Communications MCQ Set 1
1. In CDMA spread spectrum systems, chip rate is less than the bandwidth of the channel. State whether True or False.
Answer: b [Reason:] In CDMA spread spectrum systems, the chip rate is typically much greater than the flat fading bandwidth of the channel. Whereas conventional modulation techniques require an equalizer to undo intersymbol interference between adjacent channels.
2. A RAKE receiver collects the __________-versions of the original signal.
a) Time shifted
b) Amplitude shifted
c) Frequency shifted
d) Phase shifted
Answer: a [Reason:] RAKE receiver attempts to collect the time shifted versions of the original signal. It is due to the fact that there is useful information present in the multipath components.
3. RAKE receiver uses separate _________ to provide the time shifted version of the signal.
a) IF receiver
c) Correlation receiver
Answer: c [Reason:] RAKE receiver uses separate correlation receivers to provide the time shifted version of the original signal for each of the multipath signal. CDMA receivers combine these time shifted versions of the original signal to transmission in order to improve the signal to noise ratio of the receiver.
4. Each correlation receiver in RAKE receiver is adjusted in______
a) Frequency shift
b) Amplitude change
c) Phase shift
d) Time delay
Answer: d [Reason:] Each correlation receiver may be adjusted in time delay, so that a microprocessor controller can cause different correlation receivers to search in different time windows for significant multipath.
5. The range of time delays that a particular correlator can search is called ________
a) Search window
b) Sliding window
c) Time span
d) Dwell time
Answer: a [Reason:] The range of time delays that a particular correlator an search is called a search window. RAKE receiver attempts to collect the time shifted version of the original signal by providing a separate correlation receiver for each of the multipath signal.
6. RAKE receiver is used for _______ technique.
Answer: a [Reason:] RAKE receiver is essentially a diversity receiver which is used specifically for CDMA. It uses the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed a chip period.
7. A RAKE receiver uses ____ to separately detect the M strongest signals.
a) Single correlator
b) Multiple correlator
c) Single IF receiver
d) Multiple IF receivers
Answer: b [Reason:] A RAKE receiver uses multiple correlators to separately detect the M strongest multipath components. Demodulation and bit decisions are then based on the weighted ouputs of the M correlators.
8. In a RAKE receiver, if the output from one correlator is corrupted by fading, all the other correlator’s output are also corrupted. State whether True or False
Answer: b [Reason:] In a RAKE receiver, if the output from one correlator is corrupted by fading, the others may not be. And the corrupted signal may be discounted through weighting process.
9. A RAKE receiver uses ____
b) Channel coding
Answer: c [Reason:] RAKE receiver is a diversity receiver. Diversity is provided by the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed chip period.
10. Interleaving is used to obtain _____ diversity.
Answer: a [Reason:] Interleaving is used to obtain time diversity in a digital communication system without adding any overhead. It provides rapid proliferation of digital speech coders which transform analog voices into efficient digital messages.
Communications MCQ Set 2
1. Which of the following distribution is used for describing statistical time varying nature of received envelope of multipath component?
a) Log normal distribution
b) Levy distribution
c) Rayleigh distribution
d) Gaussian distribution
Answer: c [Reason:] Rayleigh distribution is the most common distribution for statistical modelling. It is used to describe the statistical time varying nature of the received envelope of a flat fading signal. It also describes the envelope of an individual multipath component.
2. Envelope of the sum of two quadrature Gaussian noise signal obeys _________ distribution.
b) Inverse Gaussian
Answer: a [Reason:] It is well known that the envelope of the sum of two quadrature Gaussian noise signal obeys Rayleigh distribution. This fading distribution could be applied to any scenario where there is no line of sight path between transmitter and receiver antennas.
3. For a Rayleigh fading signal, mean and median differ by _______
a) 2 dB
b) 10 dB
c) 0.55 dB
d) 100 dB
Answer: c [Reason:] The mean and median differ by only 0.55 dB in a Rayleigh fading signal. The differences between the rms values and the other two values are higher.
4. It is easy to compare different fading distributions using mean values instead of median values. State whether True or False.
Answer: b [Reason:] By using median values instead of mean values, it is easy to compare different fading distributions which may have widely varying means. Median is used in practice since fading data is measured in the field and a particular distribution cannot be assumed.
5. For a nonfading signal component present, the small scale fading envelope distribution is ______
c) Log normal
The small scale fading envelope is Ricean when there is a dominant stationary (nonfading) signal component, such as line of sight propagation path. In such a situation, random multipath components arriving at different angles are superimposed on a stationary dominant signal.
