Minimization of Radiation Exposure from Mobile Phone Base Stations by Power Controlling and Coordinated Multipoint Joint Transmission

MINIMIZATION OF RADIATION EXPOSURE FROM MOBILE PHONE BASE STATIONS BY POWER CONTROLLING AND COORDINATED MULTIPOINT JOINT TRANSMISSION

Molla Md. Zubaer

A thesis submitted in partial fulfillment of the requirements for the

Degree of

MASTER OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING

Department of Electrical and Electronic Engineering

Bangladesh University of Engineering and Technology

July 2017

Approval Certificate

The thesis titled “Minimization of Radiation Exposure from Mobile Phone Base Stations by Power Controlling and Coordinated Multipoint Joint Transmission” submitted by Molla Md. Zubaer, Roll No – 0411062256, Session: April, 2011 has been accepted as satisfactory in partial fulfillment of the requirements for the degree of Master of Science in Electrical and Electronic Engineering on 31 July, 2017.

Board of Examiners

…………………………………… 1. Dr. Md. Forkan Uddin Chairman Associate Professor, Department of EEE, BUET, Dhaka (Supervisor)

…………………………………… 2. Dr. Quazi Deen Mohd Khosru Member Professor and Head, Department of EEE, BUET, Dhaka (Ex-Officio)

…………………………………… 3. Dr. Satya Prasad Majumder Member Professor, Department of EEE, BUET, Dhaka

…………………………………… 4. Dr. Md. Saiful Islam Member Professor, IICT, BUET, Dhaka (External)

DECLARATION

I, hereby declare that this thesis is based on the results found by myself. Materials of work found by other researchers are mentioned by reference. This thesis, neither in whole nor in part, has been previously submitted for any degree.

Signature of the Candidate

…………………….

Molla Md. Zubaer

DEDICATION

To the people and creations, who are affected by radiation exposure.

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ACKNOWLEDGEMENT

First and above all, I praise the Almighty Allah, the Creator and the Guardian and to whom I owe my very existence. I am grateful to Him for providing me the opportunity and granting me the competence, fortitude and courage to proceed with my research. His constant grace and compassion was with me throughout my life and even more during this whole time of my research period.

I would like to express my deepest thanks to my supervisor Dr. Md. Forkan Uddin, Associate Professor, Department of EEE, BUET, for accepting me as his student and for the thoughtful guidance, warm encouragement, critical comments and corrections he has given me throughout my tenor of research. I am exceptionally lucky to have a caring, considerate and attentive supervisor like him. He is a man of knowledge and principles, who has enormously guided me to reach my goal successfully with my research.

I would also like to express my gratitude to the members of my thesis examination board, Dr. Quazi Deen Mohd Khosru, Dr. Satya Prasad Majumder and to Dr. Md. Saiful Islam for their excellent advices, detailed reviews and comments on my research work.

I am thankful to my colleagues who have helped and provided me sufficient support for successful completion of my research.

Most significantly, my research would not be completed without the continuous support and encouragement of my family members. I would like to express my heartfelt thanks to my family.

ABSTRACT

Radiation hazard from rapidly increasing BSs of cellular communication network is a burning issue of the day. A significant number of studies have been done on the effect of electromagnetic radiation. The number of research work on measuring the power density from mobile phone BSs is also notable whereas the research on the reduction of radiation from mobile phone BSs is insignificant. There are few ideas available to reduce radiation i.e., power control, use of distributed antenna system (DAS) and coordinated multi point joint transmission (CoMP-JT). However, there is hardly any study on how to use these techniques and how much radiation can be reduced by applying these techniques. In this thesis, the problem of reduction of radiation from mobile phone BSs is considered. For the study, a hexagonal cellular system with downlink traffic and frequency division duplex (FDD) technique is considered. First, the traditional cellular system is considered where all the BSs use the same transmission power. An analytical model is used to determine radiation exposure to the cell phone users. Applying this model in a typical cellular network, it is found that the radiation exposure from mobile phone BSs is significant. The effects of antenna height and traffic intensity on the radiation exposure are also studied. The safe distances under various standards are also determined for a given BS transmit power.

Next, power control and user scheduling problems are considered for the reduction of radiation exposure to the users. An algorithm is developed for configuring the transmit power and user scheduling to reduce the power density to the users by satisfying the required downlink traffic of the users. The performance of the algorithm is evaluated and demonstrated that the algorithm is very effective in reducing power density to the mobile phone users. It is found that the radiation density under proposed algorithm is approximately 1/106 times of that of the traditional system. Finally, CoMP-JT technique is considered to reduce radiation exposure from mobile phone BSs. An algorithm is developed to configure the transmit power and CoMP-JT scheduling to reduce radiation exposure to the mobile phone users by satisfying the traffic requirement of the users. The performance of the developed algorithm is evaluated and found to be effective. It is also found that the developed algorithm can help in the reduction of a significant amount of radiation exposure when the edge users are located at equidistance from the BSs.

