# SAFETY ZONE DETERMINATION FOR WIRELESS CELLULAR TOWER - A CASE STUDY FROM TANZANIA

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SAFETY ZONE DETERMINATION FOR WIRELESS CELLULAR

TOWER - A CASE STUDY FROM TANZANIA

Christina P.Nyakyi1, Salehe I. Mrutu2, Anael Sam3, Justinian Anatory4

1MSc. Student, 2 PhD. Student, 3 Senior Lecturer, The Nelson Mandela African Institute of Science and Technology,

Arusha, Tanzania.nyakyic @nm-aist.ac.tz, mrutui @nm-aist.ac.tz, [email protected]

4 Associate Professor, The University of Dodoma, Dodoma, Tanzania, anatory @ engineer.com

Abstract

Safety zone determination for wireless cellular towers has attracted attention from many researchers in the last decade. This is caused

by the rapid growth of the wireless cellular industry which has led to the installation of towers even in the residential areas. There are

many reports and ongoing researches regarding the biological and thermal effects of wireless cellular electromagnetic fields

exposures to people. Cancer, hyperthermia, neural and behaviour effects of people exposed to these electromagnetic fields have been

reported.

This motivates the research to determine safety zones from wireless cellular towers to assure safety to those living in the vicinity of

these towers. A model for safety zone determination is developed. The model takes the received power at the object, power transmitted

by the transmitter and gain of the transmitter as inputs to determine the safe distance from the radiation of a wireless cellular

transmitter. The power density received by the object and its geographical location from the radiation source are measured using the

selective radiation meter. Transmitted power and the gain of the transmitter together with the height of the tower were obtained from

the respective wireless cellular network operator. Based on the geographical location of the object, the distance from the radiation

source was calculated using the haversine formula. These inputs are then used to determine the safety zone based on the standards

and guidelines developed by WHO and ICNIRP.

Keywords - Safety zone; Power density; wireless cellular tower; Exposure limits.

----------------------------------------------------------------------***------------------------------------------------------------------------

1. INTRODUCTION

Almost all guidelines and recommended limits on human

exposure to GSM electromagnetic fields are given in terms of

The growth of the wireless cellular industry coupled with the

Specific Absorption Rate (SAR) defined by (1).

proliferation of cellular mobile applications has led to more

installation of wireless cellular transmission towers/base

2

station antennas even in residential areas, schools, markets,

E

hospitals, and other densely populated areas. This raises the

SAR

public concern regarding the safety of population exposed to

m

(1)

such networks with aggregated radiations[1].

Where

- Conductivity of body tissue,

Interaction of GSM electromagnetic fields and humans should

include all particularities of

human body which has very

E - Root mean square of intensity of electrical field at

unusual electromagnetic properties values such as electric

considered point

permittivity and electric conductivity[1].

m-mass density of tissue at that point

These properties are not well known and depend on activity of

person

SAR, the time rate of RF energy absorbed per unit mass, is

very difficult and complex to be measured in biological

This material is an active material at cell scale

tissues; standards permit the use of reference levels of power

Most cases, the problem is actually a coupled problem

flux density [W/m2] in free space. IEEE standard established

that is the thermal effect is one of the major effects and it

the limits for electric and magnetic fields, so called maximum

is affected by the blood circulation

permissible exposure(MPE) and similarly ICNIRP standard

The geometry is complex and generally environment of

defines reference limits for free-space incident fields as

the human body has to be taken into account

detailed in table 1 with safety limits exposure for public in

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some of the services. Getting together these limits SAR

distance from those antennas. The study based on estimated

compliance should be ensured. So as a replacement for

readings and not actual readings from the field [6].

complex SAR measurements, for compliance assessments the

above mentioned standards the simpler field measurements as

Electromagnetic radiation from mobile phone base station at

power flux density are used[2].

