Communications System Important Interview question Part 3

  1. What is cut-off frequency of filter?

Ans. The frequency at which the response is -3dB  pass through a  filter without being attenuated. A cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.

  2. What is Radio frequency or RF?

Ans. Radio frequency (RF) is a frequency or rate of oscillation within the range of about 3 Hz to 300 GHz. Radio frequency (RF) is a measurement representing the oscillation rate of electromagnetic radiation spectrum, or electromagnetic radio waves, from frequencies ranging from 300 GHz to as low as 9 kHz.

   3. What is modulation? And where is it utilized?

Ans. Modulation is the process of varying some characteristic of a periodic wave i.e. carrier with message signal amplitude variations. Radio communication superimposes this information bearing signal onto a carrier signal. The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal. Modulation is utilized to send an information bearing signal over long distance.

1. What is demodulation? Explain.

Ans. Demodulation is the act of removing the modulation from an analog signal to get the original baseband signal back. Demodulation is necessary because the receiver system receives a modulated signal with specific characteristics being modulated by modulator at transmitter and it needs to turn it to baseband.

1. What is a repeater in communication?

Ans.  A regenerative repeater amplifies and reconstructs such a badly distorted digital signal and develops a nearly perfect replica of the original at its output. 

1. What is CDMA, TDMA, FDMA ? Explain in brief.

Ans. Code division multiple access (CDMA) is a channel access method utilized by radio communication technologies. CDMA employs spread spectrum technology and a special pseudo random coding scheme (where each transmitter is assigned a code) to allow multiplexed over the same physical channel. CDMA is another pure digital technique. It is also known as spread spectrum because it takes the digitized version of an analog signal and spreads it out over a wider bandwidth at a lower power level. This method is called direct sequence spread spectrum (DSSS).

 Time division multiple access (TDMA) is a digital technique that divides a single channel or band into time slots. Each time slot is used to transmit one byte or another digital segment of each signal in sequential serial data format. This technique works well with slow voice data signals, but it’s also useful for compressed video and other high-speed data.

FDMA is the process of dividing one channel or bandwidth into multiple individual bands, each for use by a single user. Each individual band or channel is wide enough to accommodate the signal spectra of the transmissions to be propagated. The data to be transmitted is modulated on to each subcarrier, and all of them are linearly mixed together.FDMA divides the shared medium bandwidth into individual channels. Subcarriers modulated by the information to be transmitted occupy each subchannel.

1. Give reasons for distortion in the received signal?

Ans. Few reasons may be as listed below:

(i) Insufficient channel bandwidth

(ii) Random variations in channel characteristics

(iii) External interference and noise

1. Why digital communication is preferred over analog communication?

Ans. Digital communication is rugged; i.e., in this we have to find only the presence or absence of pulse at receiver, which can be done easily even in some noisy condition, whereas in analog a slight change in waveform may disrupt the whole meaning and false interpretation of the message.

Digital communication is compatible to today computers and data can be processed easily.

For analog communication, there is no way to avoid accumulation of noise and distortion along the path. Amplification is of no use since it amplify the signal and noise in the same proportion.

For optical fiber communication dramatic cost reduction achieved in the fabrication of digital circuity, digital communication is preferred.

1. What is companding?

Ans. Companding is used as a complement to the process of modulation and demodulation. In this process a voice signal is compressed, then changed from analog to digital, transmitted and converted back from digital to analog before it is expanded again. 

For audio analog signals, the amplitude of weak signals is raised and the amplitude of strong signals is decreased, thereby altering (compressing and expanding) the dynamic range of the signals. The technique is used in AM, FM and single-sideband modulation radio and is helpful in improving the quality of amplified voice and musical instrument sounds. Dolby and dbx noise reduction also employ companding. 

  10. What are optimum filter and matched filters?

Ans. Matched filter is an optimum filter is an ideal filter which process the input signal to minimise the effect of noise hence maximizing SNR. The impulse response depends upon the shape of the input pulse.

1. What is redundancy? What are its advantages?

Ans. Redundancy is the duplication of critical components or functions of a system with intention of increasing reliability of the system. The redundant bit is used for error detection of the transmitted stream.

1. What is Parity check? Explain.

Ans. Parity check is a method of detecting errors in data communication by counting the number of ones or zeros, per byte or per word, including a special check bit (parity bit) to see if the value is even or odd.

The parity check is used to detect errors in the transmitted stream of data.

1. What are vocoders?

Ans. A vocoder is an audio processor that captures the characteristics elements of an audio signal and then uses this signal to affect other audio signals. The technology behind the vocodereffect was initially used in attempts to synthesize speech.

1. What is E-1 digital system?

Ans. E-1 carrier system is used in European countries. It is a 32-channel PCM system. Out of the 32-channels, 30 channels are used for carrying user information while one is used for synchronization and the other is used for signalling, error detection and correction. The data rate of E-1 carrier is 2.048 Mbps.