CONTENTS

APPROVAL CERTIFICATE i

DECLARATION ii

DEDICATION iii

ACKNOLEDGEMENT iv

ABSTRACT v

Contents vi

List of Tables x

List of Figures xi

List of Symbols xiii

List of Abbreviation xiv

1. CHAPTER 1 : Introduction 1

1.1 Cellular Communication System 1

1.2 Base Transceiver Station (BTS) Installation and Operation 4

1.3 Frequency and power uses in cellular systems. 5

1.4 Antenna System Cellular Networks 6

1.5 Radiation Exposure From Cellular System and its Effect 6

1.6 Standards of Radiation Exposure and Practices in Different Countries 7

1.6.1 Science-based Limits 8

1.6.2 Precautionary limits 9

1.6.3 IEEE Standards 11

1.7 Radiation in Traditional Cellular System 12

1.8 Coordinated Multipoint(CoMP) 12

1.8.1 Coordinated Scheduling/Beamforming (CS/CB) 12

1.8.2 Joint Processing (JP-CoMP) 13

1.9 Reduction of Radiation in Cellular system 15

1.10 Motivation 16

1.11 Contributions 17

1.12 Outline of the Thesis 17

2. CHAPTER 2 : Related Work 18 2.1 Estimation of Radiation Exposure 18 2.2 Reduction of Radiation 19 2.3 Summary 19 3. CHAPTER 3 Radiation Exposure in Traditional Cellular System 20 3.1 Network architecture 20 3.1.1 User distribution 20 3.1.2 BS distribution 20 3.2 Traffic Model 20 3.3 Frequency distribution 21 3.4 Transmission scheme 21 3.5 Radio Frequency ( RF) Propagation Model 21 3.6 RF radiation exposure model 22 3.7 Data Rate Model 22 3.8 Analytical Model to Determine Radiation Exposure 22

3.9 Model to Calculate τ푖 23 3.10 Results 23 3.10.1 Network and system parameter 23

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3.10.2 Radiation Exposure 25 3.10.3 Variation of Radiation Exposure due to Changing the BS Antenna Height 27 3.10.4 Safe Radiation for Radiation Exposure 28 3.10.5 The Effect of Downlink Traffic on Power Density 29 3.11 Summary 31 4. CHAPTER 4: Reduction of Radiation by Power Controlling 32 4.1 System Model 32 4.2 Scheduling and Power Allocation Algorithm 32 4.2.1 Transmission Time of User 32 4.2.2 Total Transmission Time in a BS 33 4.2.3 Transmission Time Allocation to a User 33 4.2.4 Power Allocation for Transmission to a User 35 4.3 Analysis of Radiation Exposure under the Proposed Scheduling and Power Allocation Algorithm 35 4.4 Results 36 4.4.1 Comparison with the Traditional System 36 4.4.2 Comparison of Radiation of the Proposed Algorithm with another Existing Scheme 38 4.4.3 Reduction of Radiation Exposure 39 4.5 Summary 39

5. CHAPTER 5: Reduction of Radiation by CoMP-JT 40 5.1 Network Architecture 40 5.2 Selection of Users for CoMP-JT 40 5.3 Measurement of Channel Rate for Selected User without CoMP-JT 41 5.4 Measurement of Channel Rate for Selected User with CoMP-JT 41 5.5 Calculation of The Transmission Power for CoMP-JT 41 5.6 Comparison of Radiation Exposure between Individual Transmission vs . CoMP-JT Transmission. 42 5.7 Results and Discussion 42 5.8 Summary 50

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51 6. CHAPTER 6: Conclusion and Future Works 51 6.1 Conclusion 51 6.2 Future Works 52 6.2.1 Optimization of the Proposed Algorithms 52 6.2.2 Impact of Fading and Shadowing on the Proposed Algorithms 6.2.3 Impact of Sectoral Antenna and Directivity Gain on the Proposed 52 . Algorithm 6.2.4 Reduction of Radiation Exposure Based Proposed Algorithms by 52 Applying Available Modulations and Channel Capacity . REFERENCES 53

List of Tables

Table 1.1: TRX power class and RF power output

Table 1.2: Country with RF Exposure Limit

Table 1.3: Radiation exposures limits of various Organizations

Table 1.4: Radiation exposures limits of various Counties

Table 1.5: MPE Limits for Controlled Environments

Table 1.6: MPE Limits for Uncontrolled Environments

Table 1.7: Radiation exposure from Traditional Cellular system for Pt=20W and Gt=17dBi

Table 3.1: System Parameters

Table 4.1: System Parameters

Table 5.1: System Parameters

List of Figures

Fig 1.1: A Typical Cellular Network with its Subsystems.

