Top 100 Communication System Interview Questions

Que.51 What is Matched filter?

Answer:

• Matched filter:- If a filter generates an output to maximize the output peak power ratio to mean noise power within its frequency response then it is called a matched filter. In telecommunications, it is the optimal linear filter used to increase the SNR or signal-to-noise ratio in the existence of additive stochastic noise.

• These types of filters are generally used in radar, where a known signal is transmitted out & the reflected signal can be compared with the transmitted signal. 

• The best example of the matched filter is- pulse compression because the impulse response can be matched with input pulse signals. In image processing, two-dimensional matched filters are used to enhance the X-Ray or SNR observations.

Que.52 What is shot noise? When does it occur?

Answer:

• Shot noise in a communication channel is the result of random variation in the appearance of electrons and holes at the output side of the device. These random movements are the result of discontinuities in the device which is being used by the system. The shot noise generates a sound like several lead shots are striking over a metal plate or tube.

• It also occurs in p-n junction diodes, as though the movement of carriers within the diode is due to the action of an external potential. But, sometimes their random movement generates shot noise.

• Non-linearity or discontinuity in the system generates shot noise.

Que.53 What is  Differential phase-shift keying (DPSK)?

Answer:

• The DPSK stands for “Differential phase-shift keying”. It is one type of phase modulation used to transmit data by altering the carrier wave’s phase. In this, the modulated signal’s phase is moved to the element of an earlier signal. The phase of the signal tracks the low or high state of the earlier element. This kind of phase-shift keying doesn’t require a synchronous carrier on the demodulator. The binary bits input series can be changed so that the next bit depends upon the earlier bit. So, the earlier received bits in the receiver are utilized for detecting the current bit.

Que.54 Advantages and disadvantages of DPSK modulation.

Answer:

• The advantages of DPSK include the following:

1. This modulation doesn’t need the carrier signals at the end of the receiver circuit. Therefore compound circuits are not required.
2. The BW of the DPSK requirement is low evaluated to BPSK modulation.
3. Non-consistent receivers are simple and inexpensive to construct, therefore extensively used in wireless communication.

• The disadvantages of DPSK include the following:

1. The bit error rate or chance of error is high in DPSK in contrast to BPSK.
2. The interference of noise in DPSK is more.
3. This modulation employs two consecutive bits intended for its response. Thus error in the primary bit makes errors within a subsequent bit as well as consecutively error spreads.

Que.55 What is a Random process? Give example.

Answer:

• A random process X(t) is used to explain the mapping of an experiment that is random with a sample space S which contributes to sample functions X(t,λi).

• It is also known as a stochastic process.

• Example:

1. Tossing of a coin: we know that we can either get head or tail but we don’t know what we will actually get when a coin is tossed.
2. Rolling of a die: when we roll a die we know in advance that the outcome could be any number between 1 to 6 but we have no idea what that number will be.

Que.56 Explain the deterministic and non-deterministic random processes with examples.

Answer:

• Deterministic Random process: When the future values of any sample function are predicted depending on the knowledge of the past values, then the random process is known as the deterministic random process.

• A random process is the combination of time functions, the value of which at any given time cannot be pre-determined. So it is known as a non-deterministic process.

• Example:- Let’s take a random process {X(t)=A.cos(ωt+θ): t  ≥  0}. This process has a family of sine waves and depends on random variables A and θ. So it is a deterministic random process.

Que.57 What is Ergodic random process?

Answer: Every number of the random process has the same statistical behavior over the entire random process. The statistical behavior can be determined by examining only one sample function. Such processes are called Ergodic. The mean values are determined by time averages. Ergodic processes are also stationary processes.

Que.58 What is the use of random processes in communication system?

Answer:

• A random process or stochastic process is a family of random variables. In principle, this could refer to a finite family of random variables such as { X, Y, Z}, but in practice, the term usually refers to infinite families. The need for working with infinite families of random variables arises when we have an indeterminate amount of data for the model. 

• For example- In sending bits over a wireless channel, there is no set number of bits to be transmitted. To model this situation, we use an infinite sequence of random variables. As another example, the signal strength in a cell phone receiver varies continuously over time in a random manner depending on location. To model, this requires that the random signal strength depends on the continuous-time index t.

• All the communication systems generate noise as well as interfere with noise. Since to implement any circuit or device whether, in the Analog or Digital domain, we need its Mathematical Model, without which we cannot implement anything. It gives us a technique or measure to implement things in practical situations.

• So, the Random process is that branch of Mathematics that is able to find or implement the mathematical model for the noise in our communication systems.