Gaza was the study aimed to highlight relevant international

work and develop the computer tool which can simplify

In this paper we have presented results of measurements of

estimating and measuring EMF level in the city. The tool

field strength items of power flux density in the vicinity of

developed had to store the BTS parameters and coordinates in

base station antennas

a database, and then it generates tables and maps that illustrate

EMF level estimated theoretically. It doesn't define whether

Table 1: ICNIRP Safety Limits for public exposure

the level was on safety zone or unsafe distance rather than

generate the maps and tables.[7].

Service

ICNIRP

ICNIRP

Safety

Frequency

Safety

limit

Limit

Power

The level of power density varies with the increase of gain as

E-field [V/m]

Density (W/m2)

well as the power of transmitter was shown on the study done

GSM 900

41.9

4.66

by Al-Bazzaz, 2008. In this study theoretical assessment of

GSM 1800

58.4

9.05

electric field strength and power density performed on

WCDMA

61

9.87

locations tens of meters away from mobile phone base station

antennas was presented. This study also

doesn't show the

safety zone [8].

2. RELATED WORK

The study done by Singh (2012), the exclusion zone

3. METHODOLOGY

(compliance distance) from GSM, CDMA, 3G/UMTS, and

In this study we have done a field work on measuring power

WiMAX antennas have been found at 7.30, 6.076, 7.436, and

density at discrete levels by using Selective Radiation Hazard

6.861 m, respectively are deduced. However, in this study

Meter (SRM) - Narda 3006 which has the ability to allocate

values of EIRP which is transmitted and its gain as well as at

the geographical location where the object located in terms of

what height of antenna should be installed is not defined [3].

latitude and longitude as well as to quantify the value of power

density. By having these two locations for source and object

The study titled Estimation of peak power density of

then the discrete distance was obtained by using the

electromagnetic radiation in near and far fields for 2G and 3G

`haversine' formula. This formula is more applicable in

base station in Albania done by Cela et.al (2012), this study

calculating the great-circle distance between the two points.

discussed on the safety distance from cellular base stations

The haversine formula is given by equation (2), (3) and (4).

operating at 900MHz, 1800MHz and 2100MHz has been done

by using a simplified theoretical way considering ideal

conditions for wave propagation. The study also hasn't

actual

Givenby a

2

;

Sin

Co

s .Co

s

2

Sin

1

2

readings from the field as well as identify physical dimension

2

2 ,

(2)

for the surface even the time taken for capture the entire

readings [4].

c

a

2

.

2 tan a, 1 a,

(3)

The study done by Kaur et.al, 2012 on the effect of

permittivity

and

conductivity

of

tissue

on

SAR

of

d R c

.

electromagnetic radiations shows, how the Voltage Standing

,

(4)

Wave

Ratio

(VSWR)

and

return loss

from 900MHz

communication frequency on simulated antenna for analyzing

Where; is latitude, is longitude, R is Earth's radius (mean

its effect in terms of variation in specific absorption rate

radius = 6,371km). All angles are in radians

(SAR) of EMF radiation in human tissue at different

permittivity and conductivity has different effects accordingly.

3.1 Measurement of Power Density from Cellular

this study does not indicate whether the permittivity and

Towers

conductivity are affected by distance variation from the source

[5].

Undertaking the measurements by SRM, the operating

The study done by Kamo et al.(2011) shows; the relationship

procedure is followed. The equipment was installed with

between theoretical values with exposure limits for both public

software which has the ability of downloading the data as well

and occupational from cellular base stations, FM, UHF and

as to identify the geographical location in terms of latitude and

WiMAX antennas as well as suggest the possible safety

longitude with the help of building GPS. On setting the

frequency span GSM 900MHz was on the range of 890MHz to

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960MHz, where GSM 1800MHz was on the span of 1790MHz

up to 1880MHz the Resolution Bandwidth (RBW) was 17.5

and 450 respectively according to the manufacturer guidelines.

The standard was observed for the Europe as per setting on the

equipment.

