# Top 100 Communication System Interview Questions

## Que.26 What is the maximum frequency deviation allowed in commercial FM broadcasting?

Answer: The nature of broadcast of the given frequency band 88 – 108 MHz is FM radio. Commercial FM broadcasting uses a maximum modulating frequency of 75 kHz and a minimum modulating frequency of 15 kHz.

### Que.27 How the selectivity of a receiver can be improved?

Answer: Selectivity is the ability offered receiver to produce the desired signal at output and reject the undesired signal.  As the selectivity of the receiver is evaluated by the relative intensity of a signal received from an external source, selectivity can be improved by using the cascaded tuned circuit.

### Que.28 The most commonly used modulation scheme for telegraphy is?

Answer: FSK modulation is the most commonly used scheme for telegraphy.

### Que.29 Why do we need sampling in communication?

Answer: The signals we use in the real world, such as our voices, are called “analog” signals.  To process these signals in computers, we need to convert the signals to a “digital” form.  While an analog signal is continuous in both time and amplitude, a digital signal is discrete in both time and amplitude.  To convert a signal from continuous-time to discrete time, a process called sampling is used. The value of the signal is measured at certain intervals in time. Each measurement is referred to as a sample.

### Que.30 How many samples are necessary to ensure we are preserving the information contained in the signal?

Answer: If the signal contains high-frequency components, we will need to sample at a higher rate to avoid losing information that is in the signal.  In general, to preserve the full information in the signal, it is necessary to sample at twice the maximum frequency of the signal.  This is known as the Nyquist rate. The Sampling Theorem states that a signal can be reproduced exactly if sampled at a frequency F, where F is greater than twice the maximum frequency in the signal.

### Que.31 What happens if we sample a signal at a frequency that is lower than the Nyquist rate?

Answer: When sampling signal frequency is lower than the Nyquist rate, it will exhibit a phenomenon called Aliasing.  Aliasing is the presence of unwanted components in the reconstructed signal.  These components were not present when the original signal was sampled.  In addition, some of the frequencies in the original signal may be lost in the reconstructed signal.  Aliasing occurs because signal frequencies can overlap if the sampling frequency is too low.

Que.32 How can signals be prevented from aliasing?

Answer: To prevent aliasing, an anti-aliasing filter is used which is basically a low pass filter, which filters out the unwanted components before sampling the signal.  Because we are filtering out high-frequency components and letting lower frequency components through, this is known as low-pass filtering.

### Que.33 What are uniform and non-uniform quantization?

• The quantized levels in the uniform quantization process are equally spaced. The uniform quantization is further categorized as mid-rise type uniform quantization and mid-tread type uniform quantization. Both the uniform quantization processes are symmetric about the respective axis.
• The quantized levels in the non-uniform quantization process are unequally spaced. The relation between such quantization is generally logarithmic due to the non-linear nature of the signal.

### Que.34 Application of quantization in communication.

• In digital communication, this process is used in analog to digital converter by using a sampler and quantizer. The quantization process is also used in various fields, such as signal processing, control systems, image processing, science, and linguistics.
• In digital signal processing, quantization maps the large set of the input values to the small set of the output values with a finite number of elements. The device that performs the quantization function is known as a quantizer.
• In image processing, quantization reduces the number of discrete values in the signal. We can also say that it compresses the input signal and produces the compressed signal at the output. For example, reducing the number of colors required to represent a digital image.

### Que.35 What is commanding?

• Companding is a type of non-uniform quantization and is used to increase the strength of weak signals. It reduces the data rate of the input signal by varying the gap between the two adjacent quantization levels. The unequal quantization level makes it similar to the non-uniform quantization process.
• Companding is created from the combination of two words, compression and expanding. The signal is passed through the compressor at the transmitting end while it is passed through the expander at the receiving end. The compressor compresses the signal and improves the quality of transmission. But, it introduces distortion in the signal. The expander is used at the receiving end to undo the distortion introduced by the compressor. The inverse distortions of the two processes (compression and expanding) help in generating the output signal without distortion.

### Que.36 What is Bandpass modulation?

• Bandpass modulation is the modulation scheme in which carriers are transmitted through free space. For example- ASK, PSK, and FSK . in this scheme, one bit is transmitted in a specific time instant through free space.
• In this scheme, as the message signal switches between two possible amplitudes, the corresponding respective parameter of the carrier signal will be switched between two possible values, and for the same reason, these schemes are called signalling, switching, or keying.

### Que.37 How does QAM modulation work?

• Quadrature amplitude modulation is a combination of two signaling schemes ASK and PSK. ASK to save the energy of signals and PSK to lower the probability of error.
• the QAM signal comes with two carriers. Each has the same frequency but differs in phases by 90 degrees, or one-quarter of a cycle, which is the basis for the term quadrature.
• One signal is called the I-in-phase signal, and the other is called the Q- quadrature-phase signal. Mathematically, one of the signals can be represented with a sine wave and the other by a cosine wave. The two modulated carriers combine at the source for transmission.
• Basic signals exhibit only two positions which allow the transfer of either a 0 or 1. Using QAM there are many different points that can be used, each having defined values of phase and amplitude. This is known as a constellation diagram. The different positions are assigned different values, and in this way, a single signal is able to transfer data at a much higher rate. Constellation diagram for a 16QAM signal showing the location of the different points
• As shown above, the constellation points are typically arranged in a square grid with equal horizontal and vertical spacing. Although data is binary the most common forms of   QAM, although not all, are where their constellation can form a square with the number of points equal to a power of 2 i.e. 4, 16, 64 . . . . , i.e. 16QAM, 64QAM, etc.
• By using higher-order modulation formats, i.e. more points on the constellation, it is possible to transmit more bits per symbol. However the points are closer together and they are therefore more susceptible to noise and data errors.

