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Digital Microwave Communication Principles
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Foreword
This course is developed to meet the requirement of Huawei Optical
Network RTN microwave products.
This course informs engineers of the basics on digital microwave
communications, which will pave the way for learning the RTN series
microwave products later.
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Learning Guide
Microwave communication is developed on the basis of the
electromagnetic field theory.
Therefore, before learning this course, you are supposed to have
mastered the following knowledge:
Network communications technology basics
Electromagnetic field basic theory
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Concept and characteristics of digital microwave
communications
Functions and principles of each component of digital microwave
equipment
Common networking modes and application scenarios of digital
microwave equipment
Propagation principles of digital microwave communication and
various types of fading
Anti-fading technologies
Objectives
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Designing Microwave Transmission Links
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Optical fiber burying and land
occupation required
disaster and easy to be recover
Outdoor optical fiber maintenance required
and hard to recover from natural disaster
Limited frequency resources (frequency
required
and not affected by external factors
Transmission quality greatly affected by
climate and landform
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Microwave
Microwave is a kind of electromagnetic wave. In a broad sense, the
microwave frequency range is from 300 MHz to 300 GHz. But In
microwave communication, the frequency range is generally from 3
GHz to 30 GHz.
According to the characteristics of microwave propagation,
microwave can be considered as plane wave.
The plane wave has no electric field and magnetic field
longitudinal components along the propagation direction. The
electric field and magnetic field components are vertical to the
propagation direction. Therefore, it is called transverse
electromagnetic wave and TEM wave for short.
1
The wave with the radio frequency between 300 MHz and 300 GHz (or
the wavelength between 1 meter and 1 millimeter) is called
centimeter wave in microwave.
TEMTransverse Electric and Magnetic
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Large capacity: > 100M
Small and medium
Analog microwave communication system
1
Before 1980, analog microwave had been playing a predominant role
in communication. Since 1990, digital microwave technologies have
been developing rapidly. 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.
It has been more than 50 years since microwave technologies
developed. As a radio transmission scheme where the microwave
frequency band signal adopts ground line-of-sight (LOS)
propagation, microwave technologies have experienced the transition
from analog microwave to digital microwave. The analog microwave
and coaxial cable carrier transmission system are the two major
methods used in the early stage for long haul transmission.
The earliest TV program transmission among cities adopts the
microwave transmission channels. The small and medium capacity
digital microwave equipment (8.34 Mbit/s) developed in 1970s has
turned a new leaf for the digitalization of microwave. In late
1980s, the successful development of SDH digital microwave leads to
the emerging of the Nx155 Mbit/s large capacity digital microwave
system. Speaking of analog microwave, it was no longer used to
construct networks in the end of 1980s, and now is used only in
mountain stations owned by State Administration of Radio, Film and
Television of China.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Digital microwave communication is a way of transmitting digital
information in atmosphere through microwave or radio frequency
(RF).
Microwave communication refers to the communication that use
microwave as carrier .
Digital microwave communication refers to the microwave
communication that adopts the digital modulation.
The baseband signal is modulated to intermediate frequency (IF)
first . Then the intermediate frequency is converted into the
microwave frequency.
The baseband signal can also be modulated directly to microwave
frequency, but only phase shift keying (PSK) modulation method is
applicable.
The electromagnetic field theory is the basis on which the
microwave communication theory is developed.
Concept of Digital
1
The original communication does not contain the concept of network.
Instead, it is point-to-point communication. There were no
switches. It was manual switch at the beginning, then stored
program control (SPC) switch, and time division and space division
technologies were adopted later. The current complex networks are
all derived from the primal simple networks.
The microwave transmission media is located in the troposphere
which is the lowest layer. Above troposphere, there is the
stratosphere, the use of which is now under research.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Generally-used frequency bands in digital microwave
transmission:
7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (defined by ITU-R
Recommendations)
3.3 GHz
Regional network
8
5
4
3
2
10
20
1
30
40
50
1
(1) For long haul PDH microwave links (the distance between
stations is generally longer than 15 km), 8 GHz frequency band is
recommended. If the distance between stations is not longer than 25
km, 11 GHz frequency band can also be used. The specific frequency
band shall be determined based on the local weather conditions and
microwave transmission cross-section.
(2) For short haul PDH microwave links (generally used in the
access layer and the distance between stations is shorter than 10
km), 11/13/14/15/18 GHz frequency band is recommended.
(3) For long haul SDH microwave links (the distance between
stations is generally longer than 15 km), 5/6/7/8 GHz frequency
band is recommended. If the distance between stations is not longer
than 20 km, 11 GHz frequency band can also be used. The specific
frequency band shall be determined based on the local weather
conditions and microwave transmission cross-section.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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In each frequency band, subband frequency ranges,
transmitting/receiving spacing (T/R spacing), and channel spacing
are defined.
f0 (center frequency)
Here are a few concepts about microwave frequency band
setting.
After selecting the microwave frequency band, configure the RF
channels, that is, divide the frequency band into several smaller
sub-bands to provide the spectrum required by the transmitter. We
call these sub-bands channels. These channels are usually indicated
by their respective center frequencies and sequence number. The
channel bandwidth depends on the spectrum of the signal
transmitted, or depends on the capacity and the modulation scheme
employed. Therefore, when configuring RF channels, follow the
principles listed below:
(1) Make full use of the limited radio frequency band.
(2) For one radio station, there must be enough spacing between the
transmit frequency and receive frequency, to avoid serious
interference on the receiver brought by the transmitter.
(3) In a multi-channel system, there must be enough frequency
spacing between two adjacent channels, to avoid mutual
interference.
(4) There must be enough protection spacing at the edges of the
allocated frequency band, to avoid interference with the adjacent
frequency bands.
(5) Most RF channels are configured with equal spacing.
