2.1. Evolution

There is a need for communication among the people since ancient period where the drums, beacon fires are used as a signal to gather the people. Due to the technological and social reforms, the way of communication started with the heliograph that uses the sunrays as the signal. Due to the development in the electronics, telegraph and landline telephone became the mode of communication. To have entertainment and to know the occurrences of the society, leads to the invention of radio and television. With an aim to provide the required information at any time emerged the field of Internet, which made many changes in the life of an individual. Till the invention of wired communication, there is no harmful effect on the society and human beings. With the advancement in the field of electromagnetics and mobile phones, wireless communication combines both voice communication and Internet. Recent advancements in the mobile phone make user to communicate with the machines in our home through IoT. Though it seems advantageous and giving a belief of living a comfortable life the effect of RF exposure is adverse on human beings and also on the nature.

World Health Organization (WHO) has confirmed that the cell phone radiation is considered to be a carcinogenic to humans. Many surveys show that teenagers are mostly addicted to mobile phones (^{Lin, 2010a}). Based on the study conducted on students from different engineering, medical, pharmacy colleges, the effect of the use of mobile phones leads to lack of sleep, strain in eyes, headache, migraine, irritability caused during phone calls, restlessness, eye strain, continuous watching and skipping of meals, hearing problem and sometimes road accident because of the use of phones while driving etc. Another common impact is digital thumb because of continuously using the thumb to type in the small screen of mobile phones. The effect of mobile phones’ radiation is having more impact on person with age below 20 (^{Lin 2006}; ^2010b). One of the suggestions to minimize the effect is to reduce the time of use of mobile phones, else the radiation may cause cancer or adverse health issues (^{Lin, 2013}).

2.2. Types of study

The human beings are not only the victim of RF exposure the animals are also affected. The effect of RF exposure and mobile phones on rats are studied (^{Kesari & Behari, 2008}; ^{Sekar & Kalkan, 2002}) The study based on this is known as In Vivo. The human cells affected by RF exposure have genetic effect or non genetic effect and the study on human cells are known as In-Vitro. Apart from the genetic and non-genetic effect on the human cells effect on the epidemic layer of the human is known as Epidemiological study.

2.3. Parameters for RF exposure

Even though the phone is not used for talking or any other entertainment it continuously sends the signal to nearer base station for continuous connection. In that case the exposure to electric field, magnetic field, electromagnetic field, contact currents causes biological effect on human beings and other living organisms. According to IEEE ICES SCC 28, there is a minimal effect on health in the frequency range from 3kHz to 5MHz, the effect is adverse for the frequency range 100kHz to 300GHz. The Basic Restriction (BR) values, Specific Absorption Values (SAR), Maximum Permissible Exposure (MPE) values are specified to know the standard allowable value. The effect of RF exposure leads to thermal and non-thermal effect. The thermal effect on the tissues causes burns spots, headache, blindness if eyes tissues are affected. The non-thermal effect includes damage in DNA, immune system, change in chemical and electrical signals in the cell membrane. The biological effect is not only on the humans but also on the birds in the environment. Recently, while testing the 5G technology in Netherlands many birds died due to the effect of the microwave radiation.

As the number of users is increasing day by day, the power required by the base station to cover a very large distance is high. When the power is increased, the person near the base station is exposed more and effect is more. Nowadays people are more interested in choosing the service provider, who gives the fastest access. This requires more bandwidth requirement and it should compete with ever growing users. This leads to the use of higher frequency bands, which provides larger bandwidth. When frequency is increased it the wavelength of the wave is decreased resulting in absorption of the wave by the human body. The demand for higher bandwidth and faster data is achieved with the help of optical communication. With the help of optic fiber this is transmitted to a large distance, effect of electromagnetic interference that finds applications in aircraft and medical field. Though optical communication provides larger data rate and high bandwidth, the technology is not wireless. If it is made wireless then the data sent is lost and presence of small obstacle makes the data lost. Radio waves are mostly used for the wireless communication mainly because of its bending property when there is an obstacle. According to Dr. Gerard Hyland, Physicist, nominee for Nobel Prize in Medicine, “Existing safety guidelines for cell phone towers are completely inadequate.” Though the power radiated by the cell tower is limited within the standard value, the time duration for exposure causes more energy to be absorbed.

From Eq.1 it is well known that the energy absorbed by the person when exposed to 1mW power for 1000s is equal to exposure of 1W of power for 1s.

IEEE ICES SCC 28 defines two tier levels namely upper tier level and lower tier level.

