planning for dummies
TRANSCRIPT
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Ive been involved with radio for ages. My career
started when I was a teenager, growing up in Norway.
I set up an antenna to receive Radio Luxembourg
(1.440 kHz) on my fathers ancient Tandberg Slvsuper 5.
I stretched a 30-metre copper wire from our house to
the nearest hill, and spent rainy days lying on the sofa
listening to rock-n-roll. That winter we had a huge
thunderstorm. You can guess the rest. My father had toreplace his beloved old Tandberg and I learned my first
hard lesson about radio. Luckily, radio links have come
a long way (literally!) since then and so have I!
Ive now been invited to share some of my hard-earned
knowledge with you. I hope you enjoy this guide.
Preface
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Introduction
p l a n n i n g f o r d u m m i e s
3
This book is for you, even if
you have no experience
whatsoever of radio relay
planning and want to install
a link today. The intention
is to give you a brief
introduction to the mysteries of
radio relay planning, so that you will be able
to design and bring into service a radio link of your own.
The focus of this book is on high frequency equipment, 15 GHz and
upwards, systems such as Nera CityLink, which has become a potent
competitor to fibre optic and leased lines in enterprise networks. By reading
the following pages you will become a competent radio planner. This will
certainly pay you back since radio gives the least headaches per kilometre.
Just install it and forget it.
How to use the book
Youll find the information has been organised
into chapters with the most important topics
covered first.
The chapters may be read in any
order, so it is up to you where and
what you read first. I have also
taken the opportunity of adding in
some anecdotal stories to clarify a
point and lighten the read.
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Icons
This icon marks useful hints for the planning of radio links.
This icon appears alongside topics that cover possible dangers
to be avoided.
Topics marked with this icon are advanced subjects containing
valuable information worth knowing.
This icon marks topics included to answer some common
questions and add a little spice to the guide.
Further reading
There is much more to radio relay planning than can be covered in
this guide. If you wish to cover the subject in greater detail, NERA
Telecommunications offers a more comprehensive publication entitled
Planning Line-of-sight Radio Relay Systems, by Ingvar Henne and
myself. This is also aimed at readers who need basic engineering
information on the subject.
Further information is available from the reports
and recommendations given by the ITU-R(International Telecommunication Union
Radiocommunication), in particular the
Recommendation ITU-R P.530-7, Propagation
data and prediction methods required for the
design of terrestrial line-of-sight systems. The web
site at www.itu.ch will help you to find and order
the recommendations you need. There are also a
number of published works on radio wave
propagation and radio relay planning; see the
bibliography at the end of the guide.
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Radio Relay Systems
What are they?
The main purpose of a radio link is to transport data and voice traffic from
one place to another. The radio link uses the air as the transport medium to
send encoded electro-magnetic waves. A typical link consists of two radios
and two antennas separated by a distance from a couple of hundred metres
up to about 20 kilometres. Usually each radio is split into an indoor and an
outdoor unit with a cable in between. The reason for doing so is to ensure
maximum output power.
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How they work
The data and voice
traffic is fed into the radio
either using an electrical or
optical line. In the radio the
digital signals are coded into
analogue signals and converted
to microwaves with a typical length
of a few centimetres. Microwaves are used
because they are able to propagate high bit rates
safely through the air. The microwaves are sent using a
highly directive parabolic shaped antenna. At the other end the signals
are received and restored to the digital format. This works both ways,
simultaneously, of course.
Effectively, the link can be looked upon as just another fibre optic cable,but it is less expensive, easier to install and can be relocated fast.
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You cannot bend microwaves
Have you ever installed a TV antenna? If so, you may have noticed that
it is possible to receive a signal even though you cannot see the TV
transponder. The reason is that the electromagnetic waves have the ability
to bend around the terrain when the wavelength is long compared to the
height variations of the terrain. However, microwaves, which are only
centimetres in length, are small relative to the surroundings and hence do
not have this bending property. In order to establish a radio link you must
have line of sight between the two radio position sites.
I have seen the light
The simplest way to determine whether you have line of sight, is to useyour own eyes or binoculars. If the other site is not easily spotted, a
mirror with a diameter of 20cm or less can be used for verification.
Position yourself at your chosen site and a colleague at your target site.
