MICROWAVE COMMUNICATION SYSTEMS AND DESIGN Flashcards
A telecommunication network consists of
customer
premise equipment (CPE), switching nodes, and
transmission links
Four different ways by which we can convey
signals from one switching node to another
- Radio
- Fiber optic cables
- Coaxial cable
- Wire pair
The sizes, capacities, ranges, and operational frequency
bands for ______________ vary greatly
radio systems
Only two system types meet the necessary broadband requirements of the long-distance network
- line-of-sight (LOS) microwave
- satellite communications.
ADVANTAGES OF RADIO SYSTEMS
* Less expensive compared to fiber optic cable
* No requirement for right-of-way
* Less vulnerable to vandalism
* Not susceptible to “accidental” cutting of the link
* Often more suited to crossing rough terrain
* Often more practical in heavily urbanized areas
* Can work as a backup to fiber-optic cable links.
DISADVANTAGES OF RADIO SYSTEMS
* Limited information bandwidth
* Requires licensing
* Greatly affected by changes in atmospheric
condition, acts of nature
Microwave point-to-point communication can be
achieved by a single connection links made by a
number of intermediate repeaters with or without partial payload drop-insert
MICROWAVE BASICS
In microwave basics, the transmitted information can be
voice, data, or
video as long as it is in a digital format
MICROWAVE BASICS
A typical digital microwave radio consists of three basic components:
- A digital modem for interfacing with digital terminal equipment
- A radio frequency (RF) unit for converting a carrier signal from the modem to a microwave signal
- An antenna used to transmit and receive the signal
The combination of these three components is called a
radio terminal
Two terminals are required to establish a microwave communications link, commonly referred to as a
microwave hop or microwave link.
We can feed the data and voice traffic into the radio using
an electrical or optical interface
In the radio, the _________ are coded into analog
signals and converted to microwaves
digital signals
are sent using a highly directive
parabolic shaped antenna. At the other end, the
signals are received and restored to the digital
format
microwaves
microwave radio provides a transmission medium for digital traffic of standard capacities typically ranging from 1.544 Mbps (1T1) to 45 Mbps (1DS3) in North America, and from 2.048 Mbps (1E1) to 34 Mbps (16E1) based on ITU standards.
Plesiochronous Digital Hierarchy (PDH)
microwave radios are typically used for high-capacity backbone systems
Synchronous Digital Hierarchy (SDH) / Synchronous
Optical Network (SONET)
providing in the neighborhood of 1Gbps of throughput, are increasingly being used as well
New Ethernet microwave radios
Radio links may be established between any two
points within the line of sight and, depending on the frequency, geographical region, and rain statistics, the typical link distance can be up to
25 mi (40 km)
For the longer microwave link hops, additional
measures have to be taken to ensure required
reliability of the system
(e.g., space and/or frequency
diversity).
which are only centimeters (or inches)
in length, are small relative to the surroundings and hence do not have the bending property.
Microwaves
Therefore, to establish a radio link, it is important to have _______ between the two radio position sites. One or more radio paths connected in tandem form a microwave system.
radio LOS
The radio stations between two terminal stations are called
repeater stations (active or passive)
Active repeaters can be
_______________ when the signal is only filtered and amplified, with or without down and up conversions (e.g., in some analog FDM
systems),
nonregenerative
or they can be _________ when, in digital
applications, the signal is demodulated and remodulated before transmission to the next radio hop
regenerative
implemented without any active
radio components (e.g., two directional antennas connected back to back, a reflector, and so on) are also utilized
Passive repeaters
is a functionality provided in analog and
digital repeaters, where only radio-system specific control and service channels, and possibly part of the payload, are made available for local traffic and system management and maintenance
Drop-insert
The most common use of Fresnel zone information on a profile plot is
to check for obstructions that
penetrate the zone.
The ______ the frequency, the narrower the Fresnel zone and, consequently, the more vulnerability to non-LOS effects (object attenuation)
higher
is used to determine obstruction loss, and anytime the path clearance between the terrain and the line-of-sight path is less than 0.6F1 (60 percent of the first Fresnel zone radius), some knife-edge diffraction loss will occur
first Fresnel zone (n = 1)
Some engineers plot a ratio of ________ of the first Fresnel zone to add a bit of headroom for the path design.
100 percent
can be described as any criterion to ensure
sufficient antenna heights so that, in the worst case of refraction (for which k is minimum), the receiver antenna is not placed in the diffraction region.
Clearance
can be considered by applying climate-dependent clearance criteria or by properly handling diffraction-refraction fading (k-type fading).
