSK Is reduced to half as compared to BPSK.2. Because of reduced bandwidth, the information transmission rate of QPSK is higher.3. Variation in QPSK amplitude is not much. Hence carrier power almost remains constant.

8. What are the requirements for a digital modulation scheme? Maximum data rate Minimum probability of symbol error Minimum transmitted power Minimum channel bandwidth Maximum resistance to interfering signals Minimum circuit complexity

9. Sketch the waveform of ASK for binary sequence 1001011.

10. Differentiate ASK and FSK.ASKFSKa. The two frequencies are integer multiple The difference between the 2of base band frequencies and at the samefrequencies is minimum and at thetime they are orthogonal.same time they are orthogonal.

b. BW=4fbBW=1.5fbc. This is binary modulationThis is quadrature modulation

11. Define DPSK.Differential phase shift keying is differentially coherent modulation method. DPSK does not need a synchronous carrier at the demodulator. The input sequence of binary bits is modified such that the next bit depends upon the previous bit. Therefore in the receiver the previous bits are used to detect the present bit.

12. Give the equation for average probability of symbol error for coherent BPSK.Average probability of signal error,Pe = 1 / 2 erfc Eb / No13. Define QPSK.QPSK is Quadrature phase shift keying. In QPSK the phase of the carrier takes on one of the four equally spaced values Such as /4, 3/4, 5 /4 and 7 /4.14. Define Dibit.A unique pair of bits is called a dibit. Gray encoded set of dibits 10, 00, 01 & 11.15. Give the two basic operation of DPSK transmitter. Differential encoding of the input binary wave Phase shift keying hence, the name differential phase shift keying16. Define deviation ratio in MSK.The parameter h is defined byh= Tb(f1-f2)h is deviation ratio , measured with respect to bit rate 1/Tb.17. What is carrier synchronization?The carrier synchronization is required in coherent detection methods to generate a coherent reference at the receiver. In this method the data bearing signal is modulated on the carrier in such a way that the power spectrum of the modulated carrier signal contains a discrete component at the carrier frequency.

18. What are the two methods for carrier synchronization? Carrier synchronization using Mth Power loop Costas loop for carrier synchronization19. What is called symbol or bit synchronization? In a matched filter or correlation receiver, the incoming signal is sampled at the end of one bit or symbol duration. Therefore the receiver has to know the instants of time at which a symbol or bit is transmitted. That is the instants at which a particular bit or symbol status and when it is ended. The estimation ofthese times of bit or symbol is called symbol or bit synchronization.20. What are the two methods of bit and symbol synchronization? Closed loop bit synchronization 2) Early late gate synchronizer21. What are the disadvantages of closed loop bit synchronization? If there is a long string of 1s and os then y(t) has no zero crossings and synchronization may be lost. If zero crossing of y(t) are not placed at integer multiples of Tb, thesynchronization suffers from timing Jitter.22. What is called frame synchronization?Depending on bits used for encoding, the word length is defined. Thus each word container some fixed number of bits. The receiver has to know when a particular frame status and when its individual message bits status. This type of synchronization is called frame synchronization.23. Why synchronization is required?The signals from various sources are transmitted on the single channel by multiplexing. This requires synchronization between transmitter and receiver. Special synchronization bits are added in the transmitted signal for the purpose. Synchronization is also required for detectors to recover the digital data properly from the modulated signal.

PART - B

1. With block diagram explain the principle of Coherent FSK transmitter and receiver. 2. Explain the operation of QPSK transmitter and receiver. 3. Explain the generation and detection of DPSK signal 4. Explain in detail about BPSK. State merits and demerits of BPSK. 5. Draw the block diagram for generation and detection of QAM signal and explain it with signal space diagram. 6. Explain the principle of non-coherent ASK receiver and derive an expression for the probability error.

UNIT 4 ERROR CONTROL CODINGPART - A1. What is linear code? 2. What is code rate? 3. Define code efficiency. 4. What is hamming distance? 5. What is meant by systematic & non-systematic code? 6. How syndrome is calculated in Hamming codes and cyclic codes? 7. What is BCH Code? 8. What are the conditions to satisfy the hamming code? 9. Define code word & block length. 10. Give the parameters of RS codes. 11. Why RS codes are called maximum distance separable codes? 12. What are Go lay codes? 13. What are the advantages of cyclic codes?

PART B1. a) Explain the principle of Linear block codes using its functional block diagram? b) The generator matrix for a (6,3) block code is given below. Find all code vectors of this code 2. The generator polynomial of a (15,11) Hamming code is given by g(x) = 1+x+x4 . Develop encoder and syndrome calculator for this code using systematic form.3. The generator polynomial of a (7,4) cyclic code is G(p) = P3 + P+1. Find all the code in in non systematic and systematic form.4. Explain the construction of Block Code and how the error syndrome is calculated.5. Explain the difference between convolution codes and block codes.

UNIT 5 SPREAD SPECTRUM MODULATIONPART - A

1. What do you mean by direct sequence spread technique?The data sequence directly modulates the pseudo noise sequence. Let the data signal be b (t) and pseudo-noise signal be c (t). Then the modulated signal is given as, m (t) = b (t) c( t)

2. What are the advantages of spread spectrum modulation?Spread spectrum modulation spreads the message signal over wide bandwidth with the help of special code (key). It has following important advantages.i) Unwanted interference is rejected.ii) Protection against antijamming signals is also provided.iii) Multipath interference rejection.

3. Define pseudo noise sequence.Pseudo Noise (PN) sequence is a periodic binary sequence with a noise like waveform that is usually generated by means of Feedback Shift Register.

