Securi Securitty Level: internal use
MICROWAVE PRINCIPLE
ZTE CORPORATION
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
Objectives �
After this course, you should know: �
Concept and characteristics of digital microwave communications
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Functions and principles of each component of digital microwave equipment
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Common networking modes and application scenarios of digital microwave equipment
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Propagation principles of digital microwave communication and various types of fading
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Anti-fading technologies
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Procedure and key points in designing microwave transmission link
Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links
Transmission Methods in Modern Communications Networks Coaxial cable communication
Optical fiber communication
MUX/DEMUX
Microwave communication
Satellite communication
MUX/DEMUX
Microwave Communication vs. Optical Fiber Communication Microwave Communication Powerful space cross ability, little land occupied, not limited by land privatization Small investment, short construction period, easy maintenance
Optical Fiber Communication Optical fiber burying and land occupation required Large investment ,long construction period
Strong protection ability against natural disaster and easy to be recover
Outdoor optical fiber maintenance required and hard to recover from natural disaster
Limited frequency resources (frequency license required)
Not limited by frequency, license not required
Transmission quality greatly affected by climate and landform
Stable and reliable transmission quality and not affected by external factors
Limited transmission capacity
Large transmission capacity
Definition of Microwave �
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.
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According to the characteristics of microwave propagation, microwave can be considered as plane wave.
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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.
Development of Microwave Communication 155M
Transmission capacity bit/s/ch)
SDH digital microwave communication system
34/140M
PDH digital microwave communication system
2/4/6/8M
480 voice channels
Small and medium capacity digital microwave communication system
e 1990 s to now Lat Late 1990s
Analog microwave communication system 1980s 1970s 1950s
Note: Small capacity: < 10M Medium capacity: 10M to 100M Large capacity: > 100M
Concept of Digital Microwave Communication �
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 .
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Digital microwave communication refers to the microwave communication that adopts the digital modulation.
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The baseband signal is modulated to intermediate frequency (IF) first . Then the intermediate frequency is converted into the microwave frequency.
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The baseband signal can also be modulated directly to microwave frequency, but only phase shift keying (PSK) modulation method is applicable.
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The electromagnetic field theory is the basis on which the microwave communication theory is developed.
Microwave Frequency Band Selection and RF Channel Configuration (1) Generally-used frequency bands in digital microwave transmission:
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7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (defined by ITU-R Recommendations)
1.5 GHz
2.5 GHz
Regional network 3.3 GHz
Long haul trunk network
11 GHz
Regional network, local network, and boundary network
2/8/34 Mbit/s 34/140/155 Mbit/s
2/8/34/140/155 Mbit/s GHz 1
2
3
4
5
8
10
20
30
40 50
Microwave Frequency Band Selection and RF Channel Configuration (2) �
In each frequency band, subband frequency ranges, transmitting/receiving spacing (T/R spacing), and channel spacing are defined. ency range Frequ Freque Low frequency band
f0 (center frequency)
High frequency band
T/R spacing Protection spacing
T/R spacing
Channel spacing f1
Adjacent channel T/R spacing f2
fn
f1
Channel spacing f2
fn ’
Microwave Frequency Band Selection and RF Channel Configuration (3) Frequency range (7425M–7725M) f0 (7575M)
T/R spacing: 154M 28M
f1=7442
ency 7G Frequ Freque
f2=7470
F0 (MHz)
Range
f1 ’=7596
f5
f2’
f 5’
T/R Spac Spaciing
Channel Spacing
Primary and Non-
(MHz)
(MHz)
primary Stat Statiions Fn=f0-161+28n,
7425–7725
7575
154
28
Fn’=f0- 7+28n, (n: 1–5)
7575
161
7
7275
196
28
7597
196
28
7250–7550
7400
161
3.5
…
…
…
…
7110–7750
…
Digital Microwave 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.
Modula Modulattion
Digital baseband signal
Service signal transmitted
IF signal
Digital Microwave Communication Modulation (2) The following formula indicates a digital baseband signal being converted into a digital frequency band signal. �
+φ)) A*COS(Wc*t A*COS(Wc*t+ Amplitude
� � � �
Frequency
PSK and QAM are most frequently used in digital microwave.
Phase
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.
