DIGITAL COMMUNICATION TWO MARKS Q&A

DIGITAL COMMUNICATION

GNANAMANI COLLEGE OF ENGINEERING NH-7 A.K.SAMUTHIRAM, NAMAKKAL DEPARTMENT OF ELECTRONICS&COMMUNICATION ENGG, TWO MARK Q&A Sem/Branch : VI/ECE : Digital Digital communication Subject Prepared by : R.Shankar, AP/ECE UNIT -1 SAMPLING AND WAVEFORM CODING

1. Define Nyquist rate. Let the signal be band limited to ‘w’ Hz. Then nyquist rate is given as, Nyquist=2w samples/sec. samples/sec. Aliasing will not take place if sampling rate is greater than t han nyquist nyquist rate. r ate. 2. What is meant by aliasing effect? Aliasing effect take place when sampling frequency is less than Nyquist rate under such condition, the spectrum of the sampled signal overlaps with itself. Hence higher frequency components components are called aliasing effect. 3.

Define PWM. PWM is basically width of the pulse changes changes according to amplitude of the modulating signal. It is also referred r eferred to as pulse duration modulation or PDM. 4. State sampling theorem t heorem.. Sampling theorem states that a band limited signal of finite energy, which has no frequency components higher than W Hz, is completely described by specifying the values of the signal at the instants of time separated by 1/2w seconds. 5. Mention two merits of DPCM. i. Band width requirement of DPCM is less compared to PCM. ii. Quantization error is reduced because of prediction filter. iii. Numbers of bits used to represen r epresentt one sample value are also reduced compared compared to PCM. 6. What is the main difference in DPCM and DM? Dm encodes the input sample by only one bit. It sends the information about + Ѕ or - Ѕ i.e. step rise or fall. DPCM can have more than one bit for encoding the sample. It sends the information about difference between actual sample value and predicated sample value. 7. How the message can be recovered from PAM? The message can be recovered from PAM by passing signal t hrough reconstruction filter. The reconstruction filter integrates amplitudes of PAM pulses. Amplitude smoothing of reconstructed signal is done to remove amplitude discontinues due to pulses.

GCE, NAMAKKAL

Page 1


8. Define Quantization. Quantization is a process of approximation or rounding off. Assigning PCM codes to absolute absolute magnitudes is called quantization. quant ization. 9.Define Compandin Co mpanding. g. Companding Companding is the process process of compressing compressing and and expanding.With expanding.With companded systems,the higher amplitude analog signals are compressed prior to transmission and then expanded at the receiver. 10.Define granular noise.How it is reduced. When the original input signal has relatively constant amplitude,the reconstructed signal has variations that were not present in the original signal.This is called granular noise. Granular noise can be reduced by decreasing the step size. 11.Define Aperture effect. In flat top sampling,due to the lengthening of the sample, amplitude distortion as well as a delay of T/2 was introduced.This distortion is referred to as Aperture effect. 12.How aperture effect can be corrected? Aperture Apertur e effect can be be corrected by connecting connect ing an equalizer in cascade with the low low pass reconstruction filter. This equalizer has the effect of decreasing the in-band loss of reconstruction filter as the frequency increases in such a manner as to compensate for the aperture effect. 13. Write an expression for band width of binary PCM with N messages with maximum frequency of FmHz ? If ‘v’ number bits are used to code each input sample then bandwidth of PCM is given as BT >=N.v.Fm .Here v.f m is the bandwidth required by one message. 14. How is PDM wave converted into PPM system? The PDM signal is given as clock signal to monostable multivibrator. The multivibrator triggers on the falling edge .hence the PPM pulse width is produced after falling edge of PDM pulse. PDM signal represent the input signal amplitude in the form of width of pulse. A PPM pulse after this width of PDM pulse. 15. Mention the use of adaptive quantizer in adaptive digital wave form coding scheme. Adaptive quantizer changes its step size according to variance of input signal. Hence quantization error is reduced. ADPCM uses adaptive quantization. The bit rate of such schemes reduced due to adaptive adaptive quantization. 16. What do you understand from adaptive coding? In adaptive coding quantization step size and prediction filter co-efficient are changed as per properties of input signals. This quantization error and number of bits used to represent the sample value. Adaptive coding is used at low bit rates. r ates. GCE, NAMAKKAL

Page 2


17. What is the advantage of delta modulation over PCM? Delta modulation uses one bit to encode one sample. Hence bit rate of delta modulation is low compared to PCM. 18. Define Dirac comb or ideal sampling function. What is its Fourier Transform? Dirac comb is nothing but a periodic impulse train in which the impulses are spaced by a time interval of Ts seconds. The equation for the function is given By

The Fourier transform of

is given by

19. Give the interpolation formula for the t he reconstruction of the original signal g(t) From the sequence of sample values {g(n/2W)}.

Where, 2W is the bandwidth N is the number of samples. 20. Define Quadrature sampling. Quadrature sampling is used for uniform sampling of band pass signals Consider The in-phase component gI(t) and the Quadrature component gQ(t) may be respectively and then suppressing the sum-frequency components by means of appropriate low pass filter. Under the assumption that fc>W, fc>W, we find find that gI(t)&gQ(t) are both low-pass low-pass signals limited to -W
Where, E is the aliasing error. |G(f)| is the amplitude spectrum of the signal g(t)

Where, E is the bound for aliasing error. GCE, NAMAKKAL

Page 3


22. 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. 23. Define quantizing process. pro cess. The conversion of analog sample of the signal into digital form is called Quantizing process. 24. What are the two fold effects of quantizing process? 1. The peak-to-peak range of input sample values subdivided into a finite set of decision levels or decision thresholds. 2. The output is assigned a discrete discret e value selected from a finite set of representation levels are reconstruction values that are aligned with the treads of t he staircase. 25. What is meant by idle channel noise? Idle channel noise is the coding noise measured at the receiver output with Zero transmitters input. 26. What is meant by prediction error? The difference between the actual samples of the process at the time of interest predictor output are called a prediction error.

and the

27. Define delta modulation. Delta modulation is the t he one-bit version of differential pulse code modulation. 28. 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). 29. Name the types of uniform quantizer. 1. Mid tread type quantizer. 2. Mid riser type t ype quantizer. quantizer. 30. Define mid tread tr ead quantizer. quantizer. Origin of the signal lies in the middle of a tread of the staircase.

