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Digital Microwave Communication Principles

Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved. Page 2Page 2

Foreword

This course is developed to meet the requirement of Huawei Optical

Network RTN microwave products.

This course informs engineers of the basics on digital microwave

communications, which will pave the way for learning the RTN series

microwave products later.


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


ObjectivesObjectives

After this course, you will be able to explain:

Concept and characteristics of digital microwave communications

Functions and principles of each component of digital microwave

equipment

Common networking modes and application scenarios of digital

microwave equipment

Propagation principles of digital microwave communication and

various types of fading

Anti-fading technologies

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 Current Communications Networks

Optical fiber communication

Microwave communication

Satellite communication

MUX/DEMUX MUX/DEMUX

Microw

ave TE

Microw

ave TE

Coaxial cable communication


Microwave Communicationvs. Optical Fiber Communication

Powerful space cross ability, little land occupied, not limited by land privatization

Optical fiber burying and land occupation required

Small investment, short constructionperiod, easy maintenance

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)

Large transmission capacityLimited transmission capacity

Not limited by frequency, license not required

Stable and reliable transmission quality and not affected by external factors

Transmission quality greatly affected by climate and landform

Microwave Communication Optical Fiber Communication


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.

According to the characteristics of microwave propagation, microwave

can be considered as plane wave.

The plane wave has no electric field and magnetic field longitudinal

components along the propagation direction. The electric field and

magnetic field components are vertical to the propagation direction.

Therefore, it is called transverse electromagnetic wave and TEM wave

for short.


Development of Microwave Communication

Note:

Small capacity: < 10M

Medium capacity: 10M to 100M

Large capacity: > 100M

155M

34/140M

2/4/6/8M

480 voice channels

SDH digital microwave communication

system

PDH digital microwave communication

systemSmall and medium

capacity digital microwave communication system

Analog microwave communication

system

Transmission capacity bit/s/ch)

1950s

1970s

1980s

Late 1990s to now


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 .

Digital microwave communication refers to the microwave communication that adopts the

digital modulation.

The baseband signal is modulated to intermediate frequency (IF) first . Then the

intermediate frequency is converted into the microwave frequency.

The baseband signal can also be modulated directly to microwave frequency, but only

phase shift keying (PSK) modulation method is applicable.

The electromagnetic field theory is the basis on which the microwave communication

theory is developed.


Microwave Frequency Band Selection and RF Channel Configuration (1)

Generally-used frequency bands in digital microwave transmission:

7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (defined by ITU-R Recommendations)

85432 10 201 30 40 50

1.5 GHz 2.5 GHz

Long haul trunk network

2/8/34 Mbit/s

11 GHz

GHz

34/140/155 Mbit/s

2/8/34/140/155 Mbit/s

3.3 GHz

Regional network

Regional network, local network, and boundary network


In each frequency band, subband frequency ranges, transmitting/receiving spacing

(T/R spacing), and channel spacing are defined.

f0 (center frequency)

Frequency range

Channel spacing

f1f2 fn f1 f2 fn

Channel spacing

T/R spacingT/R spacing

Low frequency band High frequency band

Protection spacing

Adjacent channel T/R spacing




f0 (7575M)

Frequency range (7425M7725M)

28M

f1=7442 f5 f1=7596 f2 f5

T/R spacing: 154M

f2=7470

7G Frequency

Range

F0 (MHz) T/R Spacing

(MHz)

Channel Spacing

(MHz)

Primary and Non-

primary Stations

74257725 7575 154 28

Fn=f0-161+28n,

Fn=f0- 7+28n,

(n: 15)

7575 161 7

71107750 7275 196 28

7597 196 28

72507550 7400 161 3.5


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.

Digital baseband signal IF signal

Baseband signal rate

Channel bandw

idth

Modulation

Service signal transmitted


Digital Microwave Communication Modulation (2)

ASK: Amplitude Shift Keying. Use the digital baseband signal to change the carrier amplitude (A). Wc and remain unchanged.

FSK: Frequency Shift Keying. Use the digital baseband signal to change the carrier frequency (Wc). A and remain unchanged.

PSK: Phase Shift Keying. Use the digital baseband signal to change the carrier phase (). Wc and A remain unchanged.

QAM: Quadrature Amplitude Modulation. ). Use the digital baseband signal to change the carrier phase () and amplitude (A). Wc remains unchanged.

