Wireless and Cellular Communications

Viva Questions on Mobile Radio Propagation

Viva Questions on Mobile Radio Propagation 

Viva Questions on Mobile Radio Propagation, Mobile Radio Question Answers, Engineering Viva Questions, Viva Questions with Answers, Short Answer type Viva Questions on Mobile Radio Propagation, Mobile Radio Propagation  Viva Questions

Short Answer type Questions

Q.1.What are the two basic types of communication channels?

Ans.Basically, there are following two types of communication channels

(i)                 wired channels

(ii)               radio or wireless channels

The wired channels are stationary and predictable whereas the radio channels are extremely random and do not offer easy analysis. Even the speed of motion affects how rapidly the signal level fades as a mobile terminal moves in space. Therefore, it has been most difficult to model a radio channel in mobile radio system design. However, the model of a radio channel is achieved in a statistical manner which is based upon the measurement observed for the desired communication system or spectrum allocation.

Q.2.What is bandwidth?

Ans.Bandwidth is the range of frequencies and represented as BW = f2f1  where f1 and f2 are the first and last frequencies of the signal graph.

Q.3.Define attenuation.

Ans.It is nothing but reduction in signal strength mainly at higher frequency ranges and at receiving end this attenuated signal has reduced voltage levels.

Q.4.Differentiate guided and unguided mediums.

Ans.In a guided medium, the electromagnetic waves are guided by the transmission medium e.g., Copper coaxial cable.

In an unguided medium, the electromagnetic wave is not guided by the medium. e.g., the radiated energy is not guided in wireless communication.

Q.5. Give the significance of Eb/No.

Ans. The bit energy to output noise ratio is important in mobile communication where the bit error rate of data. It is generally expressed in decibel.

Q.6.What is known as multipath?

Ans.When a signal from a cell phone emerges, due to many obstacles it radiates in many directions and finally reaches its base station receiver and vice versa. Such multiple signals take multipaths to reach the base station or the cell phone.

Q.7.What is the propagation mechanism?

Ans.

(a)    Reflection

(b)   Diffraction

(c)    Scattering

Q.8.What are types of fading?

Ans.

(a)    Fast fading

(b)   Slow fading

(c)    Flat fading

(d)   Selective fading

Q.9.What are the advantages of radio transmission?

Ans.

(a)    It is cheap and simple in design.

(b)   Infrared transmission is not interfered with by electrical devices.

(c)    The radio transmission can cover a larger geographical region.

Q.10.Define refractive index.

Ans.At the interface of two materials (m1 and m2) of different densities, there will be bending or refraction taking place.

Q.11.Write a short note on reflection.

Ans.Reflection is the act of reflection or a means to turn back. This reflection with respect to electromagnetic waves will occur whenever an incident wave hits a boundary of any two media. In this case, some of the total incident power of the signal will not enter the second media.

Q.12.When does Rayleigh fading occur?

Ans.Rayleigh fading will take place when there are many paths between the transmitter and receiver.

Q.13.What is dBm0?

Ans.It is dBm referred to as zero (0) transmission level point.

Q.14.What is known as scattering?

Ans.When the radio wave impinges on a surface that is rough, then the reflected energy spreads out in all directions and it is known as scattering. It generally occurs when1 the medium of wave travel comprises many objects with smaller dimensions when compared to the signal wavelength (λ) and also if the number of objects per unit volume is large it increases the scattering effect.

Q.15.List the material properties over which the Fresnel zone reflection coefficient depends on.

Ans.The reflection coefficient (G) depends on the material’s properties namely,

(i)                 Wave polarization

(ii)               Frequency of propagation of the wave

(iii)             Angle of incidence

Q.16.Write a short note on reflection.

Ans.The reflection mechanism takes place when an electromagnetic wave impinges on an object that has larger dimensions than the wavelength (λ) of the propagating wave. Reflection occurs from buildings, walls, and surfaces of the earth.

Q.17.Write a short note on diffraction.

Ans.Diffraction is a propagating mechanism that will take place when the radio path available between transmitter and receiver (T-R) is obstructed by any surface with sharp irregularities or edges and it creates secondary waves due to diffraction.

Q.18.When the reflection of the radio signal will take place?

Ans.The reflection mechanism takes place when the electromagnetic wave travels from one medium to another medium that has different electrical properties. For example, if a wave enters from medium 1 to medium 2 and in case medium 2 is a perfect conductor then the entire radio energy will get reflected back into medium 1.