Fig. 1 shows the different levels indicating the safety measure. Below 100kHz the safety factor is not required for upper tier level as electro stimulation has no adhere effects but requires safety measures in lower tier level. 100kHz being thermal crossover frequency above which the thermal effect dominance is based on the average time of exposure. If the duration is less than the average timing then energy density, specific absorption rate (SAR) values shows their time dependence. The safety factor is 10 to 17 dB for upper tier and additional 7 dB for lower tier, if the exposure time is greater than the average time.

Figure 1 Levels for safety measure. 

2.4. Levels

Human bodies should be exposed to the limited value of sound, heat and other stimuli. In the same way, the limit for RF exposure should also be within the limit. The different levels used for setting the limit for RF exposure. Some of them are:

● Specific Absorption Rate:

SAR value tells the value or amount of power absorbed by the body due to RF emission of the mobile phone. The unit for SAR is watts per kilogram. The normal value used is 1.6W/kg but it is extended to 4W/kg.

● Basic Restriction value and Maximum Permissible value:

Basic restriction value is the minimum value to which human body can be exposed. This value can be extended to a certain limit called Maximum Permissible value.

The Table 1 shows the SAR value for different types of the mobile phone (^{RF SAFE n.d.}). The values are given separately absorbed by head issues and body issues. The effect of SAR on the body is reduce to <20% if the mobile phone is kept at a distance of 5 mm away from the body (^{Gandhi, 2019}).

2.5. Positive impact

Inspite of many negative effects on humans and animals (^{Relova et.al , 2010}), there is also a positive impact of RF power when used under reduced power (^{Aly, Deris, & Zaki, 2008}). The positive impact of mobile phones is increasing along with its negative effect. The main reason for the high use of mobile phones is that everything is available in the mobile phone with the help of the Internet. The manufactures are interested in integrating every possible device and making any device communicating among each other. This enables the social link among the people and enables marketing easier (^{Hollebeek , Glynn, & Brodie, 2014}).

The influence of mobile phones has also an effect in the field of medicine. The growth has increased in such a way that mobile applications are developed and the status of the patients is addressed to the doctors at the right time. The advantage of detecting deficiency for pregnant women using mobile applications is mentioned (^{Dunsmuir et al., 2014}). They are connected to the health centers through the app and provide the suggestions after analysing various data from the user.

In the field of medicine, RF is used in a controlled manner for cancer treatment. Though RF exposure may cause cancer or tumor cells, by the controlled power of microwave frequencies the cancer cells die. As the microwaves have very high frequency, the time period will be less. This fast switching of positive and negative field cause quick heating of food, which is used in the microwave oven.

2.6. Negative impact

The mobile phone has become the vital tool for knowing the problem and solving it or communicating at the right time to the right person. In spite of these advantages, the negative impact of the use of mobile phones is increasing day by day as the number of users has increased and time of use of mobile phones is increased. To provide the effective communication the information that is received should be the same as the transmitted value. This is achieved by increasing the power of the signal. The negative impact of RF exposure on the human health is divided into 4 groups namely

Though the microwaves are used for cooking the food, (^{Sakurai, Narita, Shinohara, & Miyakoshi, 2013}) the impact of using the inductive heating cook top on pregnant women. The exposure of the magnetic field at 23 kHz and its effect on the fetus derived astroglia cell line showed that there is no change in the gene if exposed up to 6 hours.

2.6.2. Biological effect and health issues

The accidents occur mainly due to distraction of the user while driving. The main unknown impact of the RF exposure is the health issues due to prolonged exposure to RF radiation. Finite Difference Time Domain (FDTD) method is used for analysing the electromagnetic wave when half wave dipole antenna is used as the microwave source (^{Chen & Wang, 1994}). The head is modeled as the material with the dielectric constant, conductivity as that of the tissues in the head. For a transmitted power of 0.6W, the SAR of the model is 1.23W/kg to 2.63W/kg. The lower level fixed by Federal Communication Commission is 1.6W/kg. To avoid the impact at higher

According to (^{Miyakoshi, 2013}), the genetoxic effects include

• DNA strand breaks - Under alkaline and neutral conditions the DNA when exposed to RF exposure (X rays)

• Micronucleus formation- If RF exposure > 10W/kg the DNA splits and forms micro nucleus. Figure 2 shows the formation of splitting of nucleus and formation the new one.

Figure 2 Micronucleus Formation. 

Micronucleus formation due to RF exposure (^{Miyakoshi, 2013}),

● Chroma aberration and mutation - Due to MN formation and DNA strand breakage the genetic pattern is changed.

The non- genetic effect includes,

• Change in the cell proliferation cycle

• Gene expression

• Damage in the immune system.

The fertility of the human beings both male and female is affected by the RF hazards. Based on the study conducted (^{Al-Quzwini, Al-Taee, & Al-Shaikh, 2016}), the reason for sub-fertility in the male is also due to work and environmental hazard. 29% of males were affected by sub fertility as they are living near the mobile base stations or working near that region. From the analysis (^{Karger, 2005}), the effect of microwave on the brain causes the change in the permeability if the microwave radiation SAR is above 7.5 W/kg. The changes in the EEG, secretion of melatonin in pineal gland, Calcium efflux is caused due to microwave absorption.