A*COS(Wc*t+)

Amplitude Frequency Phase

PSK and QAM are most frequently used in digital microwave.

The following formula indicates a digital baseband signal being converted into a digital frequency band signal.


Microwave Frame Structure (1) RFCOH

RFCOH

ATPC64 kbit/s

DMY64 kbit/s

MLCM11.84 Mbit/s

RSC864 kbit/s

WS2.24 Mbit/s

XPIC16 kbit/s

ID32 kbit/s

INI144 kbit/s

FA288 kbit/s

15.552 Mbit/s

SOH Payload

STM-1 155.52 Mbit/s

171.072 Mbit/s

RFCOH: Radio Frame Complementary Overhead RSC: Radio Service ChannelMLCM: Multi-Level Coding Modulation INI: N:1 switching commandDMY: DummyID: IdentifierXPIC: Cross-polarization Interference CancellationFA: Frame AlignmentATPC: Automatic Transmit Power Control WS: Wayside Service


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.Multiframe 3564 bits

Basic frame 2

1776 bits (148 words)

FS

6 bits

Basic frame 1

1776 bits148 words

FS

6 bits6 bits

C1IIC1IIC1IIC1II

C2IIbIIIIIIII

IIIIIIIIIIII

IIIIIIIIIIII

IIIIIIIIIIII

IIIIIIIIIIII

C1IIC1IIC1IIC1II

C2IIbIIaIIIII

IIIIIIIIIIII

IIIIIIIIIIII

IIIIIIIIIIII

IIIIIIIIIIII

12 bits (the 1st word) 12 bits (the 148th word)

I: STM-1 information bitC1/C2: Two-level correction coding monitoring bitsFS: Frame synchronization a/b: Other complementary overheads


Questions

What is microwave?

What is digital microwave communication?

What are the frequently used digital microwave frequency bands?

What concepts are involved in microwave frequency setting?

What are the frequently used modulation schemes? Which are the most frequently used modulation schemes?


Contents







Microwave Equipment Category

System Digital microwave

PDH SDH

Split-mount radio

Trunk radio

All outdoor radio

Small and medium capacity (216E1, 34M)

Large capacity (STM-0, STM-1, 2xSTM-1)Capacity

Structure

(Discontinued)

Analog microwave

MUX/DEMUX Mode


Trunk Microwave Equipment

High cost, large transmission capacity, more stable performance, applicable to long haul and trunk transmission

RF, IF, signal processing, and MUX/DEMUX units are all indoor. Only the antenna system is outdoor.

SDH microwave equipment

BRU: Branch RF Unit

MSTU: Main Signal Transmission Unit (transceiver, modem, SDHelectrical interface, hitless switching)

SCSU: Supervision, Control and Switching Unit

BBIU: Baseband Interface Unit (option)(STM-1 optical interface,C4 PDH interface)

P

M1

M2


All Outdoor Microwave Equipment

All the units are outdoor.

Installation is easy.

The equipment room can be saved.

All outdoor microwave equipment

IF and baseband processing unit

IF cable

RF processing unit

Service and power cable


Split-Mount Microwave Equipment (1) The RF unit is an outdoor unit (ODU).

The IF, signal processing, and

MUX/DEMUX units are integrated in

the indoor unit (IDU). The ODU and

IDU are connected through an IF cable.

The ODU can either be directly mounted onto the antenna or

connected to the antenna through a

short soft waveguide.

Although the capacity is smaller than the trunk, due to the easy installation

and maintenance, fast network

construction, its the most widely used

microwave equipment. Split-mount microwave equipment

Antenna

ODU (Outdoor Unit)

IF cable

IDU (Indoor Unit)


Split-Mount Microwave Equipment (2)

Unit Functions

Antenna: Focuses the RF signals transmitted by ODUs and increases the signal

gain.

ODU: RF processing, conversion of IF/RF signals.

IF cable: Transmitting of IF signal, management signal and power supply of ODU.

IDU: Performs access, dispatch, multiplex/demultiplex, and

modulation/demodulation for services.


Split-Mount Microwave Equipment Installation

antenna (separate mount)

ODU IF cable

Separate Mount

Soft waveguide

IDU IF port

antenna (direct mount)

ODU

IDU

Direct Mount

IF cable

IF port


Microwave Antenna (1)

Antennas are used to send and receive microwave signals.

Parabolic antennas is common type of microwave antennas.

