digital microwave communication principles-a

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.
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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
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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.
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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
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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.
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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
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(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.
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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.
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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
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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).
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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.
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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.
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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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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.
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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
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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.
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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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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.
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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?
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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.
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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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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.
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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.
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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)
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The relay efficiency of the reflector is higher than that of back-to-back antennas.
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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?
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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.).
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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
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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).
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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.
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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.
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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
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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
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Contents
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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.
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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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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
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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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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.
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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.
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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.
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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
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Contents
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Category
Effect
Power reduction
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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.
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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.
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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.
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Dh calculation in space diversity
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.
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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.
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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
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Contents
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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.
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.
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.
: 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.
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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
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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.
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.
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.
Input
Input
There is special network planning software, and the commonly used is CTE Pathloss.
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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

digital microwave communication principles-a

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