Fading and Fade Margins

Question 1

___ 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.

Answer 1

Fading

Question 2

Fading types normally considered when planning microwave point-to-point paths are___

Answer 2

Multipath fading
Rain fading
Refraction-diffraction fading (k-type fading)

Question 3

All fading types are strongly dependent on the ____ and are estimated as the probability of exceeding a given (calculated) fade margin.

Answer 3

path length

Question 4

A special type of fading is a fading due to the___

Answer 4

interference

Question 5

____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.

Answer 5

MULTIPATH FADING

Question 6

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.

Answer 6

multipath

Question 7

____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.

Answer 7

Multipath propagation

Question 8

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.

Answer 8

Upfade

Question 9

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 ____.

Answer 9

null or downfade

Question 10

Under _____ conditions, the direct signal may be attenuated and/or distortion increased to the point where frequency selective notches result and dispersive fading occurs.

Answer 10

fading

Question 11

Multipath fading is normally more active over ______ than over land.

Answer 11

bodies of water

Question 12

Multipath fading is normally most active ____ and ______ (late spring and early autumn).

Answer 12

during early and later summer

Question 13

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.

Answer 13

flat

Question 14

1._____ path inclination reduces the effects of flat fading.
2.____ path clearance (i.e., lowering antennas) will reduce the effect of flat fading.

Answer 14

1. Increasing
2. Reducing

Question 15

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.

Answer 15

flat fading

Question 16

____ 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.

Answer 16

Flat fading

Question 17

____ implies amplitude and group delay distortions across the channel bandwidth produced by the multipath nature of the transmission media.

Answer 17

Frequency-selective fading

Question 18

The ______ is a measure of the receiver’s capability to suppress the time-delayed signal.

Answer 18

equipment signature

Question 19

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.

Answer 19

signature

Question 20

The phase difference between the direct and indirect signals causes a ____ at one of the frequency positions inside the spectrum bandwidth.

Answer 20

notch (dip)

Question 21

The difference between the highest and lowest notch frequency is the _____, expressed in megahertz.

Answer 21

signature bandwidth

Question 22

The _____ (DFM in dB), is the measure of a receiver’s ability to resist dispersive fading.

Answer 22

dispersive fade margin

Question 23

The _____ is defined as the average depth of multipath fade that causes an outage, independent of thermal and interference fade margins,

Answer 23

dispersive fade margin

Question 24

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.

Answer 24

due to rain

Question 25

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.

Answer 25

increases

Question 26

Refraction-diffraction fading, also known as ____, is characterized by seasonal and daily variations in the Earth-radius factor k.

Answer 26

k-type fading

Question 27

For ____ k values, the Earth’s surface becomes more curved, and terrain irregularities, manmade structures, and other objects may intercept the Fresnel zones.

Answer 27

low

Question 28

For ____ k values, the Earth’s surface gets close to a plane surface, and better LOS (lower antenna heights) is obtained.

Answer 28

high

Question 29

_____ in microwave systems is caused by the presence of an undesired signal in a receiver.

Answer 29

Interference

Question 30

______ (in decibels) accounts for receiver threshold degradation due to interference from adjacent channel transmitters in one’s own system.

Answer 30

Adjacent-channel interference fade margin (AIFM)

Question 31

______ 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.

Answer 31

Interference fade margin (IFM)

Question 32

_____ is the fade margin applied to multipath fade outage equations for a digital radio link.

Answer 32

Composite fade margin (CFM)

Question 33

______, the difference between the normal (unfaded) RSL and the BER = 1 × 10−3 DS1 loss-of-frame point

Answer 33

TFM = thermal (flat) fade margin

Question 34

______, 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.

Answer 34

DFM = dispersive fade margin

Question 35

______. Receiver threshold degradation due to co-channel interference.

Answer 35

IFM = interference fade margin

Question 36

______. This is usually a negligible parameter except in frequency diversity and N + 1 multiline systems.

Answer 36

AIFM = adjacent-channel interference fade margin

Question 37

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.

Answer 37

longer

Question 38

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.

Answer 38

operational user requirement

Question 39

_____ is defined as a second containing one or more bit errors

Answer 39

ES (errored second)

Question 40

_____ is defined as a 1-sec period during which the BER is worse than 1 × 10^−3

Answer 40

SES (severely errored second)

Question 41

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.

Answer 41

Vigants model