Microwave antenna diameters includes: 0.3m, 0.6m, 1.2m, 1.8m,2.0m, 2.4m, 3.0m, 3.2metc.

Parabolic antenna


Different frequency channels in same frequency band can share one antenna.

Microwave Antenna (2)

TxRx

TxRx

Channel

Channel

1

1

nn

1

1

n

n


Antenna Adjustment (1)

Side viewSide lobe

Main lobeHalf-power angle Tail lobe

Top view

Main lobe

Side lobe

Half-power angle Tail lobe


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 12 dB is lost, reflection interference will be avoided.


Side lobe position

AGCVoltage

detection point

VAGC

Main lobe position

Angle



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.

CorrectWrongWrong


Split-Mount Microwave Equipment Antenna (1) Antenna gain

Definition: Ratio of the input power of an isotropic antenna Pio to the input power of a

parabolic antenna Pi when the electric field at a point is the same for the isotropic antenna

and the parabolic antenna.

Calculating formula of antenna gain:

Half-power angle

Usually, the given antenna specifications contain the gain in the largest radiation (main lobe)

direction, denoted by dBi. The half-power point, or the 3 dB point is the point which is

deviated from the central line of the main lobe and where the power is decreased by half. The

angle between the two half-power points is called the half-power angle.

Calculating formula of half-power angle:

Half-power angle

D )70~65( 005.0

2DPP

Gi

io


Cross polarization discrimination

Suppression ratio of the antenna receiving heteropolarizing waves, usually, larger than 30 dB.

XdB10lgPo/Px

Po: Receiving power of normal polarized wave

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 Antenna (2)


Split-Mount Microwave Equipment ODU (1)

ODU system architectureUplink IF/RF conversion

Frequencymixing

Sidebandfiltering

Poweramplification

RFattenuation

ATPC Powerdetection

RF loop

Localoscillation

(Tx)

Localoscillation

(Rx)

Frequencymixing

Filtering Low-noiseamplification

Bandpassfiltering

Alarm and controlDownlink RF/IF conversion

Supervision andcontrolsignal

IFamplificat

ion

IFamplification


Specifications of Transmitter

Working frequency band

Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13 GHz and

higher frequency bands are used in the access layer (e.g. BTS access).

Output power

The power at the output port of a transmitter. Generally, the output power is 15 to

30 dBm.



Local frequency stability

If the working frequency of the transmitter is unstable, the demodulated effectived

signal ratio will be decreased and the bit error ratio will be increased. The value

range of the local frequency stability is 3 to 10 ppm.

Transmit Frequency Spectrum Frame

The frequency spectrum of the transmitted signal must meet specified

requirements, to avoid occupying too much bandwidth and thus causing too much

interference to adjacent channels. The limitations to frequency spectrum is

called transmit frequency spectrum frame.




Specifications of Receiver

Working frequency band

Receivers work together with transmitters. The receiving frequency on the local

station is the transmitting frequency of the same channel on the opposite station.

Local frequency stability

The same as that of transmitters: 3 to 10 ppm

Noise figure

The noise figure of digital microwave receivers is 2.5 dB to 5 dB.


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.

Selectivity

Ability of receivers of suppressing the various interferences outside the passband,

especially the interference from adjacent channels, image interference and the

interference between transmitted and received signals.

Automatic gain control (AGC) range

Automatic control of receiver gain. With this function, input RF signals change within a

certain range and the IF signal level remains unchanges.




ODU specifications are related to radio frequencies. As one ODU cannot cover an entire frequency band, usually, a frequency band will be divided into several subbands and each subband corresponds to one ODU. Different T/R spacing corresponds to different ODUs. Primary and non-primary stations have different ODUs.

Types of ODUs = Number of frequency bands x

Number of T/R spacing x Number of subbands x 2

(ODUs of some manufacturers are also classified by capacity.

f0(7575M)

Frequency range (7425M7725M)

Subband A

7442

T/R spacing: 154M

7498

Subband B Subband C Subband A Subband B Subband C

Non-primary station Primary station

ODUs are of rich types and small volume. Usually, ODUs are produced by small manufacturers and integrated by big manufacturers.


Split-Mount Microwave Equipment IDU

Cable interface

From/to ODU

Tx IF

Rx IF

Modulation

Demodulation

Microwave frame

multiplexing

Microwave frame

demultiplexing

Cross-connection

Tributary unit

Line unit

IF unit

Service channel

Service channel

DC/DC conversion

Supervision and control

O&M interface

Power interface


Questions

What types are microwave equipment classified into?

