Part 2
What is cellular?
How does mobile radio work? What are the GSM , UMTS and LTE cellular standards ? How high is the exposure to high-frequency electromagnetic fields in the cell phone base stations and in the end devices, the cell phones? Here you will find information on the basic aspects of mobile communications.
Radio frequency electromagnetic fields
In mobile radio, high-frequency electromagnetic fields are used for the wireless transmission of voice and data.
In the electromagnetic spectrum, the high-frequency fields in the frequency range between about 100 kilohertz and 300 gigahertz located
The intensity or strength of the fields is either in shape
the magnetic field strength (unit of measurement: amperes per meter, A / m ) or
the power density (unit: watts per square meter, W / m 2 )
specified. The power density is the product of electrical and magnetic field strength.
In mobile radio, high-frequency electromagnetic fields are used for the wireless transmission of voice and data. As waves, they spread in free space at the speed of light and can transmit energy and information over long distances.
Frequency and wavelength
In the electromagnetic spectrum, the high-frequency fields are located in the frequency range between about 100 kilohertz (abbreviated " kHz "; 1 kHz = 1,000 Hertz ) and 300 gigahertz (abbreviated " GHz "; 1 GHz = 1,000,000,000 Hertz ) (see figure). Hertz (abbreviated " Hz ") is the unit of measurement for frequency , that is, for the number of vibrations per second.
Frequency and wavelength are firmly connected with each other via the speed of propagation and describe the wave character of the fields. The wavelengths are small at high frequencies and long at low frequencies. For typical mobile radio frequencies between 400 megahertz (abbreviated " MHz "; 1 MHz = 1,000,000 Hz )) and 3 gigahertz , they are, for example, 75 to 10 centimeters.
Field strength
The intensity or strength of the fields is either in shape
the electric field strength (unit: volts per meter, V / m ) or
the magnetic field strength (unit of measurement: amperes per meter, A / m ) or
the power density (unit: watts per square meter, W / m 2 )
specified. The power density is the product of electrical and magnetic field strength.
The high-frequency fields at a certain measuring point depend, among other things, on the distance to the system, the transmit power of the system, the type, orientation and downward inclination of the transmit antennas, as well as the signal attenuation due to vegetation and buildings.
Spread of fields
With increasing distance from the source, in the case of mobile radio from the antenna of a mobile radio transmission system or a mobile device, the field strengths and power densities quickly decrease. For example, the power density in free space along the direction of propagation decreases with the square of the distance, that is, it decreases to a quarter at twice the distance.
In the real environment, the propagation conditions are often much more complicated:
Objects that are in the direction of propagation can reflect, diffract or even completely or partially absorb high-frequency electromagnetic fields. How much these effects influence the spread depends, among other things, on the shape, size and material of the objects.
The antennas of mobile radio transmitters do not transmit evenly in all directions, but have preferred directions. This contributes to the fact that the field strengths in the vicinity of a transmitter can be different despite the same distance from the source.
Statements on the field intensity at a specific location can only be made on the basis of measurements or simulation calculations that take the relevant propagation conditions into account. The field strengths cannot usually be deduced from the distance from a transmitter alone.
The fact that the field intensity decreases with the distance from the antenna basically also applies to mobile phones and smartphones. Therefore, for example, the head is exposed to significantly lower field strengths when using hands-free devices than during phone calls in which the cell phone or smartphone is held directly to the ear. This effect can be observed not only in the case of permanently installed hands-free devices, for example in motor vehicles, but also when using wired and wireless headsets.
GSM standard
In order to achieve a nationwide coverage with mobile radio applications, the areas to be served are divided into so-called radio cells, each of which is covered by fixed radio transmission systems (base stations). As a rule, three sectors, each enclosing an angle of 120 °, are supplied with one or more radio channels from one location. Three antennas offset from one another are then attached to one antenna location.
Base stations and radio cells
The base stations communicate with the mobile devices via high-frequency electromagnetic fields. The base stations are connected to a central exchange via cables or radio relay. The radiated power of the systems is typically in the range of 10 to 50 watts. It depends, among other things, on the size of the respective radio cell, the typical radius of which can range from a few ten meters for so-called picocells to 35 kilometers for macro cells in rural areas, and the number of people who make calls simultaneously. Today, especially in cities, there is a well-developed mobile network coverage with small cell networks. New base stations will continue to be built to increase capacity and close dead spots.UMTS or LTE .