According to the description of microwave relay system RF channel
configuration in ITU-R F.746-3, equal spacing is the basic scheme
employed first for RF channel configuration. The frequently used
channel spacing is 2.5 MHz and 3.5 MHz, which belong to North
American system and European system respectively. For 3.5 MHz
channel spacing scheme, it is expected that the channel spacing
will be further divided into 1.75 MHz, to support the small
capacity transmission requirement of 1xE1 or 2xE1.
The following are the common parameters that are related to RF
channel configuration:
XS: the RF spacing between the center frequencies of the adjacent
RF channels of the same polarization direction in the same
transmission direction.
YS: the RF spacing between the center frequencies of closest go
channel and return channel.
ZS: the RF spacing between the center frequency of the outermost RF
channel and the frequency at the edge of the frequency band. If the
frequency spacing at the lower end is different from that at the
upper end of the frequency band, then Z1S is used to indicate the
frequency spacing at the lower end, and Z2S is used to indicate the
frequency spacing at the upper end.
DS: the spacing between the transmit and receive duplex
frequencies. Within a specified channel, the spacing between a pair
of fn and fn' is constant.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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7G Frequency Range
Fn=f0-161+28n, Fn’=f0- 7+28n, (n: 1–5)
7575
161
7
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Communication Modulation (1)
Digital baseband signal is the unmodulated digital signal. The
baseband signal cannot be directly transmitted over microwave radio
channels and must be converted into carrier signal for microwave
transmission.
Digital baseband signal
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Communication Modulation (2)
ASK: Amplitude Shift Keying. Use the digital baseband signal to
change the carrier amplitude (A). Wc and φ remain unchanged.
FSK: Frequency Shift Keying. Use the digital baseband signal to
change the carrier frequency (Wc). A and φ remain unchanged.
PSK: Phase Shift Keying. Use the digital baseband signal to change
the carrier phase (φ). Wc and A remain unchanged.
QAM: Quadrature Amplitude Modulation. ). Use the digital baseband
signal to change the carrier phase (φ) and amplitude (A). Wc
remains unchanged.
The following formula indicates a digital baseband signal being
converted into a digital frequency band signal.
A*COS(Wc*t+φ)
PSK and QAM are most frequently used in digital microwave.
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RSC: Radio Service Channel
MLCM: Multi-Level Coding Modulation
INI: N:1 switching command
WS: Wayside Service
1
In a digital microwave system, to transmit the digital information
of orderwire, wayside services, bits employed by ATPC, and channel
switching, additional bits that are called RFCOH will be added into
the main data stream coming from the SDH MUX equipment. Suppliers
plan the frame structure according to transmission rate, modulation
schemes, error correction methods, and types of required additional
information. Therefore, different suppliers may have different
microwave frame structures. This figure shows the frame structure
that employs multilevel coded modulation (MLCM).
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Microwave Frame Structure (2)
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
alignment.
I: STM-1 information bit
FS: Frame synchronization
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The SDH frame is a block structure, composed of bytes, and has a
fixed sequence. The microwave frame is different from the SDH
frame. The microwave frame is composed of bits and the arrangement
is irregular depending on application.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Questions
What are the frequently used digital microwave frequency
bands?
What concepts are involved in microwave frequency setting?
What are the frequently used modulation schemes? Which are the most
frequently used modulation schemes?
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What is microwave?
Microwave is a kind of electromagnetic wave. In a broad sense, the
frequency range of microwave is 300 MHz to 300 GHz. In microwave
communication, the frequency range generally is from 3 GHz to 30
GHz. According to the characteristics of microwave propagation,
microwave can be considered as plane wave. The plane wave has no
electric field and magnetic field longitudinal components along the
propagation direction. The electric field and magnetic field
components are vertical to the propagation direction. Therefore, it
is called “transverse electromagnetic wave” or TEM for short.
What is digital microwave communication? Digital microwave
communication is a way of transmitting digital information in
atmosphere on microwave or radio frequency (RF). It adopts the
digital modulation scheme. The baseband signal is processed in the
Intermediate Frequency (IF) unit. Then the signal is converted into
the microwave frequency band through frequency conversion.
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. Higher or lower
bands may also be employed along with the development of
technologies but the application is rare. Different bands are
applied to different fields.
What concepts are involved in microwave frequency setting?
The concepts include central frequency, transmit/receive spacing,
channel spacing and protection spacing.
What are the frequently used modulation schemes? Which are the most
frequently-used?
The frequently-used modulation modes are ASK, FSK, PSK and QAM. The
most frequently-used are PSK and QAM.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Designing Microwave Transmission Links
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Large capacity
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High cost, large transmission capacity, more stable performance,
applicable to long haul and trunk transmission
RF, IF, signal processing, and MUX/DEMUX units are all indoor. Only
the antenna system is outdoor.
SDH microwave equipment
SCSU: Supervision, Control and Switching Unit
BBIU: Baseband Interface Unit (option) (STM-1 optical interface, C4
PDH interface)
P
M1
M2
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Installation is easy.
All outdoor microwave equipment
IF cable
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Split-Mount Microwave Equipment (1)
The RF unit is an outdoor unit (ODU). The IF, signal processing,
and MUX/DEMUX units are integrated in the indoor unit (IDU). The
ODU and IDU are connected through an IF cable.
The ODU can either be directly mounted onto the antenna or
connected to the antenna through a short soft waveguide.
Although the capacity is smaller than the trunk, due to the easy
installation and maintenance, fast network construction, it’s the
most widely used microwave equipment.
Split-mount microwave equipment
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Unit Functions
Antenna: Focuses the RF signals transmitted by ODUs and increases
the signal gain.
ODU: RF processing, conversion of IF/RF signals.
IF cable: Transmitting of IF signal, management signal and power
supply of ODU.
IDU: Performs access, dispatch, multiplex/demultiplex, and
modulation/demodulation for services.
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1
The installation of the split-mount radio contains two parts,
indoor installation and outdoor installation. Indoor installation
is similar to case-shaped equipment installation. So we focus on
outdoor installation. Outdoor installation includes installing the
antenna and ODU. There are two methods. One is direct installation
and the other is separate installation.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Antennas are used to send and receive microwave signals.