Microwave Frame Structure (1) �
RFCOH 171.072 Mbit/s 15.552 Mbit/s RFCOH
STM-1 Mbit/s SOH
155.52 Payload
MLCM DMY XPIC ATPC WS RSC INI 11.84 Mbit/s 64 kbit/s 16 kbit/s 64 kbit/s 2.24 Mbit/s 864 kbit/ s144 kbit/s RFCOH: Radio Frame Complementary Overhead RSC: Radio Service Channel MLCM: Multi-Level Coding Modulation INI: N:1 switching command DMY: Dummy ID: Identifier XPIC: Cross-polarization Interference Cancellation FA: Frame Alignment ATPC: Automatic Transmit Power Control WS: Wayside Service
ID FA 32 kbit/s 288 kbit/s
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.
6 bits
Multiframe 3564 bits
FS
Basic frame 1
FS
Basic frame 2
6 bits
1776 bits(148 words)
6 bits
1776 bits (148 words)
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12 bits (the 1st word) I: STM-1 information bit C1/C2: Two-level correction coding monitoring bits FS: Frame synchronization a/b: Other complementary overheads
12 bits (the 148th word)
Questions �
What is microwave?
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What is digital microwave communication?
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What are the frequently used digital microwave frequency bands?
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What concepts are involved in microwave frequency setting?
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What are the frequently used modulation schemes? Which are the most frequently used modulation schemes?
Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links
Microwave Equipment Category System
Digital microwave
Analog microwave
MUX/DEMUX Mode
PDH
SDH
Capacity
Small and medium capacity (2–16E1, 34M)
Large capacity (STM-0, STM-1, 2xSTM-1) (Discontinued)
Trunk radio Structure
Split-mount radio All outdoor radio
Trunk Microwave Equipment BRU: Branch RF Unit
• 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.
P
MSTU: Main Signal Transmission Unit (transceiver, modem, SDH electrical interface, hitless switching)
M1 M2
SCSU: Supervision, Control and Switching Unit
BBIU: Baseband Interface Unit (option) (STM-1 optical interface, C4 PDH interface)
SDH microwave equipment
All Outdoor Microwave Equipment • All the units are outdoor.
RF processing unit
IF cable
• Installation is easy. IF and baseband processing unit
• The equipment room can be saved.
Service and power cable
All outdoor microwave equipment
Split-Mount Microwave Equipment (1) �
The RF unit is an outdoor unit (ODU). The IF, signal processing, and MUX/DEMUX units are
Antenna
integrated in the
IF cable
indoor unit (IDU). The ODU and IDU are connected through an IF cable. �
The ODU can either be directly mounted
ODU (Outdoor Unit )
onto the antenna or connected to the antenna through a short soft waveguide. �
Although the
IDU (Indoor Unit)
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
Split-Mount Microwave Equipment (2) �
Unit Functions �
Antenna: Focuses the RF signals transmitted by ODUs and increases the signal gain.
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ODU: RF processing, conversion of IF/RF signals.
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IF cable: Transmitting of IF signal, management signal and power supply of ODU.
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IDU: Performs access, dispatch, multiplex/demultiplex, and modulation/demodulation for services.
Split-Mount Microwave Equipment – Installation arate Mount Sep Sepa
Direct Mount antenna (direct mount)
antenna (separate mount)
ODU
Soft waveguide IF cable
IF cable
ODU
中频口
IDU
IDU IF port
IF port
Microwave Antenna (1)
Parabolic antenna �
Cassegrainian antenna
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.
Microwave Antenna (2) �
Different frequency channels in same frequency band can share one antenna.
Channel Tx Rx
Tx Rx
Channel
1
1
1
1
n
n
n
n
Antenna Adjustment (1) Side lobe Side view Half-power angle
Main lobe
Tail lobe
Side lobe Top view Half-power angle
Main lobe
Tail lobe
Antenna Adjustment (2) 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. �
AGC Voltage detection point VAGC
Angle Side lobe position Main lobe position
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.
Wrong
Wrong
Correct
Split-Mount Microwave Equipment – Antenna (1) �
Ant enna gain Ante �
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.
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Calculating formula of antenna gain:
G=
P io Pi
D * ⎜ =2 ⎣
Half-power ang anglle �
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.
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Calculating formula of half-power angle:
⎝ 0.5 = (65 0 ~ 70 0 )
⎣ D Half-power angle
Split-Mount Microwave Equipment – Antenna (2) �
Cross polarization discrimination Suppression ratio of the antenna receiving heteropolarizing waves, usually, larger than 30 dB.