On measuring technique the very high lobe should be detected

so that the maximum power density can be obtained as shown

on the figure 1.The detection of main lobe is done by visual

and sweeping the measuring tool to detect the appropriate

direction of the very high lobe.[9]

Figure 1: Radiation Pattern of Cell Tower Antennas

Table 2 and 3 shows the measurements obtained for GSM900

and GSM1800 antenna at the field area

Table 2: Mbauda Cellular tower, Tanzania - Actual data readings on 05/29/2013 at 10hrs

Power Density [W/m]

Latitude[S]

Longitude[E]

Distance[m]

900MHz

1800MHz

322'50.5'

3639'34.0'

974.30

054.01

00.00

322'50.8'

3639'34.0'

936.52

281.43

09.27

322'51.3'

3639'34.0'

445 .00

267.25

24.71

322'51.6'

3639'34.0'

004.06

049.59

33.98

322'52.1'

3639'34.0'

012.01

053.35

49.42

Table 3: Mrombo Cellular tower, Tanzania - Actual data readings on 05/29/2013 at 14hrs

Power Density [W/m]

Latitude [S]

Longitude [E]

Distance [m]

900MHz

1800MHz

325'09.2'

3639'25.0'

41.30

259.90

00.00

325'09.5'

3639'24.8'

64.00

148.60

09.27

325'09.7'

3639'24.6'

35.80

198.20

15.44

325'10.0'

3639'24.4'

59.30

146.50

24.71

325'10.2'

3639'24.1'

46.01

118.90

30.89

The values in table 2 and 3 are very small compared to the

CASE 1: Single Tower

limit exposure provided by the ICNIRP as shown on table 1

above. The measured value readings vary with respect to

Consider the case of single tower in a free space, a case

distance and time, which lead to stochastic readings.

common in rural/remote area.

3.2 Derivation of Model Equations

In this study we derive the power density received at

destination in free space path loss for one and two antenna and

generalize with n-antenna and consider the case when the

height of object h0 is defined. We study the effect when the

distance of object increases and the height changes.

Figure 2: Single tower with an object.

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The power density at point on the ground at a distance, x

If the equation (5) is modified to consider the height, h0, of an

metres from the tower is given by the equation:

object (like a house or human), then power density at the top

of the house is given by:

Pt Gt

Pd

Pt Gt

R2

4

(5)

Pd

r 2

4

(8)

P

Where;

d means power density from cellular tower in

W/m2.

2

2

2

(h

r

x

h )

But

o

(9)

pt

G

and

t are power and gain from transmitters in dBm and

Then equation (8) become;

dB respectively and R the distance from transmission tower to

object at ground level in metre. When the height of the tower h

P G

t

t

in metre and the distance of the point x in metre from the

P

d

tower are known, then we have;

4

2

2

x h ho

(10)

R2 x2 h2

(6)

Figure 4. Shows the variation of power density at the top of an

object of 3 metres height for antenna with Pt = 16dBm,

Thus, we have;

20dBm, and 25dBm. Field data for GSM900 and GSM1800

has been plotted together to enable comparison with

theoretical values. The graph shows that power densities from

Pd PtGt

the field were less than the theoretical power densities

x2

4

h2

(7)

produced by an antenna of Pt = 25dBm.

Figure 4.3 shows the power density of Pt = 16dBm and

-3

x 10

20dBm with a gain Gt = 17dB. These values were used

1.8

Pt = 16dBm

because many service providers of cellular network use Pt in a

1.6

Pt = 20dBm

range of 16dBm to 20dBm

Pt = 25dBm

1.4

GSM900

GSM1800

-4

2

x 10

1.2

4.5

m/

W

Pt = 16dBm

ni

1

4

Pt = 20dBm

ytisne 0.8

3.5

Drew

2

o

3

0.6

m/

P

Wni 2.5

0.4

ytisne 2

0.2

Drewo 1.5

0

P

0

50

100

150

Distance from base to object in metres (m)

