Exposure due to base station
The exposure due to base stations has been a topic of interest for the past many years as the mobile network has experienced exponential growth. The number of base stations have increased both in the urban and the rural regions. The density of the number of base stations is observed to be higher in the urban regions than in the rural regions largely due to more number of subscribers and a higher population density. The coverage range of base stations in the urban regions is smaller compared to the coverage range in the rural region and with a network of lesser number of base stations in the rural regions, they are required to transmit at a higher power (within the permissible levels) in order to cover a large area. Also in urban areas various microcells and picocells are used to provide enhanced coverage in indoor and high congestion locations.
The exposure assessment due to the base station needs to make a distinction between the rural and urban scenario along with the types of communication systems in use (GSM, DCS, UMTS, 4G/LTE). The exposure due to the base station in an urban region can be complicated with the existence of different types and sizes of buildings and surrounding infrastructure. Typically, the RF levels decrease as the square of the distance from the source (inverse square law). This is largely valid for a big open space with negligible reflections from the surrounding environment enabling line of sight path between the base station and the mobile phones within the network which is often attributed to the rural regions. In an urban region, based on the location of the base station and the surroundings, the communication signals will experience reflection and diffraction leading to superposition of the fields creating regions with high and low field intensity. The field strength levels inside a building can be from 1 to 100 times lower than those observed outside depending on the type of building construction . Additionally, exposure can vary significantly within the building where in some cases the exposure disparity will exist between the upper and lower floors of the same building. Also in the urban areas, it not uncommon to find a diurnal pattern in the exposure from the base stations such that low values are observed night while high values are observed during the rush hours of city life .
Biological effects of RF exposure
The radio frequency exposure due to electromagnetic fields in the frequency range of a few kilohertz to tens of Gigahertz is classified under the non-ionizing radiation category. This implies that the energy of these fields is too low to break chemical bonds or cause the hazardous effects attributed to those due to ionizing radiation from X-rays. There are however established hazards such as the induced thermal heating in the tissues from excessive exposure to electromagnetic fields at high power levels. Since this is largely a non-contact effect, it is also one of the most studied effect of RF exposure. The mere heating of the tissue do not imply an adverse biological effect. Also the heating effect could be seen due to the internal mechanisms of the body and/or due to the physiological strain experienced by the body . As the scientific studies undertaken till have not been able to sufficiently alleviate the concerns, there exists a possibility to further explore the potential hazards due to RF exposure. To address these concerns various research groups, government bodies and private agencies have been involved in funding and conducting research related to explore the possibility of potential health effects due to EMF exposure.
As the sources of RF energy do not cause ionizing effects, many laboratory studies conducted to observe the effect of RF exposure at the cellular level imply there is no direct evidence of RF exposure causing genotoxic effects such as DNA mutations and associated effects . Also studies conducted on animals, specifically on rodents are yet to provide conclusive evidence for the growth of tumors from RF exposure due to mobile phones [10-12].
Received Signal Quality
RxQual is designated a value between 0 and 7, where each value corresponds to an estimated number of bit errors in a number of bursts. According to the GSM technical specification GSM 05.08, RxQual value reflects the equivalent bit-error rate (BER) before channel decoding summarized in the table below.
The RxQual measurement is performed over all the received frames (104 TDMA frames) within a measurement period of 480 ms. The reported value is the average of the BER received over all the frames and is designated a value from one of the eight RxQual levels (0 through 7); where 0 signifies the lowest BER i.e. best performance and 7 signifies the worst case with a high BER. RxQual has two variants, RxQual_FULL and RxQual_SUB. When the measurement is performed over all the received frames, it is considered as RxQual_FULL. If some frames (out of the 104 frames) are not included in the measurement averaging process, the measurement is considered as RxQual_SUB. The mobile phone measurement report includes both the FULL and SUB values for RxQual.
SAR and OTA Characterization
A set of mobile phones of various form factors and models are used to perform various experiments. To perform a comprehensive analysis, the mobile phones are initially characterized for SAR and OTA for both GSM and DCS bands. The characterization is performed at the lower, middle and upper frequencies (reference frequencies) for the free space and the phone positioned in the right cheek configuration against the SAM phantom. The SAR characterization is performed using a standard dosimetry test facility while TRP characterization is performed using the compact reverberation chamber. The SAR and TRP characterizations are provided in the table below where both SAR and TRP characterization is performed at traffic channels (TCH) 975, 38, 124,
Table of contents :
LIST OF FIGURES
LIST OF TABLES
RF EXPOSURE CONCERNS
1.3.1 Occupational exposures
1.3.2 Exposure due to base station
1.3.3 Exposure due to mobile phones
1.3.4 Biological effects of RF exposure
OVER THE AIR (OTA) PERFORMANCE CHARACTERIZATION
OVERVIEW OF POWER CONTROL IN GSM AND UMTS
CHARACTERIZATION OF MEASUREMENT LOCATIONS
SOFTWARE MODIFIED PHONES
IDENTIFICATION OF THE MEASUREMENT PARAMETERS
2.2.1 Mobile station power level
2.2.2 Received Signal Level
2.2.3 Received Signal Quality
SAR AND OTA CHARACTERIZATION
UPLINK POWER CONTROL OBSERVATIONS
CHARACTE RIZATION OF INDOOR ENVIRONMENT FOR A REAL-LIFE SCENARIO
EFFECT OF HAND PHANTOM ON THE REAL-LIFE PERFORMANCE OF MOBILE PHONES
OUTDOOR MEASUREMENTS: STATIONARY SCENARIO
OUTDOOR MEASUREMENTS: MOBILITY SCENARIO
NUMERICAL CHARACTERIZATION OF MOBILE PHONES
ANTENNA DESIGN AND RF EXPOSURE
NUMERICAL ANALYSIS USING SAM AND HAND PHANTOM
SAR DISTRIBUTION COMPARISON USING FLAT PHANTOM
DESIGNING EXTERNAL MODIFICATION FOR THE MOBILE ANTENNA
3.5.1 SAR distribution comparison at DCS using SAM phantom
EXPERIMENTAL VALIDATION OF THE SAROTA INDEX
SELECTION OF DUT
SELECTION OF CONTROLS
MEASUREMENTS AND RESULTS
GSM 900 MHZ BAND WITH CHANNEL LOCK
SPATIAL VERIFICATION AT GSM
DCS BAND WITH CHANNEL LOCK
SPATIAL VERIFICATION AT DCS
SPATIAL VERIFICATION AT UMTS IS CURRENTLY UNDER INVESTIGATION
CONCLUSION AND PERSPECTIVES