Clearance
To summarize all this, we can say that, for normal propagation conditions, the following two clearance criteria have to be satisfied:
* The antenna must have clearance of 60 percent or greater at the minimum k suggested for the certain path.
* The antenna must have clearance of 100 percent or greater at k = 4/3
Radio transmission engineers introduced the ________ to compensate for the refraction in
the atmosphere.
Earth-radius factor k
Applying appropriate k-values to the true-Earth
radius, an ____________ is geometrically
obtained and, consequently, straight rays
equivalent-Earth radius
Using the _________________, ray paths
between transmitters and receivers near the Earth’s surface can be approximated by straight lines over a spherical Earth.
effective Earth’s radius model
The model is most useful for terrestrial paths and is not used for paths through the entire atmosphere.
effective Earth’s radius model
is defined as the factor that is multiplied by the actual Earth radius, E, to give the effective Earth radius Ee
effective Earth radius factor, k,
The mean Earth radius is in average
6,371 km.
For standard refractivity conditions, k = ????,
and this value should be used whenever a local value is not provided. It is important to keep in mind that lower k-values will lower the LOS; in other words, they will demand higher antenna heights
= 1.33 = 4/3
is a calculation involving the gain and loss factors associated with the antennas, transmitters, receivers, transmission
lines, and propagation environment, used to determine the maximum distance at which a transmitter and receiver can successfully operate
link budget
The purposes of the ____________ are to generate the carrier frequency that is to be used for the
communication, to modulate this carrier frequency with the desired information, and finally, to amplify the signal so that it attains a sufficiently high power level so that it may travel the desired communication distance to the receiver
transmitter
amplifies the received signal (which is at
this point much weaker than when it was
transmitted), filters out any undesirable signals
(interfering signals) that the receiver picked up and, finally, detects the existence of information in the carrier frequency
receiver
The purpose of ______________ is to interconnect
the antenna with the transmitter/receiver.
transmission lines
Transmission lines between the radio equipment and the antenna may consist of ___________.
coaxial cabling or a
(flexible) waveguide
makes it possible to utilize a common antenna for both the transmitter and receiver. The transmitter and receiver can, for example, be connected to the same antenna either via a duplex filter or a transmitter/receiver switch.
antenna-coupling unit
Together, feeder cable losses, antenna-coupling
losses, and any additional losses (depending on the radio configuration) constitute
branching losses
adapts the generated signal to the surrounding environment (to the propagation medium) and directs the radio waves that are to be transmitted towards the receiving station. When
receiving, the antenna receives the signal from the desired direction and sends it to the receiver.
antenna
is the signal level at which the radio runs continuous errors at a specified bit rate. Specifications are listed for the 10^−3
bit error rate (PDH radios) or 10^-6 bit error rate
receiver sensitivity threshold
is defined as the difference between the transmitter output power and the
receiver threshold. Lowering the system gain will
reduce the fade margin. System gain can be used to reduce antenna sizes or improve the path availability.
System gain (in decibels)
For example, 99.999 percent system availability (five minutes of outage per year) will degrade to 99.980percent (two hours of outage per year) if the modulation is changed from 16 QAM to 128 QAM without recovering the system gain reduction and all other conditions remaining unchanged
The gains from the antenna at each end are added to this gain, with larger antennas providing higher gain. The free-space loss of the radio signal as it travels over the air is then subtracted from the system, and the longer the link, the higher the loss. These calculations result in a _________ for the link
“fade margin”
is the difference between the received
signal and receiver threshold value (or sensitivity) for given BER, typically 10-6 or 10-3
.
Fade margin
is a methodical, systematic, and
sometimes lengthy process that includes the following main activities:
Microwave link design
- Loss/attenuation calculations
- Fading and fade margins calculations
- Frequency planning and interference calculations
- Quality and availability calculations
Prediction models for the purpose of performing
fading predictions are ________ i.e., they are not
founded on theoretical considerations but are only built upon observation and experience.
empirical
often provide a fair description of the fading process for distances and frequencies that lie within the data ranges for which measurements have actually been collected.
Empirical prediction models
The loss/attenuation calculation is composed of three main contributions:
propagation, branching, and
“miscellaneous” (or other) losses.
contribution comes from the
losses due to the Earth’s atmosphere and terrain—e.g., free-space as well as gas, precipitation (mainly rain), ground reflection, and obstacles.
propagation losses
contribution comes from the
hardware required to deliver the transmitter/receiver output to the antenna—e.g., waveguides as well as splitters and attenuators
branching losses
contribution has a somewhat unpredictable and sporadic character, e.g., sandstorms and dust storms as well as fog, clouds, smoke, and moving objects crossing the path. In addition, poor equipment installation and less than
perfect antenna alignment (field margin) may give rise to unpredictable losses.
