85
ELECTRONIC
S AND ELECTRIC
AL ENGINEERING
ISSN 1392 – 1215 2011. No.
6(112)
ELEKTRONIKA IR ELEKTROTECHNIKA
SIGNAL TECHNOLOGY
T 121
SIGNAL
Ų
TECHNOLOGIJA
Noise Level Estimation in th
e Shortwave F
requency Range
E. Lossmann, M. A. Meister, U. Madar
Dept. of Radio and Communications Engineering,
Tallinn University of Technology,
Ehitajate tee 5, 19086 Tallinn, Estonia, phone +372 620
2360, e-mail:
eriklos@lr.ttu.ee
Introduction
The presence of noise is th
e fundamental principle of
wireless communication which must be taken into account
always when setting the parameters of radio system such as
sensitivity, modulation type and coding, but also choosing
location of the receiving site etc. Generally,
electromagnetic noise is classi
fied according to its source –
atmospheric or man-made noise. It is common to apply the
recommendation ITU-R P.372-9 for estimating the
environmental character of background noise. The
recommendation defines five typical environments in the
context of RF (
Radio Frequency
) reception.
At lower frequencies of short-wave band, the
atmospheric noise predomin
ates. The main causes of
atmospheric noise are thunders
torms, occurring mostly in
the tropical regions of the
Earth. Electromagnetic noise,
developed by these storms, uses much the same
propagation mechanisms as
the skywave. Temporal
grouping of noises depends on daily changes in the
ionosphere, time of the year, and solar activity. Total
atmospheric noise level at a receiving site is in tight
correlation with local weathe
r conditions. For example the
local thunderstorm may increase the noise levels by about
10 dB, compared to a silent period [1].
Level of a man-made noise is less dependent on the
number of people, living in a certain area, than the
technical sophistication of loca
l infrastructure and lifestyle.
Power supplies of some lighting equipment, starting
systems for electrical motors, generators, different impulse
power supplies and big computer farms contribute to the
level of local noise environm
ent. Because of this, the
background noise levels in peak hours can raise
substantially over the top level set by the standards [2].
As the infrastructure in Estonia has changed greatly
within the last decade and
the level of technological
sophistication has increased significantly also in rural
areas, it is appropriate to explore how those 5 different
noise environments described in the recommendation ITU-
R P.372-9 can be empirically identified and which
particular sources of noise they mainly depend on. The
monitoring was carried out to evaluate short-wave radio
communication sites with respect to the quality of
reception and to get an over
view of spectral occupancy
within the frequency range from 1.5 to 15 MHz.
Measurements
Since short-wave range is not in the focus of
commercial interest, there is a lack of comprehensive data
about spectrum usage and electromagnetic noise level in
Estonia. In order to determine the character of background
noise in the prospective shor
twave reception sites and to
gather data about spectral occupancy in general the current
survey was initiated by the
Estonian Defense Forces.
We present several samples of the measured data and
give an outline of monitoring equipment in this paper, but
the more detailed description of the research procedure has
been proposed by the authors in [3].
Right choice of the antenna is critical to the
measuring of interferences, because the directivity,
polarization and gain character
istics affect significantly the
results of the measurement [4]. Also the stability of power
supply has crucial importance. We conducted comparative
measurements, using power from the battery to check the
noise level induced by the local mains power. The
monitoring receiver in use was Rohde & Schwarz ESMB
with the active monopole (rod) antenna and the laptop
computer equipped with the monitoring application
ARGUS (Fig. 1).
Fig. 1.
Layout of the monitoring system
The RF band was scanned using 5 kHz frequency
step and with receiver bandwidth 4 kHz (Fig. 2).
Measurement results were analyzed using software
package MATLAB.
86
Fig. 2.
Parameters related to the measurements
Recommendation ITU-R P.372-9
External noise is among the most important factors
determining the noise floor when estimating the signal-to-
noise ratio in short-wave range. Certainly it is most
advantageous to operate on short-wave channel with no
interference at the distant end receiver in the optimum
operating frequency, yet this is not commonly feasible.
The useful radio signal has to compete with the
disturbances in the radio channel at every given moment,
considering theoretically unlim
ited number of noise and
interference sources. In addition to the local sources of
background noise and interference one also needs to take
into account noises, originating thousands of kilometers
away, as ionosphere is equally well providing propagation
of signal, noise and diverse interferences.
ITU-R P.372-9 gives the common methodology for
specification of noise electromagnetic pattern in four
environmental categories plus galactic noise. While
predicting the expected noise levels, the characteristic
trends with frequency, time of day, season, and the
geographical location are taken into account explicitly.