Parabolic antennas and cassegrainian antennas are two common types
of microwave antennas.
Microwave antenna diameters includes: 0.3m, 0.6m, 1.2m, 1.8m,2.0m,
2.4m, 3.0m, 3.2metc.
Parabolic antenna
Cassegrainian antenna
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Different frequency channels in same frequency band can share one
antenna.
Microwave Antenna (2)
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During antenna adjustment, change the direction vertically or
horizontally. Meanwhile, use a multimeter to test the RSSI at the
receiving end. Usually, the voltage wave will be displayed as shown
in 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, 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 face a little bit
upward. Though 1–2 dB is lost, reflection interference will be
avoided.
Antenna Adjustment (2)
Side lobe position
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Antenna Adjustment (3)
During antenna adjustment, the two wrong adjustment cases are show
here. One antenna is aligned to another antenna through the side
lobe. As a result, the RSSI cannot meet the requirements.
Correct
Wrong
Wrong
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– Antenna (1)
Antenna gain
Definition: Ratio of the input power of an isotropic antenna Pio to
the input power of a parabolic antenna Pi when the electric field
at a point is the same for the isotropic antenna and the parabolic
antenna.
Calculating formula of antenna gain:
Half-power angle
Usually, the given antenna specifications contain the gain in the
largest radiation (main lobe) direction, denoted by dBi. The
half-power point, or the –3 dB point is the point which is deviated
from the central line of the main lobe and where the power is
decreased by half. The angle between the two half-power points is
called the half-power angle.
Calculating formula of half-power angle:
Half-power angle
1
We can see that when the antenna diameter is determinate, the
higher the operating frequency is, the smaller the half-power angle
is. When the operating frequency is determinate, the bigger the
antenna diameter is, the smaller the half-power angle is. And the
smaller the half-power angle is, the more the energy is
concentrated and the better the directional quality is.
G=20log 7.33×D×F
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XdB10lgPo/Px
Antenna protection ratio
Attenuation degree of the receiving capability in a direction of an
antenna compared with that in the main lobe direction. An antenna
protection ratio of 180° is called front-to-back ratio.
1
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Specifications of Transmitter
Working frequency band
Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13
GHz and
higher frequency bands are used in the access layer (e.g. BTS
access).
Output power
The power at the output port of a transmitter. Generally, the
output power is 15 to
30 dBm.
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Local frequency stability
If the working frequency of the transmitter is unstable, the
demodulated effectived
signal ratio will be decreased and the bit error ratio will be
increased. The value
range of the local frequency stability is 3 to 10 ppm.
Transmit Frequency Spectrum Frame
The frequency spectrum of the transmitted signal must meet
specified
requirements, to avoid occupying too much bandwidth and thus
causing too much
interference to adjacent channels. The limitations to frequency
spectrum is
called transmit frequency spectrum frame.
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Specifications of Receiver
Working frequency band
Receivers work together with transmitters. The receiving frequency
on the local
station is the transmitting frequency of the same channel on the
opposite station.
Local frequency stability
The same as that of transmitters: 3 to 10 ppm
Noise figure
The noise figure of digital microwave receivers is 2.5 dB to 5
dB.
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To effectively suppress interference and achieve the best
transmission quality, the
passband and amplitude frequency characteristics should be properly
chosen. The
receiver passband characteristics depend on the IF filter.
Selectivity
Ability of receivers of suppressing the various interferences
outside the passband,
especially the interference from adjacent channels, image
interference and the
interference between transmitted and received signals.
Automatic gain control (AGC) range
Automatic control of receiver gain. With this function, input RF
signals change within a
certain range and the IF signal level remains unchanges.
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– ODU (6)
ODU specifications are related to radio frequencies. As one ODU
cannot cover an entire frequency band, usually, a frequency band
will be divided into several subbands and each subband corresponds
to one ODU.
Different T/R spacing corresponds to different ODUs.
Primary and non-primary stations have different ODUs.
Types of ODUs = Number of frequency bands x Number of T/R spacing x
Number of subbands x 2
(ODUs of some manufacturers are also classified by capacity.
f0(7575M)
Subband B
Subband C
Subband A
Subband B
Subband C
Non-primary station
Primary station
ODUs are of rich types and small volume. Usually, ODUs are produced
by small manufacturers and integrated by big manufacturers.
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1
The IDU implements the functions including service access, service
grooming, multiplexing/demultiplexing, and modulation/demodulation.
Thus the IDU is the main part of a set of microwave equipment. If
we consider the IF board as the line board of optical network
equipment, then the IDU is very much similar to Huawei case-shaped
optical transmission equipment. An IDU contains service boards
(SDE, SD1, SLE, SL1, PH1, PO1), cross-connect, power and clock
board (PXC), system control and communication board (SCC). This
figure shows the internal functional module structure of an
IDU.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Questions
What types are microwave equipment classified into?
What units do the split-mount microwave equipment have? And what
are their functions??
How to adjust antennas?
What are the key specifications of ODU transmitters and
receivers?
Can you describe the entire signal flow of microwave
transmission?
1
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 washed out
and seldom used.
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.
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, grooming, multiplexing/demultiplexing and
modulation/demodulation of services.
How to adjust antennas?
The objective of antenna adjustment is to align the main lobe of
the local antenna to the main lobe of the opposite antenna.
First fix the opposite antenna and then adjust the local antenna in
the elevation or leveling direction. During elevation or leveling
adjustment, use a multimeter to test RSSI at the receiving end and
find at least three maximum values with the middle value being the
biggest. The peak point of the voltage wave indicates the main lobe
position in the elevation or leveling direction. Large-scope
adjustment is unnecessary. Perform fine adjustment on the antenna
to the peak voltage point.
The elevation and leveling adjustment methods are the same.
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.
What are the key specifications of antennas?
Antenna gain, half-power angle, cross polarization decoupling,
immunity, etc.
What are the key specifications of ODU transmitters and
receivers?
