Transcript of OTF100001 Digital Microwave Communication Principle ISSUE 1.01
Digital Microwave Communication PrincipleDigital Microwave
Communication Principle
Foreword
This course is developed for the requirement from OptiX RTN
equipments.
This course mainly introduce the basic knowledge of digital
microwave communication. Engineers can have a basic to understand
the further OptiX RTN equipments after finish the course.
This course informs engineers of the basics on digital microwave
communications, which will pave the way for learning the OptiX RTN
series products later.
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reserved.
Learning Guide
Before this course, you may refer to these references first:
SDH Principle
Electromagnetism Basics
The current type of digital microwave communication system is
mainly SDH. Microwave communication is developed on the basis of
the electromagnetic field theory. Therefore, for some basic
knowledge to understand some of the course content, you are
supposed to refer to the materials about SDH principles,
fundamentals of the network communications technology and the basic
theory of the electromagnetic field.
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reserved.
Objectives
Upon completion of this course, you will be able to:
Describe the concept and characters of digital microwave
communication
Describe the theory and function of every parts in the digital
microwave system
List the networking application for digital microwave systems
List the fadings in microwave propagation
List the common technologies of antifading
After the course, trainees should clear and describe the concept
and features of digital microwave communication, this is the basics
to understand the advantage of the wireless communication. Trainees
also need to clear the type and structure of the microwave
transmission equipment and it’s network applications. Finally,
depends on the understanding the fading of microwave propagation,
trainees should list out the common used antifading technologies,
this part is important for the further study.
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Contents
Microwave Propagation and Antifading Technologies
There are four chapters in this course. Chapter one is focus on the
digital microwave communication concepts and features. Some of the
definition is important for us to understand the microwave
equipment.
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Transmission Method for Communication
Coaxial Cable
A basic function of communications is to transmit information from
one end to the other end, as shown in the following figure. This
information may be voice, image or other information. As show in
the figure, there are four means to implement this function:
Coaxial cable communication, Optical fiber communication, microwave
communication and satellite communication. And the latter three
means are the mainly three means of communication. Of them, optical
fiber communication is wired and information is transmitted in the
fiber, so the transmission channel is of good quality and the
transmission capacity can be huge. Microwave communication and
satellite communication are wireless. The transmission channel is
rather complex and the transmission capacity is limited.
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Fiber and Microwave transmission
needed, avoid the private land
Optical cable construction,
large land used.
Outside cable maintenance,
natural disaster influence
license
Microwave (MW) Definition
Radio frequency range is from 300MHz to 300GHz.
Be regard as plane wave.
The electric field and magnetic field exist at vertical of
transmission direction of plane wave. So it is called as Transverse
Electric and Magnetic field wave (TEM).
Wavelength of the electric wave used in microwave communication is
from 1 centimeter to 1 decimeter, it also be called as centimeter
wave. And is a limited frequency band of all electromagnetic wave
frequency scope.
Based on the features of transmission , microwave can be regard as
plane wave.
Microwave propagation media in aerosphere is troposphere
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Digital MW communication concepts
The communication that use microwave as carrier is microwave
communication.
The microwave communication with digital baseband signal is Digital
microwave communication.
There is an intermediate frequency between digital baseband signal
and radio frequency signal.
Digital information carried by MW, and transmitted in space.
Baseband signal is usually processed at intermediate frequency, and
then be converted into radio frequency band by frequency
conversion. And it also can be modulated at radio frequency band,
but modulation method is only PSK.
The base theory of microwave communication is electromagnetic
theory.
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Developing of MW communication
Note: capacity less than 10M is considered as low capacity, from
10~100M is medium capacity, and more than 100M is large
capacity.
155M
34/140M
2/4/6/8M
Analog MW
1990’s to now
A rough trend of microwave communication development is the transit
from analog microwave communication to digital microwave
communication and the increase of transmission capacity. As shown
in the figure, the analog microwave communication system originated
from 1950s and the initial transmission capacity was 480 voice
channels only. In 1970s, digital microwave communication systems of
small and medium capacity appeared and the transmission capacity
reached 2/4/6/8M. In 1980s, the PDH digital microwave system
appeared and the transmission capacity was greatly raised to
34/140M. From 1980s to date, the SDH digital microwave system has
been rapidly developing. Apart from the progress of technologies,
the characteristic of digital signal, that is, keeping a good
signal-to-noise ratio, is the key factor that ensures the long haul
transmission capability.
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Frequency Band and Radio Channel
The common frequency bands :
7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (by ITU-R rec. )
2
8
34
Mbit/s
2
8
34
140
155
Mbit/s
3.3
GHz
34
140
155
Mbit/s
(1) PDH MW more than 15kmrecommend to use 8GHz, less than 25Km,
11GHz also can be used, relating to the local condition.
(2) PDH MW more than 10km recommend to use 11GHz13GHz14GHz15GHz and
18GHz.
(3) SDH MW more than 15km recommend to use 5GHz6GHz7GHz and 8GHz,
less than 20 km, also can use 11GHz, relating to local
condition.
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Frequency Band and Radio Channel (cont.)
The central frequency, T/R spacing and channel spacing are defined
in every frequency band.
f0(central freq.)
Frequency scope
Channel spacing
Protection
spacing
Fn’s are used for the transmitting frequency of high site (Primary
site)
Fns are used for the transmitting frequency of low site
(Non-primary site)
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Frequency Band and Radio Channel (cont.)
f0(7575M)
The frequency arrangement in one frequency band has different
types.