• As we know that noise is actually a probabilistic measure, and is modeled as a random variable, then, in turn, modeled as a random process because of its time-varying nature. The Gaussian Noise is the most famous model for noise in communication systems today because it almost resembles the probability density of the noise occurring in the communication systems.

Que.59 Why do we use the gaussian random variables to represent the noise in the communication system?

Answer:

• This may be explained using Central Limit Theorem. Additive noise by a channel is actually a combination of so many noises throughout the channel from different sources. For example, noise may come from a star in another galaxy, which is radiating in working frequency. These noises can be considered as infinite in number. Each of them has its own distributions, but they make groups of noises having similar distributions.

• By Central Limit Theorem, when independent identically distributed random variables are added, their sum tends toward a Gaussian distribution (Normal Distribution). So noise groups having the same distributions sum up to a Gaussian random variable.

• The Sum of two or more Gaussian random variables is also a Gaussian random variable.

• these all noises sum to a Normally distributed noise, even if they are distributed differently.

Que.60 What is the difference between a random process and a random variable?

Answer:

• A random process can be thought of as the collection of random variables. It can also be defined as a time-varying function that assigns the outcome of a random event or experiment to each time instant. E.g. a signal.

• A random variable is a variable whose value is the outcome of a random process. In order words when there is a random event or experiment then the outcome at any instant of time is assigned to a variable termed a random variable.

• For example, tossing a coin will either yield a head or tail. The tossing of the coin is a random experiment because you don’t know whether it will turn out to be a head or tail. After tossing, when an outcome is gotten either head or tail then it can be assigned to a variable.

• A person’s weight at a specific age is a random variable. The change in weight over the age is a random process.

Que.61 What is the need for channel coding in digital communications?

Answer: Channel coding boosts communication in three different ways:

1. To transmit a given data rate, e.g., R bits per channel use; then we can achieve such a data rate at a lower SNR; hence we can save power.
2. Now assume that we don’t want to save power, and we can operate at a certain SNR. Again, we can transmit at a higher rate (>R) by using a larger signal constellation along with a channel code.
3. we want to transmit a given data rate of R’ bits per second (it is different from R in case 1, in which its unit was bits per channel use), at a certain SNR. According to case 2, by using a channel code, we can increase the rate of bits per channel use. As a result, to achieve the target of R’ bits per second, we need a lower bandwidth.

Que.62 What is an eye pattern in communication?

Answer:

• An eye pattern provides the following information about a particular system –  
• Eye patterns are used to estimate the bit error rate and the signal-to-noise ratio.  
• The width of the eye-opening defines the time interval over which the received wave can be sampled without error from ISI.  
• The instant time when the eye-opening is wide will be the preferred time for sampling. 
• The rate of the closure of the eye, according to the sampling time, determines how sensitive the system is to the timing error.  
• The height of the eye-opening, at a specified sampling time, defines the margin over the noise.

Que.63 What is jitter? How it can be avoided?

Answer:

• “Jitter” describes timing errors within a system. Jitter is defined as the short-term variations of a digital signal’s significant instants from their ideal positions in time. Significant instants could be (for example) the optimum sampling instants.” 

• Jitter may affect the accuracy of the data being transmitted because minute variations in amplitude, phase, and frequency always occur. The generation of a pure carrier signal in an analog circuit is impossible. The signal may be impaired by continuous and rapid gain and/or phase changes. This jitter may be random or periodic. Phase jitter during a telephone call causes the voice to fluctuate in volume.

• Minimizing jitter is possible through the use of buffers.

Que.64 What is the difference between bit rate and baud rate?

Answer:

BASIS FOR COMPARISON	BIT RATE	BAUD RATE
Basic	Bit rate is the count of bits per second.	Baud rate is the count of signal units per second.
Meaning	It determines the number of bits traveled per second.	It determines how many times the state of a signal is changing.
Term usually used	While the emphasis is on computer efficiency.	While data transmission over the channel is more concerned.
Bandwidth determination	Can not determine the bandwidth.	It can determine how much bandwidth is required to send the signal.
Equation	Bit rate = baud rate x the count of bits per signal unit	Baud rate = bit rate / the number of bits per signal unit

Que.65 What are the effects of errors in data transmission and reception?

Answer: Effects of errors in data transmission and reception are- 

• Attenuation- due to error, the signal may be attenuated, ie. Information is not received properly at the other end.
• Distortion- wave at the receiving end gets distorted due to delay in transmission.
• Interference in voice communication can occur due to an error in signal.
• An echo can be produced because the sender cannot hear the voice at the receiver’s end.