“miscellaneous” losses
WHAT ARE THE PROPAGATION LOSS?
- Free-space path loss
- Vegetation Attenuation
- Gas Absorption
- Attenuation due to Precipitation
- Obstacle Loss
is continuously growing, and the rate of growth is very important. It is important to include a provision for at least ten years of vegetation growth
Vegetation Attenuation / Vegetation
Vegetation Attenuation
* For the case in which the foliage depth is less than 400 m, the loss is given by
CHECK THE PPT FOR THE FORMULA
- This relationship is applicable for frequencies in the range 200 MHz to 95 GHz
is a radio wave propagation model that
estimates the path loss due to the presence of vegetation on a point-to-point telecommunication link and gives slightly different results
Weissberger’s modified exponential decay model, or simply, Weissberger’s model
SEE THE FORMULA IN THE PPT
This model is found to be applicable to cases in which the ray
path is blocked by dense, dry, in-leaf trees found in temperate-latitude forests.
GAS ABSORPTION
A major difference in propagation through the real atmosphere versus free space is that there is
air present.
account for approximately 99 percent of the total volume of the atmosphere.
Nitrogen and oxygen molecules
Since the absorption bands of nitrogen are located far from the microwave radio communications region of the spectrum,
the atmosphere is considered to be composed of a mixture of two “gases”:
dry air (oxygen molecules) and water vapor
(water molecules)
are separately calculated and then summed to give the total specific attenuation.
Specific attenuation (in dB/km) for water vapor and oxygen
is strongly dependent on frequency, temperature, and the absolute or
relative humidity (RH) of the atmosphere
specific attenuation
can take the form of rain, snow, hail, fog, and
haze. All of these consist of water particles (haze can also consist of small solid particles). Their distinctions lie in the distribution of the size and form of their water drops.
Precipitation
is ,however, the main contributor in the
frequency range used by commercial radio links. Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz
Rain attenuation
The main parameter used in the calculation of rain attenuation is __________, which is obtained from cumulative distributions
rain intensity (rain rate)
The specific attenuation of rain is dependent on many parameters, such as
the form and size distribution of the
raindrops, polarization, rain intensity, and frequency.
is not included in the link budget and is used only in the calculation of rain fading
The contribution due to rain attenuation
It is important to notice that__________ increases
exponentially with rain intensity (mm/hr) and that horizontal polarization gives more rain attenuation than vertical polarization
rain attenuation
is the mechanism responsible for obstacle
loss/attenuation.
Diffraction
is also known in the literature as diffraction loss or diffraction attenuation.
obstacle loss
One powerful but simple method for calculation of obstacle loss is the ____________ , which is based on the knife-edge approximation
single-peak method
(another definition of the path clearance) will
depend on the geometry of the path
Parameter 𝒗
Notice that 𝒗 will be positive for ______________ and negative for __________________________
obstructed paths , clear LOS paths
SEE THE FORMULA AND EXAMPLE IN THE PPT
Let’s assume a single tree on an 18-GHz, 5-mi-long path. The distance of the tree to the first site is 1 mi. The obstacle height is 30 ft above center of the Fresnel’s zone and k=4/3. Calculate
attenuation due to the diffraction.
𝑨 𝒗 = 𝟐𝟏. 𝟖𝟗𝟔 𝒅B
If a ___________ is considered, the values given
in figure are reasonable approximations. Having an obstacle free 60 percent of first Fresnel zone gives 0 dB obstruction loss.
knife-edge approximation
In case of more than one obstacle, the well-known approach by ___________ assumes that all the obstacles can be replaced with one equivalent knife-edge obstacle
Bullington
The ____________ sums all the individual, single knife obstacle losses
Epstain-Peterson method
The__________ determines the largest obstacle and focuses on the calculations related to it.
Deygout method
The ___________ tends to overestimate the true path losses when there are a large number of obstacles (edges) or when they are very close together.