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XdB=10lgPo/Px
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Po: Receiving power of normal polarized wave
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Px: Receiving power of abnormal polarized wave
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.
Split-Mount Microwave Equipment – ODU (1) ODU system architecture Uplink IF/RF conversion IF amplificat ion
Frequency mixing
Sideband filtering
Local oscillation (Tx)
ATPC
Local oscillation (Rx)
Supervi sion and control signal
IF amplification
Filtering
Frequency mixing
RF attenuation
Power amplification
Power detection
RF loop
Low-noise amplification
Downlink RF/IF conversion Alarm and control
Bandpass filtering
Split-Mount Microwave Equipment – ODU (2) �
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).
put � Out Outp
power
The power at the output port of a transmitter. Generally, the output power is 15 to 30 dBm.
Split-Mount Microwave Equipment – ODU (3) � 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.
� Transm Transmiit
ame Frequency Spectrum Fr Fra
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.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.
Page 35
Split-Mount Microwave Equipment – ODU (4) �
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.
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Local frequency stability
The same as that of transmitters: 3 to 10 ppm
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Noise figure
The noise figure of digital microwave receivers is 2.5 dB to 5 dB.
Split-Mount Microwave Equipment – ODU (5) �
Passband
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.
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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.
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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.
Split-Mount Microwave Equipment – ODU (6) Frequency range (7425M–7725M) T/R spacing: 154M Subband A
7442
Subband B
f0(7575M)
Subband C
Subband A
Subband B
Subband C
ODUs are of rich types and small volume. Usually, ODUs are produced by small manufacturers and integrated by big manufacturers.
7498
Non-primary station 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.
Primary station
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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.
Split-Mount Microwave Equipment – IDU Service channel IF unit Tributary unit
Crossconnec tion
Microwave frame multiplexing Microwave frame demultiplexing
Line unit
O&M interface Power interface
Service channel Supervision and control
DC/DC conversion
Modulat ion
Demodu lation
Tx IF
Rx IF
From/to ODU
Questions �
What types are microwave equipment classified into?
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What units do the split-mount microwave equipment have? And what are their functions??
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How to adjust antennas?
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What are the key specifications of antennas?
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What are the key specifications of ODU transmitters and receivers?
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Can you describe the entire signal flow of microwave transmission?
Summary �
Classification of digital microwave equipment
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Components of split-mount microwave equipment and their functions
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Antenna installation and key specifications of antennas
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Functional modules and key performance indexes of ODU
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Functional modules of IDU
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Signal flow of microwave transmission
Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links
Common Networking Modes of Digital Microwave Ring network
Chain network
Add/Drop network Hub network
Types of Digital Microwave Stations • Digital microwave stations are classified into Pivotal stations, add/drop relay stations, relay stations and terminal stations.
Add/Drop relay station
Relay station Terminal station Pivotal station
Terminal station
Terminal station
Types of Relay Stations
Passive
• Back-to-back antenna • Plane reflector
Relay station
Active
• Regenerative repeater • IF repeater • RF repeater
Active Relay 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.
Passive Relay Station �
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.
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:
(km)
1
Ls = 142.1 + 20log d1d2 − 20log a
d 2(km) a = A co 2 “a” is the effective area (m2) of the flat reflector.
Passive Relay Station (Photos)
Passive relay station (plane reflector)
Passive relay station (parabolic reflectors)
Application of Digital Microwave Complementary networks to optical networks (access the services from the last 1 km) Special transmission conditions (rivers, lakes, islands, etc.)
BTS backhaul transmission
Microwave application Emergency communications (conventions, activities, danger elimination, disaster relief, etc.)
Redundancy backup of important links VIP customer access
Questions �
What are the networking modes frequently used for digital microwave?
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What are the types of digital microwave stations?
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What are the types of relay stations?
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What is the major application of digital microwave?
Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links
Contents 4. Microwave Propagation and Anti-fading Technologies �
4.1 Factors Affecting Ele Elecctric Wave Propagation
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4.2 Various Fading in Microwave Propagation
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4.3 Anti-fading Technologies for Digital Microwave
Key Parameters in Microwave Propagation (1) �
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.