4. Explain why FH spread spectrum is not affected by near far problem.In direct sequence spread spectrum there is single frequency band in which communication takes place. If noise or jamming signal is transmitted in this frequency band, then it is difficult to isolate noise and signal at the receiver. This is called near far problem. In FH spread spectrum, the transmission takes place in multiple interference is present in one frequency band, it does not affect signal in other frequency bands. Hence near-far problem does not exist in FH spread spectrum.

5. What is meant by Spread spectrum?The spread spectrum modulation can be defined in two parts as follows.1. The transmitted data sequence occupies a much more bandwidth than the minimum required bandwidth and,2. The spectrum spreading (i.e. increase of signal bandwidth) at the transmitter and dispreading at the receiver is obtained by special code which is independent of the data sequence (message signal).The first part of definition given above is satisfied by other modulation techniques like frequency modulation, PCM etc. but they do not satisfy second part. That is they do not use frequency spectrum spreading and dispreading. The bandwidth requirement of spread spectrum modulation techniques is thus very high.Spread spectrum modulation is used for secured communication like military applications. Noise interference has minimum effect on transmission. Unwanted receiverscannot detect the message.

6. What is frequency hop spreading?In frequency hop spread spectrum, the frequency of the carrier hops randomly from one frequency to another frequency.

7. What are the types of FH-SS?Slow frequency hoppingFast Frequency Hopping

8. What is slow frequency hopping?If the symbol rate of MFSK is an integer multiple of hop rate (multiple symbols per hop) then it is called slow frequency hopping.

9. What is Fast Frequency Hopping? If the hop rate is an integer multiple of symbol rate (multiple hops per symbol) then it is called fast frequency hopping.

10. What are the two function of fast frequency hopping?1. Spread Jammer over the entire measure of the spectrum of Txed signal.2. Retuning the Jamming signal over the frequency band of Txed signal.

11. What is Jamming Margin? Jamming Margin= Average Interference Power(J)/Average Signal Power(Ps)12. What is called jamming effect?In the frequency band of the interest, somebody else transmits the signals intentionally since these signals the in the frequency band of transmission, they interface the required signal. Hence it becomes difficult to detect the required signals. This is called jamming effect.13. What is Anti jamming?With the help of spread spectrum method, the transmitted signals are spread over the mid frequency band. Hence these signals appear as noise. Then it becomes difficult for the jammers to send jamming signals. This is called antijamming.14. What are the three codes used for the anti-jamming application?1. Golay code (24, 12)2. Expurgated Golay (24, 11)3. Maximum length shift register code.

15. What are the applications of Spread spectrum modulation? The spread spectrum has the ability to resist the effect of intentional jamming. Previously this antijam capability was used in military application .Some commercial applications also use spread spectrum because of its antijam capability. Low probability of intercept is an application of spread spectrum in military .In this case, the signal spectral density is kept small such that the presence of the signal is not detected easily. Spread spectrum is used in mobile communications. This is because the spread spectrum signal has than ability to resist the effects of multipath fading. Because of wide spectrum only small portion of the signals is in fade. Spread spectrum is also used in selective calling. In this, the central station communicates with the number of different receiving points.

16. List the disadvantages of direct sequence systems It requires wideband channel with small phase distortion It has long acquisition time The pseudo-noise generator should generate sequence at high rates This system is distance relative

17. List the advantages of direct sequence systems This system has best noise and antijam performance Unrecognized receivers find it most difficult to detect direct sequence signals It has best discrimination against multipath signals

18. List the advantages of frequency hopping systems These systems bandwidth are very large They can be programmed to avoid some portions of the spectrum They have relatively short acquisition time. The distance effect is less

19. List the disadvantages of frequency hopping systems Those systems need complex frequency synthesizers They are not useful for range and range rate measurement They need error correction

20. Define slow frequency hoppingWhen several symbols are transmitted in one frequency hop (slot), then it is called slow frequency hopping. This means the symbol rate is higher than hop rate.

21. Define fast frequency hoppingWhen several frequency hops take place to transmit one symbol, then it is called fast frequency hopping. This means the symbol rate is less than hop rate.

22. What is processing gain?Processing gain is defined as the ratio of the bandwidth of spreaded signal to the bandwidth of the unspreaded signal

23. What are the advantages of spread spectrum modulation?Spread spectrum modulation spreads the message signal over wide bandwidth with the help of special code. It provides security of the message so that unwanted users cant read the message. Unwanted interference is rejected. Protection against antijamming signals is also provided. Multipath interference rejection.

24. State the correlation property of maximum length sequence.An autocorrelation of the sequence is periodic and binary valued.

25. State Run Property?In each m sequence one half of the runs are of each kind are of length one , one fourth are of length 2, one eighth are of length 3 and so on.26. Give the correlation property of random binary sequence. The autocorrelation function of a maximum length sequence is periodic and binary valued. This property is called the correlation property.27. What is the advantage of a spread spectrum technique?The main advantage of spread spectrum technique is its ability to reject interference whether it be the unintentional interference of another user simultaneously attempting to transmit through the channel (or) the intentional interference of a hostile transmitter to jam the transmission.

PART B

1. Explain in detail the transmitter and receiver of DS SS technique.

2. Explain in detail the transmitter and receiver of FH SS technique.

3. Explain in detail the characteristics of PN sequence

4. Write notes on1. Cell splitting2. Cell sectorisation3. Frequency Reuse 5. Describe the data communications modems and explain where they are used in data communications circuits(16)

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