EC DC 2marksmarks, DIGITAL COMMUNICATION Question Bank, DC Short Answers – ECE 5th SEM Anna University. 0. By vish on. 2. DIGITAL COMMUNICATION. State sampling theorem. If a finite –energy signal satellite communication two marks question and answers QUESTION BANK. Subject Code: EC Subject Name: Digital Communication Techniques. Year/Sem: III/IV. UNIT I PULSE MODULATION. PART A (2 Marks).

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Hence higher frequencies take the form of lower frequencies. Amplitude reconstruction signal is done to remove amplitude discontinuities due to pulses. Write an expression for bandwidth of binary PCM with N messages each with a maximum frequency of fm Sith. Hence a PPM pulse of fixed width is produced after falling. PDM represents the input signal amplitude in the form of width of the pulse.

Hence quantization error qith significantly reduced due to the adaptive quantization. ADPCM uses adaptive quantization. The bit rate of such schemes is reduced due to. In adaptive coding, the quantization step size and prediction filter coefficients are changed as per properties of input signal. This reduces the quantization error and number of bits to represent the sample value.

Adaptive coding is used for speech coding at low bits rates. The quantization levels are fixed depending upon the number of bits. Here v is the number of bits used to represent samples in PCM. Hence signal to quantization noise ratio in PCM depends upon the number of bits or quantization levels.

EC Digital Communication – Two Marks and 16 Marks with Answers – All Units

Step size is made high if slope of the input signal is high. This avoids slope overload distortion. This is made physically possible by allowing ISI in the transmitted in controlled manner.

This ISI is known to. The sensitivity of the system to timing error is determined by the rate of closure digihal the eye as the sampling time is varied. This makes one sweep of beam equal to T b seconds. The transmission characteristics keep on changing.

Therefore adaptive equalization is used. The width of the eye opening defines. When the signal is passed through the channel distortion is introduced in terms of 1 amplitude 2 delay this distortion creates problem of ISI. In the raised cosine spectrum, the frequency response P f decreases towards zero gradually That is there is no abrupt transition.

The nyquist bandwidth is the minimum transmission bandwidth for zero ISI.

EC2301 Digital Communication Question Bank – 16 Mark Questions | 5th Semester ECE Anna University

Mention the need of optimum transmitting and receiving filter in ba s communicatjon data transmission. When b t changes level, phase of the carrier is changed. Hence the technique is called Differential PSK. The locally generated carrier is in exact phase with the transmitted carrier.


The impulse response of. What is the value of maximum signal to noise ratio of the matched filter? When it becomes maximum?

This equation shows that error probability depends only on energy but not on ef2301 of the signal. In this method, the receiver carrier need not be phase locked with transmitter carrier.

Hence it is called envelope detection. Write the expression for bit error rate for coherent binary FSK. Bit error rate for coherent binary FSK is given as. Multiplexing communiication can be used for bandwidth conservation. Transmission can be done by communicatioon wireless channel also. The decoding of BCH codes is comparatively simpler.

The n, k RS code takes the groups of m- bit symbols of incoming binary data stream. Thus convolutional codes generate a string of encoded bits for input message string.

Constraint length is the number of shift over which the single message bit can influence the encoder output. It is expressed in terms of message bits. Coding gain is used as a basis of comparison for different coding methods. To achieve the same bit error rate the coding gain is defined as.

The non zero output of the produce YH. The decoding delay is small in convolutional codes since they operate o smaller blocks of data. The storage hardware required by convolutional decoder is less dkgital the block sizes are smaller. Since only one previous digial bit is stored. Because of software implementation of decoding schemes they a re quite flexible compared to hardware implementation of other schemes.

This code can correct upto 3 errors.

But Golay code cannot be generalized to other combinations of n and k. Constraint length is the number of shifts over which the single message bit can influence the encoder output. This expressed in terms of message bits. This special code is known.

Pseudo noise sequence is a noise like high frequency signal. The sequence is not completely random, but it is generated by well defined logic. Hence it is called. In direct sequence spread spectrum modulation, the pseudo-noise sequence is directly modulated with data sequence.

Thus pseudo-noise sequence acts as high frequency carrier and data sequence acts as low frequency modulating signal. The pseudo-noise sequence and data sequence are applied to a product modulator. The output of product modulator can be used directly or it can further generate BPSK signal for long distance communication. These frequency slots are selected with the help of pseudo-noise sequence. Selection of frequency slots is called frequency hopping. The bandwidth of frequency hop spread spectrum is very much large compared to direct sequences.


Frequency hop spread spectrum is of two types: I slow frequency hopping and ii fast frequency hopping. Jamming margin is the ratio of average power of interference J to average power of data signal P. This secrecy capability of spread spectrum is used in military as well as in many commercial applications. When the PN sequence has the length of 2 m – 1, it is called maximal length sequence.

KWhat is the period of the maximal length sequence generated using 3 bit shift register. The frequency of the carrier is changed hopped according to bits of PN sequence. Spread spectrum systems are essentially synchronous.

The pseudo noise sequences generated at the receiver and the transmitted must be same and. The synchronization of the spread spectrum systems can be considered in two parts: Antijamming with the help of direct sequence spread spectrum signals. Low delectability signal transmission or low probability intercept. Where is the Menu? How to request Study Material? What is meant by aliasing effect?

This interference of the frequency components is called as aliasing effect. PWM is basically pulse width modulation. Width of the pulse changes according to amplitude of the modulating signal.

It also referred as pulse duration modulation or PDM. A bandlimited signal of finite energy, which has no frequency components higher than W Hz, may be completely recovered from the knowledge of its samples taken at the rate of 2W samples per second. Mention the merits of DPCM. Quantization error is reduced because of prediction filter 3.

DM encodes the input sample by one bit. DPCM can have more than one bit of encoding the sample. It communicarion the information. How the message can be recovered from PAM?

In other words, the position of the PPM pulse depends upon input signal amplitude.

Mention the use of adaptive quantizer in adaptive digital waveform co d ing schemes. Adaptive quantizer commhnication its. The bit rate of such schemes is reduced due to adaptive quantization.

What do u understand from adaptive coding? What is meant by quantization?