GCE, NAMAKKAL

Page 4


31. Define mid-riser quantizer. Origin of the signal lies in the middle of a riser of the staircase

32. Define quantization error. Quantization error is the difference d ifference between the output and input values of quantizer. 33. What do you mean by non-uniform quantization? Step size is not uniform. Non-uniform quantizer is characterized by a step size that increases as the separation from the origin of the transfer characteristics is increased. Non-uniform quantization is otherwise called as robust quantization. 34. Draw the quantization error for t he mid tread and mid-rise type of quantizer. For mid tread type:

35. What is the t he disadvantage disadvantage of uniform quantization over the non-uniform quantization? SNR decreases with decrease in input power level at the uniform quantizer but non uniform quantization maintains maintains a constant SNR for wide range of input power levels. levels. This type t ype of quantization is called as robust ro bust quantization. 36. What do you mean by companding? Define compander. The signal is compressed at the transmitter and expanded at the receiver. This as companding. The combination of a compressor and expander is called a compander.

is called called

37. Draw the block diagram of o f compander? Mention the types t ypes of companding? Block diagram:

38. What is the need for speech coding at low bit rates? The use of PCM at the standard rate of 64 Kbps demands a high channel bandwidth for its transmission, so for certain applications, bandwidth is at premium, in which case there is a definite need for speech coding at low bit rates, while maintaining acceptable fidelity or quality of reproduction.

GCE, NAMAKKAL

Page 5


39. Define ADPCM. It means adaptive differential pulse code modulation, a combination of adaptive quantization and adaptive prediction. Adaptive quantization refers to a quantizer that operates with a time varying step size. The autocorrelation function and power spectral density of speech signals are time varying functions of the respective variables. Predictors for such input should be time varying. So adaptive predictors are used.

40. What are the components of a Digital Communication System? Sampler, Quantizer, Encoder, Modulator, Decoder, Channel, Demodulator, Reconstruction Filter are the components of a Digital D igital Communication Communication System. 41. What are the t he advantages of Digital Communication? Ruggedness to channel noise and other interferences. Flexible implementation of digital hardware system .Coding of digital signal to yield extremely low error rate and high fidelity. Security of information. 42. What are the different types of PTM systems? There are two kinds of PTM schemes they are: Pulse duration or pulse width or pulse length length modulation (PDM (Or) PWM (Or) (PLM) Pulse position modulation 43. What is pulse duration modulation (PDM)? The method in which the samples samples of the message message signal signal are used to vary the width of the individual individua l pulses. This is referred referr ed to as pulse duration modulation. modulat ion.

duration

44. What is pulse position modulation? In PPM, the position of a pulse relative relative to its unmodulated time of occurrence is varied in accordance with the message signal. 45. What is pulse digital d igital modulation scheme? It is is the modulation modulation in which the message and also the carrier are in discrete These are classified as Pulse code modulation, Delta modulation

form.

46. What are the t he advantages of digital representation of analog signals? Ruggedness to transmission noise and interference Efficient regeneration of the coded signal along the transmission path. The possibility of a uniform format for the different kinds of o f baseband signals. signals. 47. Discuss Noise effect in PCM. The performance of a PCM system is influenced by two major sources of noise. Transmission Noise: Noise: Which is introduced anywhere anywhere transmitter output and the receiver receiver input. It is also named as channel Noise. Quantizing noise: This is introduced in the transmitter tr ansmitter and is carried along to the receiver output. 48. What delta modulation and give the comparison between DM and DPCM? Delta modulation is one bit (or) two level version of DPCM. They are similar except for for two important difference namely, namely, the use of one bit quantizer in delta modulator and the prediction filter is replaced by a single delay element.

GCE, NAMAKKAL

Page 6


49. Discuss the noise effect in delta modulation? In delta modulation we observe quantization noise .there are two major sources of quantizing error in DM systems. They are Slope over load distortion, Granular noise 50. Define multiplexing. Multiplexing may be defined as a technique which allows many user to share a common communication channel simultaneously. There are two major types of multiplexing techniques; Frequency Frequency division multiplexing (FDM), Time division multiplexing (TDM). 51. Give the different types of multiplexer. There are basically two types of multiplexer Low speed multiplexer High speed multiplexer 52. State band pass sampling theorem? The band pass signal x(t) whose maximum band width is 2w can be completely represented into and recovered from its samples. if it is sampled at the minimum rate of o f twice the bandwidth. 53. What are the two t wo limitation of delta modulation? 1. Slope overload distortion. d istortion. It occurs due to limited step size and fast variation in the signal. 2. Granular noise It occurs due to too large step size and very small amplitude variations in the input signal. 54. Why compressors are used in PCM? 1. With linear quantization, the signal to quantization noise ratio reduces at low signal levels. 2. Compression uses nonlinear quantization. It improves the signal to quantization noise ratio at low level signals. 55.

A band pass signal has has the spectral spectral has the spectral spectral range that extends from 20khz to 82khz. Find the acceptable range of sampling frequency f s? Here bandwidth, Bw = 82khz-20khz = 62 khz f s= 2×bw = 2×62khz = 124khz 8

56. What is the SNR of PCM system if number of quantization levels is 2 ? 8 υ Quantization levels= 2 =2

υ=8 (s/n)db= 4.8+6υdb =4.8+6×8 =52.8db

GCE, NAMAKKAL

Page 7


UNIT-2 BANDLIMITED SIGNALLING

1. What is PAM? PAM is the pulse amplitude amplitude modulation. modulation. In pulse amplitude 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. 2. What is ISI? Inter Symbol Interference arises when the communication channel is dispersive. Inter symbol interference is caused by overlapping tails of the pulse with adjacent pulses. The residual effect due to the occurrence of pulses before and after the sampling instant t i is i s called ISI. It is a major source of bit errors in the reconstructed data stream at the receiver. 3. Define Eye pattern. Eye pattern is defined as the synchronized superposition of all possible realizations of the signal of interest viewed within a particular signaling interval. The interior region of the eye pattern is eye opening. 4. What is the information provided by the eye pattern? The width of the eye opening defines the time interval over which the received signal can be sampled without error from ISI. The sensitivity of the system to timing error is determined by the rate of change of eye as the sampling time is varied. 5. What is correlative coding? Correlative level coding is used to transmit a base band signal with the signaling rate of 2Bo the Chanel of bandwidth Bo. This made physically possible by allowing ISI transmitted controlled manner. manner. This ISI is known to the receiver. The correlative coding coding is implemented implemented by duo binary signally signal ly and modified duo binary signaling. 6. What is an intersymbol interference in base band binary PAM system? In base band binary PAM symbol are transmitted one after another. These symbols are separated by sufficient time duration. The transmitter channel and receiver acts as filter to this base band data. Because of the filtering characteristics transmitted PAM pulses are spread in time. Let the transmitted wave form be represented as ,

∞ X(t)= ∑ AK g(t -kTb) k=-∞

7. Define duobinary base band PAM system. Duo binary encoding reduces the maximum frequency of the base band signal. The word ‘duo’ means to double the transmission capacity of the binary system. Let the PAM signal ak represents kth bit. Then the encoder generates the new wave form as Ck=ak+ ak-1 Thus two successive bits are added to get encoded value of kth bit. Hence Ck becomes a correlated signal even though ak is not correlated. This introduces intersysmbol in the controlled manner to reduced the band width.