Differentiation of radio cells
Since several calls are usually held simultaneously in a radio cell, their radio signals must be distinguishable from each other. The first distinction is made by the frequency of the high-frequency fields.
which is divided into several narrow frequency bands (frequency channels). Adjacent cells use different frequency bands, otherwise the conversations would interfere with each other.
Biological
effects of high-frequency fields through energy absorption and heating
·
High-frequency
electromagnetic fields are absorbed by the body ("absorbed") and can
have different effects there.
·
The strength of the
energy absorption depends on the strength and frequency of the
electromagnetic fields, but also on the properties and structures of the
biological tissue.
·
The effects of force and
the thermal effects of the high-frequency fields are clearly proven and
physically defined.
·
The decisive factor for
the biological effect of high-frequency fields is the energy absorbed -
"absorbed" - by the body. The basis for this is the specific
absorption rate ( SAR , unit of measurement: watts
per kilogram - W / kg ). It indicates the power (energy per
time) that is absorbed per kilogram of tissue.
·
Living
beings, including humans, contain many electrically charged particles and polar
molecules. Polar molecules, such as the water molecule, are electrically
neutral as a whole, but carry a negative partial charge at one end and a
positive partial charge at the other end.
·
Electric
and magnetic fields exert a force on electrically charged or polar particles so
that they move. In a high-frequency electromagnetic field , the particles move very quickly in time with the frequency . They rub against each other and heat is generated.
·
They
align themselves in the field or hike. However, such non-thermal effects cannot be
triggered by fields from radio applications, since their field strength is not
sufficient for this.
Heat effect
The
thermal effect is decisive for the possible health effects of high-frequency
fields in humans. If high-frequency fields act on the body, it can
compensate to a certain extent the so-called thermoregulation for the energy
absorbed and converted into heat:
·
If the body is only
warmed up locally, the blood can usually dissipate the additional heat.
· If the whole body is warmed up, the skin is supplied with more blood and the heat is released by evaporation on the surface of the skin (sweating).
Health
effects can be expected if certain threshold values are exceeded and the
body's heat regulation is disturbed. In animal experiments, for example,
health effects were demonstrated if the body temperature had increased
significantly by more than 1 ° Celsius over a longer period:
·
Metabolic processes were
disturbed,
·
there were changes in
behavior and
·
Disorders of embryonic
development have been observed.
Long-lasting
overheating in the eye area favors the development of cataracts and other eye
diseases. The brain and testicles are also particularly sensitive to heat.
Depth of penetration
An
important factor in the effect of high-frequency fields on living things is the
depth of penetration. It is heavily frequency-dependent:
·
Electromagnetic fields
in the megahertz range, such as those used for broadcasting, have penetration
depths of 10 to about 30 centimeters.
·
In contrast,
in mobile communications with frequencies around 1 gigahertz ( GHz )
a thousand times higher , the radiation only penetrates a few
centimeters into the tissue.
·
At frequencies above
10 gigahertz ,
such as occur with radar devices, the penetration depth is less than 1
millimeter.
·
At even higher frequencies,
high-frequency fields only act on the surface of the skin.
"Microwave listening"
A
special effect of high-frequency electromagnetic fields is caused by short,
strong pulses. Under certain conditions, these are perceptible as hums or
clicks. One speaks of "microwave hearing". The currently
accepted mechanism of "microwave hearing" is based on the
thermoelastic properties of the tissue. Tissue areas of the brain are
warmed up by short, powerful high-frequency signals and consequently
expand. This stimulates mechanical waves in the tissue that are in the
audible range and stimulate the inner ear.
In
order to generate audible stimuli in this way, very high energy values per
individual pulse are necessary. If the energy of the individual pulses is
limited, such effects cannot occur. The fields of radio and television
transmitters as well as those of mobile radio cannot cause "microwave
hearing". Perception is possible in the immediate vicinity of
powerful radar systems.
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