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Modulation modes for Digital MW
The microwave carrier is digital modulated by the baseband
signal.
Digital base band signal
Signal
rate
Channel
bandwidth
modulation
Service
signal
Digital baseband signal is the un-modulated digital signal. The
baseband signal cannot be directly transmitted over microwave radio
channels but must be converted into frequency band signal in order
to implement microwave transmission. That is, digital modulation is
performed on the carrier.
Generally, the digital baseband signal is the service signal to be
transmitted. After the carrier is modulated by the digital baseband
signal, the band of the carrier signal will expand to a certain
extent and the occupied bandwidth is the channel bandwidth.
After the digital baseband signal is modulated into the IF signal,
it still cannot be directly transmitted over the air link but must
be converted once more into the signal of a higher frequency.
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Modulation modes for Digital MW (cont.)
The frequency carrier signal can be described as:
Amplitude Shift Keying (ASK): A is variable, Wc and φ are
constant
Frequency Shift Keying (FSK): Wc is variable, A and φ are constant
Phase Shift Keying (PSK): φ is variable, A and Wc are
constant
Quadrature Amplitude Modulation (QAM): A and φ are variable, Wc is
constant
A*COSWc*t+φ
Amplitude
Frequency
Phase
PSK and QAM are commonly used in digital MW
At present, PSK and QAM are commonly used in digital microwave
communication.
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MW Frame Structure
RFCOH
ATPC
64Kb/s
DMY
64Kb/s
MLCM
11.84Mb/s
RSC
864Kb/s
WS
2.24Mb/s
XPIC
16Kb/s
ID
32Kb/s
INI
144Kb/s
FA
288Kb/s
15.552Mb/s
SOH
Payload
DMY: Dummy ID: Identification
XPIC: Cross polarization interference counteract FA: Frame
synchronization
ATPC: Automatic transmitter power control WS Wayside services
In the digital microwave system, to transmit digital orderwire
information, wayside service information, ATPC information, error
correction bits and channel switching information, additional bits
that are called RFCOH (Radio Frame Complementary OverHead) are
inserted into the main data stream coming from the SDH MUX
equipment. Vendors plan the frame structure according to the
transmission rate, modulation schemes, error correction methods,
and types of required additional information. Therefore, different
vendors may have different microwave frame structures.
The above figure shows the frame structure that employs Multi-Level
Coded Modulation (MLCM). The microwave frame structure is actually
the SDH frame structure with RFCOH added outside. RFCOH is used to
complete some functions needed for microwave transmission. The
parts in the RFCOH have specific meanings and purposes:
MLCM (Multi-Level Coding Modulation) is used for error
correction.
DMY means dummy overhead.
XPIC (Cross-Polarization Interference Cancellation) uses two
mutually orthogonal polarized waves to transmit two channels of
information so as to improve the transmission capacity of the
link.
ATPC (Automatic Transmit Power Control) is a technology used to
automatically adjust the transmit power at the transmit end
according to the received level at the receive end.
WS (Wayside Service) is a technology used to improve the link
transmission capacity by using some idle bytes in the microwave
frame to transmit service signals.
RSC (Radio Service Channel) is used for orderwire communication
between microwave stations.
INI (N:1 switching command) is used to instruct channel protection
switching in the multi-channel protection system.
ID (Identifier) is used to identify a specific microwave
link.
FA (Frame Alignment) is used for alignment and identification of
the microwave frame.
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MW Frame Structure (cont.)
RFCOH and STM-1 data are blocked by multi-frame, there are six rows
in a multi-frame, 3564 bits per rows. A multi-frame consists of two
sub-frames, and 1776 bits for one row in a sub-frame. The other 12
bits are used as FS.
Multi-frame 3564bit
Sub-frame 2
12bit first unit
12bit 148th unit
ISTM-1 date bit C1,C2: 2 Level error correction monitor bit FS:
Frame sync. a,b: other RFCOH
RFCOH is multiplexed into the STM-1 data and a block multiframe is
formed. Each multiframe has six rows and each row has 3564 bits.
One multiframe is composed of two basic frames. Each basic frame
has 1776 bits. The remaining 12 bits are used for frame
synchronization (FS).
Every basic frame comprises various bits: I indicates the STM-1
information bit, C1 and C2 indicate two-level correction coding
monitoring bits, FS indicates the frame synchronization word, and a
and b are other complementary overheads.
The SDH frame is a block structure composed of bytes and has a
fixed sequence. The microwave frame is different and is composed of
bits. The arrangement is irregular depending on the specific
application.
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Questions
What are the frequency bands commonly used in digital MW?
What are the concepts in digital MW frequency band arrangement
?
What modulation modes is commonly used? What modulation modes are
used in digital MW?
What is microwave?
Microwave is a kind of electromagnetic wave, the frequency range of
is 300 MHz to 300 GHz. It is considered as plane wave.
What is digital microwave communication?
Digital microwave communication adopts the digital modulation
scheme. The baseband signal is processed in the Intermediate
Frequency (IF) unit and be converted into the microwave radio
frequency band.
What frequency bands are commonly used in digital microwave
communication?
According to ITU-R Recommendations, the common frequency bands
include 7G/8G/11G/13G/15G/18G/23G/26G/32G/38G.
What concepts are involved in microwave frequency setting?
The concepts include central frequency, transmit/receive spacing,
channel spacing and protection spacing.
What are the common modulation schemes? Which are the most
frequently-used?
ASK, FSK, PSK and QAM. The most frequently-used are PSK and
QAM.