GCE, NAMAKKAL

Page 8


8. What is the main disadvantage of duo binary scheme? The output of duo binary encoder is given as Ck =ak + ak -1 Let ak represents an estimate of ak at the decoder. Then above equation becomes C k =a =ak + a k -1.Hence -1.Hence ak can be obtain as a k =c =ck + ak -1 -1 .This shows that if c k is received with error then a k will have error. This error will propagate the output sequence. This is the main draw back of duo binary encoding. 9. What are eye patterns? Eye pattern is used to study the effect of ISI base band transmission i. Width of eye opening defines the interval over which the received wave can be sampled without error from ISI. ii. The sensitivity of the system to timing error is determined by the rate of closer of the eye has the sampling time is varied. iii. Height of the eye opening at sampling time t ime is called margin over noise.

10. What is the value of maximum signal to noise ratio of the matched filter? When it become maximum. Maximum signal to noise ratio of the matched filter is the ratio of energy of the signal to psd of white noise Pmax=E/No/2 This maximum value of occurs at the end of bit duration Tb. 11. What is the effect of thermal noise on PCM system? The effect of thermal noise on PCM system is to cause the matched filter detector to make an occasional error in determining whether a binary 1 or binary 0 was transmitted. If the thermal noise is white and Gaussian, the probability of error depends on the ratio E b / η. 12. What is precoding? A practical means of avoiding the error propagation phenomenon is to use precoding before duobinary coding. The precoding operation performed on the binary data sequence {b k } converts into another binary sequence {d k } defined by d k = bk + dk-1, where the symbol + denotes modulo- two addition. 13. What is modified duobinary signaling? Modified duobinary signaling or Class IV partial response technique, which involves a correlation span of two binary digits. This special form of correlation is achieved by subtracting amplitude modulated pulses spaced 2Tb seconds apart. 14. What is meant by forward and backward estimation? AQF: Adaptive quantization with forward estimation. Unquantized samples of the input signal are used to derive the forward estimates. AQB: Adaptive quantization with backward estimation. Samples of the quantizer outputs are used to derive the backward estimates.

GCE, NAMAKKAL

Page 9


APF: Adaptive prediction with forward estimation, in which Unquantized samples of the input signal are used to derive the forward estimates of the predictor coefficients. APB: Adaptive prediction with backward estimation, in which Samples of the quantizer output and the prediction error are used to derive estimates of the predictor coefficients. 15. What are the limitations of forward estimation with backw backward ard estimation?  Side information  Buffering  Delay 16. How are the predictor coefficients determined? For the adaptation of the predictor coefficients the least mean square (LMS) algorithm is used. 17. Define adaptive sub band coding. It is a frequency domain coder, in which the speech signal is divided in to number of sub bands and each one is coded separately. It uses non masking phenomenon in perception for a better speech quality. The noise shaping is done by the adaptive bit assignment. 18. What are formant frequencies? frequencies? In the context of speech production pro duction the formant frequencies are the resonant frequencies of the vocal tract tube. The formants depend on the shape and dimensions of the vocal tract. 19. What is the bit rate in ASBC? Nfs= (MN) (fs/M) Nfs->bit rate M->number of sub bands of equal bandwidths N->average number of bits fs/M->sampling rate for each sub band 20. Define Adaptive filter. It is a nonlinear estimator that provides an estimate of some desired response without requiring knowledge of correlation functions, where the filter coefficients are data dependent. A popular filtering algorithm is the t he LMS algorithm. 21. Define data signaling Rate. Data signalling rate is defined as the rate measured measured in terms bits per second which data are transmitted. Data signaling rate Rb=I/Tb Where Tb=bit duration.

(b/s) at

22. 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. 23. State NRZ unipolar format. In this format binary 0 is represent by no pulse and binary 1 is represented by the positive pulse. GCE, NAMAKKAL

Page 10


24. State NRZ polar format. Binary 1 is represented by a positive pulse and binary 0 is represented by a negative pulse. 25. State NRZ bipolar format. Binary 0 is represented by no pulse and binary one is represented by the alternative positive and negative pulse. 26. State Manchester format. Binary the first half bit duration negative pulse and the second half bit duration positive pulse. Binary first half bit duration positive pulse and the second half bit duration negative pulse. 27. What is the width of the eye? It defines the time interval over which the received waveform can be sampled without error from intersymbol intersymbol interference. 28. What is sensitivity of an eye? The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied. 29. What is margin over noise? The height of the eye opening at a specified sampling time defines the margin over noise. 30. How eye pattern is obtained? 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. 31. Mention the need of optimum o ptimum transmitting and receiving filter base band data transmission. When binary data is transmitted over base band channel noise interference with it. Because of this noise interference error introduced in signal detection. Optimum filter perform to functions while receiving the noisy signal. i. Optimum filter integrate a signal during the bit interval and check the output at the time instant where signal to noise ratio is maximum. ii. Transfer function of the optimum filter is selected so as to maximize signal to noise ratio. iii. Optimum filter minimizes the probability of error. iv. 32. How is eye pattern obtained on the CRO? Eye pattern can be obtained obtained on the CRO CRO by applying the signal tonone of the input channels channels giving an external external trigger of (1/Tb)hz.this makes one sweep of beam equal to ‘T b seconds. 33. What is the condition for zero inter symbol interference? Zero ISI can be obtained obta ined if the transmitt ed pulse satisfies satisf ies the following condition: Time domain: Þ[(i-k)]= {1 for i=k 0 for i≠k Frequency domain: ∞ = ∑ p (f - nf b) =Tb k=-∞

GCE, NAMAKKAL

Page 11


34. From the eye pattern, how is the best t ime for sampling determined? It is preferable to sample sample the instant at which eye is open widest. At this instant chances of error are minimum. 35. What is the purpose of using an eye pattern? Eye pattern can be used for: 1. To determined an interval over which the received r eceived wave can be sampled sampled without error due to ISI. 2. To determine the sensitivity of the system to timing t iming error. 3. The margin over the noise is determined from the eye. 36. Why do you need adaptive equalization in a switched telephone network? In a switched switched telephone network the distortion depends upon upon 1. Transmission Transmission characteristics of o f individual links. 2. Number of links in connection. Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. The adaptive equalization is used. 37. What is an ideal nyquist channel? P(t)=sin(2πBot)/2πBot Such pulses have the spectrum of, P(f)={1/2B0 for –b0
GCE, NAMAKKAL

