**SHORT QUESTIONS WITH ANSWERS**

**Q.1. What is Baseband binary data transmission system ? **

**Ans. **The data transmission system which makes use of a baseband channel for transmitting a binary data is known as baseband data transmission i.e., the system using a channel whose bandwidth is equal to the bandwidth of the message signal, called a baseband channel is referred to as baseband data transmission system.

**Q.2. What is a Baseband binary PAM system? Explain. **

**Ans. **Pulse amplitude modulation scheme is a system in which the amplitude of discrete pulses (carrier) vary along with the message signal. Thus, baseband binary PAM system is the scheme in which the amplitude transmitted pulses take any one of the two amplitude levels i.e., either 1 (or) 0.

**Q.3. Draw the block diagram showing the elements of baseband binary PAM systems. **

**Ans. **Figure 6.55 show the elements of a baseband binary PAM system.

**diagram**

**figure 6.55.**

**Q.4. What is the drawback of binary PAM system? **

**Ans. **Following are the drawbacks:

(i) The bandwidth required for baseband binary PAM is approximately Hz, in order to transmit data at a rate of r* _{b}* bits/sec, with zero ISI.

(ii) If the bandwith available is exactly Hz, then the binary PAM system transmits data at a rate r

*bits/sec only if it uses an ideal low pass filter at the transmitter and receiver.*

_{b}(iii) Thus, such filters are physically unrealizable and if any systems are approximated to these filters then those systems are extremely sensitive to the perturbations (or) disturbances in rate, timing (or) channel characteristics.

This is the drawback baseband binary PAM system.

**Q.5. What do you mean an Eye Diagram ?**

**Ans.**The performance of baseband PAM system depends on the amount of ISI and channel noise. The distribution of ISI and channel noise in the system can be readily observed by displaying the received waveform Y(t) on an oscilloscope using a sweep rate that is a fraction of the symbol rate r

_{s}. The resulting display shape resembles a human eye and is widely known as the Eye pattern of the system.

**Q.6. What is Equalization?**

**Ans.**The process of correcting channel induced distortion is called equalization.

**7. What is the effect of ISI? Explain.**

**Ans. **Following are the effects of ISI

(i) In the absence of ISI and noise, the transmitted bit can be decoded correctly at the receiver.

(ii) The presence of ISI will introduce errors in the decision at the receiver output.

(iii) Hence, the receiver can make an error in deciding whether it has received a logic 1 or a logic 0.

**8. What is Remedy to Reduce ISI?**

**Ans. **Remedy to Reduce ISI

(i) It has been proved that the function which produces a zero intersymbol interference is a sinc function. Hence, instead of a rectangular pulse if we transmit a sine pulse then the ISI can be reduced to zero.

(ii) This is known as Nyquist Pulse Shaping. The sine pulse transmitted to have a zero ISI has been shown in figure 6.56 (a).

(iii) Further, we know that Fourier transform of a sine pulse is a rectangular function. Hence, to preserve all the frequency components, the frequency response of the filter must be exactly flat in the pass band and zero in the attenuation band as shown in figure 6.56 (b).

**diagram**

**figure 6.56.**

**Q.9. What is Nyquist’s criterion for distortless transmission? Explain. **

**Ans. **We have observed that in absence of the ISI, we have

y(t_{i}) = a_{i}

This expression shows that under these conditions, the i^{th} transmitted bit can be decoded correctly In order to minimize the effects of ISI, we have to design the transmitting and receiving filters properl. The transfer function of the channel and the shape of transmitted pulse are generally specified. Therefore, it becomes the first step towards design of filters. From this information we have to determine the transfer functions of the transmitting and receiving filters, to reconstruct the transmitted data sequence {b_{k}}. This is achieved by first extracting and then decoding the corresponding sequence of weights from the output y(t).