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Contents
Microwave Propagation and Antifading Technologies
In the chapter two, it mainly introduce the types and structures of
microwave equipments, also include the functions of every parts in
the system. Understanding of this chapter is the basics for the
further study.
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Types of Digital MW Equipment
Digital MW
Discontinued
Microwave equipment may be classified in different ways.
By system, it may fall into digital microwave equipment and analog
microwave equipment. At present, the latter is already
discontinued.
By capacity, it may fall into microwave equipment of small and
medium capacity and microwave equipment of large capacity. Small
and medium capacity refers to 2 – 16 E1s or 34M, and large capacity
refers to STM-0, STM-1 and 2 x STM-1.
By structure, it may fall into trunk microwave equipment,
split-mount microwave equipment and all outdoor microwave
equipment.
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Trunk MW Equipment
SDH MW Equipment
MSTU: Main signal transceiver unit (transceiver, modem, SDH
electric interface, hitless module)
SCSU: surveil, control, switch unit
BBIU: baseband interface unit (optional: STM-1 optical interface,
C4 PDH interface)
P
M1
M2
…
High cost, high transmission capacity, high device stability, is
suit for long distance and backbone transmission.
RFIFsignal processmultiplex units and some other units are all
indooronly antenna and feeder are outdoor.
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All-outdoor MW Equipment
All-outdoor MW equipment
IF cable
RF signal processing unit
Service and power cable
The all outdoor equipment is composed of four parts: 1) Outdoor
part including the antenna and RF processing unit. The antenna
completes directional transmitting and convergence receiving of RF
signals and enlarges the transmission distance. The RF processing
unit transmits and receives RF signals and converts RF signals into
IF signals. 2) Intermediate frequency (IF) cable, which connects
the RF processing unit with the IF and baseband processing unit and
supplies power to the RF processing unit. 3) IF and baseband
processing unit, which processes IF and baseband signals. 4)
Service and power cable, which completes service access and
supplies power to the whole equipment.
All the units of all outdoor microwave equipment are outdoor. The
installation is easy and the equipment room is saved, but
transmission capacity is generally small.
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Split-mount MW Equipment
split-mount MW equipment
(ODU)
IF Cable
Indoor Unit
RF and antenna parts are outdoor and other parts are indoor. Indoor
and outdoor units are connected by a cable.
RF unit can mount to antenna directly or with a soft
waveguide.
Transmission capacity of the split-mount microwave is contrasting
small, and easy to install and maintenance.
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Split-mount MW Equipment (cont.)
Antenna: focus RF signal sent by ODU, enlarge signal gain
ODU: RF signal processingconversion between IF signal and RF
signal.
IF cable: Transmission for IF service signal , ODU management
signal and supply power for ODU.
IDU: service access and distribute, multiple, modem and so
on.
Antenna: Focuses the RF signals transmitted by ODUs and increases
the signal gain, thus enlarging the transmission distance.
ODU: Implements RF processing to realize IF/RF conversion of
signals.
IF cable: Transmits IF signals and IDU/ODU communication signals
and also supplies power to ODUs.
IDU: Performs access, grooming, multiplexing/demultiplexing and
modulation/demodulation of services.
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Split-mount MW Equipment - Installation
Direct installation
IF cable
IF interface
The installation of the split-mount microwave equipment comprises
two parts: indoor installation and outdoor installation. Indoor
installation is the IDU installation, IDU is generally installed in
a universal cabinet. Outdoor installation includes installing the
antenna and ODU. There are two methods. One is direct installation
and the other is separate installation. As its name implies,
separate installation means that the antenna and the ODU are
separated and connected via a soft waveguide. This mode is used
when there is little installation space on the tower, as shown in
the left figure. Direct installation means that the ODU is directly
mounted behind the antenna so as to avoid the loss caused by the
soft waveguide, as shown in the above right figure.
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Antenna
The antenna propagates the electric wave from transmitter
into one direction, and receive the electric wave. Paraboloid
antenna and Kasai Green antenna are usually used.
The common diameter of antenna are: 0.3, 0.6, 1.2, 1.8, 2.4, and
3.0m, etc.
Paraboloid antenna
Kasai Green antenna
Antennas are used to send out the electric wave energy launched by
transmitters directionally or send the electric wave energy
received into receivers. Parabolic antennas and Cassegrainian
antennas are two common types of microwave antennas.
The two types of antennas have the same reflection plane but
different feeder sources. The fields to which they are applied also
vary. Generally, a parabolic antenna has the feeder source in front
of it. The RF signal is transmitted via the waveguide to the feeder
source, then reflected once by the paraboloid and finally sent out
by the feeder. A parabolic antenna is generally small and can only
transmit a short distance. A Cassegrainian antenna has the feeder
source behind it. The RF signal is reflected once by the
small/medium paraboloid of the antenna and then twice by the
paraboloid. A Cassegrainian antenna is generally big and can
transmit a big distance.
The microwave antenna diameter generally includes 0.3 m, 0.6 m, 1.2
m, 1.8 m, 2.0 m, 2.4 m, 3.0m, 3.2 m, etc.
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Antenna (cont.)
Several channels in one frequency band can share one antenna.
Tx
Rx
Tx
Rx
Channel
Channel
1
1
n
n
1
1
n
n
The performance and frequency of a microwave antenna are related.
Generally different antennas shall be chosen for different
frequency bands and the channels in the same frequency band may
share one antenna.
