This document provides an overview and installation manual for Wi-Fi point-to-point links. It discusses key considerations for installing wireless links such as antenna basics, installation height, azimuth, elevation, polarization, and performance measurements. The document provides guidance on determining antenna height using tools like GPS or a ruler. It also describes how to perform coarse and fine alignment of the antenna azimuth to maximize signal strength between the two points, using methods like binoculars or a compass. Overall, the document aims to help ensure wireless links are installed properly to achieve expected performance.

1. Project Owner: Björn Pehrson
Project Coaches: Bruce Zamaere
Erik Eliasson
Ntareme Hervé
Siraj Rathore
Project expert advisors: Anders Comestedt
Bernt Sundström
Robert Olsson
SomaliREN Fall 2011
Wi-Fi Point-to-Point Links Overview and Installation Manual
Alex Chyrkov <chyrkov@kth.se> 24 ECT Cr
Belgis Chial <belgis@kth.se> 30 ECT Cr
Huang Jin <huangj@kth.se> 15 ECT Cr
Mikael Rapp <mikrap@kth.se> 18 ECT Cr
Saroj Sharma <sarojs@kth.se> 15 ECT Cr
Srikant Narayan <srikant@kth.se> 15 ECT Cr
Vlad Ioan Bratu <bratu@kth.se> 15 ECT Cr
Total 132 ECT Cr

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Contents
Contents .................................................................................................................................................. 2
Revision History....................................................................................................................................... 3
1. Introduction...................................................................................................................................... 4
1.1. Overview of Wi-Fi Point-to-Point Links.................................................................................... 4
2. Antenna Installation and Alignment................................................................................................. 5
2.1. Antenna Basics........................................................................................................................ 5
2.2. Installation Height .................................................................................................................... 5
2.3. Azimuth.................................................................................................................................... 6
2.4. Elevation (Tilt).......................................................................................................................... 8
2.5. Polarization.............................................................................................................................. 8
3. Performance Measurements ........................................................................................................... 9
3.1. Measurement Parameters and Test Procedure ...................................................................... 9
3.2. The iperf tool.......................................................................................................................... 10
4. References .................................................................................................................................... 11

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Revision History
Version Date Changes
1.0 September 26
th
, 2011 First version of the document started
1.1 November 3
rd
, 2011

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1. Introduction
Designing point-to-point wireless links poses many challenges, starting with finding a line of sight
(LOS) between the considered end points, determining the reliability of the link and fade margin
needed, to the physical installation of the equipment.
While planning software combined with a path survey will give the necessary information
regarding antenna placement and height, expected reliability and performance, only proper
physical installation of the equipment will ensure that the performance will be as predicted.
Some key aspects to consider when installing a point-to-point wireless link are providing the
necessary electrical energy to supply the equipment, aligning the antennas and installing proper
grounding and lighting protection. After installation, measurements have to be made, such as
throughput, latency, loss or jitter in order to verify that the link is performing as expected.
1.1. Overview of Wi-Fi Point-to-Point Links
Originally the 802.11 Wi-Fi standard was designed for providing Wireless LAN access with a
limited range. However, with good planning and high gain directional antennas mounted on tall
masts the Wi-Fi technology can be used for long-range links [1].
The success of the Wi-Fi technology, combined with fast deployment at reduced costs make long
distance Wi-Fi point-to-point links the most cost effective solution for rural or remote areas [2].
Alternative solutions such as fiber optics, satellite or terrestrial microwave all come at a higher
cost. Fiber optics offer high throughput for long distances, but require large investments in
infrastructure. Satellite connections are also expensive and come with limited throughput and
large delays. The final option, terrestrial microwave, offers good throughput and also high
reliability. They also allow fast deployment, but also with high cost, which includes equipment and
frequency licenses.
There are a few limitations to consider when using Wi-Fi for long distances links. First is the fact
that they require Line of Sight (LOS) between the end-points. This can be achieved with proper
calculation of the antenna heights and by using the advantages provided by terrain elevation.
Secondly, by operating in the unlicensed band the links will be prone to interference caused by
other transmitters in the same band and. However, considering that these links will be installed
mainly in rural regions, interference will be less present. Also the interference can be limited by
using the 5GHz band.