T
O
R F1
P d1
�
d2
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).
Key Parameters in Microwave Propagation (2) �
�
Formula of the first Fresnel zone radius:
F1 = 17.32
d (1km) ⋅ d 2 (km) f (GHz ) ⋅ d (km)
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.
Key Parameters in Microwave Propagation (3) �
A
ance Clear Cleara
M h3 h1
hc
B
hp
h5
hs
h4
h6 d1
h2
d2 d
�
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.
Factors Affecting Electric Wave Propagation – Te Terrra raiin �
The reflected wave from the ground surface is the major factor that affects the received level.
Straight line Reflection
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Straight line Reflection
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.
Factors Affecting Electric Wave Propagation – Te Terrra raiin �
Different reflection conditions of different terrains have different effects on electric wave
propagation. Terrains are classified into the following four types:
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Type A: mountains (or cities with dense buildings)
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Type B: hills (gently wavy ground surface)
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Type C: plain
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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 – Atmosphere �
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.
Contents 4. Microwave Propagation and Anti-fading Technologies �
4.1 Factors Affecting Electric Wave Propagation
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4.2 Various Fading in Microwave Propagation
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4.3 Anti-fading Technologies for Digital Microwave
Fading in Microwave Propagation Fading: Random variation of the received level. The variation is irregular and the reasons for this are various. �
Fading mechanism
Fading time
Received level
Influence of fading on signal
Free Space Transmission Loss �
Free space loss: A = 92.4 + 20 log d + 20 log f (d: d: km, f: GHz). If d or f is doubled, the loss will increase by 6 dB.
d GTX
PTX = Transmit power
GRX
PRX = Receive power G = Antenna gain
f
Power level
A0 = Free space loss M = Fading margin
G A0
PTX
PRX
G M Receiving threshold
Distance
Absorption Fading �
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. 10dB
1dB
0.1dB
0.01dB 60GHz
23GHz
12GHz
7.5GHz
Atmosphere absorption curve (dB/km)
1GHz
Rain Fading �
For frequencies lower than 10 GHz, rain loss can be ignored. Only a few db may be
added to a relay section.
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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.
K-Type Fading (1) �
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 Re , �
Re= 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.
Re
R
K-Type Fading (2) �
Microwave propagation
k > 1: Positive refraction
k = 1: No refraction
k < 1: Negative refraction
K-Type Fading (3) �
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= ∞ 4/3 1 2/3
Ground surface
Actual earth radius (r)
2/3 1 4/3 k= ∞
Ground surface
Equivalent earth radius (r·k)
Multipath Fading (1) 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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Smooth ground surface.
Ground surface
Multipath Fading (2) �
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.
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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.
Multi Multi--path Fading – Frequency Selective Fading
Flat
Selective fading
Normal
Frequency (MHz)
Multi Multi--path Fading – Flat Fading Up fading
Received level in free space
Threshold level (-30 dB)
1h
Signal interruption
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 nonuniform 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
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.
闪 Scintillation 烁 衰 落 示fading 意 图
Summary �
The higher the frequency is and the longer the hop distance is, the more severe the fading is.
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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.
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In sunny days without wind, atmosphere is non-uniform and atmosphere subdivision easily forms and hardly clears. Multipath transmission often occurs in such conditions.
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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.
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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.
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Rain and fog weather causes much influence on high-frequency microwave.
Contents 4. Microwave Propagation and Anti-fading Technologies �
4.1 Factors Affecting Electric Wave Propagation
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4.2 Various Fading in Microwave Propagation
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crowave 4.3 Anti-fading Technologies for Digital Mi Mic
Anti-fading Technologies for Digital Microwave System (1) Category
Equipment level countermeasure
System level countermeasure
Effect
Adaptive equalization
Waveform distortion
Automatic transmit power control (ATPC)
Power reduction
Forward error correction (FEC)
Power reduction
Diversity receiving technology
Power reduction and waveform distortion
Anti-fading Technologies for Digital Microwave System (2) �
Frequency domain equalization
Multipath fading Signal frequency spectrum
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Slope equalization Frequency spectrum after equalization
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.