Fig 1.2: Covering area of different types of cells.

Fig 1.3: Cell and frequency reuse

Fig 1.4 - Coordinated Scheduling/Beamforming concept

Fig 1.5 : Coordinated Scheduling/Beamforming concept

Fig 1.6: CoMP-JT Procedure

Fig 3.1: Network Scenario

Fig 3.2: Power Density of Users from their Respective BSs.

Fig 3.3: Power Density for Radiation from all the BSs

Fig 3.4: Difference of aggregated power density and power density of the respective BS. for Radiation from all the BSs

Fig 3.5: Comparison of power density at each user location considering for BS antenna height = 10m and 20m

Fig 3.6: Power density varying the distance between the user and BS with BS transmit power 320 watts.

Fig 3.7: Power density by varying the distance between the user and BS with BS transmits power 20 watts.

Fig 3.8: Power density for different traffic demand by the users for Frequency Bandwidth of 10 MHz

Fig 3.9: Power density for different traffic demand by the users for Frequency Bandwidth of 5 MHz

Fig 4.1: Comparison of Power Density to the users for system with and without power control

Fig 4.2: Reduction of Radiation Exposure by power controlling and Scheduling Algorithm

Fig 4.3: Reduction of Radiation Exposure by Power Controlling and Moderate Exposure from Conventional System

Fig 4.4:Reduction of Radiation Exposure by Power Controlling and Scheduling Algorithm

Fig 5.1: The network layout for CoMP-JT

Fig 5.2: Transmission Power in CoMP-JT Mode for Common Users of BS 9,13,14 with m=3

Fig 5.3: Comparison of Power Density for with and without CoMP-JT from BS 9, 13 and 14 for frequency Reuse Factor m=3

Fig 5.4: Transmission Power in CoMP-JT mode for Common Users of BS 9,13,14 with Frequency Reuse Factor m=1

Fig 5.5: Comparison of Power Density for with and without CoMP-JT from BS 9, 13 and 14 for freq reuse factor m=1

Fig 5.6: Transmission Power in CoMP-JT mode for Common Users from BS 9 and 5 compared to 20W in Non-CoMP-JT mode for freq reuse factor m=1

Fig 5.7: Comparison of Power Density for with and without CoMP-JT from BS 9 and 5 for freq reuse factor m=2

Fig 5.8: Distance of Common Users from BS 9 and 10

Fig 5.9: Comparison of Power for Commom Users of BS 9&10 with and without CoMP- JT

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List of Symbols

Symbol Meaning Section

W Watt 1.3

MHz Mega Hz 1.6.1

kHz Kilo Hz 1.6.3

GHz Giga Hz 1.6.3

푁 Number of users 3.1

퐵 Number of base station 3.1

푃푟 Received power 3.5

푑 Distance 3.5

훾 Path loss exponent 3.5

퐺 The antenna gain 3.5

푑0 Far field crossover distance 3.5

퐾0 Constant path loss factor 3.5

휆 Operating wavelength 3.5

The factors for the directions of horizontal 퐹(휑) 3.6 plane

퐹(휃) The factors for the directions of vertical plane 3.6

R Data rate 3.6

τ The source transmitting time per second 3.6

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List of Abbreviation

1G - First Generation 2G – 2nd Generation 3G – 3rd Generation 4G – 4th Generation 5G – 5th Generation AuC - Authentication Center BB- Base Band BS - Base Station BSC - Base Station Controller BSS - Base Station Subsystem BTS - Base Transceiver Station CB- Coordinated Beamforming CDMA - Code Division Multiple Access CoMP- Coordinated Multi-Point CS- Coordinated Scheduling DAS- Distributed Antenna System DCS -Dynamic Cell Selection EIR - Equipment Identity Register FDD- Frequency Division Duplex GSM - Global System For Mobile Communications GW- Gateway HLR - Home Location Register ICI- Inter-Cell Interference JP- Joint Processing LTE -Long-Term Evolution, alternative nomenclature 4G LTE MPE- Maximum Permissible Exposure MS - Mobile Station MSC - Mobile Switching Center NSS - Network Switching Subsystem xiv

