Roland HintermayerROHDE & SCHWARZ 2009 Copyright ©

Roland Hintermayer ROHDE&SCHWARZ Copyright© 14.04.2009 2 of 20

1 Introduction...........................................................................................................................3

2 Tasks......................................................................................................................................4

2.1 Frequency Occupancy Measurements ....................................................................................... 5 2.1.1 Frequency Channel Occupancy............................................................................................................. 6 2.1.2 Frequency Band Occupancy.................................................................................................................. 7 2.1.3 Execution with 3D Waterfall Diagram for Visualization ...................................................................... 8 2.1.4 Measurement Value Statistic................................................................................................................. 9 2.1.5 Transmission Statistic ......................................................................................................................... 10

2.2 Monitoring Technical Transmitter Parameters...................................................................... 11 2.2.1 General Short-Term Monitoring of Technical Transmitter Parameters .............................................. 11 2.2.2 Deviation Measurements of FM Broadcast Transmitters.................................................................... 11 2.2.3 Long-term Monitoring of Technical Transmitter Parameters ............................................................. 12

2.3 Field Strength Measurements................................................................................................... 13 2.3.1 Stationary Field Strength Measurements below 30 MHz.................................................................... 13 2.3.2 Stationary Field Strength Measurements between 30 MHz and 500 MHz ......................................... 13 2.3.3 Stationary Field Strength Measurements above 500 MHz.................................................................. 13

2.4 Processing of Interference......................................................................................................... 15 2.4.1 Interference due to Co-Channel Emissions ......................................................................................... 15 2.4.2 Interference due to Out-of-Channel Emissions ................................................................................... 16 2.4.3 Interference due to Intermodulations................................................................................................... 16

2.5 Identification of Unlicensed Stations........................................................................................ 18 2.5.1 Manual Observation ............................................................................................................................ 18 2.5.2 Automatic Scan of Unassigned Channels............................................................................................ 19 2.5.3 Search for a Specific Illegal Station .................................................................................................... 19

3 Further information............................................................................................................20

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1 Introduction

The radio spectrum is a scarce natural resource

� which must be used effectively

� with increasing demands and

� requests on high quality of transmissions

There is a national and international interest on this resource due to common use.

To protect this scarce resource there is a need to

� observe the appropriate technical parameters and operating conditions stated in the

transmitter license to prevent co-located radio communication system from producing mutual

interference

� identify transmitter to interdict its operation or provide the necessary assistance to eliminate

the problem

o Transmitter operates without a license

o Transmitter ignores license stipulations such as frequency, output power, bandwidth, etc

o Transmitter causes interference to other services due to technical faults or deviation from

the technical parameters given in the license.

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2 Tasks

In accordance with these requests the following tasks results:

� Planning and management of transmitters

� Frequency occupancy measurements

� Monitoring of technical transmitter parameters

� Field strength measurements

� Investigation of interference due to co-channel emissions, out-of-channel emissions and

intermodulations

� Identification of unlicensed stations

To fulfil all these tasks there are different measurements requested

� Frequency

� Field strength and power flux-density

� Spectrum occupancy

� Bandwidth

� Modulation

� Radio direction finding and location

� Identification

� Signal analysis

All these tasks can only be done efficiently with the aid of a computer controlled system.

Main parts of such a system are:

� Spectrum Management System

� Monitoring System

� Geographic Information System

� Digital Audio System

These systems help the operators to execute their tasks. Regular procedure can be done

automatically by the system and for manual operation the operators will be supported on their

missions.

The responsibilities related to the Spectrum Management task are out of the scope of this article and

will not be described here.

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2.1 Frequency Occupancy Measurements

Frequency bands for private company networks require careful planning and licensing as limited

amount of available channels often has to be shared by different users (even in the same area).

Number of users or stations cannot be used to estimate whether a certain channel is „full“. It

depends on how often and for how long each station transmits.

True occupancy of a frequency can only be measured and has to be performed by a responsible

organisation like monitoring services.

Results of the occupancy measurements are basis for frequency selection in the licensing process

for these bands.

According to relevant ITU Recommendations, occupancy measurements normally run over a period

of 1 week, 24 hours per day to ensure that all peaks and off-peak traffic are covered.

Frequency is considered occupied when a received signal exceeds preset threshold in the order of

around 1 µV/m.

Detailed measurement of various parameters is not required to determine occupancy of a frequency.

The measurements have to be made in the centre of a coverage area and test equipment usually

installed in a measurement vehicle, which is left alone during the whole period. It is feasible to

measure all channels used in the measurement area at the same time due to large expense

involved.

The speed of the receiver is critical when multiple channels have to be scanned, so that „revisit time“

for one frequency can be short enough to catch short emissions at least once.