What units do the split-mount microwave equipment have? And what are their functions??

How to adjust antennas?

What are the key specifications of antennas?

What are the key specifications of ODU transmitters and receivers?

Can you describe the entire signal flow of microwave transmission?


Summary

Classification of digital microwave equipment

Components of split-mount microwave equipment and their

functions

Antenna installation and key specifications of antennas

Functional modules and key performance indexes of ODU

Functional modules of IDU

Signal flow of microwave transmission


Contents







Common Networking Modes of Digital Microwave

Ring network Chain network

Add/Drop network

Hub network


Types of Digital Microwave Stations

Terminal station

Terminal station

Terminal station

Pivotal station

Add/Drop relay station

Relay station

Digital microwave stations are classified into Pivotal stations, add/drop relay stations, relay stations and terminal stations.


Types of Relay Stations

Relay station

Back-to-back antenna Plane reflector

Active

Passive

Regenerative repeater IF repeater RF repeater


Radio Frequency relay station An active, bi-directional radio repeater system without frequency shift. The RF relay station directly amplifies the signal over radio frequency.

Regenerator relay station A high-frequency repeater of high performance. The regenerator relay station is used to extend the transmission distance of microwave communication systems, or to deflect the transmission direction of the signal to avoid obstructions and ensure the signal quality is not degraded. After complete regeneration and amplification, the received signal is forwarded.

Active Relay Station


Parabolic reflector passive relay station The parabolic reflector passive relay station is composed of two

parabolic antennas connected by a soft waveguide back to back.

The two-parabolic passive relay station often uses large-diameter

antennas. Meters are necessary to adjust antennas, which is time

consuming.

The near end is less than 5 km away.

Passive Relay Station


Plane Reflector Passive Relay Station

Plane reflector passive relay station: A metal board which has smooth surface, proper effective area, proper angle and distance with the two communication points. It is also a passive relay microwave station.

Full-distance free space loss:

a is the effective area (m2) of the flat reflector.

L d d as 1421 20 201 2. log log

a A c o s 2

d1(km)

(km)d2


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)

BTS backhaul transmission

Redundancy backup of important links

VIP customer access

Emergency communications

(conventions, activities, danger elimination, disaster relief, etc.)

Special transmission conditions (rivers, lakes,

islands, etc.)

Microwave application


Questions

What are the networking modes frequently used for digital microwave?

What are the types of digital microwave stations?

What are the types of relay stations?

What is the major application of digital microwave?


Contents







Contents


4.1 Factors Affecting Electric Wave Propagation

4.2 Various Fading in Microwave Propagation

4.3 Anti-fading Technologies for Digital Microwave


Fresnel Zone and Fresnel Zone Radius Fresnel zone: The sum of the distance from P to T and the distance from P to R

complies with the formula, TP+PR-TR= n/2 (n=1,2,3, ). The elliptical region encircled by the trail of P is called the Fresnel zone.

Key Parameters in Microwave Propagation (1)

ROT

P

F 1

d 2d 1

Fresnel zone radius: The vertical distance from P to the TR line in the Fresnel

zone. The first Fresnel zone radius is represented by F1 (n=1).


Formula of the first Fresnel zone radius:


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.

)()(

)()(32.17 211 kmdGHzf

kmdkmdF



Clearance

Along the microwave propagation trail, the obstruction from buildings, trees, and

mountain peaks is sometimes inevitable. If the height of the obstacle enters the first Fresnel

zone, additional loss might be caused. As a result, the received level is decreased and the

transmission quality is affected. Clearance is used to avoid the case described previously.

The vertical distance from the obstacle to AB line segment is called the clearance of the

obstacle on the trail. For convenience, the vertical distance hc from the obstacle to the

ground surface is used to represent the clearance. In practice, the error is not big because

the line segment AB is approximately parallel to the ground surface. If the first Fresnel zone

radius of the obstacle is F1, then hc/ F1 is the relative clearance.

A

Bh1h2

dd1 d2

hphc

hs

M Fh3

h4h5

h6


Factors Affecting Electric Wave Propagation Terrain

The reflected wave from the ground surface is the major factor that affects the received level.