Key specifications of transmitters: Operating frequency band,
output power, local oscillator frequency stability, transmit
frequency spectrum frame, etc.
Key specifications of receivers: Operating frequency band, output
power, local oscillator frequency stability, noise figure,
passband, selectivity, AGC range, etc.
Can you describe the entire signal flow of microwave
transmission?
We may take the process of microwave transmission from the transmit
end to the receive end to describe the signal flow of microwave
transmission:
In the transmit end, the service access unit completes the access
of the digital baseband signal, then the signal forms the microwave
frame at the multiplexing unit, the microwave frame signal is
modulated at the modulation unit into the IF signal, and the IF
signal is sent to the ODU. After the ODU implements frequency
mixing of the IF signal with the local transmit oscillator, the IF
signal enters the sideband filter to become the RF signal. The
converted RF signal is then amplified via the power amplifier and
finally sent out via the antenna.
In the receive end, the antenna transmits the RF signal upon
receipt of it to the ODU. The ODU first implements filtering to
filter out some interference signals and then implements low-noise
preamplification to improve the level of the received weak RF
signal. The amplified signal undergoes frequency mixing with the
local receive oscillator, and is then filtered to become the IF
signal. The IF signal is then amplified and sent to the IDU. The
IDU first demodulates the IF signal to get the digital baseband
signal. Till now, the signal is still a complete microwave frame
structure. The digital baseband signal is then sent to the
multiplexing unit, where overheads and service signals are
separated. The overheads are sent to the control unit and the
service signals are sent to the cross-connect unit for service
dispatching.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Summary
Components of split-mount microwave equipment and their
functions
Antenna installation and key specifications of antennas
Functional modules and key performance indexes of ODU
Functional modules of IDU
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Designing Microwave Transmission Links
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Terminal station
Terminal station
Terminal station
Pivotal station
Digital microwave stations are classified into Pivotal stations,
add/drop relay stations, relay stations and terminal
stations.
1
Terminal station: It refers to the microwave station that transmits
services only in one direction.
Relay station: It refers to the microwave station that transmits
services in two directions and is required added to solve the
problem existing in the microwave line of sight communication. The
relay station is classified into two types, active relay station
and passive relay station.
Add/Drop relay station: It refers to the microwave station that
transmits services in two directions and adds/drops transmitted
services.
Pivotal station: It refers to the microwave station that transmits
services in three or more than three directions and transfers the
services in transmission channels in different directions. It is
also called the HUB station.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Radio Frequency relay station
An active, bi-directional radio repeater system without frequency
shift. The RF relay station directly amplifies the signal over
radio frequency.
Regenerator relay station
A high-frequency repeater of high performance. The regenerator
relay station is used to extend the transmission distance of
microwave communication systems, or to deflect the transmission
direction of the signal to avoid obstructions and ensure the signal
quality is not degraded. After complete regeneration and
amplification, the received signal is forwarded.
Active Relay Station
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Parabolic reflector passive relay station
The parabolic reflector passive relay station is composed of two
parabolic antennas connected by a soft waveguide back to
back.
The two-parabolic passive relay station often uses large-diameter
antennas. Meters are necessary to adjust antennas, which is time
consuming.
The near end is less than 5 km away.
Passive Relay Station
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Plane Reflector Passive Relay Station
Plane reflector passive relay station: A metal board which has
smooth surface, proper effective area, proper angle and distance
with the two communication points. It is also a passive relay
microwave station.
Full-distance free space loss:
“a” is the effective area (m2) of the flat reflector.
(km)
(km)
1
The relay efficiency of the reflector is higher than that of
back-to-back antennas.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Application of Digital Microwave
Complementary networks to optical networks (access the services
from the last 1 km)
BTS backhaul transmission
VIP customer access
Microwave application
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Questions
What are the networking modes frequently used for digital
microwave?
What are the types of digital microwave stations?
What are the types of relay stations?
What is the major application of digital microwave?
1
What are the networking modes frequently used for digital
microwave?
The frequently-used networking modes include ring network,
point-to-point chain network, hub network and add/drop
network.
What are the types of digital microwave stations?
Digital microwave stations are classified into pivotal stations,
add/drop relay stations, terminal stations, and relay
stations.
Pivotal station: A station located in the backbone link to
communicate with other stations in various directions.
Add/drop station: A station located in the middle of the link to
add/drop tributaries and communicate with the two stations in two
directions of the backbone link.
Terminal station: A station located at either end of the link or at
the endpoint of a tributary link.
Relay station: A station located in the middle of the link without
adding/dropping voice channels.
What are the types of relay stations?
Relay stations fall into passive relay stations and active relay
stations. There are two types of passive relay stations:
Back-to-back antenna and plane reflector. Active relay stations
include regenerator stations, IF repeaters and RF repeaters.
What are the major applications of digital microwave?
Digital microwave is mainly used for complementary networks to
optical networks (the last mile access), BTS backhaul transmission,
redundancy backup of important links, VIP customer access,
emergency communications (large conferences, disaster relief, etc.)
and special transmission conditions (rivers, lakes, islands,
etc.).
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Designing Microwave Transmission Links
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Contents
4.1 Factors Affecting Electric Wave Propagation
4.2 Various Fading in Microwave Propagation
4.3 Anti-fading Technologies for Digital Microwave
1
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Fresnel Zone and Fresnel Zone Radius
Fresnel zone: The sum of the distance from P to T and the distance
from P to R complies with the formula, TP+PR-TR= n/2 (n=1,2,3, …).
The elliptical region encircled by the trail of P is called the
Fresnel zone.
Key Parameters in
Microwave Propagation (1)
Fresnel zone radius: The vertical distance from P to the TR line in
the Fresnel zone. The first Fresnel zone radius is represented by
F1 (n=1).
1
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Key Parameters in
Microwave Propagation (2)
The first Fresnel zone is the region where the microwave
transmission energy is the most concentrated. The obstruction in
the Fresnel zone should be as little as possible. With the increase
of the Fresnel zone serial numbers, the field strength of the
receiving point reduces as per arithmetic series.