Page 12


UNIT-3 PASS BAND DATA TRANSMISSION

1. Define ASK. In ASK carrier is switched on when binary ‘1’ is to be transmitted transmitted and it is switched off when binary ‘0’ transmitted ASK is also called on-off keying. 2. What is meant by DPSK? In DPSK input sequence is modified. Let input sequence be d(t) and output sequence b(t). How b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0. When b(t) changes level phase of the carr ier is changed. And as started above b(t) changes changes it level only if d(t)=1. Hence the technique technique is called differential PSK. PSK. 3. Explain coherent detection det ection.. In coherent detection the local carrier generated at the receiver phase locked locked with carrier carr ier at the transmitter. transmitter. The detection is is done done by correlating received received noisy noisy signal and locally generated carrier. The coherent detection is a synchronous detection. 4. What is the difference between PSK&FSK? In PSK phase of the carrier is switched according to input bit sequence .In FSK frequency of the carrier is switched according to input bit sequence.FSK needs double of the bandwidth of PSK . 5. What is mean by coherent ASK? In coherent ASK correlation is used to detect the signal. Locally generated `carrier is correlated with incoming ASK signal. The locally generated carrier is in exact phase with the transmitted carrier. carr ier. Coherent ASK is also called synchronous synchro nous ASK. 6. What is the major advantage of coherent PSK over coherent ASK? ASK is on off signaling where as the modulated carrier is continuously transmitted in PSK. Hence peak power requirement require ment is more in ASK whereas it it is reduced in case of PSK. 7. What are the properties of matched filter? The signal to noise ratio of the matched filter depends only upon the ratio of the signal energy to the psd of white white noise at the filter filter input 1) The The output output signal signal of a matched filter is proportional to a shifted version of the autocorrelation function of the input signal to which the filter is matched. 8. Why do we go for Gram-Schmidt Gram-Schmidt Orthogonali Ort hogonalization zation procedure? Consider a message signal m. The task of transforming an incoming message mi=1, 2,…..M, into a modulated modulated wave wave si(t) si(t) may be divided into into separate separate discrete discrete time & continuous continuous time operations. The justification for this separation lies in the Gram-Schmidt Orthogonalization procedure which permits the representation of any set of M energy signals, {si(t)}, as linear combinations combinat ions of N orthonormal orthono rmal basis functions, where N ≤M.

GCE, NAMAKKAL

Page 13


9.

What is matched filter receiver? A filter whose impulse response is a time reversed & delayed version of some signal π j (t) then it it is said to be matched matched to π j (t) correspondingly, the optimum receiver based on the detector is referred to as the matched matched filter receiver.

10. What is maximum likelihood detector? Maximum likelihood detector computes the metric for each transmitted message compares them and then decides in favor of maximum. The device for implementing the decision rule i.e.; set ^m = mi if In [ fx(x/mk)] is maximum for k=i is called maximum –likelihood detector and the decision decision rule is called maximum likelihood. 11. Define antipodal signals. A pair of sinusoidal signals that differ only in a phase shift of 180 degrees are referred to as antipodal signals. 12. Explain how QPSK differs from PSK in term of transmission bandwidth and bit Information it carries? For a given bit rate 1/Tb, a QPSK wave requires half the transmission bandwidth of the corresponding binary PSK wave. Equivalently for a given transmission bandwidth, a QPSK wave carries twice as many bits of information as the corresponding binary PSK wave. 13. Give the equation for average probability of symbol error for coherent coherent binary PSK. Average probability probability of signal s ignal error, Pe = 1 / 2 erfc √Eb Eb / No 14. Give the signal space characterization of QPSK.

15. Define QPSK. QPSK is Quadriphase –shift keying. In QPSK the phase of t he carrier takes on one of o f the four

equally spaced values Such as π/4 , 3π/4, 5π/4 and 7π/4.

16. Define Dibit. A unique pair of bits is called a dibit. Gray encoded set of dibits 10, 00, 01 & 11 17.

Give the transmi t ransmitted tted signal of Non-coherent binary FSK. 2Eb/Tb Cos(2πf i t) , 0 ≤  t ≤Tb Tb Si(t) = {√2Eb/Tb O, elsewhere fi = nc+ i/ Tb 18. Give the two basic operation of DPSK transmitter. 1. Differential encoding of the input binary wave 2. Phase –shift keying hence, the name differential phase shift keying. ke ying.

GCE, NAMAKKAL

Page 14


19. Define deviation ratio in MSK? The parameter h is defined by h= Tb(f1-f2) h is deviation ratio , measured with respect respect to bit rate 1/Tb.



20. Define MSK signal in interval 0 ≤ t≤ Tb. S(t) = √2Eb/Tb Cos [ 2π f 1t + θ(0)] for symbol 1 √2Eb/Tb Cos [ 2π f 2 t + θ(0)] for symbol 0 21. What is nominal carrier frequency in MSK? Nominal carrier frequency is the arithmetic mean of the two frequencies f1 and f2 and it is given as fc = ½ (f1 + f2) Where f1 is the frequency for symbol –1 f2 is the frequency for symbol – 0 22. 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 spectrum of the modulated modulated carrier signal contains a discrete component component at the carrier frequency. frequency. 23. What are the three broad types of synchronization? 1. Carrier synchronization synchronization 2. Symbol & Bit synchronization synchronization 3. Frame synchronization. 24. What are the two methods for carrier synchronization? synchronization? th Carrier synchronization using M Power loop Costas loop for carrier synchronization synchronization 25. What is called symbol or bit synchronization? 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 of these times of bit or symbol is called symbol or bit synchroniza s ynchronization. tion. 26. What are the two methods of bit and symbol synchronization? Closed loop bit synchronization Early late gate synchronizer 27. What are the t he disadvantages of closed loop bit synchronization? If there is a long string of 1’s and o’s 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, the Synchronization Synchronization suffers from timing Jitter.

GCE, NAMAKKAL

Page 15


28. In minimum shift keying what is the relation between the signal frequencies and bit rate? Let the bit rate be f b and frequency of carrier be f 0. 0.the higher and lower MSK signal frequencies are given as, fH= f 0+(f b /4) fL=f 0+(f b /4) 29. Write the expression for bit error rate for coherent binary FSK? The bit error rate of coherent binary FSK is given as Pe=1/2erfc √(0.6 /n0) 30. What is the error probability probability of MSK MS K and DPSK? Error probability of MSK: Pe=1/2erfc√(eb /n0) Error probability of DPSK: (-eb/n0)

Pe= 1/2e

31. Highlight the major difference between a QPSK signal and a MSK signal. MSK signal have continuous phase in all the cases, cases, whereas whereas QPSK signal

abrupt phase phase shift of π/2 or π.

has

32. Compare the probability of error of PSK with w ith that of FSK? BPSK: Pe= 1/2 erfc√(eb /n0) BFSK: Pe=1/2 erfc √(0.6eb /4n0)  For the fixed value of eb /n0, error probability probability of o f BPSK is less than BFSK.  For the given probability probability of error, error, the eb /n0 of BPSK is 3db less compared to that BFSK. 33. 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. 34. What is noncoherent modulation? Whenever it is impractical to have knowledge of the carrier phase at the receiver, noncoherent modulation is employed. One of these two signals is sent over an imperfect channel that shifts the carrier phase by an unknown amount. Let g 1(t) and g2(t) denote the phase shifted versions of s 1(t) and s2(t), respectively. It is assumed that the signals g 1(t) and g2(t) remain orthogonal and have the same energy E, regardless of the unknown carrier phase. Such a signaling scheme is referred as noncoherent modulat ion.