Deygout method
The recommended models for diffraction loss are given by the
_______________
ITU-R Recommendation P.526
The loss due to reflection on the
ground is dependent on the __________ of the
ground and the__________
total reflection
coefficient, phase
shift
obtained for a phase angle of 0°, and the lowest value (Amin) is for a phase angle of 180°.
highest value (Amax) of signal strength
is dependent on the frequency,
grazing angle (angle between the ray beam and the horizontal plane), polarization, and other ground properties.
reflection coefficient
The__________________ is very small—usually less than 1°
grazing angle of radio-relay paths
is not automatically included in the link budget. However, when reflection cannot be avoided, the fade margin may be adjusted by including this contribution as “additional loss” in
the link budget
contribution resulting from reflection loss
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 as follows:
Multipath fading
- Flat fading
- Frequency-selective fading
Rain fading
Refraction-diffraction fading (k-type fading)
All fading types are strongly dependent on the path length and are estimated as the probability of exceeding a given (calculated) fade margin
A special type of fading is a fading due to the
interference
is the dominant fading mechanism for
frequencies lower than approximately 10 GHz
Multipath fading
A reflected wave causes a phenomenon known as ______ , meaning that the radio signal can travel multiple paths to reach the receiver.
multipath
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
Multipath propagation gives rise to two kinds of signal degrading effects, i.e.,
flat fading and frequency selective
fading.
is due to thermal noise and interference.
flat fading effect
both flat and selective fading typically
occur in _________
combination.
Two scenarios of multipath are
upfade
null or downfade.
If the two signals reach the receiver in phase, then the signal is amplified. This is known as an _________
upfade
can also occur when the radio wave is trapped within an atmospheric duct
Upfades
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
null or downfade.
Under ___________ , the direct signal may be attenuated and/or distortion increased to the point where frequency selective notches result and dispersive fading occurs.
fading conditions
Such distortion results in ISI (intersymbol interference) in the demodulator, an increase in data signal BER, and a possible loss of data signal recovery
Some important facts about multipath fading are as follows:
- Multipath fading is normally more active over bodies of water
(lakes, sea, and so forth) than over land. - It is important to avoid ground reflection.
- Multipath fading is normally most active during early and later
summer (late spring and early autumn). - In radio links of typical length and in temperate climates, multipath activity lasts approximately three months
Some important facts about multipath fading are as follows:
* 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 flat instead of frequency selective.
* Increasing path inclination reduces the effects of flat fading.
Reducing path clearance (i.e., lowering antennas) will reduce the effect of flat fading
is a reduction in input signal level where all
frequencies in the channel of interest are equally affected. Flat fading 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.
Flat fading
is assumed to follow the Rayleigh distribution.
Deep
flat fading
implies amplitude and group delay
distortions across the channel bandwidth produced by the multipath nature of the transmission media.
Frequency-selective fading
Frequency-Selective Fading and Dispersive Fade Margin
It particularly affects medium- and high-capacity radio links (>32 Mbps). The sensitivity of digital radio equipment to frequency-selective fading can be described by the ______________________
signature curve of the equipment
is a measure of the receiver’s capability to suppress the time-delayed signal.
equipment signature
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)
By changing the phase difference between the direct and indirect signals, the notch frequency will change inside the bandwidth. At every notch frequency, the signal is attenuated until the threshold for a specific BER (10−3 and/or 10−6) is exceeded
Frequency-Selective Fading and Dispersive Fade Margin
The figure illustrate the sensitivity (also known as _________________and expressed in dB) of the receiver as a function of the notch frequency.
the notch
depth
The difference between the highest and lowest notch frequency is the ____________ , expressed in megahertz
signature bandwidth
The _____________ , is the measure of a
receiver’s ability to resist dispersive fading.
dispersive fade margin (DFM in dB)
To measure the DFM of a digital radio receiver, manufacturers simulate multipath fading conditions either in the field or at the
factory
Frequency-Selective Fading and Dispersive Fade Margin
is defined as the average depth of
multipath fade that causes an outage, independent of thermal and interference fade margins, and can be approximated using
the following formula:
dispersive fade margin
The extent of the _______________________ is primarily a function of the form and the size distribution of the raindrops.
_______ starts increasing noticeably at about 10 GHz and, for frequencies above 15 GHz, rain fading is normally the dominant fading mechanism
attenuation due to rain
Rain fading
It enters into this equation because it is a measure of the average size of the raindrops. When the rain rate increases (i.e., it rains harder), the raindrops are larger, and
thus there is more attenuation
rain rate
usually in cells accompanying thunderstorm
activity and weather fronts, has a great impact on path availability above 10 GHz
Heavy rainfall
increases dramatically with frequency and then with path length.
Rain outage
In the design of a microwave communications link, microwave engineers include a margin to compensate for the effects of rain at a given level of availability
Rain Fading
is characterized by seasonal and daily variations in the Earth-radius factor k.