1
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Clearance
Along the microwave propagation trail, the obstruction from
buildings, trees, and mountain peaks is sometimes inevitable. If
the height of the obstacle enters the first Fresnel zone,
additional loss might be caused. As a result, the received level is
decreased and the transmission quality is affected. Clearance is
used to avoid the case described previously.
The vertical distance from the obstacle to AB line segment is
called the clearance of the obstacle on the trail. For convenience,
the vertical distance hc from the obstacle to the ground surface is
used to represent the clearance. In practice, the error is not big
because the line segment AB is approximately parallel to the ground
surface. If the first Fresnel zone radius of the obstacle is F1,
then hc/ F1 is the relative clearance.
1
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– Terrain
The reflected wave from the ground surface is the major factor that
affects the received level.
Smooth ground or water surface can reflect the part of the signal
energy transmitted by the antenna to the receiving antenna and
cause interference to the main wave (direct wave). The vector sum
of the reflected wave and main wave increases or decreases the
composite wave. As a result, the transmission becomes unstable.
Therefore, when doing microwave link design, avoid reflected waves
as much as possible. If reflection is inevitable, make use of the
terrain ups and downs to block the reflected waves.
Straight line
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Different reflection conditions of different terrains have
different effects on electric wave propagation. Terrains are
classified into the following four types:
Type A: mountains (or cities with dense buildings)
Type B: hills (gently wavy ground surface)
Type C: plain
Type D: large-area water surface
The reflection coefficient of mountains is the smallest, and thus
the mountain terrain is most suitable for microwave transmission.
The hill terrain is less suitable. When designing circuits, try to
avoid smooth plane such as water surface.
Factors Affecting Electric Wave Propagation
– Terrain
1
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Troposphere indicates the low altitude atmosphere within 10 km from
the ground. Microwave antennas will not be higher than troposphere,
so the electric wave propagation in aerosphere can be narrowed down
to that in troposphere. Main effects of troposphere on electric
wave propagation are listed below:
Absorption caused by gas resonance. This type of absorption can
affect the microwave at 12 GHz or higher.
Absorption and scattering caused by rain, fog, and snow. This type
of absorption can affect the microwave at 10 GHz or higher.
Refraction, absorption, reflection and scattering caused by
inhomogeneity of atmosphere. Refraction is the most significant
impact to the microwave propagation.
Factors Affecting Electric Wave Propagation – Atmosphere
1
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Contents
4.1 Factors Affecting Electric Wave Propagation
4.2 Various Fading in Microwave Propagation
4.3 Anti-fading Technologies for Digital Microwave
1
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Fast fading
Slow fading
Up fading
Down fading
Flat fading
Free space propagation fading
Fading: Random variation of the received level. The variation is
irregular and the reasons for this are various.
1
There are several kinds of fading according to the causes.
1) Flat fading: The signal has the same level fading depth in the
transmission bandwidth and the power is reduced.
2) Frequency-selective fading: Waveform distortion caused by the
frequency selectivity of fading.
Direct wave is used in microwave propagation. The field strength at
the receive point is the superposition of direct wave and
ground-reflected wave.
3) The propagation medium is the low-level aerosphere, ground and
ground object along the path.
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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reserved.
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Free Space Transmission Loss
Free space loss: A = 92.4 + 20 log d + 20 log f
(d: km, f: GHz). If d or f is doubled, the loss will increase by 6
dB.
Power level
1
Free space is an infinite space filled up with even and ideal
propagation medium, in which electromagnetic wave is not affected
by the factors such as blocking, reflection, diffraction,
scattering, and absorption.
The concept of level fading contains a threshold level, a receive
level, and a margin usually reserved. The margin may be not much
for small-capacity systems. But for the current large-capacity
digital microwave system, larger margin is required. The receive
level in free space can be calculated by this formula:
Pr(dBm)=Pt+Gt+Gr-Lf-Lt-Lr-Lb
Lt/Lr: loss of transmit/receive feed line
Lb: loss of branch system
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reserved.
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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.
Statistic shows 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.
Atmosphere absorption curve (dB/km)
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For frequencies lower than 10 GHz, rain loss can be ignored. Only a
few db may be added to a relay section.
For frequencies higher than 10 GHz, repeater spacing 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 and higher, the relay distance is
limited in few kilometres due to rain loss.
High frequency bands can be used for user-level transmission. The
higher the frequency band is, the more severe the rain
fading.
Rain Fading
1
Water droplets in rain or fog can cause scattering or absorption
attenuation for electromagnetic wave.
7G and 8G microwave can transmit for over 100 km.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Atmosphere refraction
As a result of atmosphere refraction, the microwave propagation
trail is bent. It is considered that the electromagnetic wave is
propagated along a straight line above the earth with an equivalent
earth radius of , = KR (R: 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. The K value may change within a comparatively large
range. This can affect line-of-sight propagation.
R
1
What is the earth radius? In microwave, the earth radius used is
6370 km. The circumference of earth is over 40,000 km.
For the purpose of calculation, the concept of equivalent earth
radius is used. Electromagnetic wave is considered as a straight
line. The actual earth radius "a" is equivalent to "ae". The basic
principle is that the clearance between the radial and the ground
remains the same.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Equivalent earth radius
In temperate zones, the refraction when the K value is 4/3 is
regarded as the standard refraction, where the atmosphere is the
standard atmosphere and Re which is 4R/3 is the standard equivalent
earth radius.
K-Type Fading (3)
Ground surface
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Multipath fading: Due to multipath propagation of refracted waves,
reflected waves, and scattered waves, multiple electric waves are
received at the receiving end. The composition of these electric
waves will result in severe interference fading.
Reasons for multipath fading: reflections due to non-uniform
atmosphere, water surface and smooth ground surface.
Down fading: fading where the composite wave level is lower than
the free space received level. Up fading: fading where the
composite wave level is higher than the free space received
level.