Smooth ground or water surface can reflect the part of the signal energy transmitted by the

antenna to the receiving antenna and cause interference to the main wave (direct wave). The vector

sum of the reflected wave and main wave increases or decreases the composite wave. As a result,

the transmission becomes unstable. Therefore, when doing microwave link design, avoid reflected

waves as much as possible. If reflection is inevitable, make use of the terrain ups and downs to block

the reflected waves.

Straight line

Reflection

Straight line

Reflection


Different reflection conditions of different terrains have different effects on electric

wave propagation. Terrains are classified into the following four types:

Type A: mountains (or cities with dense buildings)

Type B: hills (gently wavy ground surface)

Type C: plain

Type D: large-area water surface

The reflection coefficient of mountains is the smallest, and thus the mountain terrain

is most suitable for microwave transmission. The hill terrain is less suitable. When

designing circuits, try to avoid smooth plane such as water surface.

Factors Affecting Electric Wave Propagation Terrain


Troposphere indicates the low altitude atmosphere within 10 km from the ground. Microwave antennas will not be higher than troposphere, so the electric wave

propagation in aerosphere can be narrowed down to that in troposphere. Main effects

of troposphere on electric wave propagation are listed below:

Absorption caused by gas resonance. This type of absorption can affect the

microwave at 12 GHz or higher.

Absorption and scattering caused by rain, fog, and snow. This type of

absorption can affect the microwave at 10 GHz or higher.

Refraction, absorption, reflection and scattering caused by inhomogeneity of

atmosphere. Refraction is the most significant impact to the microwave

propagation.

Factors Affecting Electric Wave Propagation Atmosphere


Contents






Fading in Microwave Propagation

Fading mechanism

Absorption fading

Rain fading

Scintillation fading

K-type fading

Duct type fading

Fading time Received level

Influence of fading on signal

Fast fading

Slow

fading

Up fading

Dow

n fading

Flat fading

Frequency selective fading

Free space propagation fading

Fading: Random variation of the received level. The variation is irregular and the reasons for this are various.


Free Space Transmission Loss

Free space loss: A = 92.4 + 20 log d + 20 log f

(d: km, f: GHz). If d or f is doubled, the loss will increase by 6 dB.

Power level

PTX = Transmit power

G = Antenna gain

A0 = Free space loss

M = Fading margin

PTX

Distance

GTX GRX

PRX

A0

MReceiving threshold

G

d

G

f

PRX = Receive power


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.

Atmosphere absorption curve (dB/km)1GHz7.5GHz12GHz23GHz60GHz

0.01dB

10dB

1dB

0.1dB


For frequencies lower than 10 GHz, rain loss can be ignored. Only a few db may

be added to a relay section.

For frequencies higher than 10 GHz, repeater spacing is mainly affected by rain

loss. For example, for the 13 GHz frequency or higher, 100 mm/h rainfall causes a

loss of 5 dB/km. Hence, for the 13 GHz and 15 GHz frequencies, the maximum relay

distance is about 10 km. For the 20 GHz frequency and higher, the relay distance is

limited in few kilometres due to rain loss.

High frequency bands can be used for user-level transmission. The higher the

frequency band is, the more severe the rain fading.

Rain Fading


Atmosphere refraction As a result of atmosphere refraction, the microwave propagation trail is bent. It is

considered that the electromagnetic wave is propagated along a straight line above

the earth with an equivalent earth radius of , = KR (R: actual earth radius.)

The average measured K value is about 4/3. However, the K value of a specific

section is related to the meteorological phenomena of the section. The K value may

change within a comparatively large range. This can affect line-of-sight propagation.

ReRe

R e R

K-Type Fading (1)


Microwave propagation

k > 1: Positive refraction

k = 1: No refraction

k < 1: Negative refraction

K-Type Fading (2)


Equivalent earth radius In temperate zones, the refraction when the K value is 4/3 is regarded as the standard refraction, where the atmosphere is the standard atmosphere and Re which is 4R/3 is the standard equivalent earth radius.

K-Type Fading (3)

4/3 1

2/3

Actual earth radius (r)

Ground surface

2/3

4/31

k =

Equivalent earth radius (rk)

Ground surface

k =


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

Smooth ground surface.

Multipath Fading (1)

Ground surface


Multipath fading is a type of interference fading caused by multipath transmission.

Multipath fading is caused by mutual interference between the direct wave and

reflected wave (or diffracted wave on some conditions) with different phases.