2.6.4. Cancer

The negative impact of high RF radiation leads to deadly diseases such as cancer (^{Lin, 2005}; ²⁰⁰⁹; ^2010c). As the use of mobile phones is widely spread to communicate, ear and head are the most part of the body. To analyse the effect of RF radiation the TARO model has been developed for the brain.

Figure 3 shows the TARO model used for analysing the brain. The different regions are listed in the Table 2 that gives the SAR values for different regions of the brain:

Figure 3 TARO model for brain. 

Table 2 SAR values of the phone. 

Regions	Size in cm3
Temporal Parietal Frontal Occipital Cerebellum Brain stem	340 352 352 96 123 81

Based on the experiment (^{Varsier, Wake, Taki, & Watanabe, 2009}) on the TARO model of the brain by the cell phone radiation. The effect of the brain cancer depends on the way the phone used and the type of mobile phone. The results shows that the temporal lobe is the most affected part. The antenna position also affects the cerebellum part and the analysis on the different frequency bands such as 800MHz and 1500MHz is analysed

According to the study (^{Lin, 2005}) on the tumor of Auditory nerve due to the use of mobile phones, the effect is high if their use lasts for more than 10 years. The study was conducted before the wide spread of the use of digital phones. But with the mixed use of the analog and digital phones, the effect has been studied.

3.1. Reduction in power by coding

The advancement in the field of electronics and communication and coding has made mobile communication system an efficient and ever growing technology in all aspects. Mobile communication uses the radio waves for wireless communication. Because of insufficient bandwidth, the new technology based on 5G uses the millimeter wave for the transmission. The RF exposure is there because the continuous connection between the base station and the mobile phone. The base station acts as a central access point through which the call gets connected to the destination user. To get the new calls from the base station, the mobile phone continuously transmits the signal to indicate it as alive under the current base station. Lots of research is done in terms of reducing the power of the data to be sent in the wireless medium. The lower power limit is given by the Shannon’s limit, which tells the minimum power used for the efficient transmission of the data for the required bandwidth. The main purpose of increasing the power is to extract the data back from the erroneous channel. Some of the methods are implemented to reduce the power level reduced for the same error rate. Error Control Coding techniques are used so that additional bits are used. This method helps in spreading the energy to be retrieved in the extra bits. This also enables to check whether the data sent is affected by noise.

All techniques used in the mobile phone should reduce the power required to transmit the signal. The main challenge in the mobile communication is to retrieve the exact data from the low power signal. The suitable coding technique is used so that the better error correction is achieved with the same power. The different kinds of code such as

The recent development of polar codes and LDPC codes are used in 5G technology to improve the coding gain of the system. The reduction of power is improvement in the coding gain, (^{Sun, Cho, & Zhang, 2017}) proposed different post-processing methods in polar codes decoding method. The coding gain for the linear error correcting codes such as Hamming codes, Golay codes were defined for different lengths (^{Jovanovic & Budisin, 1984}). The maximum coding gain for Hamming codes is achieved for (127,120) at 2.76db where 120 message bits are taken and the 7 additional parity bits are added as redundant bits making a total of 127 bits. (^{Tahir, Schwarz, & Rupp, 2017}) suggested that for larger data length, the LDPC and Turbo codes are similar characteristics. The convolutional coding performs well lower data length but performance deteroiates at larger message and the complexity of the system increases. Once the data to be sent is used with the reduced power another major role played is the antenna in the power radiation. The main reason for increase in power at the transmitter side is that some amount of power is radiated as backlobe or minor lobe. This is the unwanted power which is absorbed by the user of the phone. Research is done to reduce the back lobe of the antenna by using reflector that reflects the backwards radiated power.

3.2. Power Absorbing materials

The importance of RF exposure is prevented by the research in the chemical field for designing a material that absorbs the RF power. This is used for hospitals or electromagnetic protected area. Different nano materials such as carbon fibers, sulfides, aluminum oxides, polymers, nitrides, metals, alloys their reflection characteristics under different frequencies are discussed (^{Green & Chen, 2019}). Based on the requirements the corresponding materials of specified length is used.

The use of FeP nano particles that enables the absorption of the microwave frequency is proposed (^{Green et al., 2018}). This nanoparticle absorbs more power at the 2.5GHz, which is widely used as Wifi range. This is suitable only for those under the safety-absorbing layer. The advancement in the field of frequency selective materials several filters can be designed to absorb the RF power of particular frequency.

3.3. General precautions