The measurement results should be stored for later evaluation that displays the occupancy of each

channel in separate graphics over time

To successfully complete these tasks, the following prerequisites are needed:

� Measurement of time and level with capability of storage;

� Data evaluation in numeric and graphical formats according to relevant ITU standards.

There are different ways to view and evaluate the results from an occupancy measurement,

depending on user‘s preference and aim behind the measurement.

Common variants include:

� Frequency Channel Occupancy;

� Frequency Band Occupancy;

� Execution with 3D Waterfall Diagram for Visualization;

� Measurement Value Statistic

� Transmission Statistic

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2.1.1 Frequency Channel Occupancy

Graphic displays statistics about one of the measured frequency channels.

Frequency occupancy (in %) for one channel depending on time of the day can be seen at a glance.

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2.1.2 Frequency Band Occupancy

Graphic displays occupancy of whole frequency band measured (all channels) vs. time.

Top half of graphic shows percentage of time when defined threshold is exceeded for each

frequency measured.

Bottom half shows a line between measurements exceeding this threshold over time.

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2.1.3 Execution with 3D Waterfall Diagram for Visualization

Graphic displays occupancy of whole frequency band measured (all channels) vs. time in 3D

waterfall diagram.

Easier to comprehend as compared with frequency band occupancy statistic.

However, it becomes unusable when too many frequently occupied channels have been measured.

Allows only a rough estimation of level during transmission.

Exact readings are not possible.

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2.1.4 Measurement Value Statistic

Graphic results in detailed statistical display of the probability that a certain level on a frequency is

reached or exceeded.

Top window (Histogram) shows how many percent of the values measured have a certain level

exactly.

Bottom window (Distribution Diagram) shows how many percent of the measurements have values

exceeding a certain level.

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2.1.5 Transmission Statistic

Interesting to find out the average duration of transmissions on a channel and the duration of the

pauses between them.

To help determine whether frequency can take additional traffic, especially on shared data link

frequencies like Packet Radio Network.

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2.2 Monitoring Technical Transmitter Parameters

For all radio services, the radio regulatory authority of each country sets limits for technical

parameters of transmitters such as output and radiated power, frequency offset or degree of

modulation.

Exceeding any of the above mentioned parameters will result in poorer transmission quality or

reduced coverage. May cause interference to neighbouring channels in some cases.

Radio Monitoring Services therefore have to be able to measure these parameters.

There are 3 categories

� General short-term monitoring of technical transmitter parameters;

� Deviation measurements of FM broadcast transmitters;

� Long-term monitoring of technical transmitter parameters.

2.2.1 General Short-Term Monitoring of Technical Transmitter Parameters

Measurement of frequency offset, deviation and degree of modulation (key technical parameters) for

most transmissions is a key function of the monitoring system.

Modulation becomes distorted in a receiver expecting nominal values when transmitter exceeds any

of the above mentioned parameters beyond a certain value.

Broader bandwidth will cause interference to neighbouring channels.

Useable coverage area will decrease as receiver‘s squelch will not consider transmitted frequency

being inside its channel.

To successfully complete these tasks, the following prerequisites are needed:

� Automatic measurement of level, deviation (FM), modulation depth (AM), and offset for all

receivable emissions in a predefined list of channels and comparison against predefined

limits;

� Continuous measurement of level, deviation/ modulation depth, offset, and bearing

(localization) if possible of all emissions on the frequency in question in case of emissions

exceeding the limits;

� Transmission of audio;

� List of licensees on the frequency in question inside measurement area.

2.2.2 Deviation Measurements of FM Broadcast Transmitters

Continuous program of most FM sound broadcast transmitters does not allow to transmit test signals

to carry out preventive measurements.

Measurements have to be performed using the varying normal program modulation. (Measurement

of 1 hour generally acceptable)

Most critical is the deviation which affects the resulting bandwidth and must not exceed a maximum

value. (In Germany: 75 kHz)

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Changes in audio level from the studio line into the transmitter, and internal drift of preset

parameters may result in a maximum deviation exceeding the limit.

A considerable margin to the point where transmitter leaves its linear range and produces distortion

is usually catered for.

However, higher occupied RF bandwidth may cause potential interference to neighbouring channels,

and reduces planning margins for reuse of same frequency.

Broadcast agencies and radio monitoring services must be able to measure the maximum deviation

of FM sound broadcast transmitters during normal operating conditions.

To successfully complete these tasks, the following prerequisites are needed:

� Measurement of level and deviation on pre-defined frequencies;

� Level of emissions on neighbouring channels;

� Used channels in measurement area.

2.2.3 Long-term Monitoring of Technical Transmitter Parameters

Broadcasting agencies and media companies are obliged to continuously monitor transmitter

parameters to ensure proper operation of the network.

