International Conference on Power and Telecommunication (ICEPT 2009)
On a Model for Evaluation of Existing Field due to Cell-Clusters in GSM Networks
M. A. K. Adelabu ([email protected], [email protected] )
A. I. Mowete ([email protected], [email protected] )
ABSTRACT
This presentation describes the development
of an empirical model for use in the
determination of ambient electric field
strength. This is a required input for the
process of interference management in the
Nigerian
radio-communications
environment. First, it gives a representative
review of theoretical models available in the
open
literature
for
radio
channel
assignments; and then using the main
attributes of these models, explains how
experimental models, specific to any given
electromagnetic environment, may be
developed. Preliminary results obtained
from the use of field measurements taken in
the Nigerian environment typified by
carefully selected areas are presented, and a
comparison of these results with those
presented by published theoretical models
are shown to support the feasibility of the
development of a general empirical model.
1.
achieve the required quality of service, radio
resources have to be optimally deployed.
This often leads to overlapping coverage of
served area from different cell clusters. As
an improvement on existing baseline data,
operators apply Geographic Information
System in mapping the signal strength
available, the minimum requirement and the
maximum allowable levels to guarantee
compatible operation with other existing
networks, and other services such as
Industrial, Scientific and Medical (ISM),
Electronic Article Surveillance (EAS)
systems, automotive industry, and electric
power.[2]
Suffice to say that frequency
(cell) planning usually carried out aims at
resolving this and other propagation issues.
However, existing situations sometimes
deviate from propagation characteristics
based on assumed models in an environment
with little theoretical basis, or empirical
modeling. Such characteristics often times
are predicated on models obtained from
experiments elsewhere.
INTRODUCTION
Like many other countries in the world,
mobile communication networks have
resulted in a quantum leap in the availability
of telecommunication services in Nigeria.
As at October 2008, 58.99 million active
telephone lines are in use in Nigeria [1], of
which 57.6million are mobile lines provided
by five operators using GSM technology and
the rest mainly provided by operators using
CDMA technology. Provision of adequate
link for these lines requires substantial
deployment of radio resources, a situation
that resulted in high density of masts
especially in major urban centres. To
A. I. Mowete & M. A. K. Adelabu
This paper presents the reports of spot
measurements of electric field strength and
power density of radiation from GSM Base
Transmitter Stations spread across Lagos
and other states in South-western Nigeria
that were taken over a period of six months
between 2005 and 2006. Use was made of
the GPS to identify the geographic location
of each BTS and determine the distance
between the point at which the readings
were taken and the BTS sites. These
readings cover a wide variety of geographic
features – from coastal plains to hilly
terrains. The obtained readings were used to
assess on one hand the impact of the
International Conference on Power and Telecommunication (ICEPT 2009)
increase in level of electromagnetic
radiation due to the presence of the BTS
masts, and also to establish baseline data to
develop the model for evaluating the field
strength of radio propagation present in the
environment as an associated problem of
Interference Management. As a follow up to
the measurements taken in 2005/2006, new
measurements were carried out on selected
site (Faculty of Engineering Car Park) in
2009. These were taken to compare with
previous readings taken at the same site
since several new BTSs have been installed
by most of the operators at various locations
nearer the faculty to provide greater
coverage of the site.
One other purpose served by the survey is as
a basic environmental survey and impact
assessment study informed by the rate at
which BTSs are ubiquitously sited in order
to make mobile telephony available with
appropriate Quality of Service (QoS); while
at the same time maintain a radiation level
not exceeding prescribed standard values by
regulatory bodies such as IEEE, ICNIRP,
CEPT, IEC-SON [2]
2.
EVALUATION
OF
SIGNAL
STRENGTH AND CELL COVERAGE
In general, the generated electric
field strength denoted by E at a distance d
from a transmitting antenna admits the
expression, [3, 5]
30 P
(1)
d
where P represents the radiated power (or
EIRP-Effective Isotropic Radiated Power) at
the measurement location.
E
Based on this expression, several
propagation models [7] have been proposed
for use in the determination of the cell radius
for a given mast height and other BTS
A. I. Mowete & M. A. K. Adelabu
radiation parameters. When eqn 1 is applied
to configuration of BTS-MS as in fig. 2, the
electric field is related to distance as [3];
 

GT    'PT
 2 
E (d ) 
for d ≤ do (2a)
d
60GT    'PT

..
d
d 
E (d )  E (d o ) o 
 d 
for d > do
 2
GT (   ' )
.. (2b)
GT ( o   ' )
Where PT is transmitted power, 0 =  is
elevation angle for d = d0. GT() is the
directional gain of antenna. α is the power
decay exponent; (α ≥ 2) and do is the
transition (reference) distance depicting the
migration from near-field to far-field zones.
