61. Which of the following is not a channel parameter?
Coherence time
RMS delay spread
Doppler spread
Bandwidth
Answer.4. Bandwidth
Explanation
Every channel can be characterized through a parameter which is the ratio of the power in the dominant path and scattered path. Channel parameters are coherence bandwidth, RMS delay spread and Doppler spread.
Any of the characteristics or attributes of a signal, such as the signal amplitude, magnitude, frequency, phase, duration, shape, polarization, modulation, level, or irradiance. Signal parameters are bandwidth and symbol period. Different types of fading occur depending on the relation between channel and signal parameters.
62. Which of the following do not undergo free space propagation?
Wired telephone systems
Wireless line of sight radio links
Microwave line of sight radio links
Satellite communication system
Answer.1. Wired telephone systems
Explanation
Wired telephone systems do not undergo free space propagation.
Space wave propagation is defined as the mode of propagation in which the radio waves transmitted from the transmitting antenna reach the receiving antenna after traveling or propagating through space. It is why the waves here are also known as space waves and propagation as line-of-sight communication.
The free space propagation model assumes a transmit antenna and a receive antenna to be located in an otherwise empty environment.
EM signals when traveling through wireless channels experience fading effects due to various effects. But in some cases, the transmission is with no obstruction and direct line of sights such as in satellite communication, microwave, and wireless line of sight radio links.
63. Which of the following is not a multipath channel parameter that can be determined from power delay profile?
RMS delay spread
Doppler spread
Mean excess delay
Excess delay spread
Answer.2. Doppler spread
Explanation
The power delay profile (PDP) gives the intensity of a signal received through a multipath channel as a function of time delay. The time delay is the difference in travel time between multipath arrivals. The mean excess delay, excess delay spread, and RMS delay spread are some multipath channel parameters. They can be determined from a power delay profile.
Doppler spread is a measure of spectral broadening caused by the time rate of change of mobile radio channel. Doppler spread refers to the widening of the spectrum of a narrow-band signal transmitted through a multipath propagation channel. It is due to the different Doppler shift frequencies associated with the multiple propagation paths when there is relative motion between the transmitter and the receiver.
64. What is the dimension of object as compared to wavelength of propagating wave when reflection occurs?
Small
Same
Very small
Large
Answer.4. Large
Explanation
In telecommunications media, the Reflected propagation is Sometimes part of the transmitted wave travels to the receiver by reflection off a smooth boundary whose edge irregularities are only a fraction of the transmitted wavelength.
Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave.
Reflections occur from the surface of the earth and from buildings and walls.
65. What is the main disadvantage of the RF pulse system?
Simplicity
Interference and noise
Not real-time
Complexity
Answer.2. Interference and noise
Explanation
Pulsed radiofrequency is the technique whereby radio frequency (RF) oscillations are gated at a rate of pulses (cycles) per second (one cycle per second is known as a hertz (Hz)).
Radio Frequency (RF) interference is caused by transmitters on the same or similar frequency to the one you’re receiving.
The main problem with RF pulse system is that it is subject to interference and noise. This is due to the wide passband filter required for multipath time resolution.
66. Which of the following explains the concept of diffraction loss?
Archimedes’ Principle
Fresnel zone
Principle of Simultaneity
Pascal’s Principle
Answer.2. Fresnel zone
Explanation
The Fresnel Zone is the area around the visual line-of-sight that radio waves spread out into after they leave the antenna. The Fresnel zone construction provides physical insight into the interpretation of Fermat’s principle which states that, if the optical path length of a light ray is varied in a neighborhood about the true path, there is no change in path length.
The concept of diffraction loss is a function of the path difference around an obstruction. It can be explained by Fresnel zones. Fresnel zones represent successive regions where secondary waves have a path length from Tx to Rx which are nλ/2 greater than the total path length.
67. Which of the following is not an effect caused by multipath in radio channels?
Random frequency modulation
Time dispersion
Rapid changes in signal strength
Power of base station
Answer.4. Power of base station
Explanation
In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.
Multipath results in an error in pseudo-range measurements and thus affects the positioning accuracy since the multipath signal takes a longer path than the direct signal. To verify the effects of multipath propagation on positioning performance, an experiment was conducted under controlled conditions.
Rapid changes in signal strength over a small travel distance are caused due to multipath. It causes random frequency modulation due to varying Doppler shifts on different multipath signals. Time dispersion is also caused by multipath propagation delays.
68. Which of the following is not a practical path loss estimation technique?
Determination of percentage of coverage area
Hata model
Log distance path loss model
Log-normal shadowing
Answer.3. Log distance path loss model
Explanation
Path loss models describe the signal attenuation between a transmit and a receive antenna as a function of the propagation distance and other parameters. The log-distance path loss model is a radio propagation model that predicts the path loss a signal encounters inside a building or densely populated areas over a distance.
Log-normal shadowing, log distance path loss model, and determination of the percentage of coverage area are practical pat loss estimation techniques. Hata model is only valid for the exterior environment.
69. Which of the most widely used model for signal prediction in urban areas?
Okumura model
Attenuation factor model
Ericsson Multiple Breakpoint Model
Log distance path loss model
Answer.1. Okumura model
Explanation
The Okumura model is a radio propagation model that was built using the data collected in the city of Tokyo, Japan. The model is ideal for use in cities with many urban structures but not many tall blocking structures. The model served as a base for the Hata model.
70. _______ techniques can be used to improve system performance in fading channels.
Diversity Technique
Channel Coding
Equalization Technique
None of the above
Answer.1. Diversity Technique
Explanation
Diversity techniques can be used to improve system performance in fading channels.
Instead of transmitting and receiving the desired signal through one channel, L copies of the desired signal through 3 M, different channels are received. The idea is that while some copies may undergo deep fades, others may not.
There are several different kinds of diversity, which are commonly employed in wireless communication systems.
When the equalizer is used to counter the effects of time dispersion (ISI), diversity is used to reduce the depth and duration of the fades experienced by a receiver in a local area. This may be applied both at the base station and the mobile station.
There are several types of diversity techniques like space diversity, frequency diversity, time diversity, and polarization diversity of which space diversity.
