otf100001 digital microwave communication principle issue 1.01

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

otf100001 digital microwave communication principle issue 1.01

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