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2. Antenna Installation and Alignment
2.1. Antenna Basics
The antenna is a very important component in a wireless link or wireless systems. Its main role is
to convert electrical energy into electromagnetic waves and vice versa. While there are many
types of antenna types, they are all characterized by their gain and radiation pattern.
The gain shows how well an antenna can concentrate the radiated power in a specific direction or
how well it can convert this power back to electrical energy when receiving. Antenna gain is
usually measured in dBi, which shows the gain compared to an isotropic antenna. The isotropic
antenna is an antenna that radiates power uniformly in all directions.
The radiation pattern is a graphical representation that shows the radiated power of the antenna
as a function of direction. While the radiation pattern has a 3D shape, it is commonly represented
only in horizontal plane (azimuth) and vertical plane (elevation).
Figure 1: Example of radiation pattern of a 25 dBi antenna from Ubiquity NanoBridge M5
As it can be seen in the above image, high gain antennas have a narrow beam width and for that
reason antenna alignment plays a major role in the performance of point to point links
Beside height, there are three adjustments that are usually made for aligning an antenna:
azimuth, elevation and polarization.
2.2. Installation Height
Antenna height is critical for point to point links and usually is determined with the help of
software tools. Failing to respect the planned height of installation might lead to the obstruction of
the Line of Sight which will result in failure to communicate.
While most of the times the antennas will be mounted on towers or masts where the heights of
different sections are known, there might be situations where the height must be determined.

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The most straightforward option is using a GPS receiver and placing it on the antenna to measure
the height. Another options is to use the height of a person that is standing beside the mast or
tower as a measure and count how many measures will be needed to reach the antenna
When possible, a ruler can be used to estimate the height. The first step is standing at a
measured distance from the structure the antenna is mounted on and placing the ruler at eye
level as in the image bellow.
The antenna height is given by the formula:
h2 = h1*(d1/d2)
Figure 2: Approximating antenna height using a ruler
2.3. Azimuth
The azimuth represents the orientation of the antenna in the horizontal plane, measured in
degrees in a clockwise direction, with North representing 0 degrees, East 90 degrees, South 180
degrees and West 270 degrees. The value of the azimuth is usually determined using planning
software, especially for long distance links where the end point cannot be viewed through a
binocular.
There are two steps involved when doing the azimuth alignment. First there is the coarse-tuning
which involves adjusting the azimuth roughly in the required direction in order to have sufficient
signal for fine tuning. Once there is some level of received signal strength the antenna can be fine
tuned so that the signal strength will be maximized.
Coarse tuning or pre-alignment can be done in different ways, depending on the conditions. If the
link is relatively short and the weather is clear, binoculars can be used to view the distant end-
point. The binoculars should be placed on the top edge of the antenna and the azimuth adjusted
until the distant antenna is centered as viewed through the binoculars.

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Figure 3: Antenna top-view
In the case that the link distance is long and the distant end-point cannot be seen, a compass
must be used for azimuth pre-alignment. However, in order to use the compass method, azimuth
information must be obtained first from a link simulation program.
The compass should have indications for azimuth, in degrees and also a sight and a mirror, in
order to view objects while looking at the compass indication in a mirror.
Fig.4: Sighting compass
When using a compass, the angle between the magnetic north (the one shown by the compass)
and the true north must be taken into account. This angle, known as magnetic declination [3] can
be determined from maps and also from online tools [4].
Another issue to consider is the fact that magnetic compasses are influenced by large metallic
structures such as towers and can give inaccurate readings.
During our long-haul Wi-Fi experiment, we have been using the end point coordinates received
from GPS on a smartphone and GPS software which showed the direction pointed by the phone
in real time. We consider this to be a simple and reliable method suitable for most cases, as the
phone is not affected by magnetic declination, metallic objects etc.
Another measure that aided us in aligning the antennas when it was already dark was using
powerful flashlights and lasers. However, even the GPS by itself should be enough.
After the pre-alignment is completed, fine adjustments of the azimuth must be made in order to
achieve the highest signal strength. This is done by measuring the received signal level (RSL).
The tools for RSL measurements are usually provided with the radio units through a management
interface, which can be accessed in different ways, such as a HTTP based GUI, Telnet, SSH or
Serial Port, depending on the manufacturer of the units.
Figure 5: Example of web-based alignment tool provided with Ubiquity NanoBridge M5

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By observing the RSL the antennas can be finely adjusted in order to obtain the maximum signal
level possible, considering the given conditions. One antenna should be kept fixed while the other
is adjusted. When the maximum signal is obtained the antenna can be securely fixed in place
using the mounting brackets.
2.4. Elevation (Tilt)
The elevation angle shows the position of the antenna in respect to the vertical plane. A 0
o
elevation angle means that the antenna is parallel to the vertical position. A positive elevation
angle means that the antennas points in an upward direction while a negative elevation angle
means that the antenna points in a downward direction.
For most links an elevation angle of 0
o
is recommended, if the antennas are placed at equal
heights. However, if the antennas are mounted at significantly different heights, the elevation
angle must be adjusted in order to ensure proper alignment.
Usually the mounting hardware provided with the antenna has indications for the elevation angle,
so after the angle is determined it can be adjusted.
Figure 6: Elevation angle, antenna side-view
The value of the elevation angle can be determined using planning software, in the same way
that the azimuth angle is determined.
2.5. Polarization
Antenna polarization represents the orientation of the electrical field in respect to the Earth
surface. There are three major types of polarization, vertical horizontal and circular. Vertical
polarization means that the electrical field is perpendicular to the Earth surface while horizontal
polarization means that the electrical field is parallel to the Earth surface. In circular polarization
the electrical field is radiated both in the horizontal plane and the vertical plane (and all planes in
between), in a rotating manner clockwise (right hand circular - RHC) or counterclockwise (left
hand circular - LHC).
It is important that both antennas in a point to point link use the same polarization, because a
misalignment in polarization will severely degrade signal strength and make communication
impossible. Polarization should be adjusted before mounting the antennas on the mast.
Most products have the polarization clearly marked on the antenna. Also there is the case when
antennas are dual-polarized, such as the Ubiquity NanoBridge M5 which has both vertical and
horizontal polarization, thus creating a MIMO system with improved data rates.
On reflector antennas, the polarization can be identified by looking at the antenna feed.