Multipath fading grows more severe when the wave passes water surface or

smooth ground surface. Therefore, when designing the route, try to avoid smooth

water and ground surface. When these terrains are inevitable, use the high and low

antenna technologies to bring the reflection point closer to one end so as to reduce

the impact of the reflected wave, or use the high and low antennas and space

diversity technologies or the antennas that are against reflected waves to overcome

multipath fading.

Multipath Fading (2)


Frequency (MHz)

R

e

c

e

i

v

e

d

p

o

w

e

r

(

d

B

m

)

Normal

Flat Selective fading

Multipath Fading Frequency Selective Fading


1h

Received level in free space

Threshold level(-30 dB)

Signal interruption

Up fading

Multipath Fading Flat Fading


Duct Type Fading

Due to the effects of the meteorological conditions such as ground cooling in the night, burnt warm by the sun in the morning, smooth sea surface, and anticyclone, a non-uniform structure is formed in atmosphere. This phenomenon is called atmospheric duct.

If microwave beams pass through the atmospheric duct while the receiving point is outside the duct layer, the field strength at the receiving point is from not only the direct wave and ground reflected wave, but also the reflected wave from the edge of the duct layer. As a result, severe interference fading occurs and causes interruption to the communications.

Duct type fading


Scintillation Fading

When the dielectric constant of local atmosphere is different from the ambient due to the particle clusters formed under different pressure, temperature, and humidity conditions, scattering occurs to the electric wave. This is called scintillation fading. The amplitude and phase of different scattered waves vary with the atmosphere. As a result, the composite field strength at the receiving point changes randomly.

Scintillation fading is a type of fast fading which lasts a short time. The level changes little and the main wave is barely affected. Scintillation fading will not cause communications interruption.

Scintillation fading


The higher the frequency is and the longer the hop distance is, the more severe the

fading is. Fading is more severe at night than in the daylight, in summer than in winter. In the

daylight, sunshine is good for air convection. In summer, weather changes frequently. In sunny days without wind, atmosphere is non-uniform and atmosphere subdivision

easily forms and hardly clears. Multipath transmission often occurs in such conditions. Fading is more severe along water route than land route, because both the reflection

coefficient of water surface and the atmosphere refraction coefficient above water

surface are bigger. Fading is more severe along plain route than mountain route, because atmosphere

subdivision often occurs over plain and the ground reflection factor of the plain is

bigger. Rain and fog weather causes much influence on high-frequency microwave.

Summary


Contents






Category Effect

Equipment level countermeasure

Adaptive equalization Waveform distortion

Automatic transmit power control (ATPC) Power reduction

Forward error correction (FEC) Power reduction

System level countermeasure Diversity receiving technology

Power reduction and waveform distortion

Anti-fading Technologies for Digital Microwave System (1)


Signal frequency spectrum

Multipath fadingSlope 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.

Frequency domain equalization



Time domain equalization Time domain equalization directly counteracts the intersymbol interference.


Before

T T T

AfterC-n C0 Cn

Ts-Ts-2Ts Ts-Ts-2Ts


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

ReceiverDemodulator

ATPC

Receiver

ATPC

Transmitter Modulator

Demodulator



ATPC: The output power of the transmitter automatically traces and changes with the

received level of the receiver within the control range of ATPC.

The time rate of severe propagation fading is usually small (


Anti-fading Technologies for Digital Microwave System (6) ATPC adjustment process (gradual change)

ATPC dynamic range-72

-55

-45

-35

-25

102857545

31

21

Received level (dB

m)

Link loss (dB)

High level

Low level

Transmitter output level (dB

m)



Cross-polarization interference cancellation (XPIC)

In microwave transmission, XPIC is used to transmit two different signals

over one frequency. The utilization

ratio of the frequency spectrum is

doubled. To avoid severe interference

between two different polarized signals,

the interference compensation

technology must be used.

Frequency configuration of U6 GHz frequency band (ITU-R F.384-5)

30MHz80MHz 60MHz

340MHz

1 2 3 4 5 6 7 8

680MHz

V (H)

H (V)

1 2 3 4 5 6 7 8

30MHz 80MHz 60MHz340MHz

680 MHz

1 2 3 4 5 6 7 8

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

Shape of waveguide interface

Electric field direction

Horizontal polarization

Vertical polarization


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.

Frequency diversity (FD)Space diversity (SD)

Hf1

f2



Frequency diversity Signals at different frequencies have different fading characteristics. Accordingly,

two or more microwave frequencies with certain frequency spacing to transmit and

receive the same information which is then selected or composed, to reduce the

influence of fading. This work mode is called frequency diversity.