6. Ricean distribution degenerates to ________ distribution when the dominant component fades away.
a) Log normal
Answer: c [Reason:] Ricean distribution degenerates to Rayleigh distribution when the dominant component fades away. As the dominant signal becomes weaker, the composite signal resembles a noise signal which has an envelope that is Rayleigh.
7. The envelope of a bandpass noise is ______
a) Uniformly distributed
Answer: b [Reason:] The envelope of only bandpass noise is Rayleigh distribution. Rayleigh distribution is a continuous probability density function for positive random variables.
8. The envelope of a sinusoid plus bandpass noise has ____
a) Uniformly distributed
Answer: c [Reason:] The envelope of a sinusoid plus bandpass noise has Ricean distribution. In probability theory, Ricean distribution is the probability distribution which has magnitude of a circular bivariate normal random variable with potentially non-zero mean.
9. What do you call an attenuation that occurs over many different wavelengths of the carrier?
a) Rayleigh fading
b) Ricean fading
c) Wavelength fading
d) Slow fading
Answer: d [Reason:] Slow fading does not vary quickly with the frequency. It originates due to effect of mobility. Slow fading is the result of signal path change due to shadowing and obstructions such as tree or buildings etc.
10. Which of the reception problems below that is not due to multipath?
a) Delayed spreading
b) Rayleigh fading
c) Random Doppler shift
d) Slow fading
Answer: d [Reason:] Slow fading arises when the coherence time of the channel is large relative to the delay requirement of the application. Slow fading is caused by events such as shadowing, where a large obstruction such as a hill or large building obscures the main signal path between the transmitter and the receiver.
Communications MCQ Set 3
1. Which of the following mechanism do not impact propagation in mobile communication system?
Answer: d [Reason:] Reflection, diffraction and scattering are the three basic propagation mechanism which impact propagation in mobile communication system. Large scale propagation model and small scale fading and multipath propagation are described by the physics of reflection, diffraction and scattering.
2. What is the dimension of object as compared to wavelength of propagating wave when reflection occurs?
d) Very small
Answer: a [Reason:] Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagation wave. Reflection occurs from the surface of the Earth and from buildings and walls.
3. When does the wave propagating from one medium to another gets partially reflection and partially transmitted?
a) Both mediums have same electrical properties
b) Both mediums have different electrical properties
c) Both mediums have same magnetic properties
d) Both mediums have different magnetic properties
Answer: b [Reason:] When a radio wave propagating in one medium impinges upon another medium having different electrical properties. The wave is partially reflected and partially transmitted.
4. What is the case of reflection, in course of second medium being a perfect dielectric?
a) Loss of energy during absorption
b) Total energy reflected back to first medium
c) No loss of energy in absorption
d) Total energy transmitted into second medium
Answer: c [Reason:] If the plane wave is incident on a perfect dielectric, part of the energy is transmitted into the second medium and part of the energy is reflected back into the first medium. There is no loss of energy in absorption.
5. What is the case of reflection, in course of second medium being a perfect conductor?
a) Loss of energy during absorption
b) Total energy reflected back to first medium
c) Partly transmission and reflection
d) Total energy transmitted into second medium
Answer: b [Reason:] If the second medium is perfect conductor, then all incident energy is reflected back into the second medium. There is no loss of energy during absorption.
6. Which of the following relates the incident and reflected & transmitted wave?
a) Fresnel transmission coefficient
b) Scattering coefficient
c) Diffraction coefficients
d) Fresnel reflection coefficient
Answer: d [Reason:] The electric field intensity of the reflected and transmitted waves may be related to the incident waves in the medium of origin through the Fresnel reflection coefficient. It is equal to the ratio of the amplitude of the reflected wave to the incident wave, with each expressed as phasors.
7. Reflection coefficient is not a function of __________
a) Material property
b) Diffraction loss
c) Wave polarization
d) Angle of incidence
Answer: b [Reason:] The reflection coefficient is a function of the material properties, and generally depends upon the wave polarization, angle of incidence and frequency of propagating waves. It is a parameter that describes how much of an electromagnetic wave is reflected by an impedance discontinuity in the transmission medium.