1

0.5

Figure 3: Power Density with fixed height of object ho=3m in

0

various Pt

0

20

40

60

80

100

120

140

160

180

200

Distance from base to object in metres (m)

To compute the total power density absorbed by an object of

height h0, we use the idea of calculus, and show that the total

Figure 2: Power density versus distance of a point for Pt =

power density is given by

16dBm, 20dBm and a gain Gt = 17dB

Total Power Density =

The graph shows that power density decrease with the increase

ho

P G

t

t

h

h h

0

in distance of a point from the tower, and at some point the

P

d

arctan arctan

power density attains a constant minimum value.

4

0

x

x (11)

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Remember that this formula is applicable only when h0 is

R2 h2 x2

defined otherwise the total power for h0 = 0 is computed from

1

1

1

(12)

equation (5).

2

2

2

Figure 5. Shows the total power density at the top of various

R

h

x

2

2

2

(13)

object heights from 3m to 10m. It can be seen that total power

density increases with the height of an object

The power density for each antenna from the two towers is

given by

P1G

0.7

1

P

d 1

,

2

Pt = 16dBm

4 R1

(14)

Pt = 20dBm

2

0.6

m/

W

P2 G

ni

2

0.5

P

y

d ,2

ti

2

4

s

R2

n

(15)

eDr 0.4

ewo

By substituting equation (12) and (13) into equation (14) and

P

de 0.3

(15) then it becomes

talu

P G

1 1

m

u

Pd 1

,

4 h2 x2

c 0.2

cA

1

1

l

(16)

atoT 0.1

2

2

Pd ,2 4 P G

h2 x2

03

4

5

6

7

8

9

10

2

2

(17)

Height of object in metres (m)

.

The average power density at a point from the two antennas is

Figure 4: Total Power Density for object with height from

given by

h0=3m to 10m

1

1 1

2

2

P

P

P

P G P G

d

d 1

,

d ,2

4 2

2

2

2

h

x

h

CASE 2: Two Towers

1

1

x

2

2 (18)

Consider two towers that generate and transmit power at P1

Figure 7. Shows that power density depends on the distance of

and P2. Let the respective height of towers be h1 and h2 and

an object from the antenna. That is, power density decreases as

x1 and x2 be the horizontal distance of the object from the two

the density increases

towers respectively. The power density at a particular point on

the ground will be the sum of the power densities from the two

antennas.

.

Figure 6: Graph of Power Density at a object from two

Figure 5: Power Density to an object from two towersFrom

antennaWhen the height of object is considered then the

Pythagoras theorem, we have

expression (18) is derived as shown on equation (19) and its

relation is illustrated in the graph figure 8.

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n

1

P G

i i

1

P1G1

P2G2

P

d, i

P

d P

d 1

,

P

d,2

(19)

4

2

i 1 2

x h h

i

i

0

4

2

2

h

1 h0 2

x

h

2 h0

2

x

1

2

(24)

Note: The equation (24) is a general expression which can be

Figure 8 shows the variation of power density at the top of an

used to determine the power density at any point for an object

object of height 3m situated at a distance x1 and x2 from the

with height 0 to h0 provided that power transmitted and the

two antennas of height h1 and h2 respectively.

gains of antenna as well as the distance from the transmitter

are known.

3.3 Computation and Validation of safety zone

There are a number of national and international regulations

standards and recommendations dealing with electromagnetic

exposure in the radio frequency range. The limits are generally

very similar and usually based on recommendation from the

World Health Organization (WHO) and the International

Commission on Non-Ionizing Radiation Protection (ICNIRP)

guidelines. It is shows that the limits have been set with a wide

margin in order to protect people from any known negative

health effects of both short and long term exposure to

electromagnetic field. Basic restrictions on exposure are

provided for both occupational exposure and public exposure.