### Que.38 Which is better BPSK or QPSK?

• BPSK represents binary input 1 and 0 w.r.t. change in carrier phase by 180 degrees, while QPSK represents two bits using complex carrier symbols each having a 90-degree shift with one another. BPSK is considered to be a robust modulation scheme compared to the QPSK as it is easy for the receiver to receive the original bits.
• In the BPSK demodulator, only two decision points are required to retrieve the original binary information. In the QPSK demodulator, four decision points are needed. With BPSK, higher distance coverage can be achieved from the base station cellular cell or fixed station to the mobile subscribers compared to QPSK.
• QPSK has the advantage of having a double data rate compared to BPSK. This is due to the support of two bits per carrier in QPSK compare to one bit per carrier in the case of BPSK.

### Que.39 What type of modulation do cell phones use.

• QAM is the most widely used modulation scheme in radio communication as it is able to carry higher data rates as compared to other modulation schemes and our cell phone uses radio communication.

### Que.40 What are FSK and PSK?

• FSK:  In frequency-shift keying, binary 1 is represented by a high-frequency carrier, and a low-frequency carrier represents binary 0. NRZ coding is used for FSK generation.
• PSK: while in phase-shift keying, binary 1 is represented by the actual carrier, and binary 0 is represented by the 180o phase shift of the carrier signal.

### Que.41 Why was DPCM introduced in digital communications?

Answer: For the samples that are highly correlated, when encoded by the PCM technique, leave redundant information behind. To process this redundant information and to have a better output, it is a wise decision to take a predicted sampled value, assumed from its previous output, and summarize them with the quantized values. Such a process is called as Differential PCM or DPCM technique.

### Que.42 What are the advantages of delta modulation?

Answer: The advantage of delta modulation is that this process transmits only one bit either 0 or 1 per sample. So it requires lower channel bandwidth than PCM. And signaling rate is also small because it transfers only one bit per sample. So the drawback of PCM is removed by using this modulation.

### Que.43 Why is adaptive delta modulation preferred over delta modulation?

• The signal-to-noise ratio of ADM is better than that of DM because of the reduction in slope overload distortion and idle noise.
• Because of the variable step size, the dynamic range of ADM  is wider than DM.
• Utilization of bandwidth is better in ADM than DM.
• In delta modulation, the step size cannot be varied. It remains fixed for the entire signal. However, in the case of ADM, the step size can be varied according to the variation of the signal.
• In delta modulation, slope overload distortion and granular noise can be present but in ADM, quantization noise is present. The other types of errors are mainly avoided.
• It has a wider dynamic range than delta modulation.

### Que.44 What are the real-life applications of adaptive delta modulation?

• It is effectively used in audio communications.
• It can be used in systems that require improved voice quality.
• It is used in voice coding.
• It is used in television signal transmission

### Que. 45 Advantage of M- ary signaling scheme over binary signaling scheme.

• The word binary represents two bits.
• M simply represents a digit that corresponds to the number of conditions, levels, or combinations possible for a given number of binary variables.
• This is the type of digital modulation technique used for data transmission in which instead of one bit, two or more bits are transmitted at a time. As a single signal is used for multiple-bit transmission, the channel bandwidth is reduced.

### Que.46 What are the disadvantages associated with M- array signaling scheme?

Answer: The disadvantage is that system complexity and the probability of error increased.

### Que.47 What is white noise in analog communications?

Answer: This is also called thermal noise or Gaussian noise or Johnson noise. Occur due to thermal agitation. Atom due to thermal agitation gains energy and move in a random fashion and collides with each other, this produces heat corresponding to thermal noise or white noise.

### Que.48 What is narrow-band noise? What is its significance?

Answer: When a white noise signal is passed through a band pass filter, the resulting noise is called narrow-band noise. It is used to find the effect of white noise on AM, DSB, and SSB.

### Que.49 Advantages of using synchronous detector.

Answer: Though a synchronous detector is a complex circuitry, whatever be the modulation technique, if the detector is a synchronous detector, then the figure of merit (FOM) is equal to 1. Which implies that it nullifies the effect of noise. So, the noise performance of the synchronous detector is superior.

### Que.50 What is solar noise?

Answer: Solar Noise: Solar noise is generated by the sun. As Sun is a large body with extremely high temperatures thus it emits or releases high electrical energy in noise form over a broad frequency range. However, the intensity of the produced noise signal changes timely. This is so because the temperature change of the sun follows 11 years of the life cycle. Hence large electrical disturbances occur after a period of every 11 years. While in other years the noise level is comparatively low.

GO TO THE NEXT PAGE Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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