3. PROPAGATION CHARACTERISTICS
It is well established that propagation from
source to destination or victim, where the
recipient can be a desired or an unintended
one can follow a direct path or through
single/multiple reflections. Such arriving
signals having experienced some fading and
phase delay would (vectorially) add up,
thereby
weakening
or
sometimes
strengthening the signal at the receiver.
3.1 Multipath Propagation
Multipath propagation occurs as rays bounce
off objects in the propagating environment
thereby creating reflected signal paths
between the source (mobile base station or
broadcasting tower) and the receiver (user
terminal or other receivers). It could
therefore be said that multipath propagation
International Conference on Power and Telecommunication (ICEPT 2009)
occurs due to multiple replicas of the same
signal arriving with different arrival times at
the receiver. The reflected signals arrive at
the receiver with random amplitudes and
phase shifts because of the different paths
followed by the arriving signals. In dense
urban environment and sometimes range of
hills, where multipath propagations occur,
the range of times of arrival of the “replica
signals” can be very significant [5, 6].
Multipath propagation consists of the direct
dominant signal and a Raleigh distribution
of the reflected signals with various
amplitudes and phase shifts (fig.1). This
results to random signal fades as some of the
reflected signals destructively cancel the
others while some constructively add to the
others over brief periods. The degree of
cancellations (called fading) depends on the
Delay Spread of the reflected signals. The
components of Multipath phenomenon can
result into the fading effect that can lead to
signal impairment. [5,6]
3.1.1 Fading
Fading occurs because of the reflected
signals that arrive from different paths out of
phase at the receiver. These out of phase
signals, depending on their arrival times,
randomly cancel or add to each other. The
fading due to multipath can be frequency
selective in which case the channel must
have introduced time dispersion resulting in
the delay spread exceeding the symbol
period. Flat fading can also occur and in
such case, there is no time dispersion and
the delay spread is less than the symbol
period.[5,6]
Fading signal amplitude is statistically
described as Rayleigh or Ricean [7].
Rayleigh fades are due to Non-Line-ofSight (NLOS) signal component present in
the received signal. When there is the
presence of Line-of-Sight (LOS) signal
A. I. Mowete & M. A. K. Adelabu
component in the received signal, the fading
is regarded as Ricean [7]. Generally, if there
is no LOS path to a receiver (radio,
television or mobile station), the fading is
Rayleigh. Fig. 1 shows the typical scenario
of the existing radio propagation
environment. In addition to the various
BTSs, one can add for a cosmopolitan city
like Lagos the increasing number of radio
and TV broadcasting stations some of which
have non-negligible harmonic content in the
operational
frequencies
of
mobile
telephony/communication. The presence of
this array of signals at the MS for example,
is sometimes manifested in the weakening of
the desired signal and strengthening of
interfering/undesired content.
4. FIELD DETERMINATION MODEL
Fig.1 shows the scenario describing
multipath propagation from which, the value
of the field strength E at any given point
admits the expression
L
L
NL
i
j
i j
Ei   Ei   Eij
(3)
L - no of elements per antenna, N – no of
propagation rays associated with each
element.
Where Ei admits the general form defined in
eqn.(2.2) The second part is due to the
presence of signals radiated from other
masts within radius of significant reception.
These contributions from other BTSs and
sometimes from high power, low frequency
broadcasting transmitters, occasionally
result in weakening of the desired signal.
Interference management characteristic of
this model especially in a overlapping
coverage environment derives from the
relationship of cell radius with mast height
proposed by [3] as
R  0.0257 H 3  11.5H 2  40.4.
(4)
International Conference on Power and Telecommunication (ICEPT 2009)
Eqns 3 and 4 come in handy in explaining
the seemingly poor signal reception in some
areas that are well covered by a cluster of
cells without a dominant cell Ei. These
models were used to verify the
characteristics of the values of parameters of
cell coverage from different networks in
faculty of engineering, with the electric field
strength and power density measured over a
twenty-four period first in 2006 and recently
(2009) after the installation of new BTSs
nearer the faculty by some of the operators.