Que.66 What is bit error rate (BER)?

Answer:

• It is the rate at which error occurs in a transmission system or we can say it is the total number of errors that occurs in a string of a stated number of bits.

• If the medium between transmitter and receiver is good and signal to noise ratio is high, then the bit error rate will be small, however, if noise is detected in a system, then the bit error rate will be considered.

• It is a key parameter for measuring the performance of a wired or wireless data channel. When transmitting data from one point to another point over a wired or wireless or radio link, the key parameter is how many errors will appear in data at the remote end.

• Thus, BER assesses the full end-to-end performance of a system including the transmitter, receiver, and medium between the two.

Que.67 What are the factors affecting the bit error rate?

Answer: BER is affected by a number of factors, these factors are:

1. Interference: interference in a system is usually because by external factors and cannot be changed by the system design, however by reducing the bandwidth of a channel, it can be reduced.
2. Increased transmitted power: by increasing the power level of the system, power per bit is increased.
3. Reduced bandwidth: reducing bandwidth reduces the BER as the data transmission is reduced.

Que.68 How to avoid inter-symbol interference?

Answer: There are different methods to reduce ISI are:

• Equalization
• Nyquist criterion
• Eye pattern

Que.69  What is Pre-emphasis and de-emphasis mechanism? Where do we use it?

Answer:

• Pre-emphasis: it is the process of artificial boosting of high-frequency components of a message signal in order to increase the signal-to-noise ratio.

• It is done in the transmitter before modulation.

• De-emphasis: it is the reverse of pre-emphasis to get back the original message signal. Low pass filters are used in the de-emphasis circuit.

• This is done after modulation in the receiver.

• Pre-emphasis and de-emphasis are used to improve the fidelity of the receiver in FM transmission of audio signals.

Que.70 Why pre-emphasis and de-emphasis mechanisms are not required in AM receivers?

Answer: In AM, the audio signal to be transmitted is having only low frequencies, for which the signal-to-noise ratio is high. Hence, PE & DE mechanisms are not required.

Que.71 Why do we calculate the FOM of the receiver?

Answer:

• A figure of merit is a quantity used to characterize the performance of a device, system, or method, relative to its alternatives. In engineering, Figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application.

• Figure Of Merit is a measure of the noise performance of a receiver. It basically represents how much noise is being added by an amplifier to the signal. However, the signal-to-noise ratio is the measure of the amount of signal present with respect to the surrounding noise.

Que.72 What is intermodulation noise? How it can be removed?

Answer:

• Intermodulation noise is a special type of cross-talk. The signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal. This type of noise is similar to harmonics in music. On a multiplexed line, many different signals are amplified together, and slight variations in the adjustment of the equipment can cause intermodulation noise. A maladjusted modem may transmit a strong frequency tone when not transmitting data, thus producing this type of noise.

• Intermodulation noise is often caused by improper multiplexing. Changing multiplexing techniques (e.g., from FDM to TDM) or changing the frequencies or size of the guardbands in FDM can help.

Que.73 Explain the checksum technique of error detection.

Answer: A checksum (typically one byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum. The receiver calculates its own checksum in the same way and compares it with the transmitted checksum. If the two values are equal, the message is presumed to contain no errors. The use of checksum detects close to 95 percent of the errors for multiple-bit burst errors.

Que.74 What are the different sources of error in communication? How these errors can be avoided?

Answer:

Que.75 Why do we calculate the signal-to-noise ratio in communication?

Answer:

• SNR refers to the ratio between the power of the desired output signal and the background noise, which is described as-

SNR = \frac{V_{signal}}{V_{noise}}

where V_signal and V_noise are the measured signal voltage and noise voltage, respectively. High SNR is desired for a wide range of applications. Reduction of noise is used to increase the SNR. 

• SNR of a system should be as high as possible to reduce the effects of noise. 

• So, signal to noise ratio determines the strength of the signal with respect to noise present in a system, so that if noise is more in a system, effective measures can be taken to reduce it. Higher the SNR, the better the quality of the signal. It’s most often expressed as a measurement of decibels (dB). Higher numbers generally mean a better specification since there’s more useful information (the signal) than unwanted data (the noise).

• For example, when an audio component lists a signal-to-noise ratio of 100 dB, it means that the audio signal level is 100 dB higher than the noise level. Therefore, a signal-to-noise ratio specification of 100 dB is considerably better than one that is 70 dB or less.

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