Use the mirror to catch the sun and reflect it towards the target site.
To steady your aim, point the suns rays at the ground, then move the
mirror carefully upwards until the rays hit the target site. A mobile
phone or walkie-talkie will help you and your colleague communicate
to determine whether the rays were clearly visible at the target site.
Line of sight
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It is usually best to use the mirror at the lowest situated site since it
is much easier to spot and direct the reflected ray to something that
is visible against a background. It is worth remembering that the sun
is simplest to catch and redirect at the most northerly site in the northern
hemisphere and the most southerly site in the southern hemisphere.
If, like me, you live in a place where the sun does not shine that
often, you can use a camera flash instead to ensure line of sight.
A normal camera flash is easily spotted up to about 10 kilometres.
When setting up a link you should try to avoid transmitting over flat
areas or water. If you do there is a possibility of having a reflected
signal in addition to the original signal which can disturb the
communication. If you have to transmit over such areas, you should try
to conceal the antenna from the reflection point or point the antennas
slightly upwards to minimise the effect of the reflection.
When planning radio relays, it is common to use terrain profiles.
These are drawings showing terrain obstacles, (taken for instance from
a 1:50.000 map) between the sites. On this drawing the Fresnel zone
is superimposed in order to determine antenna heights. If the signal
path is long, the actual terrain heights have to be increased slightly
(mostly towards the middle of the path), to compensate for a minute
degree of bending of microwaves in air and also for the curvature ofthe earth. This added height is called earth-bulge.
As mentioned earlier the radio links transport traffic using electro-
magnetic waves. Since the wavelength of microwaves is small we
can treat the waves as a ray, but we need a little extra space around
the line of sight in order to get as much energy as possible from one site to
another. This space is cigar shaped and is named the Fresnel zone.
The extent of the Fresnel zone varies with the frequency of the signal
and the distances from the sites.
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The size of the Fresnel zone is a function of the frequency, the path
length and the distance from the sites. The illustration below gives theFresnel zone size at the middle of the signal path.
400
375
350
325
Altitude[m]
0.0 5.0 15.010.0Distance [km]
2
12
9
8
7
6
5
4
3
10
11
1 2 3 4 5 9876 13121110 20191817161514
10 GHz
15GHz
20GHz
25GHz
30GHz
35GHz
40GHz
Path Length [km]
Fresne
lzone
[m]
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It is a sad fact that nearly everything in life is limited, and that goes for
the transmitting range of a radio link as well. When microwave power is
transmitted from one site it gets diluted in the air and is received as a
very weak signal at the other end. The signal strength drops with the
square of the distance and if the sites are too far apart it is not possible
to discriminate the signal from the background noise. In addition radiolinks are susceptible to rain and that must also be taken into account
when designing the radio link.
To establish losses and gains a link budget must be drawn.
For convenience engineers use dBm in calculating signal power.
dBm relates to Watt logarithmically providing a simple calculation
(0 dBm = 1 mW, and consequently 1 W = +30 dBm).
A Nera CityLink example budget
Output power: +20 dBm
Antenna gain: +39 dB
Free space loss (10 km, 18 GHz): -138 dB
Antenna gain: +39 dB
Received level: -40 dBm
The lowest power level the Nera CityLink can function with is -73 dBm
which provides about 33 dB margin. These excessive dBs are termed
fading margin and help to maintain high performance and availability
in rainy conditions.
10
Link budget
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FREE SPACE LOSS:
Imagine that you put up
an aerial that transmits
power in all directions.
The power transmitted will
be diluted in the air and the
power received at a distance
(r) will be proportional to the
transmitted power divided by the area of a sphere with radius r.
From electromagnetic theory it can be shown, (dont ask me how!), that
maximum radiated energy from a point source is inversely proportional to the
square of the frequency. Putting this together we get a formula for calculating free
space loss as a function of distance and frequency. The graph shows the results
for the actual Nera CityLink frequencies.
ANTENNA GAIN: You may wonder about the notion antenna gain.How is it possible to get gain from a passive device? Has the perpetum
mobile been accidentally invented? No of course not. The antenna gain is
to do with the way we define and calculate free space loss. In the free space loss
calculation, antennas radiating in all directions (omnidirectional) were
considered, but microwave
antennas with their
parabolic design are able
to focus the power in a
certain direction. The more
directive the antenna is, the
higher the antenna gain
will be. The antenna gain
is proportional to the
square of the diameter
and frequency.