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Antenna Aligning
Side view
Main lobe
Main lobe
The radiated power of an antenna is basically distributed to the
main lobe, the side lobe and the rear lobe. The main lobe has the
highest power, which extends to the two sides.
17.psd
18.psd
Antenna Aligning
Correct
Wrong
Wrong
The objective of antenna adjustment is to align the main lobe of
the local antenna to the main lobe of the opposite antenna.
During antenna adjustment, change the direction vertically or
horizontally. Meanwhile, use a multimeter to test RSSI at the
receiving end. Usually, the voltage waveform will be displayed as
shown at the lower right corner. The peak point of the voltage wave
indicates the main lobe position in the vertical or horizontal
direction. Large-scope adjustment is unnecessary. Perform fine
adjustment on the antenna to the peak voltage point.
When antennas are poorly aligned, only a small voltage may be
detected in one direction. In this case, perform coarse adjustment
on the antennas at both ends, so that the antennas are roughly
aligned.
The antennas at both ends that are well aligned will face a little
bit upward. Though 1–2 dB is lost, reflection interference will be
avoided.
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Antenna Specifications
Antenna gain
The input power ratio of isotropic antenna (Pio) to surface antenna
(Pi) when getting the same electric field intensity at the same
point.
It can be calculated by formula( unit: dB) :
Half power angle (3 dB beam width)
From the main lobe deviates to both sides, the points where the
power decrease half are half power point. The angle between the two
half power points is half power angle.
Approximate calculation formula
D: Antenna diameter, λ: Wavelength η: Usability coefficient
When the “D” is constantthe higher frequencythe smaller half power
anglewhen the frequency constantthe bigger diameterthe smaller half
power angle. The smaller half power anglethe higher power
centralization in specific direction.
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Antenna Specifications (cont.)
Cross polarization discrimination (XPD)
The suppressive intensity of power received from expected
polarization (Po) to the other polarization (Px). It should more
than 30db. Formula is:
XdB10lgPo/Px
Antenna protection ratio
It is the ratio of the receiving attenuation in antenna other lobes
to the receiving attenuation in antenna main lobe. The 180 degree
antenna protection ratio also be called as the front / rear
protection ratio.
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Outdoor Unit
Working frequency band:
One ODU can cover one frequency band or some part of a frequency
band.
Output power:
The power at the output port of transmitter.
The typical range of power is from 15 to 30 dBm.
ODU is used to convert IF and RF signals, also works as the band
pass filter and RF/IF amplifier.
Working frequency band:
The backbone microwave usually use the frequency bands of 6,7 and 8
GHz.
The 11, 13 GHz and above frequency bands are used in access layer
(eg, BTS access).
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Outdoor Unit (cont.)
Frequency stability
The oscillation frequency stability of microwave device is from 3
to 10 ppm.
Transmitting frequency spectrum frame
Frequency stability
If the work frequency of transmitter is not stable, the amplitude
of demodulated effective signal will descend, and bit error ratio
will increase.
Transmitting frequency spectrum frame
Transmitting signal mask must comply with some limitation, lest to
occupy more bandwidth and bring serious interference to adjacent
channels.
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Outdoor Unit (cont.)
Work frequency band:
The receiving frequency of local station is the same with the
remote station.
Frequency stability
Noise Figure
The noise figure of digital microwave receiver is from 2.5 to
5dB.
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Outdoor Unit (cont.)
Passband
The typical value is 1 to 2 times of transmission code element
rate.
Selectivity
Automatic gain control (AGC) range
Automatic control the gain to keep the same IF output power level
when receiving RF power level shift in a range because of
fading.
Passband
To effectively suppressing interference and getting optimal signal.
It depends on the band pass filter
Selectivity
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Indoor Unit
Processing RFCOH
Cable interface
From/to ODU
Tx IF
Rx IF
The IDU implements the functions including service access, service
dispatching, multiplexing/demultiplexing, and
modulation/demodulation. Thus the IDU is the main part of a set of
microwave equipment.
In the transmit direction, the service signal undergoes microwave
frame multiplexing to form the complete microwave frame structure
(but the signal is still digital baseband signal). After being
modulated, the signal is converted into the IF signal and then
transmitted via the IF cable to the ODU.
In the receive direction, the process is just completely
reverse.
Moreover, the IDU is monitored and controlled via the O&M
interface and the system is supplied power via the power
interface.
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Questions
What are the classification of digital MW equipment
What components are there in the split-mount digital MW
equipmentWhat are the functions of them?
What are the main parameters of antenna
What are the parameters of ODU transmitter and receiver
What types are microwave equipment classified into?
Trunk, all-outdoor and split mount.
What units do the split-mount microwave equipment have? And what
are their functions?
The split-mount microwave equipment is composed of four parts:
Antenna, ODU, IF cable and IDU. Antenna focuses the RF signals
transmitted by ODUs and increases the signal gain, thus enlarging
the transmission distance. ODU implements RF processing to realize
IF/RF conversion of signals. IF cable transmits IF signals and
IDU/ODU communication signals and also supplies power to ODUs. IDU
performs access of services.
What are the main parameters of antenna?
Gain, half power angle, cross polarization discrimination,
protection ratio.
What are the parameters of ODU transmitter and receiver?
Working frequency band, output power, frequency stability,
transmitting frequency spectrum frame, noise figure, selective,
passband, automatic gain control range.
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Summary
Functions of the components in split-mount digital MW
equipment
Parameters of antenna
Parameters of ODU
Function of IDU
Contents
Microwave Propagation and Antifading Technologies
Chapter three is mainly introduce the networking application of
microwave equipment, it helps the reader to understand the digital
microwave transmission network and hardware configuration in
further study.