Anti-fading Technologies for Digital Microwave System (3) �
Time domain equalization Time domain equalization directly counteracts the intersymbol interference. �
T C-n
… C0
T
…
T Cn After
-2Ts
-Ts
Ts
-2Ts
-Ts
Ts
Anti-fading Technologies 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 Modulator
Transmitter
ATPC
Demodulator
Receiver
Receiver
Demodulator
ATPC
Transmitter
Modulator
Anti-fading Technologies 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.
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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.
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Effects of ATPC: � Reduces the interference to adjacent systems and over-reach interference �
Reduces DC power consumption
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Reduces up fading
� BER
Improves residual
Anti-fading Technologies For Digital Microwave System (6) �
ent process (gradual change) ATPC adjustm adjustme
-25 High level -35
31
-45
Low level 21
-55
ATPC dynamic range -72 45
75 Link loss (dB)
85
102
Anti-fading Technologies for Digital Microwave System (7) Cross-polarization interference cancellation (XPIC) �
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680MHz 30MHz
340 MHz
80MHz
60MHz
In microwave transmission, XPIC is
used to transmit two different signals
1
2
3
4
5
6
7
8
1’
2’
3’
4’
5’
4’
5’
6’
7’
8’
V (H)
over one frequency. The utilization ratio of the frequency spectrum is doubled. To
H (V)
avoid severe interference between two different polarized signals, the interference compensation technology
680 MHz 30MHz
must be used.
340MHz
80MHz 1
2
3
1X
2X
3X
4
5
6
60MHz 7
1’
8
2’
3’
6’
7’
V (H)
Horizontal polarization H (V)
Vertical polarization de interface Shape of wavegui waveguid
4X
5X 6X
7X
8X
1X’ 7X’
2X’ 8X’
3X'
4X’
5X’ 6X’
Frequency configuration of U6 GHz frequency band (ITU-R F.384-5)
8’
Anti-fading Technologies for Digital Microwave System (8) �
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. �
f1 H
Space diversity (SD)
f2
Frequency diversity (FD)
Anti-fading Technologies for Digital Microwave System (9) �
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.
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Disadvantages: The utilization ratio of frequency bands is low. f1 f2
Anti-fading Technologies For Digital Microwave System (10) � 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. �
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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 �
Anti-fading Technologies for Digital Microwave System (11) �
Rx
Dh calculation in space diversity
Tx
Dh h1
d �
Approximately, Dh can be calculated according to this formula:
Dh =
(nl+l/2)d
l: wavelength d: path distance h1: height of the antenna at the transmit end
2h1
Anti-fading Technologies for Digital Microwave System (12) �
Apart from the anti-fading technologies introduced previously, here are two frequently
used tips: �
Method I: Make use of some terrain and ground objects to block reflected waves.
Anti-fading Technologies for Digital Microwave System (13) �
Method II: high and low antennas
Protection Modes of Digital Microwave Equipment (1)
Hybrid coupler
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. �
Protection Modes of Digital Microwave Equipment (2) �
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. �
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ent vendors support different spec Products of differ differe speciifica tions.
ch1 ch2 ch3
M1 M2
M1 M2
M3
M3
ch1 ch2 ch3
chP
P
P
chP
Switching control unit
RFSOH
Switching control unit
Protection Modes of Digital Microwave Equipment (3) Configuration
ction Mode Prote Protec
marks Re Rem
Application Terminal of the network
1+0
NP
Non-protection
1+1
FD
Channel protection
1+1
SD
Equipment protection and channel protection
Intrafrequency
1+1
FD+SD
Equipment protection and channel protection
Interfrequency
N+1
FD
Equipment protection and channel protection
Interfrequency
Interfrequency
Select the proper mode depending on the geographical condition and requirements of the customer
Large-capacity backbone network
Questions �
What factors can affect the microwave propagation?
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What types of fading exists in the microwave propagation?
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What are the two categories is the anti-fading technology?
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What protection modes are available for the microwave?
Summary �
Importance parameters affecting microwave propagation
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Various factors affecting microwave propagation
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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)
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Anti-fading technologies
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Anti-fading measures adopted on the equipment: adaptive equalization, ATPC, and XPIC
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Anti-fading measures adopted in the system: FD and SD
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Protection modes of the microwave equipment
Contents 1. Digital Microwave Communication Overview 2. Digital Microwave Communication Equipment 3. Digital Microwave Networking and Application 4. Microwave Propagation and Anti-fading Technologies 5. Designing Microwave Transmission Links
Contents 5. Designing Microwave Transmission Links �
on Line 5.1 Basis of Designing a Microwave Transmissi Transmissio
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5.2 Procedures for Designing a Microwave Transmission Line
Basis of Designing a Microwave Transmission Line �
Requirement on the point-to-point line-of-sight communication
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Objective of designing a microwave transmission line
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Transmission clearance
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Meanings of K value in the microwave transmission planning
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.