GCE, NAMAKKAL

Page 16


35. What are the needs for data compression? The data compression compression is employed in two t ypes ypes of situat ions: a. In source coding where the permitted coding alphabet cannot cannot exactly represent the information source. b. I n f o r m a t i o n trans transmi missi ssion on at at a ra rate gre greate aterr than than the the cha chann nnel el capaci capacity. ty. 36. When M-ary signaling schemes are preferred pre ferred over binary signaling schemes and why? wh y? M-ary signaling schemes are preferred over binary signaling schemes when t he requirement requirement is to conserve bandwidth bandwidth at the expense of increased power. In binary PSK, bandwidth required in inversely proportional to T b, whereas in M-ary, the bandwidth required is inversely proportional to nT b, reduction in bandwidth by the factor n = log2M over binary PSK.

UNIT-4 ERROR CONTROL CODING

1.What are the error detection detection and correction capabilities of hamming codes? The minimum distance (dmin) of hamming codes is ‘3’. Hence it can be used to detect double errors or correct signal signal error. Hamming Hamming codes are basically basically linear block block codes with dmin=3. 2. How syndrome is calculated in hamming codes and cyclic codes? In hamming codes the syndrome is calculated as, S=YHT T Here Y is the received and H is the transpose transpose of parity check matrix. In cyclic codes the syndrome vector polynomial is given as, S(p)=rem[Y(p)/G(p)] Here Y(p) is received vector polynomial and G(p) is generated polynomial . 3. What is BCH code? BCH codes are most extensive and powerfull error correcting cyclic codes. The decoding of BCH codes is comparatively comparatively smaller. For For any positive inter inter ‘m’ and and ‘t’ (where t<2 m-1) there exists a BCH code with following parameter: Block length:N=2 m-1 Number of parity bits: n-k<= mt. Minimum distance: dmin>=2t+1. 4. What is RS code? These are non binary BCH codes. The encoder for RS codes operates on multiple bits simultaneously. simultaneously. The (n,k) RS code take the group of m-bit symbols of the incoming binary data stream. stream. It takes such ‘k’ number of symbols in one block. Then the encoder adds (n-k) redundant symbols to form the code word of ‘n’ symbols. Rs codes have: m-1 Block length: N=2 symbols Message size: K symbols Parity checks size: N-k=2t symbols Minimum distance: d min=2t+1 symb s ymbols. ols.

GCE, NAMAKKAL

Page 17


5. What is the difference between block code and convolution code? Block code take ‘k’ number number of message message bit bit simultaneously and from n bit bit code vector. vector. This code vector is also called block. Convolutional code takes one message bit at a time and generates two or more encoded bit. Thus convolutional code generates a string of encoded bit for input message string. 6. Define free distance and coding gain? Free distance is the minimum distance between code vectors. It is also equal to minimum weight of code vector. Coding Coding gain is used as bases of comparison for different different coding method. To achieve the same bit error rate the coding gain is defined as A=(Eb /No) encoded/ (E b /No) coded For convolutional coding gain is given as A=rd /2 f 7.

What is linear code? A code is linear if the sum of any two code vectors produces another code vector. 8. What is code rate? Code rate is the ratio rat io of message bits (k) and the encoder output bits ( n). It is defined by r (i.e.) r= k/N 9. Define code efficiency. It is the ratio rat io of message bits in a block to t he transmitted bits for that block by the encoder i.e.

10. What is hamming distance? The hamming distance between two code vectors is equal to the number of elements in which they differ. For example let the t he two code vectors be X= (101) and Y= (110).These two code vectors differ in second and third bits. Therefore the hamming distance between x and Y is t wo. 11. What is meant by systematic & non-systematic code? In a systematic block code, message bit appear first and then check bits. In the non-systematic code, message and check bits cannot be identified in the code vector. 12. What are the conditions to satisfy the hamming code? 1) No. of Check bits q ≥3 q 2) Block length n = 2 –1 3) No of message bits K = n-q 4) Minimum distance dmin =3 13. Define code word & block length. The encoded block of ‘n’ bits is called code word. The no. of bits ‘n’ after coding is called block length. 14. Why RS codes are called maximum distance separable codes ? ( n,k) Linear block code for which the minimum distance dist ance equals n – k + 1 is called maximum distance separable codes. For RS code minimum distance equals n – k + 1 so it is called as maximum distance separable code 15. What are Golay codes? Golay code is the (23, 12) cyclic code whose generating polynomial is, G(p) =P11+P9+p7+P6+p5+p+ =P11+P9+p7+P6+p5+p+1 1 This code has a minimum distance of d min=7. This code can correct up to 3 errors. It is perfect code. GCE, NAMAKKAL

Page 18


16. What are the advantages of cyclic codes? 1. Encoders and decoders for cyclic codes are simple 2. Cyclic codes also detect error burst that span many successive bits. 17. What is meant by syndrome of linear block code? t The non-zero output of the produce Yh is called syndrome and it is used to detect the errors in y. syndrome is denoted by’s’ and it is given as, t S=YH 18. What is convolutional code? Fixed number of inputs bits is stored in the shift register and they are combined with the help of mod-2 adders. This operation is equivalent to binary convolution and hence it is called convolution coding. 19. Define constraint length in convolution codes. Constraint length is the number of shifts over which the single message bit can influence the encoder output. It is expressed in terms of message bits. 20. What is the difference between block codes and convolutional codes? Block codes take ‘k’ number of message bit simultaneously and form ‘n’-bit code vector. This code vector is called block. Convolutional code takes one message bit at a time and generates two or more encoded bits. Thus convolution codes generate a string of encoded bits for input message string. 21. Define channel channel data rate. It is the bit rate at the output of encoder. If the bit rate at the input of encoder is R s, then channel data rate will be, Channel data rate (Ro) = (n/k) Rs] 22. Mention any two methods used for error control coding. 1. Forward acting error correction 2. Error detection with retransmission 23. Mention the two types of errors introduced during transmission on the data. 1. Random errors 2. Burst errors 24. What are the properties of cyclic code? 1. Linear property 2. Cyclic property 25. What are the needs for error control coding? The needs for error control coding are (i) To change the t he data quality quality from problematic to acceptable one o ne (ii) To reduce r educe the required Eb /No for a fixed bit error rate. (iii) This reduction in Eb /No may be exploited to reduce the required transmitted power or reduce the hardware costs by requiring a smaller antenna size in the case of radio communications. 26. What are the t ypes of error correcting codes? The codes are classified in to block codes and convolution codes. The distinguishing feature for the classification is the presence or absence of memory in the encoders for the two codes.