Includes monitoring of predefined limits for field strength, modulation, frequency offset, bandwidth,

and transmitter breakdowns.

Alarms should be raised if there are deviations from nominal.

• Field Strength

� Tolerance range

� If field strength falls below the lower limit, transmitter breakdown is signalled.

� If field strength goes above the upper limit, impermissible transmit power is signalled.

• Frequency Offset

� Tolerance range.

� Deviation from permissible tolerance range is signalled as erroneous operation.

• Modulation and Bandwidth

� Upper limits.

� Deviation above upper limits is signalled as erroneous operation.

To successfully complete these tasks, the following prerequisites are needed:

� Measurement of level, offset and deviation on fixed frequency at predefined times;

� Alarm message to user(s) if any predefined limits exceeded.

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2.3 Field Strength Measurements

Field strength of a transmitter need to be measured by monitoring services and broadcasters for the

following purposes:

� Monitoring coverage of a known transmitter or radio network;

� Assessing the influence of propagation.

Method of performing field strength measurements depends on the frequency (different propagation

models). For that reason the measurement ranges are divided in three categories

� Stationary Field Strength Measurements below 30 MHz;

� Stationary Field Strength Measurements between 30 MHz and 500 MHz;

� Field Strength Measurements above 500 MHz.

2.3.1 Stationary Field Strength Measurements below 30 MHz

In the frequency range below 30 MHz, obstructions of receiving location and reflection usually have

no big influence on field strength measurement performed at a fixed location.

Antennas often measure magnetic component.

Transmissions comes in from the ionosphere under considerable elevation angle (rarely obstructed)

especially in the HF range.

Field strength measured will vary over time due to changing propagation conditions, therefore it is

necessary to run measurement over a longer period (e.g. 10 min) and determine actual value

statistically (maximum or median field strength).

2.3.2 Stationary Field Strength Measurements between 30 MHz and 500 MHz

In the frequency range between 30 MHz and 500 MHz, reflections and shading have great influence

on the result of stationary field strength measurements.

Propagation occurs mainly along LOS (line of sight).

Time component plays no important role, therefore 1 single measurement is sufficient.

Care must be taken to ensure reflection-free reception, stationary field strength measurements are

possible with reasonable accuracy in this frequency range.

2.3.3 Stationary Field Strength Measurements above 500 MHz

Virtually impossible to achieve a clean, reflection-free reception at only one location for frequencies

above 500 MHz.

Field strength measurements therefore have to be obtained by collecting results from multiple

locations within a small area (i.e. < wavelength) while measurement equipment is moving.

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True field strength value is later calculated from all samples taken as a probability (usually specified

as 50%, 90% or 99% of locations).

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2.4 Processing of Interference

In order to maintain a good quality and reliability of radio communications for all users, radio

monitoring services (e.g. Regulatory Authority for Telecommunication and Posts in Germany) have

to process any interference cases reported from customers quickly and efficiently.

Experience when listening to the different interference effects as well as access to a flexible

measuring and observation system helps to identify the source.

Mainly three categories of interference are considered:

� Interference due to co-channel emissions,

� Interference due to out-of-channel emissions;

� Interference due to intermodulations.

2.4.1 Interference due to Co-Channel Emissions

An overview of the spectrum concerned, bearing and localization of any emission, and demodulated

audio for identification is required.

This category contains interference resulting from emissions using the same frequency as the

interfered station.

Possible causes include:

� Unlicensed transmitters;

� Stations being received due to abnormal propagation conditions;

� Radio operators operating their equipment outside their licensed area;

� Emissions from electrical devices being not properly shielded (e.g. computer) or faulty (e.g.

antenna amplifier).

� Wanted and unwanted signal operate on the same, known frequency.

Actual task is the measurement of technical parameters such as level and frequency offset, as well

as bearing/localization of stations on this frequency.

Localization, receiving level and offset can be characteristic for a certain station.

Measurement of these parameters may help to separate between different stations.

To successfully complete these tasks, the following prerequisites are needed:

� Hardware capable of measuring level, frequency offset and bearing;

� Transmission of audio;

� List of licensed users in the receiving area.

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2.4.2 Interference due to Out-of-Channel Emissions

This category contains interference resulting from emissions normally using a frequency different

from the interfered station.

Possible causes include:

� Harmonics that are not properly attenuated, appearing on the transmit frequency;

� Vestigial sideband emissions on neighbouring channels with insufficient filtering at the

transmitter stage;

� Spurious (unwanted) emissions on other than the wanted frequency due to technical faults in

the transmitter, or antenna mismatch;

Unwanted sideband emissions tend to drift in frequency quite considerably, and they touch the

interfered channel only briefly, and allow no proper tracking with measurement equipment.