Q.19.What is the plane of incidence?

Ans.The plane of incidence is defined as a plane containing reflected, incident and also the transmitted rays.

Q.20.Give two advantages of diffraction mechanism.

Ans.

(i)                 Diffraction allows radio signals to travel around curved surfaces and to propagate beyond obstructions.

(ii)               Even when the receiver moves into a shadowed region the diffraction field will exist.

Q.21.What is the main cause for diffraction effect?

Ans.Diffraction is caused by the propagation of secondary wavelets into the shadowed regions. Also, the field strength of the diffracted wave will be a vector sum of all the electric field components of the secondary wavelets.

Q.22.List few outdoor propagation models.

Ans.

1.      Durkins model

2.      Longley-Rice model

3.      Okumura model

4.      PCS extension model

5.      Hata model

6.      Wideband PCS microcell model

Q.23.Mention any three indoor propagation models.

Ans.

(i)                 Log-distance path loss model

(ii)               Ericsson multiple breakpoint model

(iii)             Partition losses considering the same floor of a building

Q.24.What will be the level of attenuation for reinforced concrete partitions and wooden partitions?

Ans.If the partitions of a multistoried building are concrete (permanent), the attenuation from 1 room to another will be high whereas for wood like movable partitions in a building the level of attenuation of mobile radio propagation will be less.

Q.25.List any three factors influencing small scale fading.

Ans.

(i)                 Speed of mobile unit

(ii)               Multipath propagation

(iii)             Transmission bandwidth of the signal

Q.26.What are the two types of fading of multipath delay spread?

Ans.

1.      Time dispersion fading

2.      Frequency selective fading

Q.27.What are the two types of fading of Doppler spread?

Ans.

1.      Frequency dispersion fading

2.      Time selective fading

Q.28.What are the fading effects of Doppler spread?

Ans.

1.      Fast fading

2.      Slow fading

Q.29.What are level crossing rates and Rayleigh fading in multipath fading channels?

Ans.The level crossing rate (LCR) and Rayleigh fading signal predictions are important for designing error controls codes and diversity schemes that have to be implemented. The level crossing rate is also taken as an expected rate at which the Rayleigh fading envelope crosses a specified level in the positive going signal directed and the number of level crossings is denoted as Nr.

Q.30.What is slow and fast fading?

Ans.Under slow fading case, the channel impulse response varies at a rate much slower rate when compared to a transmitted baseband signal.

In the fast-fading case, the impulse response of the channel varies rapidly within the respective symbol duration. In other words, the channel’s coherence time period is smaller than the symbol time period of the signal being transmitted.

Q.31.List two parameters that describe a time varying nature of the channel.

Ans.

1.      Doppler spread

2.      Coherence time

Q.32.What is coherence bandwidth?

Ans.The coherence bandwidth represents a range of frequencies over which the channel is almost flat In other words it is the range of frequencies over which the frequency components have enough potential for proper amplitude correlation.

Q.33.What is a small-scale fading model?

Ans.The radio propagation models that characterize the fluctuations in received signals over short distances of short time periods are called as small scale fading model.

Q.34.What is a free space propagation model?

Ans.It is a propagation model that can predict the received signal strength whenever the transmitter and receiver (T-R) have a clear line of sight (LOS) path between them.

Q.35.Define path loss.

Ans.The signal attenuation is represented as path loss (PL). It is defined as the difference between the power transmitted and power received by the antenna and it is expressed in decibels (dB).

Q.36.What are the components of propagation path characteristics?

Ans.The propagation path characteristics can be divided into three components in indoor and outdoor environments. They are:

(i)                 Shadowing

(ii)               Multipath fading

(iii)             Path loss with respect to distance.

Q.37.What are the demerits of LoS wireless transmissions?

Ans.

(a)    Free space loss

(b)   Noise

(c)    Multipath fading

(d)   Refraction

(e)    Distortions

Q.38.Define multipath-propagation.

Ans.In cellular mobile communication, the received signal arrives through delayed multiple paths (due to obstacles in its wave travel) and this is termed as multipath propagation.

Q.39.What are the effects of ‘fading’?

Ans.As fading occurs,

(a)    The error rate of the transmission system will increase

(b)   There will be a significant fluctuation in BER value

Evolution of Wireless and Cellular Communications Viva Questions

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