GCE, NAMAKKAL

Page 19


27. What is discrete memory-less channel? The waveform channel is said to be memory-less, if the detector output in a given time interval depends only on the signal transmitted in that interval, and not on any previous transmission. 28. What are systematic codes? Block codes in which the message bits are transmitted in unaltered form are called systematic codes. For application requiring both error detection and error correction, the use of systematic codes simplifies implementation of the decoder. 29. Give the structure of the code word. Consider a (n,k) code word, in which k bits of the n code bits are always identical to the message sequence to be transmitted. The remaining n-k bits are computed from the message bits in accordance with a prescribed encoding rule that determines the mathematical structure of the code and these bits are referred as generalized parity check bits or simply parity bits. b0, b1, . . . , bn-k-1

m0, m1, . . . , mk-1

30. What are repetition repetit ion codes? Repetition codes represent the simplest type of linear block codes. A single message bit is encoded into a block of n identical bits, producing a (n,1) block code. Such a code allows provision for a variable amount of redundancy. There are only two code words in the code: all zero code word and all one code word. 31. State the properties of syndrome. The syndrome depends only on the error pattern and not on the transmitted code word. All error patterns that t hat differ by a code word have the same syndrome. 32. What are cyclic codes? Cyclic codes form a subclass of linear block codes. A binary code is said to be cyclic code if it exhibits two fundamental properties: 1. Linearit Linearit y property: property: The sum of any two code words in the code is also a code word. 2. Cyclic property: Any cyclic cyclic shift of a code word in in the code is is also a code word. 33. State Channel coding theorem. t heorem. The channel coding theorem states that if a discrete memory-less channel has capacity capacity C and a source generates information at a rate less than C, then there exists a coding technique such that the output of the source may be transmitted over the channel with an arbitrarily low probability of symbol error. 34. Define Hamming weight. The Hamming weight w(c) of a code vector is defined as the number of nonzero elements in the code vector. 35. Define minimum distance d min. The minimum distance d min of a linear block code is defined as the smallest Hamming distance between any pair of code vectors in the code c ode word.

GCE, NAMAKKAL

Page 20


36. Define constraint length of a convolutional code. The constraint length of a convolutional code , expressed in terms of message bits, is defined as the number of shifts over which a single message bit can influence the encoder output. 37. Define maximum likelihood decoding of convolutional codes. Let p(r|c) denote the conditional probability probability of receiving r, given that c was sent and the loglikelihood function equals log p(r|c). Then the maximum likelihood decoder or decision rule is described as follows: Choose the estimate c for which the log likelihood function log p(r|c) is maximum. 38. Define free distance of a convolutional code. The free distance of a convolutional code is defined as the minimum Hamming distance between any two code words in the code. 39. How many errors can be corrected by a convolutional code? A convolutional code with free distance d free can correct t errors if and only if d free is greater than 2t. 40. What does the term catastrophic cyclic code represent? ‘000’ is not a state of the shift register sequence in PN sequence generator, since this results in a catastrophic cyclic code i.e. once the 000 state st ate is entered, the shift register sequence sequence cannot leave leave this state. UNIT-5 SPREAD SPECTRUM SYSTEMS

1. What is pseudo noise sequence? Pseudo noise sequence is a noise like high frequency signals. The sequence is not completely random but it is generated by well defined logic. hence it is called Pseudo noise sequence. Pseudo noise sequence is used in spread spectrum spectr um communicat ion for spreading message message signal. signa l. 2. What is processing pr ocessing gain? Processing gain =bandwidth of spreaded signal\ bandwidth of unspreaded signal For DS-SS processing gain is given as PG=Tb\Tc Tb=bit period of data sequence sequence Tc=bit period of PN sequence And for FH-SS processing gain is given as, t PG=2 Here ‘t’ is the number of bit in PN sequence. 3. What is frequency hop spread spectrum? In frequency hop spread spectrum, the spectrum of a data modulated carrier is widened by changing the carrier frequency in a pseudo-random manner and this technique rely on the availability of a noise-like spreading code called pseudo-noise sequence. 4. What is spread spectrum? Spread spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. The spread spectrum is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver (operating in synchronism with the transmitter) to despread the received signal so that the original data sequence may be recovered. GCE, NAMAKKAL

Page 21


5. What is synchronization? For proper operation, a spread spectrum communication system requires that the locally generated PN sequence used in the receiver to dispread the received signal be synchronized to the PN sequence used to spread the transmitted signal in the transmitter. It consists of acquisition and tracking. 6. What is acquisition? In acquisition or coarse synchronization, the two PN codes are aligned to within a fraction of a chip in as short time as possible. PN acquisition takes place in two steps. First the receive signal is multiplied by a locally generated PN code to produce a measure of correlation between it and the PN code used in the transmitter. Next, an appropriate decision rule and search strategy is used to process the measure the correlation so obtained to determine whether the two codes are in synchronism and what to do if they are not. 7. What is the advantage of the spread spectrum Communication? The primary advantage of the spread spectrum communication is its ability to reject interference whether it is the unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission. 8. What is direct sequence spread spectrum? In direct sequence spread spectrum, two stages of modulation are used. First the incoming data sequence sequence is used use d to modulate a wide band code. This code transforms the narrow band data sequence into a noise-like wide-band signal. The resulting wide-band signal undergoes a second modulation using a phase-shift phase-shift keying technique. 9. What is tracking? t racking? Once the incoming PN code has been acquired, tracking or fine synchronization takes place. It is accomplished using phase lock techniques, which is similar to those used for the local generation Of coherent carrier references. The principle difference between them lies in the way in which phase discrimination is implemented. 10. Give the expression for the t he (SNR)0 (SNR)o = (2Tb/Tc)(SNR)I or 10 log10(SNR)o = 10 log10(SNR)I + 3 + 10 log10(PG) dB The 3 dB term on the righ r ightt hand side of the above equation accounts for the gain in SNR that is obtained through the use of coherent detection. detection. The last term 10 log10(PG) accounts for gain in SNR obtained by the use of spread spectrum. 11. Define jamming margin. The jamming margin is defined as the ratio J/P which is given by Jamming margin)dB = (Processing gain)dB – 10 log10 (Eb/No)min Where 10 log10 (Eb/No)min = The minimum value needed to support a prescribed prescribed average probability of error. 12. What is the advantage of CDMA system? The advantage of the CDMA system is that t hat collisions are not destructive; each of the signals involved in a collision would be received with only a slight increase in error rate. 13. What is near-far problem? When an unwanted user’s received power is much larger than the received power presented by the desired user, errors can occur. This problem is referred to as near-far problem and limits the utility Of DS systems to applications where each user’s received power is same.