Advantages: The effect is obvious. Only one antenna is required.

Disadvantages: The utilization ratio of frequency bands is low.

f1

f2


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.

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


Dh =(nll/2)d

2h1l: wavelengthd: path distanceh1: height of the antenna at the transmit end

h1

Tx

Rx

Dh

d

Dh calculation in space diversity


Approximately, Dh can be calculated according to this formula:


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.



Method II: high and low antennas



Protection Modes of Digital Microwave Equipment (1)

With one hybrid coupler added between two ODUs and the antenna, the 1+1 HSB can be realized in the configuration of one antenna. Moreover, the FD technology can also be adopted.

The 1+1 HSB can also be realized in the configuration of two antennas. In this case, the FD and SD technologies can both be adopted, which improves the system availability.

Hybrid coupler


N+1 (N3, 7, 11) Protection

In the following figure, Mn stands for the active channel and P stands for the standby channel. The active channel and the standby channel have their independent modulation/demodulation unit and signal transmitting /receiving unit.

When the fault or fading occurs in the active channel, the signal is switched to the standby channel. The channel backup is an inter-frequency backup. This protection mode (FD) is mainly used in the all indoor microwave equipment.

Products of different vendors support different specifications.


Switching control unit

Switching control unitRFSOH

P

M1M2M3

P

M1M2M3

ch1ch2

chP

ch3

ch1ch2

chP

ch3



Configuration Protection Mode Remarks Application

1+0 NP Non-protection Terminal of the network

1+1 FD Channel protection Inter-frequency Select the proper mode

depending on the geographical condition and requirements of the

customer

1+1 SD Equipment protection and channel protection

Intra-frequency

1+1 FD+SD Equipment protection and channel protection

Inter-frequency

N+1 FD Equipment protection and channel protection

Inter-frequency

Large-capacity backbone network


Questions

What factors can affect the microwave propagation?

What types of fading exists in the microwave propagation?

What are the two categories is the anti-fading technology?

What protection modes are available for the microwave?


Summary Importance parameters affecting microwave propagation

Various factors affecting microwave propagation

Various fading types in the microwave propagation (free space propagation fading,

atmospheric absorption fading, rain or fog scattering fading, K type fading,

multipath fading, duct type fading, and scintillation type fading)

Anti-fading technologies

Anti-fading measures adopted on the equipment: adaptive equalization, ATPC,

and XPIC

Anti-fading measures adopted in the system: FD and SD

Protection modes of the microwave equipment


Contents







Contents


5.1 Basis of Designing a Microwave Transmission Line

5.2 Procedures for Designing a Microwave Transmission Line


Requirement on the point-to-point line-of-sight communication

Objective of designing a microwave transmission line

Transmission clearance

Meanings of K value in the microwave transmission planning

Basis of Designing a Microwave Transmission Line


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 waveAntenna

D


In the microwave transmission, the transmit power is very small, only the antenna in

the accurate direction can realize the communication. For the communication of long

distance, use the antenna of greater diameter or increase the transmit power.

Requirement on a Microwave Transmission Line

3 dB

Direction demonstration of the microwave antenna

Microwave antenna

Half power angle of the microwave antenna


k = 4/3

The first Fresnel zone

Objective of Designing a Microwave Transmission Line In common geographical conditions, it is recommended that there be no

obstacles within the first Fresnel zone if K is equal to 4/3.

When the microwave transmission line passes the water surface or the desert area, it is recommended that there are no obstacles within the first Fresnel zone if K is equal to 1.


The knife-edged obstacle blocks partial of the Fresnel zone. This also causes

the diffraction of the microwave. Influenced by the two reasons, the level at the

actual receive point must be lower than the free space level. The loss caused by

the knife-edged obstacle is called additional loss.

Transmission Clearance (1)


When the peak of the obstacle is in the line

connecting the transmit end and the receive end, that

is, the HC is equal to 0, the additional loss is equal to

6 dB.

When the peak of the obstacle is above the line

connecting the transmit end and the receive end, the

additional loss is increased greatly.

When the peak of the obstacle is below the line

connecting the transmit end the receive end, the

additional loss fluctuates around 0 dB. The

transmission loss in the path and the signal receiving

level approach the values in the free space

transmission.