1. What is PN sequences?

Ans. A P-N sequence is a sequence of pseudo-random binary number generated using a linear feedback shift register. A P-N sequence is used in pseudo-random scrambler and descrambler and in DS-SS system. CDMA also uses PN sequence.

1. Why direct sequence spread spectrum secure?

Ans. Direct Sequence Spread Spectrum (DSSS) is a spread spectrum technique whereby the original data signal is multiplied with a pseudo random noise spreading code. This spreading code has a higher chip rate (this the bitrate of the code), which results in a wideband time continuous scrambled signal. The steam of information in DSSS is divided into small pieces, each associated with a frequency channel across spectrum. Data to be transmitted is combined data based on spreading ratio. The chipping code is a redundant bit pattern associated with each bit transmitted. This makes the communication safe and secure.

1. What is near-far problem?

Ans. The near–far problem or hearability problem is the effect of a strong signal from a near signal source in making it hard for a receiver to hear a weaker signal from a further source due to adjacent-channel interference, co-channel interference, distortion, capture effect, dynamic range limitation, or the like. Such a situation is common in wireless communication systems, in particular CDMA. 

This problem is commonly solved by dynamic output power adjustment of the transmitters.

1. Give uses of frequency hopping spread spectrum technology?

Ans. It used in

• Military radios cryptographic technique to send sensitive information.
• Adaptive FH-SS is used in Bluetooth technology.

1. What is noise figure? What is indicates?

Ans. The noise figure (F) is defined as the ratio of the signal-to-noise power supplied to the input terminals of a receiver or amplifier to the signal to- noise power supplied to the output or load resistor.

Noise figure (NF) and noise factor (F) are measures of degradation of the signal-to-noise ratio (SNR), caused by components in a signal chain. It is a number by which the performance of an amplifier or a radio receiver can be specified, with lower values indicating better performance.

1. . What are the methods to generate SSB?

Ans. (i) Filter method: double side band signal is passed through filter to get single side band signal.

 (ii) Phase shift method: phase shift using Hilbert transform is done to generate SSB signal.

1. What is the pilot carrier system? Explain its uses.

Ans. To get the carrier signal at the receiver in synchronism with that at transmitted end, we transit only a very small part of carrier signal along with the modulated signal. This is called pilot carrier. It makes demodulation easy.

1. What are Eigen values in frequency modulation? What it represents?

Ans. The value of modulation index at which carrier disappears completely, are called eigen values in FM signal.

1. Why local oscillator frequency is always kept higher than RF frequency of tuned station?

Ans. It is much easier to design an oscillator that is tunable over a smaller frequency ratio. Keeping local oscillator frequency higher than RF frequency, the frequency ratio is much smaller () than that keeping local oscillator frequency is always kept higher than RF frequency. So it is easy to tune the capacitor to generate local oscillator.

1. What is squelch?

Ans. Squelch or muting enables the receiver’s output to remain cut off unless the carrier is present. It avoids unnecessary loud noise to be amplified and to be present at output when no input is present.

1. Give the limitation of Shannon-Hartley theorem.

Ans. If bandwidth is infinite, then channel capacity is not infinite. It is finite and can be calculated as

                                           C ≈ 1.44 (S/N)

This is the condition which is limitation of this theorem.

1. What is mixing or heterodyning?

Ans. The operating of multiplying a signal with an auxiliary sinusoidal signal is called as mixing or heterodyning. It is done for frequency translation of the signal.

1. What are the problem in the recovery of the baseband signal?

Ans. Suppose that the auxiliary signal used for recovery differ in phase from the auxiliary signal used in the initial translation. If this phase angle is ɵ, the recovered waveform will be proportional to m(t) cos ɵ i.e., amplitude attenuates. Suppose that the recovered baseband signal is not precisely at frequency f_c but is instead at f_c + ∆f then the recovered baseband signal will be proportional to m(t) cos 2 π∆ ft, resulting a signal which will wax and wane and thus the recovery of the message signal won’t be possible. 

1. What are different types of cell and their radius in cellular communication?

Ans.

1. What are BTS and BSC? Explain.                                                                                                                                                       

Ans. The base transceiver station (BTS) contains the equipment for transmitting and receiving radio signal (transceivers), antennas, and equipment for encrypting and decrypting communication with the base station controller (BSC).

The base station subsystem (BSS) is the section of a traditional cellular telephone network which is responsible for handling traffic and signalling between a mobile phone and the network switching subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phone, paging, transmission and reception over the air interface and many other tasks related to the radio network.

1. What are 1G, 2G, 3G and 4G ? Explain with their features.

Ans. 

You Can Check the following link for complete set of interview questions : 

Brush up your Concepts with 1000+ Important Interview Questions

Thanks

The Most Comprehensive Exam Prep App. Stay Safe!