Refraction-diffraction fading, also known as k-type fading,
For ________ , the Earth’s surface becomes more curved, and terrain irregularities, manmade structures, and other objects may intercept the Fresnel zones.
low k values,
For ____________ , the Earth’s surface gets close to a plane surface, and better LOS (lower antenna heights) is obtained
high k values
in microwave systems is caused by the presence
of an undesired signal in a receiver.
Interference
When this undesired signal exceeds certain limiting values, the quality of the desired received signal is affected. To maintain
reliable service, the ratio of the desired received signal to the (undesired) interfering signal should always be larger than the
threshold value
Interference Fade Margins
accounts for receiver threshold degradation due to interference from adjacent channel transmitters in one’s own system
Adjacent-channel interference fade margin (AIFM) (in
decibels)
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. The complete expression for describing the CFM for a digital
microwave radio is given by
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.
These four fade margins are power added to derive the CFM. ( Composite Fade Margin)
TFM = thermal (flat) fade margin
DFM = dispersive fade margin
IFM = interference fade margin
AIFM = adjacent-channel interference fade margin
A major concern for microwave system users is how often and for how long a system might be out of service.
Outages and Availability
Performance prediction (related to propagation effects) principally depends on the assessment of two main propagation mechanisms:
multipath fading and/or attenuation
due to rain
The basis for the dimensioning of the links in a network is usually defined by the ______________ , which describes the required ________ of a connection and the ______ required during the available time
operational user requirement
availability
quality
is often used in order to obtain an internationally accepted availability and quality for parts of or the entire planned network
dimensioning standard developed by ITU
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)
Outages and Availability
* An outage in a digital microwave link occurs with a loss of DS1
frame sync (OOF) for more than 10 secs. DS1 frame loss
typically occurs when the BER increases beyond 1 × 10−3
𝑶𝒖𝒕𝒂𝒈𝒆 𝑼𝒏𝒂𝒗𝒂𝒊𝒍𝒂𝒃𝒊𝒍𝒕𝒚 =
𝑺𝑬𝑺/𝒕𝒙𝟏𝟎𝟎 (%)
* Availability is expressed as a percentage as follows:
𝑨 = 𝟏𝟎𝟎 − 𝑶𝒖𝒕𝒂𝒈𝒆(𝑼𝒏𝒂𝒗𝒂𝒊𝒍𝒂𝒃𝒊𝒍𝒊𝒕𝒚)
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., ____________.
Vigants model
deep fades
The average probability of multipath fading in Vigants North
American model over a long period of time (for example, one
year) due to fading is given by
SEE THE FORMULA IN THE PPT
is often assigned to spur links and on
short systems of less than about 10 tandem hops.
A relaxed 99.999 percent per-hop reliability (320 SES/yroutage) objective (floor)
For high reliability links (usually in long-haul systems with many hops in tandem), the per-hop objective may approach or exceed 99.9999 percent, allowing only 20–30 secs of per-hop
outage per year
Vigants North American Multipath Probability Model
Let’s find the probability of multipath fading outage (SES) for a
microwave path 30-mi long, at 6 GHz, average terrain
roughness of 50 ft, climate-terrain factor c = 1, and t = 40°F,
composite fade margin CFM = 36 dB (equal numerically to
thermal fade margin).
ANS. 𝑷 = 𝟏𝟎𝟐 𝒙𝟏𝟎−𝟔
, 𝑶𝒖𝒕𝒂𝒈𝒆 = 𝟔𝟓𝟑 SES/yr (99.998% outage probability)
A method for predicting the single-frequency (or narrowband) fading distribution suitable for large fade depths A in the average worst month in any part of the world (based on ITU-R
p.530-12) and for detailed link design is given as follows:
SEE THE FORMULA IN THE PPT
ITU-R Multipath Probability Model
The term _____ is provided on a 1.5° grid in latitude and longitude in ITU-R Recommendation P.453
𝒅𝑵𝟏
Term _____ is defined as the standard deviation of terrain heights (m) within a 110 × 110 km area with a 30s resolution
𝒔𝒂
The method just shown is used for small percentages of time, does not make use of the path profile, and can be used for initial planning, licensing, or design purposes.
* The general rule is that, the smoother the path, the more likely the occurrence of multipath fading.
ITU-R Multipath Probability Model
Let’s assume that we are designing a 7 GHz link in Northern
Europe (or any other region with dN1 = −200 N-units/km,
based on ITU maps) and the link length is 40 km. Antenna
heights above sea level are 200 and 350 m. We have to find
the required fade margin if the reliability requirement of the
link is 99.999 percent.
𝑨 ≈ 𝟑𝟗 𝒅B