-24-26

-22-20-18-16-14-12-10-8-6

-4-20

42

-28

6

8

-2.5-2.0-1.5-1.0-0.5 0 0.51.0 1.5 2.0 2.5

Loss caused by block of knife-edged obstacle

HC/F1

A

d

d

i

t

i

o

n

a

l

l

o

s

s

(

d

B

)


Clearance calculation

h2

d1d2

dhb

hs

hc

h1

Kddhb 210785.0

Calculation formula for path clearance

sbc hhddhdhh 1221

The value of clearance is required greater than that of the first Fresnel Zones radius.


stands for the projecting height of the earth.

bh

K stands for the atmosphere refraction factor.


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.


2

1

2

0

cos21Fh

EEV ce

E

0E

: Combined field strength when the irradiated wave and reflected wave arrive at the receive point: Field strength when the irradiated wave arrives at the received point

inthe free space transmission

: Equivalent ground reflection factor


The relation of the V and can be

represented by the curve in the figure on the

right.

In the case that is equal to 1, with the

influence of the earth considered, HC/F1 is

equal to 0.577 when the signal receiving level

is equal to the free space level the first time.

In the case that is smaller than 1, HC/F1 is

approximately equal to 0.6 when the signal

receiving level is equal to the free space level

the first time.

When the HC/F1 is equal to 0.577, the

clearance is called the free space clearance,

represented by H0 and expressed in the

following formula:

H0 = 0.577F 1 = (d1d2/d)1/2


-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

0.64

1 .04

1 .31

1 .43

1 .56

1 .76

1 .93

2 .01

2 .10

2 .26

2 .39

2 .46

2 .54

2 .66

2 .78

2 .85

3 .02

0.20.50.81

VdB

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 is not greater than 0.5, that is, for the circuit that passes the

area of small ground reflection factor like the mountainous area, city, and hilly

area, to avoid over great diffraction, the height of the antenna should be

adjusted according to the following requirements:

When K = 2/3, HC 0.3F1 (for common obstacles)

HC 0 (for knife-shaped obstacles)

The diffraction fading should not be greater than 8 dB in this case.


Meaning of K Value in Microwave Transmission Planning (2)

In the case that 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.


Step 1 Determine the route according to the engineering map.

Step 2 Select the site of the microwave station.

Step 3 Draw the cross-sectional chart of the terrain.

Step 4 Calculate the parameters for site construction.

Procedure for Designing a Microwave Transmission Line


Procedure for Designing a Microwave Transmission Line (1)

We should select the area that rolls as much as possible, such as the hilly area. We should avoid passing the water surface and the flat and wide area that is not suitable for the transmission of the electric wave. In this way, the strong reflection signal and the accordingly caused deep fading can be avoided.

The line should avoid crossing through or penetrating into the mountainous area.

The line should go along with the railway, road and other areas with the convenient transportation.

Step 1 Determine the route according to engineering map.


The distance between two sites should not be too long. The distance

between two relay stations should be equal, and each relay section should

have the proper clearance.

Select the Z route to avoid the over-reach interference.

Avoid the interference from other radio services, such as the satellite

communication system, radar site, TV station, and broadcast station.

Step 2 Select the site of the microwave station.


Over-reach interference

f1 f1 f1

f2 f2 f2The signal from the first

microwave station interferes with the signal of the same

frequency from the third microwave station.


Draw the cross-sectional chart of the terrain based on the data of each site.

Calculate the antenna height and transmission situation of each site. For the

line that has strong reflection, adjust the mounting height of the antenna to

block the reflected wave, or have the reflection point fall on the earth

surface with small reflection factor.

Consider the path clearance. The clearance in the plain area should not be

over great, and that in the mountainous area should not be over small.

Step 3 Draw the cross-sectional chart of the terrain.



Calculate the terrain parameters when the route and the site are already determined.

Calculate the azimuth and the elevation angles of the antenna, distance between sites, free space transmission loss and receive level, rain fading index, line interruption probability, and allocated values and margin of the line index.

When the margin of the line index is eligible, plan the equipment and frequencies, make the approximate budget, and deliver the construction chart.

Step 4 Calculate the parameters for site construction.


Input

Input

There is special network planning software, and the commonly used is CTE Pathloss.


Questions

What are the requirements for microwave communication?

What is the goal of microwave design?

What extra factors should be taken into consideration for microwave planning?

Can you tell the procedure for designing a microwave transmission line?

Thank You

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