Base stations communicate and transmit the signal in a designed area or ‘cell’. There are three types of cells: macrocells, microcells and picocells, these are depending upon the size and the power output of the antenna of the base station.
Macrocells provide the main structure for the base station network. It has power outputs of tens of watts and has the ability to communicate within a range of 35 kilometres. Microcells are used to improve the main network, especially where the amount of calls are relatively high. They are placed in airports, and near shopping malls. And moreover these are increasing in demand as the demand of mobile phones increases in the country. The microcells need less power to operate and their range is of a few hundred metres. Picocells base stations have a lower power output than Microcells, by a few watts.
In 1998 the International Commission on Non-Ionizing Radiation Protection (ICNIRP) published guidelines covering the exposure to the RF radiation, these were based on a study by the NRPB (National Radiological Board of the UK).
The introduction of the mobile phones to the market was a little bit modest, with less than 1% of the Salvadorian population subscribing to the services by the start of the 1990s. this was due because the market was monopolized by the only service provider at that time, Telemovil. By the end of the decade, Telecom, recently privatized and Telefonica, a Spanish company joined the market and therefore started the war of the mobile phone market. By the end of the decade there was a rapid increase on the use of these devices, by 1997, the amount doubled and by the start of the decade, in 2000, there is a relationship that 1 of every three to five Salvadorians have a mobile phone.
The active mobile telecommunication sector brings a number of economic benefits to the country in terms of employment and the tax revenue. Moreover there are other advantages, and these are that the mobile telecommunications now play an important role in the general commercial activity of the country and this makes an indirect contribution to the national economy. Although it is difficult to get accurate data because this sector is developing very rapidly. However, taken together the operators, manufacturers and sales outlets stores, probably employ about 100,000 people (this is only an estimate).
Analyzing the issue
First of all in order to analyze the issue stated above, more about RF fields from mobile phones have to be explained.
Mobile phones and their base stations transmit and receive signals using electromagnetic waves. An RF wave used for radio communication is referred to as a carrier wave. The information it carries (speech, computer data, etc) has to be added to the carrier wave in some way, this is a process called modulation. The information can be transmitted via digital or analogue signals. The carrier wave varies much faster than the signal so that the modulation produces a relatively low oscillation in the amplitude of the carrier wave, This occurs if transmitted in the analogue form. The other way of transmission is by the digital form, concerning itself from ceros and ones. Digital transmission, usually binary, offers many technical advantages over analogue transmission systems. It is less at risk to distortion by interference and electrical noise, and moreover it is replacing or has completely replaced analogue transmissions in radio, TV and mobile phones. (see figure 1.2)
Concerning mobile phones, they send and receive information by radio communication. RF signals are transmitted from the phone to the nearest base station and incoming signals are sent from the mobile phone to the nearest base station and incoming signals are sent from the base station to the phone at a slightly different frequency. An ideal network may consists of a web of hexagonal cells, each with a base station in the center (see figure 1.3), but in practice the coverage of each cell will differ according to the ground and the availability of site for the base stations.
The two main types of cellular phone technologies are the GSM (Global System for Mobile communications) acquired by DIGICEL and Personal and the CDMA acquired by Telefonica.
The GSM mostly operates in either the 900Mhz or 1800Mhz band. The digital processing uses phase modulation that results in only a very small and essentially random changes in the amplitude of the carrier wave. The phase modulation consists on two electromagnetic waves, which have the same frequency but may be out of phase. Meaning that one wave is displaced from the other so that, when one has its maximum positive value the other has its minimum negative value. When the wave is phase modulated, each digit transmitted introduces a phase charge in the carrier wave and the change produced by a one is different from the one produced by a zero. In this system, each user requires a frequency channel of 200kHz; therefore there is a maximum of 174 channels within the 35MHz bandwidth of the 900MHz and 374 channels of the 1800MHz band. The channels are distributed across the cells in order for them to not cause interference with each other. The maximum power that the GSM mobile phones are permitted to transmit by regulation are 2W in the 900MHz band and 1W in the 1800MHz band.
Another type of technology used is the CDMA (Code Division Multiple Access), ‘a digital cellular technology that uses spread-spectrum techniques’. This technology doesn’t assign a frequency to the user, not like the GSM that does so. The transmission is labeled by a coding method that is different for each user. Since all transmissions occur at the same time, the changes in amplitude of the carrier wave are random. Meaning that it differs a lot, not following a certain pattern. There are two types of CDMA, one of them is the FDD (Frequency Division Duplex), where separate 5MHz channels are used to and from the mobile phone, and the TDD (Time Division Duplex) where the same channel is used but in different time periods.
Going deeper into the mobile technology, base stations communicate to the handset and vice versa by electromagnetic waves. These are electric and magnetic waves oscillating in the same direction, between the peak, that is the highest value and zero. The size of the field can be indicated either by the magnitude of the peak value or by the average value of the wave. The properties of an electromagnetic field change with the distance from the source.
The RF power from the mobile phone is mainly transmitted by the antenna that is functioning together with the other elements inside the handset. The antenna is usually a metal helix or a metal rod of a few centimeters long. When the antenna is near the body, the radiation penetrates it but the fields inside the antenna are significantly less than for the values outside of the same. The RF radiation of the antennas in the base station is significantly more than the ones of the mobile phones, and therefore exerts a greater power. The limit to the power is set by the need of the base station to avoid RF interference, this mean with no interference the electromagnetic waves will not collide with each other making any disruption in any call. The RF intensity on the ground is not zero outside the main beam, although it appears to be, as the antenna fires a beam at a certain angle in other to communicate with the handset. The beam is just clearly the maximum intensity of the electromagnetic waves, but still RF radiation is emitted outside the range of this beam. Furthermore there are two properties of the electromagnetic waves emitted by both the mobile phones and the base stations that might be a significant issue in the interaction with the human tissue. These are: the frequency spectrum and the coherent time. The emission from a mobile phone is essentially at one frequency and the base stations might be at several different frequencies, therefore it is defined that the wave will have a relatively long coherent time.
The field penetration into the body is called ‘dosimetry’. RF fields penetrate the body to an extent that it start to decrease with the increase in the frequency. To understand the effects this might have on human tissue, the magnitude of the fields needs to be determined within various parts of the body that are exposed to such emissions.
From scientific evidence the electric and magnetic fields produced in the body by a nearby electromagnetic source causes both thermal and non-thermal biological effects. And again the effect vary with the emitted frequency of the wave, and tissues containing magnetite (Fe3O4) are being studied, as magnetite is a naturally occurring oxide of iron, and iron has electromagnetic properties induced if in a field. Magnetite is found in certain bacteria and in the cells of many animals. It has been found that animals like some species of birds and fish make use of the magnetite to provide them with magnetic sensibility, which they use in navigation. Moreover no other effects associated with electromagnetic interaction have been demonstrated in animals. Indeed it seems to be generally that any biological effect from the mobile phones is much likely to be from electric than from magnetic fields.
Looking more deeply into the effects, thermal effects are those who are caused by the rise in temperature produced by the energy absorbed from the oscillating electric fields.
Figure 1.1
Figure 1.2
Figure 1.3
Coherent time is the average time between random phase changes, which are the result of phase modulation.