120
155
145
140
135
130
125
150
0 5 10 15 20
40GHz
35GHz
30GHz
25GHz
20GHz
15GHz
Distance [km]
Freespace
loss
[db]
50
35
30
40
45
18 23 25
0.45m0.60m
1.20m
Frequency [GHz]
An
tennaga
in[dBi]
Antennadiameter
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Rain, showers later. For more than thirty
years I have had the resulting pleasure of
this weather forecast. Rain doesnt just
bother me, it also has an attenuating
effect on microwaves. If there is a rain
shower in the radio link path, some of
the energy will be lost due to scattering
and absorption by the rain drops and
the signal strength will diminish.
The attenuating effect of rain depends on
the rain intensity (number of drops and
their size) and the frequency of the
signal. The rain intensity is measured in
millimetres pr. hour [mm/h] and for
radio link purposes the intensity
measured for 0.01% of the timeat a given place is used. When you know
the frequency and rainrate for a given link
it is possible to
calculate the specific
attenuation. This figure
tells you how much
the signal strength will
be attenuated in onekilometre with rain.
0.01
100
1.0
0.1
10
0 5 10 2015 3025 4035
150 mm/h
100 mm/h
50 mm/h
25 mm/h
5 mm/h
1.25 mm/h
0,25 mm/h
Frequency [GHz]
Spec
ifica
ttenua
tion
[dB/km
]
Rain attenuation for vertical polarisation as a function of frequency and rain rate
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Availability
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The volume and intensity of rainfall varies
geographically; both are generally greateralong the equator than in the
temperate regions. As the rain
intensity increases the rain cell size
gets smaller and normally only parts
of the radio link path are affected.
p l a n n i n g f o r d u m m i e s
3
>140
>120
>100
>90
>80
>70
>60
>50
>40
>30
>20
>10 Rain intensity (mm/h)
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By multiplying the
specific rain
attenuation with the
effective path length
you get the needed
fading margin for a
link performing with
an unavailability
0.01%. Usually you
need a much better
performance, so the fading margin must be scaled to obtain
the desired unavailability figures.
0
14
12
10
8
6
4
2
16
18
1 5 10 15 302520 4035 5045
20mm/h
40mm/h
60mm/h
80mm/h100mm/h
Path Length [km]
Effec
tivepa
thleng
th[km
]
The ingredients needed for the unavailability calculation
1. Estimate the rainrate by using the world precipitation map
2. Find the specific attenuation for the given rainrate and desired
frequency
3. Use the rainrate and path length to get the effective path length
4. Multiply effective path length and the specific attenuation to get A,the fade margin for 0.01% unavailability
5. Scale the fade margin to achieve desired unavailability
50
35
30
25
20
40
45
0.000001 0.00001 0.0001 0.10.010.001Outage probability [%]
Fa
demarg
in[dB]
A=5
A=10
A=15
A=20
A=25
A=25
A=35
A=40
0
15
10
5
A = rain attenuation [dB]
for 0.01% of the time
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If you are planning a long radio link path you should stick to vertical
polarisation. When the rain intensity increases the raindrops get bigger
and change from their original spherical shape, becoming flatter due to
air resistance. With vertical polarisation most of the energy is in the
vertical plane and hence the wave sees less rain than if it were
horizontally polarised. The use of vertical polarisation may decrease
the outage time by as much as 30% compared to horizontal
polarisation.
World record of precipitation
I thought that Bergen, Norway, with its two metres of annual rainfall
was about the wettest place on earth, but take a look at this graph.
In Cherrapunji, India it rains 30 metres a year and in Cilaos,
La Reunion 2 metres fall in just a couple of days.
1 2 34 6 986 12604020
Duration
Ra
infa
ll[inc
hes]
Minutes Hours Days Months
10518 24 20 30 32 98 12 24
2000
1000800600
400
200
10080
60
40
20
10
86
4
2
1
Ra
infall[mm
]
50,800
25,40020,38015,240
10,150
5080
25402032
1524
1016
508
254
204153
102
51
2.5
R=16
.6D
0.475
FUSSEN, BAVARIAPLUMB POINT, JAMAICA
CURTEA DE ARGES, RUMANIAHOLT, MO
ROCKPORT, W VAD MANIS, TEXAS
SMETHPORT, PA
BELOUVE LA REUNIONCILAOS LA REUNION
CHERRAPUNJI, INDIA
UNIONVILLE, MD
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Objectives
Frequencies
In order to establish a link you need to
select a radio frequency for transmission.