There are other variations that could be considered only
statistically. The Recommendation gives the prediction
methodology for approximated calculations of background
noise level on the assumption that interferences due to
surplus co-channel transmissions and other sources of
impulse noise in close range are not present [5, 6].
The external noise figure
F
a
which is defined by ITU-
R P.372-9 in logarithmic notation
for the frequency
f
applies to a short vertical antenna over a perfectly
conducting ground plane. This parameter is related to
rms
noise field strength
E
n
along the antenna by
5
.
95
log
20
B
f
F
E
a
n
,
(1)
where
E
n
is in dB (above 1 μV/m), frequency
f
is
expressed in MHz, and the receiver bandwidth
B
– in dB
(where
B
is in dB-Hz).
In a real communication envi
ronment the character of
external noise power is highly impulsive and non-
Gaussian, hence fitting of probabilistic distribution of the
received random noise waveform is required. Nevertheless,
for the long-term predictions it is more convenient to use
the median level of man-made noise. For estimation of
median values of man-made noise power for different
environments and frequencies
the following expression is
given by:
dB
f
d
c
F
am
,
log
,
(2)
where frequency
f
is expressed in MHz and environmental
constants
c
and
d
are listed in the Table 1.
Table 1.
Values of
environmental constants
c
and
d
Environmental category
c d
Business (curve A)
76.8
27.7
Residential (curve
B)
72.5
27.7
Rural (curve C)
67.2
27.7
Quiet rural (curve D)
53.6
28.6
Galactic noise (curve E)
52.0
23.0
The curves illustrating the expected levels of
background noise are shown on Fig. 3.
Fig. 3.
Curves of the expected background noise level for the
frequency range from 1.5 to 15 MHz
These curves are idealized
. The recommendation also
gives table for
decile
values of average man-made noise
power expressed in dB above or below the median. These
values were measured in the 1970s and may change
considerably with day-to-day, in order to the activities
which may generate man-made noise. It shows that short-
term behaviour of the noise level can vary rather largely.
Comparison of the empirical data to the
Recommendation
In real communication en
vironment the value of
F
a
changes stochastically, as both the development of
thunderstorms and propagation conditions are changing
randomly. Usually domestic appliances and their power
supplies can cause the noise on low frequencies. The
atmospheric noise predominates at frequencies below 10
MHz, but simultaneously we can find the man-made noise
and interference pattern as well.
This is the case illustrated
on Fig. 4. Measurements were carried out on 28
th
October
2009 during daytime between 12 and 13 UTC.
Fig. 5 and Fig. 6 are examples of background noise
pattern at two rural sites and
the associated contours plot
were recorded on the 21
st
and 25
th
of October 2009.
Fig. 6 reveals broadband disturbances below 5 MHz
originated from the combinati
on of an engine-generator
and bad grounding of the communication equipment.
Fig. 7 and Fig. 8 present plots of the noise pattern in
two residential areas. Spectrum was scanned on 21
st
of
October and 10
th
of November 2009 between 1400 and
87
1500 UTC. The daily sunspot number varied from 11 to 23
during the monitoring.
Fig. 4.
Measured background noise
at a quiet rural site
Fig. 5.
Measured background noise at a rural site 1
Fig. 6.
Measured background noise at a rural site 2
We refer to it as the quiet period of solar activity.
These plots are shown to illustrate strong interferences and
man-made noise regarding the location, the time-of-day
and propagation conditions. The comparisons between
theoretical level of predicted data and those measured in
practice reveal considerable disparities [6]. Although the
noise dissemination may use either sky wave or ground
wave methods the primary sources of noise are local ones.
Fig. 9 shows measurement results performed in an
office at the University of Technology equipped with large
PC farms. Yet the ITU-R P.372-9 does not cover the
indoor noise levels. This example is present to view as a
reference to get the idea of the EMC scenario. The PC
emission dominates over the spectrum in question as it was
expected. Similar measurements were made by Weinmann
and Dostert [7].
Fig. 7.
Measured background noise at a residential site 1
Fig. 8.
Measured background noise at a residential site 2
Fig. 9.
Measured background noise at a business site inside the
building
Conclusions
The lower end of shortwave band is a very
complicated communications environment with respect to
noise and interference. There exist significant deviations
from the expected background noise level, especially at
lower frequencies from 1.5 to 4 MHz since the amplitude
of man-made noise decreases with increasing frequency.