The standard posed by ICNIRP guidance for public exposure

limits is 9.2W/m2 for GSM 1800MHz and 4.7 W/m2 for GSM

900MHz. However, these limits can be tolerable if the person

is exposed for few seconds and not for permanent exposure

[10].

Consider a case of a human lying on the bed of 2m2 the hip or

height made by the body taken to be negligible. Then, the

Figure 7: Power Density from two towers for an object at a

power absorbed is given by exposure limits times the surface

height ho=3m

area.

CASE 3: For n towers

For GSM 1800 power density;

2

2

2

W

2

.

9

W

P

4

.

18

d

m

m

n

P

PiGi

d,i

2

For GSM 900 power density;

i 1 4 Ri

(20)

2

2

2

7

.

4

W

W

P

4

.

9

d

m

m

R2 x2 h2

If time is considered then power absorbed

For

i

i

i

(21)

For 30min. exposure in GSM 1800;

W

4

.

18

30

(

)

60 sec

120

,

33

Joule

n

Pd

P

P G

i

i

d , i

2

2

i 1 4 x h

i

i

Then,

(22)

For 30min. exposure in GSM 900;

W

4

.

9

30

(

)

60 sec

920

,

16

Joule

Pd

If height of object h0 is considered

Therefore;

If the exposure provided is compared with the exposure from

n

the microwave cooker of 500W then;

P

Pi Gi

d , i

2

i 1 x2

4

h

i

i

h0 (23)

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For GSM 1800 the exposure will be =

This exposure time is high for a person who is exposed for the

33120W.sec 500W 670sec

long time. We need to look for the minimum distance for

tolerable exposure. To do this we apply the standard given by

EMF-Exposure-Guidelines-For-Sleeping-Areas[11]which

While the exposure for GSM 900 will be =

recommend the maximum exposure of 10 W/m to be of less

16920W.sec 500W

84

.

33

sec

concern as shown on the table below.

Table 4: Exposure guidelines for sleeping area

Power

density

in

No Concern

Slight Concern

Severe Concern

Extreme

microwatt

Concern

per square meter W/m

< 0.1

0.1-10

10 - 1000

> 1000

per square cm W/cm

< 0.000,01

0.000,01 - 0.001

0.001 - 0.1

> 0.1

Figure 9; shows the minimum distance where an object can

geographical limitations, geographical locations were taken

live with less concern of exposure. The distance is obtained by

and later converted to distances as it has been explained in

the point at which the graph for power densities meets the line

Section 3 subsection A. The results show that the power

Pd = 10 W/m shown by black line above zero power density.

densities depend on the distance from the antenna. That is, the

This distance is the safety distance to live. The safety zone

longer the distance the smaller the power density and that the

depends on the power transmitted by the antenna [11].

power density is maximum at the tower. The results also show

that the power density at a point from an antenna depended on

P

dBm

the application. That it, the power density from a GSM900

Thus for the power transmitted;

t

16

is 120m

application was different from the GSM1800 application.

P

dBm

And for power transmitted;

t

20

is 190m as shown

on the figure 9.

Numerical analysis reveals that the power received by the

object varies directly proportional with the power transmitted

Therefore more the power of transmitter the far the safety zone

by the source and inversely proportional with its square

occur.

distance from the source. The greater the power transmitted,

the longer the distance to the safety zone, and hence it

-4

x 10

4.5

becomes severe to live near the source.

4

Pt = 16dBm

3.5

Pt = 20dBm

4.2 Findings Regard the Safety Zone

2 m/

Min

3

W

To develop a model for cellular tower radiations several

ni 2.5

y

parameters were considered including the power transmitted

tisne

by an antenna, the gain of an antenna, the allowable exposure

2

Dre

for both GSM900 and GSM1800 and the surface area

1.5

woP

occupied by an object. The results show that the safety zone

1

will much depend on the power transmitted by the antenna

0.5

given the same antenna gain. From our study the safety zone

0

for transmitted power of 16dBm was 120m and for 20dBm

0

20

40

60

80

100

120

140

160

180

200

Distance from base to object in metres (m)

was 190m for a gain of antenna of 17dB,

Figure 8: Graph of Power densities versus distance with

To validate the model measurement taken from the field were

safety zone defined.

plotted together with the theoretical values. The data for

GSM900 were below the graph corresponding to Pt = 25dBm

4. FINDINGS, RESULTS AND CONCLUSIONS

and

those

from

GSM1800

were

below

the

graph

corresponding to Pt = 20dBm.