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3. Performance Measurements
After installation and antenna alignment, the link should be measured in order to determine the
performance. There are a number of parameters that should be accounted when measuring IP
network performance. First there is the bandwidth, which will give an indication of the speed of
the network and the amount of traffic it can sustain. Secondly, real-time applications such as
streaming video or VoIP have performance requirements that include low latency and jitter. Also
packet loss rate should be included in a performance analysis [5].
3.1. Measurement Parameters and Test Procedure
The following parameters should be tested in order to determine the performance of a point-to-
point Wi-Fi link:
Bandwidth - the maximum achievable throughput between end-points.
Latency - the total delay between end-points.
Jitter - the variation of latency, which has a direct effect on real-time services.
Packet loss - the ratio of lost packets to total transmitted packets.
Latency is measured using the ping command between the two end-points
Other parameters can be measured using iperf. The iperf utility is a client-server software tool
that can measure throughput, jitter and packet loss between two end-points, using a command
line interface [6].
The first step is to install iperf on two machines that will each be connected to one end-point of
the link. The machine can run either in client mode or server mode, depending on the test
requirements. The second step is to set-up the machines with IP address in the same network
space and to test connectivity between them using the ping command. Once there is
connectivity between the end-points, we can start measurements with iperf.
When using iperf one machine acts as a client and the other as a server, with the client
connecting to the IP address of the server. The tests can be run unidirectional or bi-directional, by
using different command arguments as presented in the next section.
Figure 7: Example of a setup to test a Wi-Fi point-to-point link using iperf

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3.2. The iperf tool
Installing iperf:
Ubuntu: to install the program if there is an Internet connection available, run
 apt-get install iperf
If there is no Internet connection, copy the tar archive and decompress it, using:
 tar -xzvf iperf-2.0.5.tar.gz
After decompressing the archive navigate tot the iperf folder and run the following
commands:
 ./configure
 make
 make install
The iperf utility is now available via the command line.
Windows: download or copy iperf.exe in a folder and use the command line to navigate to
that folder. Once the path is set to the folder containing iperf the tool is available via the
command line.
Table 1: useful iperf commands
Type of
Measurment
Server Side
Command
Client Side command Observations
Bandwidth (TCP
Test)
iperf -s iperf -c <server IP>
-f option on the client side will change the
output format (default Mb/s). (b) bits, (k)
kilobits,(g) gigabits, B(bytes),(K) kilobytes,
(M) megabytes, (G) gigabytes.
Bidirectional
Bandwidth
iperf -s iperf -c <server IP> -r
The server will connect to the client also.
Default only the bandwidth from the client
to the server is measured
Simultaneous
Bidirectional
Bandwidth
iperf -s iperf -c <server IP> -d
Jitter and Packet
Loss (UDP Test)
iperf -s -u iperf -c <server IP> -u
1. -b option should be used on the client side
to specify the desired bandwidth.
Example: iperf -c <IP addrr of
server> -u -b 10m will allocate 10Mb
bandwidth
A complete list of iperf commands can be viewed by running:
 iperf -h

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4. References
[1] Ermanno Pietrosemoli, “Setting Long Distance Wi-Fi Records: Proofing Solutions for Rural
Connectivity”, The Journal of Community Informatics, Vol. 4 No. 1 (2008) Special Issue: Wireless
Networking for Communities, Citizens and the Public Interest
[2] M. Zennaro, C. Fonda, E. Pietrosemoli, A.Muyepa S. Okay, R. Flickenger, S.M. Radicella, "On
a long wireless link for rural telemedicine in Malawi", 6th Int. Conf. on Open Access, Lilongwe,
Nov. 2008
[3] Magnetic Declination, Available: http://en.wikipedia.org/wiki/Magnetic_declination
[4] Magnetic-Declination Tool Using Google Maps, Available: http://www.magnetic-
declination.com/
[5] Geoff Huston, “Measuring IP Networks”, The Internet Protocol Journal, Vol.6 No.1, March
2003
[6] iperf, Available: http://en.wikipedia.org/wiki/Iperf; http://sourceforge.net/projects/iperf/