Communications MCQ Number 00850

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Answer: d
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.

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.

Answer: b [Reason:] The envelope of only bandpass noise is Rayleigh distribution. Rayleigh distribution is a continuous probability density function for positive random variables.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.