OSS-Operation and Support Subsystem RAN – Radio Access Network RF- Radio Frequency RRH- Remote Radio Head SINR- Signal-to-Interference-plus-Noise-Ratio SNR- signal to noise ratio SIM - Subscriber Identity Module TDD- Time Division Duplex TDMA- Time Division Multiple Access UE - User Equipment VLR - Visitor Location Register

CHAPTER 1

INTRODUCTION

1.1 Cellular Communication System

In today’s world the number of cell phone subscribers has crossed 5.2 Billion [1]. As the demand for wireless communication increased with time, cell based communication came into play to meet the shortfall of limited resources through reuse technique. Cellular network is simply a number of small power transmitters instead of a single high power transmitter, where the small power transmitter covers a certain short range, called cell. Frequency reuse methods are used to achieve maximum useful narrow radio spectrum. In cellular radio networks, a short range is covered by one base station, where other base stations are installed with short corresponding ranges. Neighboring cells require different frequencies to avoid co-channel interference; however, same frequency can be used in distant cells [2]. The capacity of the entire network is maximized by splitting the whole coverage range into several small hexagonal cells. This also, minimized the frequency reuse [2]. However, with the development of digital signal processing, modulation and multiple access technique, the same frequency is also being used in the neighbouring cells as found in CDMA, 3G and 4G systems.

HLR AUC GW

NSS EIR MSC VLR

BSC BSC

BSS BTS

Cell 1

Cell 2 Cell 4 Cell Cell 3

Cell 5 Cell 6 MS Cell 7

Fig 1.1: A Typical Cellular Network with its Subsystems [3]

1.1.1 Cellular Network Subsystem

A typical cellular network consists of the following subsystems [3] [4]. . Network and Switching Subsystem (NSS) . Base-Station Subsystem (BSS) . Mobile station (MS) . Operation and Support Subsystem (OSS)

The subsystems are illustrated in Fig. 1.1.

Network Switching Subsystem (NSS)

The cellular system architecture (like GSM) provides the main control and interfacing for the whole mobile network. The key components of the core network are discussed below [3] [4]:

(a) Mobile Services Switching Centre (MSC) This acts alike typical switching node within a PSTN or ISDN, moreover it delivers further functionality to support the requirements of a mobile user, such as, registration, authentication, call location, inter-MSC handovers, call routing to a mobile subscriber etc.

(b) Home Location Register (HLR) This database contains all the administrative information about each subscriber including their last known location. Thus, the cellular network is able to route calls to the relevant BS for the MS.

(c) Visitor Location Register (VLR) This contains selected information from the HLR that assists the selected services for the individual subscriber to be afforded.

(d) Equipment Identity Register (EIR) The EIR is the entity that decides whether a given mobile equipment may be allowed into the network. Depending upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems.

(e) Authentication Centre (AuC) This is a protected database that contains the secret key also contained in the user's SIM card. It is used for authentication and for ciphering on the radio channel.

(f) Gateway (GW) This interface connects the cellular network with the external world and with other systems. The gateway can be of different types like voice or data to the Internet.

Base Station Subsystem (BSS)

The Base Station Subsystem (BSS) section of the cellular network architecture consists of two elements [3] [4]:

(a) Base Transceiver Station (BTS): This contains the radio transmitter receivers, and their associated antennas that transmit and receive data to directly communicate through mobiles. BTS is also called Base Station (BS).

(b) Base Station Controller (BSC): It controls a group of BTSs, and is merged frequently with one of the BTSs in its group. It covers the radio resources and controls handover within the group of BTSs.

Operation and Support Subsystem (OSS) It is used to control and monitor the overall cellular network. Also, helps to control the traffic load of the BSS. However, the number of BS rises with the scaling of the subscriber population.

Mobile Station Mobile station (MS), or user equipment (UE) or cell phone is the end user equipment that connects the users with the system. It is used in RAN to make air interface with the BS. This is nothing but a small radio equipment with Subscriber Identity Module (SIM) card. It has many features to interact with the cellular network and make voice call and exchange data. The SIM contains the information that gives identity of the user to the network.

1.2 Base Transceiver Station (BTS) Installation and Operation

BTS or BS is the main functional block to connect the UE with the cellular network. From the core network, the BTS provide necessary information of the network through radio channel so that the mobile stations can get registered with cellular network. A typical FDD BTS works with two types of frequencies that is the uplink frequency and downlink frequency [2]. The downlink frequency is used to send the information from the BTS to the mobile phones and the uplink frequency is used to send information from the mobile phones to the BTS.