If you, for instance, have a radio in the
18 GHz band there are more than fifteen
different frequencies to choose from.
But you can not just pick a frequency at
will, because there may be others alreadyusing the same frequency band. Should you
accidentally choose a frequency already
16
How good should the transmission quality of the link be? As good as it
gets, but there will always be a trade off between quality and affordability.
Unless you are planning a large trunk network (which I dont recommend
with only this guide as reference!), the availability objectives for high
performance links in access networks are in the range of 99.99% to
99.999% of the time. Please refer to ITU recommendations for details on
availability if your link is part of a larger telecommunications network.
The bit-error-ratio (BER) should typically not exceed 10-6 for links carrying
data traffic (e.g., ATM- or IP-traffic), or 10-3 for telephony.
The unavailability objectives usually cover both propagation effects and
equipment effects. Yes, even Nera equipment may fail, even though it is
highly unlikely. The mutual size of the two portions is more or less up to
you, but it is common among administrations and route designers to use
30%-50% of the outage for rain.
Availability [%] Unavailability [%] Unavailability [min. annually]
99.99 0.01 5299.995 0.005 2699.999 0.001 5.2
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used by somebody in the vicinity, it may cause interference and as a result
deteriorate the transmission quality. Use of frequencies is co-ordinated and
planned. Such co-ordination is usually
handled by the regulatory
authorities or administrations in
the various countries and can
be a complicated procedure.
You must apply for the
frequencies you need using
application forms which are
typically a couple of pages
of information about the
position, frequencies,
transmitted power and
other relevant technical
details of your planned link.
But, do not despair. When
you buy a Nera radio, we
will fill in the form for you
and plan your net as part of
the service. The only thing you
have to do is to buy the radio.
Frequency bands are divided into two half-bands, one half-band for
transmission, the other half-band for reception. If you have more than
one link using the same site, all transmitters and receivers must be in
the same half-band. If not, transmitted signals may leak into nearby receiversand cause severe interference.
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Installation, bringing into service or commissioning. Whatever you
call it, the name of the game is to get up and running fast. If like me,
you only read manuals when you have to, Nera have made it simple.
The manual is thin with lots of illustrations, easy to read and right to
the point.
After you have opened the packages and checked that all items accord
to the list, the installation can start. You can begin with either the
indoor unit or the outdoor unit. As a nuts and bolts man who prefersthe outdoor life, I usually start with the antenna and the outdoor unit.
When the units have been installed and cables have been connected it
is time for commissioning, i.e. turning on the power. The Indoor unit
will respond with the message *PWR On Boot *Please wait... and after
a short while the message Starting Application, will tell you that
everything is all right.
On the cover of the Nera CityLink manual, you will find the NEWConfigurator. It is PC software for easy commissioning and
configuration of the Nera CityLink. In order to speed up the initial
configuration, the NEW Configurator contains a wizard that guides
you through the necessary steps.
The last thing you have to do is to align
the antennas. You do one antenna at
a time. A coarse alignment can bedone by using line of sight or a
compass. The fine alignment
is done by adjusting for
maximum input power using
the outdoor units audible
signal or AGC voltage.
On the air
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The ultimate test of manhood
Some years
ago I had the
pleasure of
setting up a test link
at 15 GHz to
investigate the
amount of outage dueto rain and sleet.
After the installation,
it was time for
antenna alignment
which is thought to be
tricky especially on
higher frequencies. My colleagues, who know how impractical I am, decided
to let me do the antenna alignment. They stood watching and smiling to
themselves. This made me a little uncomfortable at first, but not for long.
First of all most links are close to the horizontal due to the relative longer
length of the path compared to the height difference between sites. If the
antennas are rotated in the same direction, clockwise for instance, the
ultimate peek in received power should be easily detected. Armed with this
knowledge, a leveller and a power meter, I faced the task (and my audience).