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Common Networking Application
link
Tree
There are four common networking modes of digital microwave: Ring
network, point-to-point chain network, add/drop network and hub
network.
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Types of Digital MW Stations
The digital MW station includes terminal station, relay station and
pivotal station
Terminal station
Terminal station
Terminal station
Pivotal station
Pivotal station
Relay station
Terminal station: it is located at the both ends of a line or the
end of a branch.
Relay station: it is located at the middle of a line , without
adding or dropping service.
pivotal station: it is located at backbone, need to complete
communication in multi-direction
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Types of Relay Stations
Parabolic reflectors
Plane reflector
Regenerative relay
IF relay
RF relay
Relay stations may fall into passive relay stations and active
relay stations. There are two types of passive relay stations:
back-to-back antenna and plane reflector. The active relay stations
include regenerators, IF repeaters and RF repeaters.
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Active Relay Stations
RF direct station:
Amplifying MW signal at RF band bidirectionally without frequency
shift.
Regenerative relay station:
It extends the MW propagation distance and change direction to
round the obstacles.
RF direct station is an active, bidirectional, non-frequency-shift
RF relay system. For it amplifies signals directly on the RF, it is
called RF direct station. It can be used as a relay station that
needs not add/drop voice channels in the microwave system. It can
be used to solve the block problem caused by mountains and large
building, and it can also be inserted in the newly built and
already established microwave to increase fading margin.
Regenerative relay station is a high-frequency repeater with high
performance. Regenerative relay station is similar to back-to-back
terminal station, including an entire set of RF unit with
regenerative microwave signals. It can extend the signal
transmission path and change transmission direction to round
obstacles, but it is incapable of adding/dropping voice channels.
It can be used to break the distance limit of microwave
transmission system or divert the transmission direction to round
line-of-sight obstacles, and the signal quality is not degraded. It
receives signals, fully regenerates and amplifies the signals and
then transmits the signals.
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Passive Relay Stations
Parabolic reflectors:
It consists of two parabolic antennas which are connected back to
back with a section of waveguide.
Plane reflectors:
A metal panel with a smooth surface and effective acreage.
Plane reflectors:
A metal plane that is smooth to some extent, has proper available
area, and a suitable angle and distance to two communication
points, is also a microwave passive relay station. The station uses
the reflection function of the metal plane to change the
propagation direction of the microwave beam and round the obstacle
to achieve communication.
the efficiency of plane reflector is higher than the dual parabolic
reflectors.
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Passive Relay (actual picture)
Plane reflectors
Parabolic reflectors
The left figure above shows the plane reflector passive relay
station. It uses two reflector planes and the signal undergoes
twice reflection. The right figure above shows the parabolic
reflector passive relay station.
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Application of Digital MW
Backhaul transmission for mobile BTS
Critical link backup
Special transmission situation (river, lake, island)
VIP customer access
Complementary networks to optical networks, Complementary networks
to optical networks mean to use microwave transmission when optical
network transmission is not suitable, so as to make the network
structure more complete.
BTS backhaul transmission, BTS backhaul transmission means to use
microwave transmission as the backhaul link of the mobile BTS when
the transmission capacity of the mobile BTS is small and the use of
optical fiber transmission will bring a high cost.
redundancy backup of important links, Redundancy backup of
important links means to use microwave transmission as a backup of
some important transmission links, because microwave transmission
has quite strong ability to protect against natural disaster.
VIP customer access, VIP customer access means to use microwave
transmission for some important customers when the needed
transmission capacity is not large and the use of optical fibers
for every such customer will give rise to a high cost.
Emergency communications, Emergency communications means to use
microwave transmission for temporary communications in such
scenarios as large conferences or disaster relief, because the
microwave transmission network can be constructed in a short time
and can be quickly removed after the use.
Special transmission conditions. Special transmission conditions
mean to use microwave transmission for the terrains where optical
fiber transmission is not suitable, for example, rivers, lakes,
islands and so on.
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Questions
Which network application are commonly used by digital MW?
What types of stations are there in the digital MW system?
What types of the relay stations are there?
What are the applications for digital MW system?
Which network application are commonly used by digital
microwave?
ring network, point-to-point chain network, hub network and
add/drop network.
What are the types of digital microwave stations?
pivotal stations, terminal stations, and relay stations.
What types of relay stations are there?
passive relay stations and active relay stations.
What are the applications of digital microwave?
Refer to page 45.
Contents
Microwave Propagation and Antifading Technologies
In the chapter four, it mainly introduces all types of the fading
in the microwave propagation and the corresponding antifading
technologies. It helps the reader to understand the microwave
network design and hardware configuration. It is also the necessary
basic knowledge for the microwave equipment maintenance
operator.
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reserved.
Contents
4.2 Antifading Technologies
Factors Affect MW Propagation
The reflection from land affect receiving signal from main
direction
4 types of the landform:
A: mountainous region (or the region of dense buildings)
B: foothill (the fluctuation of ground is gently)
C: flatland
Direct
Reflection
Direct
Reflection
Part of the signal power from the transmitter antenna may be
reflected by the smooth ground or the water and interfere the main
signal which undergo the direct propagation direction. The vector
sum of reflected wave and main wave lead the sum wave augment or
reduce, so the propagation is not stable. Therefore when we design
the propagation path of microwave, we should reduce the reflected
wave, and if there is some reflected wave, we should use the
fluctuation of terrain to block the reflected wave.