4.1 Findings Regard the Measurements

Furthermore, the results show that all the measurements taken

To gain understanding of the power densities at different point

within 50m from the antenna in the field were above the

from the antenna, measurement of power density were made at

discrete distances from the transmission towers. Due to

minimum exposure of 10 W/m for sleeping areas.

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The results obtained in this study for safety zone are far

[6] R. M. Bexhet Kamo, Vladi Kolici,Sanie Cela, Algenti

greater than those obtained by Kamo et al (2012) which were

Lala, "Estimation of peak power density in the vicinity of

between 12.9 to 46.3 meters from antenna for GSM900,

cellular base stations, FM,UHF and WiMAX antennas,"

1800.respectively, the difference is due to exposure limits used

International Journal of Engineering & Technology

IJET-

where Kamo used 9W/m2 and 4.5W/m2 exposure limits from

IJENS, vol. Vol:11, 2011.

ICNIRP guidelines recommendation [6].

[7] M. Abdelati, "Electromagnetic Radiation from Mobile

Phone Base Stations at Gaza," At Islamic University of Gaza

4.3 Results and Conclusions

(Natural Sciences Series), vol. Vol.13, pp. p129-146, 2005.

[8] S. H. S. Al-Bazzaz, "Theoretical Estimation of Power

The results regarding the safety zone shows that the safety

Density Levels around Mobile Telephone Base Stations,"

zone depend on the power transmitted by the antenna for the

Journal of Science & Technology, vol. 13, 2008.

same antenna gain. The safety zone for transmitted power of

[9] M. K. T. S. A. Kumar, "Effects of Mobile Tower

16dBm was 120m and for 20dBm was 190m for a gain of

Radiations & Case Studies from different Countries Pertaining

antenna of 17dB with the fixed height of antenna 30m above

the Issue," International Journal of Applied Engineering

the ground. All field measurements taken within 50m from the

Research, vol. 7, 2012

antenna in the field were above the minimum exposure of

[10] ICNIRP, "Guidelines for limiting exposure to time-

10 W/m for sleeping areas.

varying electric, magnetic fields (up to 300GHz)," ed. Health

Physics Society, 1998, pp. 494-522.

The measurements of power densities at the field and the

[11] G. Abdel-Rassoul, et al., "Neurobehavioral effects among

model has been presented to give an estimate of the safety

inhabitants

around

mobile

phone

base

stations,"

zone for people living in close vicinity to wireless cellular

Neurotoxicology, vol. 28, pp. 434-440, 2007.

towers. Different analyses have been carried out before

estimating the safety zone. Results have shown that safety

zone depends on the power transmitted by the antenna.

However, for a power of 16dBm taken to be the smallest, the

safety zone was 120m from the tower. This call for a concern

for people living in the close vicinity and respective

authorities should ensure that people reside far from the tower

by 120m or more depending on the power transmitted to avoid

severe health effect.

REFERENCES

[1] L. Nicolas, et al., "Interactions between electromagnetic

field and biological tissues: Questions, some answers and

future trends," International Compumag Society Newsletter,

vol. 10, pp. 4-9, 2003.

[2] L. A. Mimoza Ibrani, Enver Hamiti,Ruzhdi Sefa,

"Exposure assessment in the vicinity of 900 MHz GSM base

station antenna," in The 11th WSEAS International Conference

on

COMMUNICATIONS,

Agios

Nikolaos,Crete

Island

,Greece., 2007.

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