Line propagation
Irradiated wave Antenna
D
Requirement on a Microwave Transmission Line �
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.
Direction demonstration of the microwave antenna Microwave antenna
Half power angle of the microwave antenna
3 dB
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.
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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. The first Fresnel zone
k = 4/3
Transmission Clearance (1) �
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 knifeedged obstacle is called additional loss.
Transmission Clearance (2) �
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
8 6 4 2
connecting the transmit end and the receive end, the
0 -2
additional loss is increased greatly.
-4
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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.
-6 -8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 -2.5-2.0-1.5-1.0-0.5
0 0.51.0 1.5 2.0 2.5 HC/F1
Loss caused by block of knife-edged obstacle
Transmission Clearance (3) �
Clearance calculation Calculation formula for path clearance
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h1d2 + h2 d1 hc = − hb − hs d The value of clearance is required greater than that of the first Fresnel Zone’s radius.
hc
h2
h1
stands for the projecting height of the earth. b �
h
d1d2 hb = 0.0785 K �
hs
K stands for the atmosphere refraction factor.
d1
hb
d
d2
Transmission Clearance (4) �
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.
E V= = 1+ E0
E
2
−2
h cos ce F1
: Combined field strength when the irradiated wave and reflected wave
arrive at the receive point E0 : Field strength when the irradiated wave arrives at the received point in the free space transmission : Equivalent ground reflection factor
Transmission Clearance (5) �
The relation of the V and
can
represented by the curve in the befigure on the right. � In the case that Φ is equal to 1, with the
V(dB) 10 5
influence of the earth considered, HC/F1 is equal to 0.577 when the signal receiving level is equal
0 -5
to the free space level the first time. � In the case that Φ is smaller than 1, HC/F1 is
-10
approximately equal to 0.6 when the signal
-20
receiving level is equal to the free space level the
-25
first time.
-30
=0.2 φ
-15
When the HC/F1 is equal to 0.577, the
-35
clearance is called the free space clearance,
-40
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φ
=0.5 φ =0.8 φ =1
represented by H0 and expressed in the following formula: H0 = 0.577F 1 = (λd1d2/d)1/2
Relation curve of V and Hc/F1 HC/F1=N
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)
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The diffraction fading should not be greater than 8 dB in this case.
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.
Procedure for Designing a Microwave Transmission Line �
Step 1 Determine the route according to the engineering map.
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Step 2 Select the site of the microwave station.
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Step 3 Draw the cross-sectional chart of the terrain.
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Step 4 Calculate the parameters for site construction.
Procedure for Designing a Microwave Transmission Line (1) Step 1
Determine the route according to engineering map.
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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.
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The line should avoid crossing through or penetrating into the mountainous area.
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The line should go along with the railway, road and other areas with the convenient transportation.
Procedure for Designing a Microwave Transmission Line (2) Step 2 �
Select the site of the microwave station. 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.
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Select the Z route to avoid the over-reach interference.
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Avoid the interference from other radio services, such as the satellite communication system, radar site, TV station, and broadcast station. f1
f1
f1
f2
f2
f2
Over-reach interference
The signal from the first microwave station interferes with the signal of the same frequency from the third microwave station.
Procedure for Designing a Microwave Transmission Line (3) Step 3
Draw the cross-sectional chart of the terrain.
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Draw the cross-sectional chart of the terrain based on the data of each site.
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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.
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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.
Procedure for Designing a Microwave Transmission Line (4) Step 4
Calculate the parameters for site construction.
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Calculate the terrain parameters when the route and the site are already determined.
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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.
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When the margin of the line index is eligible, plan the equipment and frequencies, make the approximate budget, and deliver the construction chart. Input
There is special network planning software, and the commonly used is CTE Pathloss. Input
Questions �
What are the requirements for microwave communication?
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What is the goal of microwave design?
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What extra factors should be taken into consideration for microwave planning?
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Can you tell the procedure for designing a microwave transmission line?