The reflectance of mountainous region is the least, and it is the
most suitable terrain for microwave propagation. And the terrain of
foothill is the second. We should avoid water or other smooth
surface when we design the microwave propagation path.
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Factors Affect MW Propagation (cont.)
Atmosphere and weather:
Atmosphere absorption mainly affect the microwave whose frequency
is over 12 GHz.
Refraction, reflection, dispersion in the troposphere.
Scattering and absorption loss caused by rain, fog and snow. It
mainly affect the microwave whose frequency is over 10 GHz.
The troposphere is the low atmosphere layer which is up to 10 km
from the ground. Because the height of microwave antenna can’t
exceed the troposphere, we just need to know the electric wave
propagates in the troposphere which can substitute the study of in
the atmosphere.
Atmosphere absorption loss because of the resonance between
aerosphere molecule and the microwave when the frequency of
microwave is close to the syntony frequency of the molecule.
Atmosphere is asymmetric, the microwave in the troposphere may
cause refraction, reflection, dispersion, etc. Among these, the
effect from refraction is the most serious.
Scattering and absorption loss caused by rain, fog and snow. It
mainly affect the microwave transmission whose frequency is over 10
GHz. The higher frequency, the more loss caused by rain, fog and
snow.
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Classification of the Fading
Scintillation fading
Fading
The receiving power level fluctuates randomly. The variation is
irregular, and the reason is various. When time conditions (such as
season, day and night) and climate conditions (such as rain, fog,
and snow) change, the temperature, temperature rate and stress of
the atmosphere, position of ground reflection, and reflection
coefficient change. These changes can cause the field strength at
the receive point to change. Such phenomenon is called radio
propagation fading. Obviously, fading is a random phenomenon.
The degree of fading is indicated by the fading factor VdB. The
reasons for fading are mainly atmosphere and ground effect.
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Free Space Fading
d = distance in km f = frequency in GHz
Power Level
PRX = Receiving power
Free space is an infinite space filled up with even and ideal
propagation media, in which electromagnetic waves are not affected
by the factors such as blocking, reflection, diffraction,
scattering, and absorption. However, this does not mean that there
is no loss when microwave is propagated in free space, because the
microwave beam will keep eradiating when it is propagated in free
space and so a certain loss will arise.
Calculation formula of free space loss: A = 92.4 + 20 log d + 20
log f, where D is in the unit of km and F is in the unit of GHz.
The loss will increase by 6 dB when d or f is doubled.
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Absorption Loss
Molecules of all substances are composed of charged particles.
These particles have their own electromagnetic resonant
frequencies. When the microwave frequencies of these substances are
close to their resonance frequencies, resonance absorption occurs
to the microwave.
Statistics show that absorption to the microwave frequency lower
than 12 GHz is smaller than 0.1 dB/km. Compared with free space
loss, the absorption loss can be ignored.
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Rain & Fog Fading
Generally, different frequency band has different loss.
less than 10 GHz, its fading caused by rain and fog is not
serious.
over 10 GHz, relay distance is limited by fading caused by
rains.
over 20GHz, the relay distance is only about several kilometers for
the rain & fog fading.
Rain & fog fading refers to the scattering or absorption of the
electromagnetic wave energy caused by rain, fog or snow.
For frequencies lower than 10 GHz, rain loss can be ignored. Only a
few decibels may be added to a relay section.
For frequencies higher than 10 GHz, relay distance is mainly
affected by rain loss. For example, for the 13 GHz frequency or
higher, 100 mm/h rainfall causes a loss of 5 dB/km. Hence, for the
13 GHz and 15 GHz frequencies, the maximum relay distance is about
10 km. For the 20 GHz frequency or higher, the relay distance is
limited in few kilometers due to rain loss.
Therefore, high frequency bands can be used for user-level
transmission. The higher frequency band, the severer rain
fading.
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K Factor Fading
A equivalent radius: Re=KR (R is the real radius of earth).
the value of K is depend on the local meteorological
phenomena
R
K in K-factor fading refers to atmosphere refraction and K-factor
fading means the fading caused by the change of atmosphere
refraction.
As a result of atmosphere refraction, the microwave propagation
trail is actually bent. It is considered that the electromagnetic
wave is propagated along a straight line above the earth with an
equivalent earth radius of Re = KR (R is the actual earth
radius).
The average measured K value is about 4/3. However, the K value of
a specific section is related to the meteorological phenomena of
the section. It may change within a comparatively large range. This
can affect line-of-sight propagation.
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Scintillation Fading
The particle cluster formed in local atmosphere for pressure,
temperature or humidity is different as other area, and the
electric wave is scattered by it.
sketch map of Scintillation fading
Scintillation fading is also called “fluctuation fading”. When the
dielectric constant of local atmosphere is different from the
ambient due to the particle clusters formed under different
pressure, temperature, and humidity conditions, scattering occurs
to the electric wave. This is called scintillation fading. The
amplitude and phase of different scattered waves vary with the
atmosphere. As a result, the composite field strength at the
receiving point changes randomly. Scintillation fading is a type of
fast fading which lasts a short time. The level changes little and
the main wave is barely affected. Scintillation fading will not
cause communications interruption.
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reserved.