Non-uniform atmosphere
Water surface
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Multipath fading is a type of interference fading caused by
multipath transmission. Multipath fading is caused by mutual
interference between the direct wave and reflected wave (or
diffracted wave on some conditions) with different phases.
Multipath fading grows more severe when the wave passes water
surface or smooth ground surface. Therefore, when designing the
route, try to avoid smooth water and ground surface. When these
terrains are inevitable, use the high and low antenna technologies
to bring the reflection point closer to one end so as to reduce the
impact of the reflected wave, or use the high and low antennas and
space diversity technologies or the antennas that are against
reflected waves to overcome multipath fading.
Multipath Fading (2)
1
The fading caused by the changes of K value. When the K value
changes, it indicates that multipath fading is caused by ground
reflection.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Threshold level
(-30 dB)
Signal interruption
Up fading
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Duct Type Fading
Due to the effects of the meteorological conditions such as ground
cooling in the 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 beams 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.
Duct type fading
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Scintillation 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.
Scintillation fading
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The higher the frequency is and the longer the hop distance is, the
more severe the fading is.
Fading is more severe at night than in the daylight, in summer than
in winter. In the daylight, sunshine is good for air convection. In
summer, weather changes frequently.
In sunny days without wind, atmosphere is non-uniform and
atmosphere subdivision easily forms and hardly clears. Multipath
transmission often occurs in such conditions.
Fading is more severe along water route than land route, because
both the reflection coefficient of water surface and the atmosphere
refraction coefficient above water surface are bigger.
Fading is more severe along plain route than mountain route,
because atmosphere subdivision often occurs over plain and the
ground reflection factor of the plain is bigger.
Rain and fog weather causes much influence on high-frequency
microwave.
Summary
1
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Contents
4.1 Factors Affecting Electric Wave Propagation
4.2 Various Fading in Microwave Propagation
4.3 Anti-fading Technologies for Digital Microwave
1
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Category
Effect
Power reduction
1
In digital microwave transmission, the receive power decrease or
waveform distortion is caused by various fading forms such as
atmosphere, ground, and climate. This may further cause the circuit
performance to downgrade. Therefore, proper anti-fading measures
shall be taken to improve the performance of the transmission
circuit system.
In addition, to apply microwave communication to the areas with so
difficult propagation conditions (such as long distance, sea
surface, and swamp) that other transmission technologies cannot
satisfy the requirements, anti-fading measures shall also be
taken.
In a digital microwave transmission system, the degradation factors
can be divided into time-variant and time-invariant factors. Level
fading, frequency selective fading, and rain fading belong to
time-variant degradation factors. And the incomplete system belongs
to time-invariant degradation factors. From the degradation
phenomenon perspective, these factors can cause waveform
distortion, or the increase of interference noise and heat
noise.
For waveform distortion, the automatic equalization technique and
various diversity combining techniques that enable the frequency
characteristic to become flat are very effective.
To reduce the interference noise, the effective techniques
are:
Interference compensation technique used for cross polarization
waves
Diversity combining technique used to increase the receive level
and decrease the interference noise
Antenna technique used to improve the antenna directivity and avoid
receiving interference electromagnetic wave.
For heat noise, these non-linear compensation techniques and
diversity combining techniques that are used to increase transmit
power or to prevent from the decrease of receive power can be
adopted.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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The frequency domain equalization only equalizes the amplitude
frequency response characteristics of the signal instead of the
phase frequency spectrum characteristics.
The circuit is simple.
Signal frequency spectrum
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Anti-fading Technologies
Before
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for Digital Microwave System (4)
Automatic transmit power control (ATPC)
Under normal propagation conditions, the output power of the
transmitter is always at a lower level, for example, 10 to 15 dB
lower than the normal level. When propagation fading occurs and the
receiver detects that the propagation fading is lower than the
minimum received level specified by ATPC, the RFCOH is used to let
the transmitter to raise the transmit power.
Working principle of ATPC
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for Digital Microwave System (5)
ATPC: The output power of the transmitter automatically traces and
changes with the received level of the receiver within the control
range of ATPC.
The time rate of severe propagation fading is usually small
(<1%). After ATPC is configured, the transmitter works at a
power 10 to 15 dB lower than the nominal power for over 99% of the
time. In this way, adjacent channel interference and power
consumption can be reduced.
Effects of ATPC:
Reduces DC power consumption
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ATPC dynamic range
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Cross-polarization interference cancellation (XPIC)
In microwave transmission, XPIC is used to transmit two different
signals over one frequency. The utilization ratio of the frequency
spectrum is doubled. To avoid severe interference between two
different polarized signals, the interference compensation
technology must be used.
Frequency configuration of U6 GHz frequency band (ITU-R
F.384-5)
30MHz
680MHz
30MHz
V (H)
H (V)
Shape of waveguide interface
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Diversity technologies
For diversity, two or multiple transmission paths are used to
transmit the same information and the receiver output signals are
selected or composed, to reduce the effect of fading.
Diversity has the following types, 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.
Frequency diversity (FD)
Space diversity (SD)
H
f1
f2
1
But as frequent recourses are becoming scarce currently and
frequency diversity functions better only when the frequency
spacing is large enough, space diversity is more often used.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Frequency diversity
Signals at different frequencies have different fading
characteristics. Accordingly, two or more microwave frequencies
with certain frequency spacing to transmit and receive the same
information which is then selected or composed, to reduce the
influence of fading. This work mode is called frequency
diversity.
Advantages: The effect is obvious. Only one antenna is
required.
Disadvantages: The utilization ratio of frequency bands is
low.
f1
f2
1
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Space diversity
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 to receive the
signals at the same frequency which are composed or selected. This
work mode is called space diversity. If there are n pairs of
antennas, it is called n-fold diversity.
Advantages: The frequency resources are saved.
Disadvantages: The equipment is complicated, as two or more suites
of antennas are required.