8. Polarized wave can be mathematically represented as sum of ________
a) Four orthogonal components
b) Two spatially adjacent components
c) Two spatially orthogonal components
d) Six orthogonal components
Answer: c [Reason:] A polarized wave may be mathematically represented as sum of two spatially orthogonal components. For an arbitrary polarization, super position may be used to compute the reflected fields from a reflecting surface.
9. The plane of incidence contains only incident rays. State True or False.
Answer: b [Reason:] The plane of incidence is defined as the plane containing the incident, reflected and transmitted waves. The incident light is polarized with its electric field perpendicular to the plane containing the incident, reflected, and refracted rays.
10. Permittivity and conductivity are insensitive to ______ for a good conductor.
a) Operating frequency
b) Polarization density
c) Electric field
d) Property of material
Answer: a [Reason:] The terms permittivity and conductivity are insensitive to operating frequency when the material is a good conductor. In the case of conductors, it is evident that electric field inside a conductor is zero. That is because free charges reside only on the surface of conductor and not inside.
11. Velocity of electromagnetic wave can be given by _______
Answer: a [Reason:] For a medium with permittivity, ∈ and permeability, μ the velocity of electromagnetic wave is given by 1/√(μ∈). It is also known as phase velocity. The velocity of light is given by 3*108 m/s.
12. The boundary condition at the surface of incidence obeys ________
a) Kepler’s law
b) Gauss law
c) Faraday law
d) Snell’s law
Answer: d [Reason:] Snell’s law is also known as Snell–Descartes law or the law of refraction. It gives a formula to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
13. The angle at which no reflection occurs in the medium of origin is called _________
a) Azimuth angle
b) Elevation angle
c) Brewster angle
d) Inclination angle
Answer: c [Reason:] The Brewster angle is the angle at which no reflection occurs in the medium of origin. It occurs when the incident angle is such that the reflection coefficient is equal to zero. The critical Brewster’s angles for diamond, glass and water are 67.5°, 57° and 53° respectively.
Communications MCQ Set 4
1. A helical antenna is used for satellite tracking because of _____
a) Circular polarization
Answer: a [Reason:] In helical antenna, the diameter and pitch of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix. It radiates circularly polarized radio waves. These are used for satellite communication.
2. Repeaters inside communications satellites are known as _______
Answer: b [Reason:] A communications satellite’s transponder is the series of interconnected units that form a communications channel between the receiving and the transmitting antennas. It is mainly used in satellite communication to transfer the received signals.
3. _______ is the geographical representation of a satellite antenna radiation pattern.
Answer: a [Reason:] The geographical representation of a satellite’s antenna radiation pattern is called a footprint or footprint map. In essence, a footprint of a satellite is the area on Earth’s surface that the satellite can receive from or transmit to.
4. The smallest beam of a satellite antenna radiation pattern is _______
a) Zone beam
b) Hemispheric beam
c) Spot beam
d) Global beam
Answer: c [Reason:] The size of the antenna that generates these beams on earth is related directly to the peak gain at the center of the spot beams and the smallest spot beam size. The spot beams are typically defined by the contours at 3 or 4 dB down from the peak power at the center of the beam.
5. _________ detects the satellite signal relayed from the feed and converts it to an electric current, amplifies and lowers its frequency.
a) Horn antenna
c) Satellite receiver
d) Satellite dish
Answer: b [Reason:] LNA detects the satellite signal relayed from the feed and converts it to an electric current, amplifies and lower its frequency. The most common device used as an LNA is tunnel diode.
6. A satellite signal transmitted from a satellite transponder to earth’s station is _________
Answer: b [Reason:] In satellite telecommunication, a downlink is the link from a satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite.
7. __________ is a loss of power of a satellite downlink signal due to earth’s atmosphere.
a) Atmospheric loss
b) Path loss
c) Radiation loss
Answer: b [Reason:] The path loss is the loss in signal strength of a signal as it travels through free space. This value is usually calculated by discounting any obstacles or reflections that might occur in its path.
8. Which of the following is the point on the satellite orbits closest to the Earth?
Answer: b [Reason:] The point where satellite is closest to the Earth is known as the perigee. Here, the satellite moves at its fastest. The high point of the orbit, when the satellite is moving the slowest is called the apogee.
9. What kind of battery panels are used in some advance satellites?
a) Germanium based panels
b) Silicon based panel
c) Gallium Phosphate solar panel array
d) Gallium Arsenide solar panel array
Answer: d [Reason:] Gallium Arsenide solar panel arrays are used for battery panels in some advanced satellites. These new types of cells allow smaller solar arrays to be used on future space missions.