Duct Type Fading
When electric waves pass the atmospheric waveguide, super
reflection occurs.
sketch map of Duct Type fading
Due to the effects of the meteorological conditions such as ground
cooling at night, burnt warm by the sun in the morning, smooth sea
surface and anticyclone, a non-uniform structure is formed in
atmosphere. This phenomenon is called “Atmospheric Duct”. If
microwave rays pass through the atmospheric duct while the
receiving point is outside the duct layer, the field strength at
the receiving point is from not only the direct wave and ground
reflected wave, but also the reflected wave from the edge of the
duct layer. As a result, severe interference fading occurs and
causes interruption to the communications.
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Multi-Path Propagation and Fading
The receiving paths includes direct path and other reflection
paths.
Multi-path fading is caused by the signals interference from
different propagation paths
Ground
Multi-path electric waves have random amplitude and phase at the
receiving point, and the level of the receiving point is the vector
sum of mutual interference of the waves, therefore, the receiving
level produces multi-path interfering fading along with this
multi-path propagation phenomenon. This phenomenon typically occurs
in hot and humid summer, for example, in the basin of the Yellow
river, it frequently occurs in July, August and September. This
phenomenon is more apt to occur in plains and water reticulation
areas than mountain areas.
Muti-path fading is more serious especially when the path through
water surface, lake and smooth ground, so we should avoid it. If it
cannot be avoided, we should reduce the influence of multi-path
reflection by adopting high-low antenna technology to adjust the
reflection point near one end, or high-low antenna technology and
space diversity technology, or antireflection antenna.
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Flat Fading
1 h
Threshold
(-30dB )
Signal interruption
Upward fading
Fast fading
Slow fading
Fading can be classified based on the field strength of the
receiving point. When the received level is higher than the free
space level for few dB, it is called upward fading, and when it is
lower than the free space level for few dB to few tens of dB, it is
called downward fading
Fading can be classified into slow fading and fast fading based on
the duration. Long-duration fading is called slow fading and the
duration is from several minutes to several hours. Short-duration
fading is called slow fading and the duration is from several
seconds to several minutes. Slow fading varies slowly, it is slowly
formed and then slowly disappears, and it is always caused by
atmospheric refraction changing slowly in a wide area. For in a
wide area (such as a section of relay circuit), atmospheric
refraction becomes bad and recovers in a relatively long time, and
then slow fading is formed. Fast fading is closely related to
multi-path propagation caused by thin layer in the atmospheric
waveguide and turbulent current. In the range of microwave, if the
paths of each ray in the previous multi-path propagation vary, the
composite signals of the rays at the receiving point may vary and
then fast fading is formed.
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Frequency Selective Fading
Frequency selective fading will cause the in-band distortion and
decrease system original fading margin.
Freq. (MHz)
Selective fading
In normal status, receiving power at every frequency range is
almost on the same level.
When the frequency selective fading occurred, the receiving power
level on some certain frequency is lower than on others, like the
gap in the above figure. At this moment, it will cause the
distortion in the time field of the demodulated signal.
The bandwidth of large capacity system is wide, this influence is
more serious. For the small capacity system, the influence can be
ignored for the small bandwidth occupation.
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reserved.
Contents
4.2 Antifading Technologies
Antifading Technologies
Adaptive Equalization
Diversity receive technologies
Wave shape distortion and Power reduction
Multi-Path fading may cause fading and distortion of the
transmission channel, which varies with the geographical
environment and time. Hence, any kind of anti-fading measure must
be adaptive.
To deal with flat fading, the automatic gain control circuit (AGE)
of the intermediate frequency amplifier in the receiver and channel
switching method are for common use.
To deal with frequency selective fading, the diversity technology
and adaptive equalization technology are adopted. The following
three measures are used for frequency selective anti-fading. These
anti-fading technologies suppress amplitude dispersion and delay
dispersion in different ranges of space, frequency and time. If
these technologies are combined, a better anti-fading effect can be
achieved.
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reserved.
Adaptive Frequency Equalization
AFE uses the frequency characteristics of an adjustable network to
compensate distortion of amplitude frequency characteristics and
phase frequency characteristics of actual channels.
A standard signal frequency spectrum, after being transmitted, will
have frequency spectrum characteristic distortion due to multipath
fading and other factors. The signals will distort accordingly. We
may equalize the distorted signal frequency spectrum via the slope
of a frequency domain to reduce the influence of signal frequency
domain distortion. This method is called “Frequency Domain
Equalization”.
Frequency domain equalization only equalizes the amplitude
frequency response characteristics of the signal instead of the
phase frequency spectrum characteristics. The circuit is
simple.
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reserved.
Adaptive Time Equalization
T
T
T
After
Equalization
C-n
C0
Cn
Ts
-Ts
-2Ts
Ts
-Ts
-2Ts
ATE is used in time domain to directly counteract inter-symbol
interference (ISI) caused by distortion of amplitude and group
delay.
A standard signal waveform, after being transmitted for a certain
distance, will be expanded or have some other distortions, which
will cause inter-symbol interference and further result in bit
errors. In this case, a time domain equalizer with multiple taps
may be used to sample the time domain signals at multiple points
and then perform weighted summation, so as to reduce the distortion
of the signal time domain waveform. This method is called “Time
Domain Equalization”.
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reserved.
Automatic Transmit Power Control
ATPC is used to reduce interference to adjacent system,
upward-fading, DC power consumption and refine characteristic of
residual error rate.
modulator
transmitter
receiver
demodulator
ATPC
receiver
ATPC
transmitter
modulator
demodulator
ATPC helps the output power of a transmitter operates in a normal
value. When the level of the remote receiver reduces, the output
power can be increased until gradually reach maximum by the
feedback from reverse communication channel.