**QUESTIONS**

- Explain the difference between bandpass transmission and passband transmission.
- Explain various PAM digital formats.
- Explain the followings:

(i) Intersymbol Interference, (ii) Eye pattern

- What is Line coding ? Explain.
- What are the properties of Line coding?
- Draw the block diagram of a baseband digital communications of various blocks.
- What is Nyquist criterion of zero ISI ? Explain.
- Why do we need to use the dicrete PAM formats?
- State the important properties of line codes.
- What is the meaning of word “RZ”.
- State true or false

(a) The dc value of unipolar RZ format is zero.

(b) The unipolar NRZ signal used two different amplitudes

(c) Polar NRZ format has a zero dc value

- In the bipolar NRZ or AMI line codes, the binary 0 is represented by

(a) alternate zeros and ones.

(b) alternate + A and -A amplituds.

(c) zero amplitudes

13 The synchronization at the receiver is better for:

(a) Bipolar RZ (b) Bipolar NRZ (c) Manchester

- Explain the various techniques to detect the baseband digital signals.
- Explain how integrator is used to detect baseband digital signals. Obtain an the expression for signal to noise ratio of Integrate and Dump receiver.
- Derive an expression for error probability for an integrator.
- Derive an expression for error probability for optimum filter.
- Derive an expression for transfer function for optimum filter.
- Explain what is matched filter. How it differs from optimum filter. Derive an expression for impulse response of the matched filter.
- Derive an expression for error probability of matched filter.
- What is a correlator? Explain is the difference and similarity between correlator and matched filter?
- Derive expressions for the error probabilities of binary PCM, ASK, PSK and FSK signals which use matched filter detection in presence of white Gaussian noise.

**PROBLEMS**

- Represent the data 10110100 using the following digital data formats with the help of neat figures:

(i) Unipolar RZ

(ii) Unipolar NRZ

(iii) Split phase Manchester.

- Given the binary sequence 1101110, draw the transmitted pulse waveform for

(i) AMI RZ signalling format, and

(ii) Split-phase (Manchester) signaling format.

- In a binary PCM system on/off signaling is used. The matched filter receiver is used for detection of the signal. Calculate the probability of error if signaling rate is doubled.

[**Hint:** If signaling rate is doubled, bit period ‘*T’* is reduced by half, hence energy is also reduced by half.]

- An FSK system transmits binary data at the rate of 2.5 x 10
^{6}bits per second. During the transmission, the white noise of zero mean and power spectral density 10^{-20}watts/Hz is added to the signal. In absence of noise, the amplitude of the received sinusoidal wave for digit 1 or 0 is 1 microvolt. Find the average probability of symbol error for current binary FSK system. - A binary PCM uses NRZ signaling has error probability of 10
^{-6}. Assume that the signaling rate is doubled, then find new value of the average probability of error.

[**Hint:** Here NRZ signaling is used. Hence energy equation and error probability equation will be different than those derived in sec. Since signaling rate is doubled, bit period and hence energy is redsuced by half. Use erf (x) table in appendix]. **[Ans. P _{e} = 10^{-3}]**

**OBJECTIVE TYPE QUESTIONS**

**Multiple Choice Questions**

- Optimum system is the one

(a) which is physically realisable

(b) which must lead to unique solution

(c) all of the two

- If in a particular digital communication system application, the receiver designed is an optimum coherent receiver, the channel noise is white and bits are transmitted using rectangular pulses then which one of the following statements would be incosistent? The receiver is

(a) a correlation receiver

(b) matched filter receiver

(c) an integrate and dump receiver

(d) a sample, hold and dump receiver

- In manchester code, the symbol rate r
_{s}and data rate r_{s}are related as

(a) r_{b} = 2r

(b) T_{s} = 2T_{b}

(c) T_{s} = T_{b}/2

(d) r_{s} = r_{b}

- Transversal equaliser uses tapped delay lines to

(a) reduce & SI

(b) reduce BER

(c) increase bit rate

(d) increase band widths

__Answers __

- (a) 2. (a) 3. (a) 4. (a)