Fading and Fade Margins Flashcards
___ is defined as the variation of the strength of a received radio carrier signal due to atmospheric changes and/or ground and water reflections in the propagation path.
Fading
Fading types normally considered when planning microwave point-to-point paths are___
Multipath fading
Rain fading
Refraction-diffraction fading (k-type fading)
All fading types are strongly dependent on the ____ and are estimated as the probability of exceeding a given (calculated) fade margin.
path length
A special type of fading is a fading due to the___
interference
____is the dominant fading mechanism for frequencies lower than approximately 10 GHz. A reflected wave causes a phenomenon known as multipath, meaning that the radio signal can travel multiple paths to reach the receiver.
MULTIPATH FADING
Typically, _____ occurs when a reflected wave reaches the receiver at the same time as the direct wave that travels in a straight line from the transmitter.
multipath
____gives rise to two kinds of signal degrading effects, i.e., flat fading and frequency selective fading. The flat fading effect is due to thermal noise and interference. Certainly, both flat and selective fading typically occur in combination.
Multipath propagation
If the two signals reach the receiver in phase, then the signal is amplified. This is known as an ____. It can also occur when the radio wave is trapped within an atmospheric duct.
Upfade
If the two waves reach the receiver out of phase, they weaken the overall received signal. If the two waves are 180° apart when they reach the receiver, they can completely cancel each other out so that a radio does not receive a signal at all. A location where a signal is canceled out by multipath is called a___ or ____.
null or downfade
Under _____ conditions, the direct signal may be attenuated and/or distortion increased to the point where frequency selective notches result and dispersive fading occurs.
fading
Multipath fading is normally more active over ______ than over land.
bodies of water
Multipath fading is normally most active ____ and ______ (late spring and early autumn).
during early and later summer
A rule of thumb is that multipath fading, for radio links having bandwidths less than 40 MHz and path lengths less than approximately 30 km (20 mi), is described as being____ instead of frequency selective.
flat
1._____ path inclination reduces the effects of flat fading.
2.____ path clearance (i.e., lowering antennas) will reduce the effect of flat fading.
- Increasing
- Reducing
A ___ is a reduction in input signal level where all frequencies in the channel of interest are equally affected. It implies barely noticeable variation of the amplitude of the signal across the channel bandwidth.
flat fading
____ is dependent on path length, frequency, and path inclination. In addition, it is strongly dependent on the geoclimatic factor (temperature/pressure variations), which is the factor that accounts for the refraction properties in the atmosphere, antenna altitudes, and the type of terrain. Deep flat fading is assumed to follow the Rayleigh distribution.
Flat fading
____ implies amplitude and group delay distortions across the channel bandwidth produced by the multipath nature of the transmission media.
Frequency-selective fading
The ______ is a measure of the receiver’s capability to suppress the time-delayed signal.
equipment signature
The _____ is therefore the level of the signal that is necessary to obtain a certain BER (currently referred to as 10−3 and/or 10−6) in the presence of an interfering signal with a predefined delay and it is measured in the laboratory.
signature
The phase difference between the direct and indirect signals causes a ____ at one of the frequency positions inside the spectrum bandwidth.
notch (dip)
The difference between the highest and lowest notch frequency is the _____, expressed in megahertz.
signature bandwidth
The _____ (DFM in dB), is the measure of a receiver’s ability to resist dispersive fading.
dispersive fade margin
The _____ is defined as the average depth of multipath fade that causes an outage, independent of thermal and interference fade margins,
dispersive fade margin
The extent of the attenuation ____ is primarily a function of the form and the size distribution of the raindrops. Rain fading starts increasing noticeably at about 10 GHz and, for frequencies above 15 GHz, rain fading is normally the dominant fading mechanism.
due to rain
The rain rate enters into this equation because it is a measure of the average size of the raindrops. When the rain rate ____ (i.e., it rains harder), the raindrops are larger, and thus there is more attenuation.
increases
Refraction-diffraction fading, also known as ____, is characterized by seasonal and daily variations in the Earth-radius factor k.
k-type fading
For ____ k values, the Earth’s surface becomes more curved, and terrain irregularities, manmade structures, and other objects may intercept the Fresnel zones.
low
For ____ k values, the Earth’s surface gets close to a plane surface, and better LOS (lower antenna heights) is obtained.
high
_____ in microwave systems is caused by the presence of an undesired signal in a receiver.
Interference
______ (in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters in one’s own system.
Adjacent-channel interference fade margin (AIFM)
______ is the depth of fade to the point at which RF interference degrades the BER to 1 × 10−3. It is affected by the frequency congestion, directivity of the interfering and victim system antennas, and so on.
Interference fade margin (IFM)
_____ is the fade margin applied to multipath fade outage equations for a digital radio link.
Composite fade margin (CFM)
______, the difference between the normal (unfaded) RSL and the BER = 1 × 10−3 DS1 loss-of-frame point
TFM = thermal (flat) fade margin
______, provided by the radio manufacturer from measurements. It is affected by the complexity of the digital modulation scheme and the types and effectiveness of the adaptive amplitude and/or baseband time domain equalization (if any) used.
DFM = dispersive fade margin
______. Receiver threshold degradation due to co-channel interference.
IFM = interference fade margin
______. This is usually a negligible parameter except in frequency diversity
and N + 1 multiline systems.
AIFM = adjacent-channel interference fade margin
The ____ the link, the more critical these factors become, since the system gain and composite fade margin determine the range and the reliability performance of a radio under various fading conditions.
longer
The basis for the dimensioning of the links in a network is usually defined by the _____, which describes the required availability of a connection and the quality required during the available time.
operational user requirement
_____ is defined as a second containing one or more bit errors
ES (errored second)
_____ is defined as a 1-sec period during which the BER is worse than 1 × 10^−3
SES (severely errored second)
The _____, used mostly in North America, is also widely used in ITU-R regions. This model was created for fade depths of 15 dB or more, i.e., deep fades.
Vigants model