GCE, NAMAKKAL

Page 22


14. What are the disadvantages of spread spectrum? The disadvantages of spread spectrum are: The capabilities of physical devices used to generate the PN spread spectrum signal impose a practical limit on the attainable processing gain which is not enough to overcome the effects of some jammers of concern. DS spread spectrum is affected by near-far pro blem. 15. Why FH spread spectrum is not affected by near-far problem? In a FH system, if the transmitter and receiver are tuned to frequency fi and the interfering signal fj

≠

fi, then the demodulator does not even know the presence of the interferer since its spectrum is not in the signal bandwidth. Thus, there is no near-far problem. 16.Define a random binary sequence. A random binary sequence is a sequence in which the presence of a binary symbol 1 or 0 is equally probable. 17. State the balance property of random binary sequence. In each period of a maximum length sequence, the number of 1s is always one more than the number of 0s. This property is called the balance property. 18. Mention about the run property. pr operty. Among the runs of 1s and 0s in each period of a maximum length sequence, one half the runs of each kind are of length one, one fourth are of length two, one eighth are of length three, and so or as long as these function represent meaningful numbers of runs. This property is called the run property. 19. 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. 20. Mention the significance of spread spectrum modulation. An important attribute attr ibute of spread-spectrum modulation is that it can provide protection against externally generated interfering (jamming) signals with finite power. The jamming signal may consist of a fairly powerful broadband noise or multitone waveform that is directed at the receiver for the purpose of disrupting communications. Protection against jamming waveforms is provided by purposely making the information bearing signal occupy a bandwidth far in excess of minimum bandwidth necessary necessary to transmit it. 21. What is called processing gain? Processing Gain (PG) is defined as the ratio of o f the bandwidth of spread message signal to the bandwidth of unspreaded data signal i.e.

22. 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.

GCE, NAMAKKAL

Page 23


23. 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 anti jamming. 24. What are the three t hree codes used for for the t he anti jamming application? 1. Golay code (24, 12) 2. Expurgated Golay (24, 11) 3.maxmum 3.maxmum length shift s hift register code. 25. What is called frequency hop spread spectrum? In frequency hop spread spectrum, the frequency of the carrier hops randomly from one frequency to another frequency. 26. What is slow frequency hopping? If the symbol rate of MFSK is an integer multiple of hop rate (multiple symbols symbols per hop) then t hen it is called slow frequency hopping 27. 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. 28. 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 frequency band of Txed signal. 29. What are the features of code co de Division multiple Accesses? 1. It does not require r equire external synchronization synchronization networks. 2. CDMA offers gradual degradation in perfor mance when the no. of users is increased But it is easy to add new user to the system. 3. If offers an external interference inter ference rejection capability. capability. 30. What is called multipath Interference? The interference caused by the interfacing of the signal form the indirect path with the signal of direct path is called multipath interference. 31. What is the advantage of a spread spectrum technique? The main advantage of spread spectrum technique is its ability to reject interference whether it is the unintentional interference of another user simultaneously attempting to transmit through the channel (or) the intentional interference of a hostile transmitter t o jam the transmission. 32. Define code word The encoded block of “n” bits is known as code word. It consists of message bits and redundant bits. 33. Define code rate. The ratio of message bits(K) bits(K) and the encoder output bits(n) is known as code rate. Usually Usually code rate is denoted by r I.e r=k\n , we find that 0
GCE, NAMAKKAL

Page 24


UNIT I 8 MARKS

1. 2. 3. 4. 5. 6.

Drive the expression for the sampling process in time domain. Explain the quantization process with PCM block diagram Explain the generation of PPM and PWM with neat circuit circuit diagram. d iagram. Explain the frame format and signaling scheme used in T1 carrier system. Obtain an expression for the processing gain of a DPCM system. Explain the principle of Delta modulation and derive an expression for average output noise power in delta modulation 7. Explain the generation of PAM signals. 8. Explain in detail about TDM. 9. Explain briefly about the effects of under sampling. 10. A PCM system uses a uniform quantizer followed by a 7-bit binary encoder. The bit rate of the 6 system is equal to 50X10 b/s. (a) What is the maximum message bandwidth for which the system operates satisfactorily? (b) Determine the output signal-to-(quantization) noise ratio when a full-load sinusoidal modulating wave of frequency 1MHz is applied to the input. 16 MARKS

1. Describe the following systems by presenting appropriate appr opriate diagrams. (i) Time Division multiplexing (ii) delta modulation 2. Draw block diagram of differential PCM and explain the function performed by each block. 3. Explain the process of quantization, encoding encoding and decoding in PCM? In what way differential PCM is better than PCM? 4. Discuss the basic issues involved in the design of a regenerative repeater for pulse code modulation. 5. What are all the t ypes of sampling technique technique and explain about any two. 6. Compare DM with ADM and explain linear prediction filter. 7. Write brief notes notes on 1) TDM 2) FDM. FDM. 8. With necessary sketches and expressions, briefly explain about flat top sampling and chopper sampling. 9. Derive the expression expression for SNR in PCM system and compare it it with delta modulation. Explain Explain how can the SNR be improved in a PCM system? 10. Explain in detail about DPCM.

GCE, NAMAKKAL

Page 25


UNIT II 8 MARKS

1. 2. 3. 4. 5.

With neat sketches, explain the duo binary signaling scheme. With neat sketches, explain the modified duo binary signaling scheme. Write briefly about eye pattern. Derive the Nyquist criterion for distortion less transmissi t ransmission. on. Explain the cross talk produced due to high frequency frequency cutoff of the channel in PAM. What is the effect of pulse width on the t he cross talk factor ? 6. Compare the power spectra of different binary formats. 7. Generate the code words for (7,4) Hamming code. 8. State and prove the t he properties of syndrome decoding. 9. Sketch the time response and frequency response of signal with raised cosine pulse spectrum. 10. Briefly explain the Viterbi V iterbi decoding algorithm. 16 MARKS

1. Discuss on the following: (i)Baseband M-array PAM transmi tr ansmission(ii) ssion(ii) Adaptive equalization 2. Derive the expression for the matched filter and pro ve any one property. 3. Prove the effect ISI using necessary block diagram and design. 4. (i) Explain briefly about linear block codes.(4) (ii) Evaluate the syndrome S for all five probable single error patterns in (5,1) Repetition Repetition code.(6) (iii) Briefly explain the Viterbi V iterbi decoding algorithm .(6) 5. Find the optimum filter frequency response How that maximizes the output SNR When the input noise is not a white noise. 6. What do you you understand understand by the term Inter Inter –Symbol Interference Interference (ISI)?Discuss in in detail the Nyquist criterion for minimizing minimizing ISI.Explain the difficulties in implementing it in a practical system. t he block diagram of an adaptive filter and explain the LMS algor ithm. 7. (i) Draw the (ii) Explain how how eye pattern patter n is used to study the performance of a data transmission system. 8. Derive and plot the power spectra of NRZ uni polar and bipolar bipolar format signals. signals. 9. Draw the block diagram of the duo binary binary and and modified signaling signaling scheme scheme and explain with and without pre coder. 10. (i)Discuss the merits and demerits of Duobinary Signalling .(6) (ii)The binary data(0 1 1 1 0 0 1 0 1 ) are applied to the input of a duobinary encoder.Construct the duobinary encoder output and corresponding receiver output,without a precoder .Suppose due to error during transmission ,the level produced by the third digit is reduced to zero,construct the new receiver output (10).