The noise originates mainly from electric motors and
ignition systems located in th
e close range of receiving
antenna. The good grounding is also very important for
shortwave communication. However, business sites with a
number of interference sources such as computer farms
and various communication systems produce equally very
high levels of background noise within the whole
shortwave spectrum.
Altogether the noise level on short waves could be
characterized relatively well by
using the data provided in
ITU-R P.372-9.
88
There is a need for more detailed knowledge about
the variations of interference levels. It is of crucial
importance to monitor shortwave band over a longer
period of time with averaging over multiple scans, to
observe seasonal changes, so
lar activity etc for long-term
channel assignment.
Acknowledgements
The authors would like to extend thanks to all the people
that participated in the meas
urements and data analysis.
We would especially like to
thank Peeter Lamster for his
dedicated support and cooperation.
References
1.
Freeman R.
Telecommunications Transmission Handbook. –
USA: Wiley, 1998. – 1232 p.
2.
Berdnikova J., Ruuben T.,
Müürsepp I., Lossmann E.
Resolution and Doppler Tolerance of Cognitive System
Waveforms // Electronics and Electrical Engineering. –
Kaunas: Technologija, 2010. – No. 7(103). – P. 101–104.
3.
Meister M.–A., Lossmann E., Madar U.,
Results of the
Practical Research for HF Communications in Estonia //
Nordic Shortwave Conference Proceedings. – Arkitektkopia
AB, Växjö, Sweden, 2010. – P. 4.2.1.–4.2.10.
4.
Straw R.D
The ARRL Antenna Book. – USA: The National
Association for Amateur Radi
o, Newington, CT, 2007. – P.
23–29 (chapter 23).
5.
Goodman J. M.
Operational communication systems and
relationships to the ionosphere
and space weather // Advances
in Space Research, 2005. – Vol. 36. – Iss. 12. – P. 2241–
2252.
6.
Bradley P. A., Damboldt T., Suessmann P.
, Propagation
models for HF radio service
planning // HF Radio Systems
and Techniques, 2000. – Eighth International Conference on
(IEE Conf. Proc. No. 474). – P.175–179.
7.
Weinmann F., Dostert K.,
Verification of background noise
in the short wave frequency range according to
recommendation ITU–R P.372 // AE
U – International Journal
of Electronics and Communications, Volume 60, Issue 3, 1
March 2006. – P. 208–216.
Received 2011 03 15
E. Lossmann, M. A. Meister, U. Madar.
Noise Level Estimation in the Shortwave Frequency Range // Electronics and Electrical
Engineering. – Kaunas: Technologija, 2011. – No. 6(112). – P. 85–88.
Noise is a composite signal by nature, changing widely over time,
and some of its components can
be controlled while the others
cannot. Although the signal-to-noise ratio (SNR
) is the main factor determining the signa
l quality at the receiving end, it is
quite
complicated to define the characteristics of the local noise, especially in the shortwave bands. This paper provides an overvie
w of the
background noise measurements to evaluate
the problematic shortwave communications s
ites in Estonia and to assess their suitabi
lity as
radio receiving sites. Extensive statistical data was collected during the study, the sources of interference were identified a
nd their
spatiotemporal influence was analyzed. Moni
toring results were compared to methods
and models, presented in recommendation ITU-
R
P.372-9. Ill. 9, bibl. 7, ta
bl. 1 (in English; abstracts
in English and Lithuanian).
E. Lossmann, M. A. Meister, U. Madar. Triukšmo lygio
į
vertinimas trumpadažni
ų
bang
ų
diapazone // Elektronika ir
elektrotechnika. – Kaunas: Technologija, 2011. – Nr. 6(112). – P. 85–88.
Kai kurias triukšmo signalo dedam
ą
sias galima kontroliuoti, kit
ų
negalima. Signalo kokyb
ę
lemia signalo ir triukšmo santykis.
Estijoje analizuojamas trumpadažni
ų
bang
ų
diapazonas ir šiame dažnyje atlikti triukšm
ų
tyrimai. Surinkti statistiniai duomenys.
Nustatyti triukšm
ų
šaltiniai, pateikta
į
takos analiz
ė
. Gauti rezultatai palyginti su ITU-R P.372-
9 rekomendacijoje apra
šytais metodais ir
modeliais. Il. 9, bibl. 7, lent. 1 (angl
ų
kalba; santraukos angl
ų
ir lietuvi
ų
k.).
http://www.ee.ktu.lt/journal/2011/06/20__ISSN_1392-1215_Noise%20Level%20Estimation%20in%20the%20Shortwave%20Frequency%20Range.pdf
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