To be honest, it was easy. In a matter of minutes the antennas were aligned
and I could talk to Hilmar on the service phone. So the joke was on them!
And what about the measurements? Well, they were run for a year and
showed that the outage was far less than expected and theory had suggested.
After doing the installation and commissioning, you will be eager to get
on air, but just a little more patience please. First of all you should
check that the radio performs as expected. You can use the built-in test
facilities in the NEW Configurator to measure the quality. The radio shouldperform with less than 10 errors in 24 hours.
-85.00
-45.00
-55.00
-60.00
-65.00
-70.00
-75.00
-80.00
-50.00Input level
Rain intensity
Non-linear time scale
Inpu
tleve
l[dBm
]
14
.44
12
.00
12
.30
12
.45
13
.25
14
.04
14
.28
14
.36
12
.25
15
.19
15
.59
0.00
40.00
30.00
25.00
20.00
15.00
10.00
5.00
35.00
Ra
inintensi
ty[mm
/h](5m
in.)
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Security has never been my strong point. But since there are so manyallegations about the lack of security when using radio, lets consider
the issue. Imagine that you want to eavesdrop a link. The first
thing you need is an antenna, preferably
a big one, and an expensive, $99.999,
spectrum analyser. In order to get a
signal you must be close to the
transmitter or in its antenna beam which
is usually very unlikely.
The next thing to do is to guess the
data rate, possible overhead, modulation
method, filtering and modulation code
based on the received spectrum and get a
device that is able to do the decoding.
Then, when you have managed to get the digital
data, you must unscramble and synchronise it before you have access to
the SDH frames. The easiest way to do all this, which can be very
complicated, is to get a receiver from the same manufacturer who provided
the transmitter you want to eavesdrop. Just looking at the antenna and the
outdoor unit can give you a clue. Also a phone call to some engineers in
the organisation owning the transmitter can be very useful.
In order to extract the information from the SDH frames you need an SDH
processor and demultiplexer. With such a device you can get access to data
traffic at various rates. Finished? Oh no, now you have to guess what kindof traffic it is. Is it voice, video or pure data? It is hard to tell by just looking
at the data stream. Usually it is both encrypted and scrambled. If you
should be lucky enough to break the code, you will still face the problems
with protocols and language.
The conclusion must be that radio for all practical purposes is secure.
In fact, due to its complex nature it is even more secure than other
competing data transporting media. If you still have an interest in
eavesdropping, I think it is better to do that while you are commuting.
The diversity in subject, content and presentation will both amuse and
amaze you.
Breaking in a rough guide
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Bibliography
There are of course a lot of books on radio wave propagation and radio
relay planning and the following titles may be of interest to you
Giger Adolf, Low-Angle Microwave Propagation: Physics and
Modeling, Artech House, ISBN 0-89006-584-5
Hall Martin P.M., Effects of the troposphere on radio
communication, IEE, ISBN 0-86341-086-3
ITU-R, Handbook Digital Radio-Relay Systems,
ITU, ISBN 92-61-06281-4
Ivanek Ferdo, Terrestrial Digital Microwave Communications,
Artech House, ISBN 0-89006-302-8
Townsend A.A.R., Digital Line-Of-Sight Radio Links,
Prentice Hall, ISBN 0-13212-622-2
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Im singing in the rain
Just singing in the rain
What a glorious feeling
Im happy again
Im laughing at clouds
So dark up above
Cause the suns in my heart
And Im ready for love
Let the stormy clouds chase
Everyone from the placeCome on with the rain
Ive a smile on my face
Ill walk down the lane
With a happy refrain
Cause Im singing
Just singing in the rain
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Ever since I saw the first for dummies book in the bookstore,
I have considered it a good idea. These books give readers who do
not know anything about a subject a fast introduction to the basic
facts. I have long been waiting for a for dummies book on
planning of radio links. With titles like sex for dummies and
narcissism for dummies emerging, I do not have high hopes fora planning book arriving. Well, instead of waiting in vain, I have
written one myself and I hope that you will find it interesting as a
door opener to the fascinating world of radio link planning.
The book covers the basics for planning radio link in the
enterprise network and covers the following topics:
Radio relay systems Objectives
Line of sight Frequencies Link budget Commissioning
Availability Security