Figs. 3 and 4 show the electric field strength
recorded in the faculty car park due to the
overlapping coverage from the BTSs located
at distances ranging from 800m to 2300m to
the faculty, while fig. 4 show the power
density in [dBm] associated with the field
strength. A comparison of the values taken
in 2006 and those taken in 2009 reveal that
increasing the number of BTSs and siting
some very close has not caused a remarkable
increase in the field strength as to ignite the
fear of high dosage of radiation. Rather, it
helps to guarantee a more stable ambient for
improved service quality.
5.
CONCLUDING REMARKS
The
values
of
the
constitutive
electromagnetic parameters recorded were
found to conform with the empirical formula
eqns 2 and 4 derived in [3] and eqn 3
proposed in this work for use in evaluating
electric field strength at distances from
antenna locations. The values of electric
field strength E and power density P were
also compared to recommended standard
values by three bodies, IEEE, FCC and
ICNIRP. New BTSs introduced to provide
coverage on both 900MHz and 1800 MHz
were shown to contribute to improving the
quality of service without drastically raising
the ambient electrosmog. The results were
compared with those obtained from other
A. I. Mowete & M. A. K. Adelabu
works reported in open literature some of
which also addressed the issue of
compliance with allowable limits of
radiation from base stations.
REFERENCE
[1].
www.ncc.gov.ng
[2]
Use of Security and Similar Devices
utilizing Electromagnetic Fields;
ICNIRP Health Physics August
2004, vol. 87 no.2
[3]
M Barbiroli, C. Carciofi, V. DegliEsposti, and G. Falciasecca (2002);
“Evaluation of Exposure Levels
Generated by Cellular Systems:
Methodology and Results”, IEEE
Trans. On Veh. Technol. Vol. 51,
pp.1322-1329. Nov. 2002
[4] M. A. K. Adelabu and A. I. Mowete
(2006): ‘A Comparison of Signal
Reception Quality from GSM Local
Networks’. Proceedings of National
Conference organised by Dept of
Electrical and Electronics Engineering,
University of Lagos. July 2006
[5] A.C. Odinma, A. M. Anyanwu and
M.A.K. Adelabu(2006): “The
Perturbation of Mobile Communications
& Interference in a Rocky
Environment”. NSE Technical
Transaction. Vol. 41 no. 2 pp 60 – 70,
April – June, 2006
[6]
A.C. Odinma, A. M. Anyanwu and
M.A.K. Adelabu(2007); “Evaluation of
Mobile Communications in a
Predominantly Rocky Area’, IEC
Annual Review of Communications, vol.
60, www.iec.org
International Conference on Power and Telecommunication (ICEPT 2009)
[7] Lee, W. C. Y. (1998); “Mobile
Communications Engineering – Theory
and Applications,” McGraw-Hill, NY;
1998.
Rocks
Dominant Reflector
Other
BS
MS
Home
BS
Sky scrapper
Dominant reflector
Scattering
Radius
Figure 1: Multipath signal arrival model [6]
θ'
θ
H
d’
MS
d
do
BTS
h
R
fig. 2 Propagation model of a mast mounted antenna.
A. I. Mowete & M. A. K. Adelabu
International Conference on Power and Telecommunication (ICEPT 2009)
Fig. 3 Electric Field Strength
300
250
200
E [mV/m] 150
Series1
Series2
100
50
0
00.00
03.00
06.00
2006
107.9
58.9
93.3
2009
222
25.7
280.7
09.00
12.00
15.00
18.00
246.3
149.4
271.3
227.3
86.3
96.3
146.8
134.2
Time of day [Hours]
Fig. 4 Electric Field Strength
120
100
80
E [dbu] 60
2006
2009
40
20
0
2006
2009
00.00
100.6
106.8
03.00
95.3
88.1
06.00
99.4
108.9
09.00
107.7
98.4
Time of day [Hours]
A. I. Mowete & M. A. K. Adelabu
12.00
103.2
99.7
15.00
108.5
103.7
18.00
107
102.6
International Conference on Power and Telecommunication (ICEPT 2009)
Fig. 5 Power Density
0
-10
-20
-30
S [dBm]
-40
2006
2009
-50
-60
-70
00.00
03.00
06.00
2006
-54.91
-61.5
-54.1
2009
-50.1
-55.4
-42.6
09.00
15.00
18.00
-52.3
-42.5
-47.9
-53.4
-54.6
-50.8
-52.6
Time of the Day [Hours]
A. I. Mowete & M. A. K. Adelabu
12.00
-47