Antenna distance: As per experience, the distance between the
diversity antennas is 100 to 200 times the wavelength in frequently
used frequency bands.
f1
f1
1
Space diversity can effectively solve the K-type fading caused by
the interference of ground-reflective wave and direct wave, and the
interference fading caused troposphere reflection.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Dh calculation in space diversity
Anti-fading Technologies
Approximately, Dh can be calculated according to this
formula:
Dh =
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Apart from the anti-fading technologies introduced previously, here
are two frequently used tips:
Method I: Make use of some terrain and ground objects to block
reflected waves.
Anti-fading Technologies
1
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Anti-fading Technologies
1
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Digital Microwave Equipment (1)
With one hybrid coupler added between two ODUs and the antenna, the
1+1 HSB can be realized in the configuration of one antenna.
Moreover, the FD technology can also be adopted.
The 1+1 HSB can also be realized in the configuration of two
antennas. In this case, the FD and SD technologies can both be
adopted, which improves the system availability.
Hybrid coupler
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N+1 (N≤3, 7, 11) Protection
In the following figure, Mn stands for the active channel and P
stands for the standby channel. The active channel and the standby
channel have their independent modulation/demodulation unit and
signal transmitting /receiving unit.
When the fault or fading occurs in the active channel, the signal
is switched to the standby channel. The channel backup is an
inter-frequency backup. This protection mode (FD) is mainly used in
the all indoor microwave equipment.
Products of different vendors support different
specifications.
Protection Modes of
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Inter-frequency
Select the proper mode depending on the geographical condition and
requirements of the customer
1+1
Intra-frequency
Inter-frequency
Inter-frequency
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Questions
What types of fading exists in the microwave propagation?
What are the two categories is the anti-fading technology?
What protection modes are available for the microwave?
1
Answer: The factors include terrain, atmosphere and climate.
What types of fading exist in microwave propagation?
Answer: Fading may fall into many types by different classification
methods.
By the mechanism of fading, fading may fall into duct type fading,
k-type 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 are the two categories of anti-fading technologies?
Answer: Equipment-level countermeasures and system-level
countermeasures.
The equipment-level countermeasures include adaptive equalization,
automatic transmit power control (ATPC) and forward error
correction (FEC).
The system-level countermeasures include the diversity receiving
technology.
What protection modes are available for microwave?
Answer: 1+1 FD, 1+1 SD, 1+1 FD+SD, N+1 FD, etc.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Summary
Importance parameters affecting microwave propagation
Various factors affecting microwave propagation
Various fading types in the microwave propagation (free space
propagation fading, atmospheric absorption fading, rain or fog
scattering fading, K type fading, multipath fading, duct type
fading, and scintillation type fading)
Anti-fading technologies
Anti-fading measures adopted on the equipment: adaptive
equalization, ATPC, and XPIC
Anti-fading measures adopted in the system: FD and SD
Protection modes of the microwave equipment
1
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Contents
Designing Microwave Transmission Links
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Contents
1
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Objective of designing a microwave transmission line
Transmission clearance
Basis of Designing a Microwave Transmission Line
1
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Requirement on a Microwave Transmission Line
Because the microwave is a short wave and has weak ability of
diffraction, the normal communication can be realized in the
line-of-sight transmission without obstacles.
D
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In the microwave transmission, the transmit power is very small,
only the antenna in the accurate direction can realize the
communication. For the communication of long distance, use the
antenna of greater diameter or increase the transmit power.
Requirement on a Microwave Transmission Line
3 dB
Microwave antenna
1
This is a diagram of the transmit power of an antenna.
Why the microwave antenna adopts a parabolic surface instead of a
round one? The principle is the same as the torch.
Adjust the receive level of the antenna to the maximum. The meters
include a multimeter and a NEC voltage regulator. The margin
between the receive level of the main lobe and the side lobe can be
over 10 dB.
When adjusting the antenna, 0.5 watt indicates 3 dB. What is the
half-power angle used for? The antenna shall be adjusted into the
range of the half-power angle.
The iron tower can shake sometimes. Check whether the shaking
affect the half-power angle or not. Generally, acquire the shaking
direction and range of the iron tower when proposing
requirements.
Adding Note 4
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Objective of Designing a Microwave Transmission Line
In common geographical conditions, it is recommended that there be
no obstacles within the first Fresnel zone if K is equal to
4/3.
When the microwave transmission line passes the water surface or
the desert area, it is recommended that there are no obstacles
within the first Fresnel zone if K is equal to 1.
k = 4/3
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The knife-edged obstacle blocks partial of the Fresnel zone. This
also causes the diffraction of the microwave. Influenced by the two
reasons, the level at the actual receive point must be lower than
the free space level. The loss caused by the knife-edged obstacle
is called additional loss.
Transmission Clearance (1)
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When the peak of the obstacle is in the line connecting the
transmit end and the receive end, that is, the HC is equal to 0,
the additional loss is equal to 6 dB.
When the peak of the obstacle is above the line connecting the
transmit end and the receive end, the additional loss is increased
greatly.
When the peak of the obstacle is below the line connecting the
transmit end the receive end, the additional loss fluctuates around
0 dB. The transmission loss in the path and the signal receiving
level approach the values in the free space transmission.
Transmission Clearance (2)
HC/F1
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Calculation formula for path clearance
The value of clearance is required greater than that of the first
Fresnel Zone’s radius.
Transmission Clearance (3)
d
1
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To present the influence of various factors on microwave
transmission, the field strength fading factor V is introduced. The
field strength fading factor V is defined as the ratio of the
combined field strength when the irradiated wave and the reflected
wave arrive at the receive point to the field strength when the
irradiated wave arrives at the receive point in the free space
transmission.
Transmission Clearance (4)
: Combined field strength when the irradiated wave and reflected
wave
arrive at the receive point
: Field strength when the irradiated wave arrives at the received
point in
the free space transmission
: Equivalent ground reflection factor
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The relation of the V and can be represented by the curve in the
figure on the right.
In the case that Φ is equal to 1, with the influence of the earth
considered, HC/F1 is equal to 0.577 when the signal receiving level
is equal to the free space level the first time.