10. A satellite battery has more power but lighter ________
Answer: a [Reason:] Lithium batteries have more power and are lighter in weight. Any mass that could be saved by the use of lighter batteries would allow a corresponding increase in the amount of useful payload equipment.
11. INTELSAT stands for ________
a) Intel Satellite
b) International Telephone Satellite
c) International Telecommunications Satellite
d) International Satellite
Answer: c [Reason:] INTELSAT is a communications satellite services provider. INTELSAT operates a fleet of 52 communications satellites, which is one of the world’s largest fleet of commercial satellites.
12. The frequency of Ku band for satellite communications is ______
a) 6/4 GHz
b) 14/11 GHz
c) 12/14 GHz
d) 4/8 GHz
Answer: b [Reason:] The Ku band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 11.7 to 12.7GHz. (downlink frequencies) and 14 to 14.5GHz (uplink frequencies).
13. The most common device used as an LNA is ________
a) Zener diode
b) Tunnel diode
d) Shockley diode
Answer: b [Reason:] The LNA must provide a relatively flat response for the frequency range of interest, preferably with less than 1 dB of gain variation. The most common device used as an LNA (low noise amplifier) is tunnel diode. It is a highly sensitive, low-noise device.
Communications MCQ Set 5
1. Scattering occurs when medium consists of objects with dimensions _______ compared to wavelength.
d) Very large
Answer: b [Reason:] Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength. But the number of obstacles per unit volume is large.
2. Scattered waves are produced at ________
a) Rough surface
b) Shadowed region
c) Smooth surface
Answer: a [Reason:] Scattered waves are produced by rough surfaces, small objects or by other irregularities in the channel. In practice, foliage, street signs, and lamp posts induce scattering in a mobile communication system.
3. The actual received signal is ______ than what is predicted by reflection and diffraction model.
d) Very weak
Answer: c [Reason:] The actual received signal in a mobile radio environment is often stronger than what is predicted by reflection and diffraction model alone. This is because when a radio wave incidence upon the rough surface, reflected energy is spread out in all directions.
4. Scattered energy in all directions provides _________ at a receiver.
b) Loss of signal
c) No energy
d) Additional radio energy
Answer: d [Reason:] Objects such as lamp posts and trees tend to scatter energy in all directions. They provide additional radio energy at a receiver. Scattering may also refer to particle-particle collisions between molecules, atoms, electrons, photons and other particles.
5. Surface roughness are often tested using ______
a) Rayleigh criterion
b) Lawson criterion
c) Barkhausen stability criterion
d) Nyquist criterion
Answer: a [Reason:] Rough surface is often tested using a Rayleigh criterion. It defines the critical height of surface protuberances for a given angle of incidence. The Rayleigh criterion is the criterion for the minimum resolvable detail. The imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another.
6. A surface is considered rough if protuberance is ______ than critical height.
d) No relation
Answer: c [Reason:] A surface is considered rough if its minimum to maximum protuberance is greater than the critical height calculated using Rayleigh criterion. It is considered smooth if protuberance is less than critical height.
7. RCS of scattering object is defined as the ratio of ______
a) Power density of signal scattered to power density of radio wave incident
b) Power density of radio wave incident to power density of signal scattered
c) Power density of incident waves to power density of reflected wave
d) Power density of reflected wave to power density of incident waves
Answer: a [Reason:] The radar cross section of a scattering object is defined as the ratio of the power density of the signal scattered in the direction of the receiver to the power density of the radio wave incident upon the scattering object. It has unit of square meters.
8. Which equation is used to calculate the received power due to scattering for urban mobile radio system?
a) Laplace equation
b) Bistatic radar equation
c) Poisson’s equation
d) Maxwell equation
Answer: b [Reason:] For urban mobile radio systems, models based on bistatic radar equation is used to compute the received power due to scattering in the far field. This equation describes the propagation of wave in free space which impinges on a scattering object and then reradiated in the direction of receiver.
9. In ionosphere propagation, waves arriving at the receiving antenna using the phenomenon of _______
Answer: a [Reason:] Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more paths. It is due to localized non-uniformities in the medium through which they pass.
10. Power density is basically termed as ________ power per unit area.
Answer: c [Reason:] Power density is the amount of power (time rate of energy transfer) per unit volume. It is also termed as radiated power per unit area. In energy transformers including batteries, fuel cells, motors, etc., power density refers to a volume. It is then also called volume power density.