GCE, NAMAKKAL

Page 26


UNIT III 8 MARKS

1. 2. 3. 4. 5. 6. 7.

Explain the Quadriphase Shift keying (QPSK) modulation. modulation. Enumerate on carrier and symb s ymbol ol synchronization. Write a note note on DPSK. Explain the technique used for generating N orthonormal basis functions. Compare the BER of coherent PSK coherent QPSK and coherent FSK. Obtain probability of error in terms t erms of Eb/No for QPSK. In a PSK system, system, the received waveforms S1(t)=A S1(t)=A Coswt,S2(t)=-A Coswt are Coherently Coherently detected with a matched filter. The value of A is 20 mV, and the bit Rate is 1 Mbps. Assume that the noise power spectral density h/2=10-11 h/2=10-11 W/Hz. Find t he probability probability of o f error Pe . 8. With necessary equations and signal space diagram, explain briefly about FSK system. 9. Discuss briefly about Minimum Shift Keying for a Continuous Phase Frequency Shift Keying(CPFSK) signal. 10. With necessary equations and signal space diagram, explain br iefly about FSK system. 16 MARKS

1. Draw the block diagram of QPSK transmitter and coherent QPSK receiver and explain their operation. 2. Draw the block diagram of MSK transmi tr ansmitter tter and explain the function of each block. block. 3. With necessary equations and signal space diagram, obtain the probability of error for coherent binary FSK systems. 4. Explain BPSK signal transmission and coherent BPSK reception with suitable diagrams. Derive an expression expression for the probability of symbol error for the scheme. 5. Describe the following: (i)Signal space dimensionality and processing gain . (8) (ii). Maximum length and Gold codes. (8) 6. Explain the MFSK and derive the pe. 7. Compare the all t ypes of Passband transmission. transmission. 8. Derive the error probability probability expression for the MPSK. 9. Give the block diagram diagram for the generation and detection of BFSK signal signal and Give a brief brief explanation of the same. 10. Explain the direct sequence sequence spread spectrum modulation modulation with coherent binary Phase shift shift keying .

GCE, NAMAKKAL

Page 27


UNIT IV 8MARKS

1. 2. 3. 4. 5. 6.

Write the generator matrix and parity check matrix of (7,4) hamming code. Briefly explain the viterbi decoding de coding algorithm. State and prove the t he properties of syndrome decoding. Give the special features of Trellis codes. Describe a decoding procedure for linear block code. Give the details of implementation implementat ion of cyclic encoder and decoder based on Linear feedback shift registers. 7. Explain the features of RS code. 8. State and explain the properties of maximal length sequence. 9. Explain the syndrome ‘S’ for all five probable single error patterns in(5,1) repetition code. 10. Generate the code words for (7, 4) Hamming code. 16 MARKS

(1) (2) 1. Consider a rate ½, non-systematic convolutional code with, g (p)={1,0,1}, g ={1,1,1}. Determine the encoder output corresponding to the data sequence {1,0,1,0,1}. If the first and the fourth bits of the encoded sequence are affected during transmission, demonstrate the error correcting capability of the viterbi algorithm Draw the block diagram of DS-SS transmitter and receiver and explain the function performed by each block in brief. 2. For a (2,1,3) convolution code with g1 = ( 1 0 1 1) and g2 = ( 1 1 1 1), design the encoder and find the following. (i) Generator matrix (ii) Transfer function matrix Compute Co mpute the coded output using both the methods assuming the input input u = (101101). 2 4 5 8 10 3. A (15,5) linear cyclic code has a generator polynomial, g(D)=1+D+D +D +D +D +D Draw block diagrams of an encoder and syndrome calculator for this code. Find the code polynomial 4 6 8 14 in syste yst ematic mat ic for fo rm, for for t he mess me ssag agee polynomial m (D)= (D )=1+ 1+D D2+D4. Is y(D)=1+D +D +D +D , a code polynomial? polynomial? If I f not, find the syndrome of y(D). 4. Explain any four characteristics of the following block codes (i) BCH codes (ii) CRC codes (iii) maximum length codes. 5. Let g(x) is the generator polynomial of a cyclic code C. Find a scheme for encoding the data sequence(d 0,d1….dk-1) into (n,k) systematic code C. 6. The generator matrix for a (6,3) block code is given below. Find all code vectors for t his code. 100 011 G= 010 : 101 001 : 110 7. Explain the construction of Block Code and Explain how error syndrome is calculated. 8. Explain the channel coding techni t echnique que used in t he Block codes. 9. What are all the message vectors can be extracted from the code vector that was generated by 1+X+X3. 10. Explain in detail about Orthogonal codes, Bi orthogonal codes and Trans orthogonal codes.

GCE, NAMAKKAL

Page 28


UNIT V 8MARKS

1. 2. 3. 4. 5. 6. 7. 8.

Write shorts notes on Probability Error. Write shorts notes on Jamming Margin. Write shorts notes on (S/N) of o f the DS System. State and explain the properties of maximal length sequences. Discuss the antijam ant ijam characteristics. With neat sketches, explain the frequency hop spread spectrum techniques. Explain about Maximum length and Gold codes. Discuss the ways in which fast hopping scheme and slow frequency hopping (spread spectrum) schemes could be used to mitigate multipath effect. 9. Obtain the expression for the processing gain in DS-SS(PSK) system. 10. Explain the principle of operation o peration of frequency hopped M-ary FSK spread spectrum spectru m system.

16 MARKS

1. Generate the PN sequence for [5, 4, 3, 1] and prove the all properties of the same. 2. List out and prove the Properties of the PN sequence. 3. Discuss the configurat ion of a Direct Sequence spread spectrum spectru m system with BPSK in detail. deta il. 4. With suitable block diagrams, discuss hopping spread spectrum spectru m systems.

the structure of the transmitter and Receiver of frequency

5. Draw the block diagram of DS- spread spectrum system system transmitter and receiver receiver and explain the funct function ion Performed by each block in brief . 6. Derive an expression for Jamming margin for direct sequence spread spectrum system with BPSK modulation. modulat ion. ii) An PN sequence is generated using a feedback register regist er of length length m = 4. The chip rate is 107 chips per sec. Find the length, chip duration and the period of the PN sequence. 7. List out the t he uses of the FH. Explain the FH techniques with the frequency spectrum. spectrum. 8. Explain the direct sequence spread spectrum spectrum modulation with coherent co herent binary phase shift keying. 9. Draw the block diagram of FHSS system and explain. 10. Discuss a suitable method for generating PN sequence. Explain with suitable diagrams, how the PN sequence is used in frequency hopping spread spectrum transmitter and receiver.

GCE, NAMAKKAL

Page 29

DIGITAL COMMUNICATION TWO MARKS Q&A

Recommend Documents