In the case that Φ is smaller than 1, HC/F1 is approximately equal
to 0.6 when the signal receiving level is equal to the free space
level the first time.
When the HC/F1 is equal to 0.577, the clearance is called the free
space clearance, represented by H0 and expressed in the following
formula:
H0 = 0.577F 1 = (λd1d2/d)1/2
HC/F1=N
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Meaning of K Value in Microwave Transmission Planning (1)
To make the clearance cost-effective and reasonable in the
engineering, the height of the antenna should be adjusted according
to the following requirements.
In the case that Φ is not greater than 0.5, that is, for the
circuit that passes the area of small ground reflection factor like
the mountainous area, city, and hilly area, to avoid over great
diffraction, the height of the antenna should be adjusted according
to the following requirements:
When K = 2/3, HC ≥ 0.3F1 (for common obstacles)
HC ≥ 0 (for knife-shaped obstacles)
The diffraction fading should not be greater than 8 dB in this
case.
1
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Meaning of K Value in Microwave Transmission Planning (2)
In the case that Φ is greater than 0.7, that is, for the circuit
that passes the area of great ground reflection factor like the
plain area and water reticulation area, to avoid over great
reflection fading, the height of the antenna should be adjusted
according to the following requirements
When K = 2/3, HC ≥ 0.3F1 (for common obstacles)
HC ≥ 0 (for knife-edged obstacles)
When K = 4/3, HC ≈ F1
When K = ∞, HC ≤ 1.35F1 (The deep fading occurs when HC = 21/2
F1.)
If these requirements cannot be met, change the height of the
antenna or the route.
1
This standard requirement shall be satisfied at the same time. If
not, and if the transmission distance is within 20 km, ensure the
conditions that the K value is 2/3 and then ensure that the K value
is infinite. If the standard requirement still cannot be satisfied,
SD need be used.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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Step 1 Determine the route according to the engineering map.
Step 2 Select the site of the microwave station.
Step 3 Draw the cross-sectional chart of the terrain.
Step 4 Calculate the parameters for site construction.
Procedure for Designing a Microwave Transmission Line
1
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Procedure for Designing a Microwave Transmission Line (1)
We should select the area that rolls as much as possible, such as
the hilly area. We should avoid passing the water surface and the
flat and wide area that is not suitable for the transmission of the
electric wave. In this way, the strong reflection signal and the
accordingly caused deep fading can be avoided.
The line should avoid crossing through or penetrating into the
mountainous area.
The line should go along with the railway, road and other areas
with the convenient transportation.
Step 1 Determine the route according to engineering map.
1
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The distance between two sites should not be too long. The distance
between two relay stations should be equal, and each relay section
should have the proper clearance.
Select the Z route to avoid the over-reach interference.
Avoid the interference from other radio services, such as the
satellite communication system, radar site, TV station, and
broadcast station.
Step 2 Select the site of the microwave station.
Procedure for Designing a Microwave Transmission Line (2)
Over-reach interference
f1
f1
f1
f2
f2
f2
The signal from the first microwave station interferes with the
signal of the same frequency from the third microwave
station.
1
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Draw the cross-sectional chart of the terrain based on the data of
each site.
Calculate the antenna height and transmission situation of each
site. For the line that has strong reflection, adjust the mounting
height of the antenna to block the reflected wave, or have the
reflection point fall on the earth surface with small reflection
factor.
Consider the path clearance. The clearance in the plain area should
not be over great, and that in the mountainous area should not be
over small.
Step 3 Draw the cross-sectional chart of the terrain.
Procedure for Designing a Microwave Transmission Line (3)
1
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Calculate the terrain parameters when the route and the site are
already determined.
Calculate the azimuth and the elevation angles of the antenna,
distance between sites, free space transmission loss and receive
level, rain fading index, line interruption probability, and
allocated values and margin of the line index.
When the margin of the line index is eligible, plan the equipment
and frequencies, make the approximate budget, and deliver the
construction chart.
Step 4 Calculate the parameters for site construction.
Procedure for Designing a Microwave Transmission Line (4)
Input
Input
There is special network planning software, and the commonly used
is CTE Pathloss.
1
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Questions
What are the requirements for microwave communication?
What is the goal of microwave design?
What extra factors should be taken into consideration for microwave
planning?
Can you tell the procedure for designing a microwave transmission
line?
1
What are the requirements for microwave communication?
Because the microwave is a short wave and has weak ability of
diffraction, the normal communication can be realized only in the
line-of-sight transmission without obstacles.
In microwave transmission, the transmit power is very small, so
only the antenna in the accurate direction can realize the
communication. The only way to implement long-haul communication is
to use the antenna of a greater diameter or increase the transmit
power of the antenna.
What is the goal of microwave design?
In common geographical conditions, it is recommended that there be
no obstacles within the first Fresnel zone if K is equal to
4/3.
When the microwave transmission line passes the water surface or
the desert area, it is recommended that there be no obstacles
within the first Fresnel zone if K is equal to 1.
What extra factors should be taken into consideration for microwave
planning?
Many factors should be considered in microwave planning. First,
select the appropriate frequency band and channel configuration
scheme according to the surrounding electromagnetic environment.
Then select the appropriate links and sites. Generally, we should
select the links with a small ground reflection factor. The
selected sites should facilitate site construction and maintenance
and ensure the line-of-sight communication between sites. Moreover,
determine the appropriate clearance according to the K value and
ground reflection factor, and then determine the mounting height
and diameter of the antennas. Finally, calculate if the circuit
indices, e.g. received level and link interruption rate, satisfy
the requirements according to the local climate conditions. Add
protection if necessary when the indices do not satisfy the
requirements.
Can you tell us the procedure for designing a microwave
transmission line?
Four steps:
Step 1: Determine the circuit route according to the engineering
map.
Step 2: Select the site of the microwave station.
Step 3: Draw the cross-sectional chart of the terrain.
Step 4: Calculate the parameters for site construction.
Thank you