Characteristics of ATPC: The output power of a microwave
transmitter can automatically trace the receiving levels at the
receive end within the range controlled by ATPC and vary with the
levels. In normal propagation conditions, the output power of a
transmitter is fixed at a low level that may be about 10–15 dB
lower than the normal level. When the level is lower than the
lowest received level specified by ATPC and the receiver detects
propagation fading in the event of propagation fading, ATPC uses
the RFCOH byte to control the peer end transmitter and thus
increase the transmitting power till a rated power value. Usually,
the time rate occurring on severe propagation fading is short; that
is, lower than 1%. After the ATPC is adopted, the transmitter
operates at the power 10–15 dB lower than the rated power in most
time (over 99%).
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XPIC
Frequency configuration in U6GHz bandITU-R F.384-5
Direction of electric field
680MHz
30MHz
V (H)
H (V)
1X 2X 3X 4X 5X 6X 7X 8X
1’ 2’ 3’ 4’ 5’ 6’ 7’ 8’
1X’ 2X’ 3X' 4X’ 5X’ 6X’ 7X’ 8X’
It use two quadrature polarized signal with same frequency to
double the capacity. To avoid serious interference between Them,
XPIC is used.
In a common microwave radio transmission system, the frequency of
two polarization waves are allocated in different interleave mode.
The interference between two polarization waves is small. However,
in SDH microwave transmission, to improve the spectrum utilization,
the co-channel or channel-insertion cross-polarization frequency
regeneration mode is adopted.
In a light-of-sight propagation route, in the event of multi-path
fading, dispersion on the nonuniform layer and ground or rain and
fog, the cross-polarization signals may severely cause interference
to co-polarization signals. Hence, the interwave interference
compensation technology of cross-polarization should be
introduced.
Cross polarization Interference Counteracter (XPIC) can be
implemented in radio frequency, intermediate frequency and baseband
frequency. The latter two frequency bands are more common. After
the XPIC is adopted, the XPI can be improved by about 20 dB.
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Diversity Reception
Diversity reception is used to minimize the effects of fading. It
includes:
Space diversity (SD)
Frequency diversity (FD)
Polarization diversity
Angle diversity
Diversity means two or multiple transmission paths are used to
transmit the same information and the receiver output signals are
selected or combined to reduce the effect of fading. Diversity
falls into space diversity, frequency diversity, polarization
diversity, and angle diversity.
Space diversity and frequency diversity are more frequently used.
Space diversity is economical and has a good effect. Frequency
diversity is often applied to multi-channel systems as it requires
a wide bandwidth. Usually, the system that has one standby channel
is configured with frequency diversity.
But as frequency recourses are becoming scarce currently and
frequency diversity functions better only when the frequency
spacing is large enough, space diversity is more often used.
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Frequency Diversity
The merit is only need one set of feeder and antenna, but its
demerit is that utilization of frequency band is low.
f1
f2
In space transmission, fading characteristic of signals with
different frequency are different. So it can combine or select two
or more signals with different frequency which carry same
information to reduce fading.
In frequency diversity systems, the correlation of two diversity
received signals (frequency correlation) should be small. Only in
this event, deep fading on two frequencies can be avoided in a
given path and good diversity effect can be implemented. The bigger
the spacing of two frequencies is, the smaller the correlation of
deep fading at the same time.
When the diversity in the same frequency uses 2% of the working
frequency as a frequency spacing, the diversity improvement effect
can be obtained.
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Space Diversity
The merit is saving frequency resource, but demerit is system is
complex and need two or more sets of feeder and antenna.
f1
f1
Signals have different multipath effect over different paths and
thus have different fading characteristics. Accordingly, two or
more suites of antennas at different altitude levels may be used to
receive the signals at the same frequency and are then combined or
selected. This working mode is called “Space Diversity”. If there
are n suites of antennas, it is called “n-fold diversity”.
The merit of space diversity is that frequency resources are saved.
The disadvantage is that the equipment is complicated and two or
more suites of antennas are needed.
Antenna distance: The distance between the diversity antennas is
100 to 200 times the wavelength in frequently used frequency
bands.
Space diversity can effectively solve the K factor fading caused by
the interference of ground-reflective wave and direct wave, and the
interference fading caused by troposphere reflection.
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Other Antifading Methods
blocking the reflected wave by some terrain or obstacles.
Make use of some terrain and ground objects to block reflected
waves. Specifically, control the location of the reflection point
so that it is near the obstacle.
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reserved.
Other Antifading Methods (cont.)
Different height antennas in one hop.
Use high and low antennas, that is, let the height of the antenna
at one end be different from the height of the antenna at the other
end, so that the reflected wave does not fall within the receive
range of the receive antenna.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
Questions
What types of the fading are there in microwave propagation?
What types of antifading technologies can be used?
What are the factors which affect microwave propagation?
Terrain, atmosphere and climate.
What types of the fading are there in microwave propagation?
By the mechanism of fading, fading may fall into duct type fading,
k factor fading, scintillation fading, rain fading, absorption
fading and free space propagation fading.
By fading time, fading may fall into fast fading and slow
fading.
By received level, fading may fall into up fading and down
fading.
By the influence of fading on signals, fading may fall into
frequency selective fading and flat fading.
What types of antifading technologies can be used?
ATE, AFE, ATPC, XPIC, SD, FD.
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reserved.
Summary
Structure and function of digital microwave equipment
Application of digital microwave communication
Microwave propagation and fading