First task is to determine actual transmitter frequency of the interferer.

Measuring equipment is then tuned to that frequency.

Station can be identified in same manner as for Co-Channel Interference described earlier.

To successfully complete these tasks, the following prerequisites are needed:

� Hardware capable of measuring level, frequency offset and bearing;

� Transmission of audio;

� Visual indication of actual occupation (level) of neighbouring frequency range;

� Fast switching between interfered and other frequencies;

� List of licensed users in the receiving area.

2.4.3 Interference due to Intermodulations

Receiver Intermodulation Effect

Intermodulation products are created whenever the 1st active stages of a receiver are exposed to

signals exceeding the linear amplifying range.

Fake emissions appear like ordinary signals to subsequent receiver stages.

Transmitter Inter-modulation Effect

Happens when old or rusty pieces of metal (e.g. lighting protection, antenna mounting etc) is

exposed to strong RF signals from > 1 transmitter.

Metal parts act as a semi-conductor and retransmits original frequencies plus inter-modulation

products.

Intermodulation effect is shown to be caused by odd harmonics of any participating frequency. (e.g.

a transmitter on 40 MHz is most likely to be a source frequency for an intermodulation frequency of 3

x 40 MHz)

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Addition and subtraction of any 2 or 3 of these frequencies is also likely to cause intermodulation

effect. (e.g. a transmitter on 20 MHz and another transmitter on 60 MHz can produce an

intermodulation frequency of 3 x 60 MHz – 3 x 20 MHz)

Vast amount of possibilities, but only one frequency is the cause of the actual problem!

There are three considerations to identify source frequencies for an intermodulation:

� All participating source transmitters must be present on air whenever the intermodulation

product is heard;

� All participating source frequencies have very high input levels at the receiving antenna of the

interfered station;

� Modulation of all participating source transmitters can be heard simultaneously on the

interfered channel.

To successfully complete these tasks, the following prerequisites are needed:

� Quick overview of all transmitter frequencies active in the area and their received levels;

� Calculation of all possible combinations of these frequencies and their harmonics that fall on

the affected frequency (intermodulation frequency);

� Transmission of audio;

� Fast switching between intermodulation frequency and all possible source frequencies found

in the calculation;

� Users of transmitting stations in the receiving area.

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2.5 Identification of Unlicensed Stations

Increasing numbers of radio stations and services using a limited RF spectrum means that a high

standard in radio communications can only be achieved when all transmitting stations operate

according to national and international regulations.

Licenses are issued for transmitters stating operating conditions and technical parameters by

national regulating authorities.

Stations operating without licenses or ignoring restrictions of licenses (e.g. Frequency, output

power) have to be identified.

Routine monitoring are required to prevent illegal stations from causing problems (e.g. interference),

and to ensure that expected revenue from licences fees are paid by legal users of RF equipment.

Most efficient method for measurements depend on situation:

� Identification of emissions in a scan if the frequency range (e.g. military band, high

frequencies with limited coverage) is reserved for users that are not active during

measurement. Any emission detected is most likely illegal;

� Identification of emissions with high probability of being illegal by skipping known licenses

stations transmitting in frequency band;

� Searching for one specific illegal station.

Three categories of perform the task are usual:

� Manual Observation;

� Automatic Scan of Unassigned Channels;

� Search for a Specific Illegal Station

2.5.1 Manual Observation

� Frequency bands (e.g. military bands) that are usually unoccupied by many licensed stations

are often subjected to illegal usage.

� Relatively easy to find illegal stations, as every transmission is a likely candidate for an illegal

station when scanning the respective range.

� Makes sense to manually observe frequency band for emissions and to identify each one of

them, if the band is known to be used illegally very often.

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2.5.2 Automatic Scan of Unassigned Channels

Only received signals on unassigned channels are likely to be illegal stations, when the frequency

band has licensed stations on certain frequencies that are expected to operate during the

observation.

Scan for all emissions excluding assigned frequencies

2.5.3 Search for a Specific Illegal Station

Not applicable to manually observed frequency range if an unlicensed station was heard once, or

transmits only rarely.

Important to still identify the illegal station to stop its operation, especially when it uses safety

relevant frequencies or interferes with important radio services.

Best solution is to place an unmanned measurement unit in the suspected area that automatically

scans the frequency band in question for emissions meeting criteria (e.g. level, offset, direction)

obtained from previous observation.

Measurement unit then alerts monitoring station once the illegal station is detected, which will then

initiate further steps.

Transmission times and measurement values of the searched station have to be recorded to build a

profile of the station, and to increase the chances of a successful identification by planning manual

observation efficiently.

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