Each BS covers a certain area. Based on the coverage, the cell area can be categorized as macro, micro, pico, nano and femto cells [5]. In macro cell the BTS equipment has the highest RF output power thus covers the largest area, micro BTS las a little lower power and cover smaller area. Similarly the pico, nano and femto cells have the BS with smaller power. Based on the need of coverage and capacity, the Macro BTS are used in rural areas and micro BTS are used in urban areas [6]. The Fig 1.2 shows the cell coverage area for different places.

Urban zone

BS BS

BSC MSC

Internet

Base station (BS)

Rural zone

Fig 1.2: Coverage area of different types of cells [6].

With the development long term evolution (LTE), BS has better facilities [7] [8] [9]. An LTE BSs has two parts i.e. the remote radio head (RRH) and the base band (BB) unit. It provides a better coordination and facility to manage the radio channels for efficient communication.

Radio Channels of BS

There are many physical and logical channels in radio access network (RAN). These are mainly categorized into control channels and traffic channels [4]. Control channels are mainly used for cell broadcast, synchronization, call processing and controlling of the radio parameters. Traffic channels are used for passing voice, video and data. Training Sequence Bits in control channels are used for synchronization of the MS channels with BS radio channels. For various BTSs, the maximum output power per carrier measured at the antenna connector after all stages of combining shall be, according to its class, defined in the following Table 1.1 [4].

BTS (GSM 900) BTS (DCS 1800)

TRX Maximum TRX Maximum

Power class Output power Power class Output power

1 320 - (<640)W 1 20 - (<40)W

2 160 - (<320)W 2 10 - (<20)W

3 80 - (<160)W 3 5 - (<10)W

4 40 - (<80)W 4 2.5 - (<5)W

5 20 - (<40)W 6 10 - (<20)W 7 5 - (<10)W 8 2.5 - (<5)W

Table 1.1: TRX power class and RF power output

1.3 Frequency and Power Uses in Cellular Systems

Cellular networks are completely based on the technique of frequency reuse, so that the narrow radio spectrum will get maximum use. In cellular radio networks, small area is covered by one BS and other BSs are installed with small overlapping areas. Neighboring cells require using different frequencies to evade interference, but the same frequency can be reused in distant cells. However same frequency band is being used in neighbouring cells in CDMA and 4G systems. In 4G system, the frequency band is divided in several resource blocks and those resource blocks are used in such a way that interference is within a limit. The typical frequency reuse and cell

positions are illustrated in fig 1.3. The entire coverage area is spited into many small hexagonal cells so as to increase the capacity of entire network and a decrease in the reuse of frequency [5].

F1 F2 F1 F2 F1 F1 F1 F1

Cells in 3G/4G System with frequency reuse Cells in 2G System with frequency reuse

Fig 1.3: Cell and frequency reuse [5]

1.4 Antenna System Cellular Networks

The original 2-way-radio cell towers were at the centers of the cells and were omni-directional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge [5]. Each tower has three sets of directional antennas aimed in three different directions with 120 degrees for each cell (totaling 360 degrees) and receiving/transmitting into three different cells at different frequencies. This provides a minimum of three channels (from three towers) for each cell. The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas.

1.5 Radiation Exposure From Cellular System and its Effect

Researchers found human beings are bio electrical systems, so the environmental exposures to EMF may fuse with fundamental biological routes in the human body and may lead to diseases, discomforts as reported in literature [10] [11] [12]. While talking through mobile phone, a high frequency signal modulated at certain low frequency i.e. pulsed may have harmful effects than an un-modulated study carrier. Modulation signals are vital components in the delivery of EMF signals to which cells, tissues, organs and individuals can respond biologically. Hence, modulation signals may interfere with normal, nonlinear biological function. However, various actions have been taken to address mobile phone radiation and health issues from time to time as the use of mobile phone became very popular [13] [14]. The radiation emitted by the fixed

infrastructure used in mobile telephone is more powerful at local ranges as the field intensities drop rapidly with the increase in distance from the base of the antenna.

The effects of EMF radiation can be two types [13] [14]:

(i) Bio effects- these are measureable responses to a stimulus or to a change in the atmosphere and are not necessarily harmful to our health.

(ii) Health effects- these are the changes which may be short term or long term. These effects stress the system and may be harmful to human health.

These effects can be broadly divided into thermal and non-thermal effects [13] [14].

Thermal Effects

The thermal effect has been largely referred to the heat that is generated due to absorption of EMF radiation. In the case of a person using a cell phone, most of the heating effect occurs at the surface of the head which leads to an increase in body temperature. Thermal effects from BS radiation are not very dangerous as the heat generation is not that significant after a certain distance from the BS where the users are usually located [13] [14].

Non-thermal Effects

The Non-thermal effects are attributed to the induced electromagnetic effects inside the biological cells of the body which is possibly more harmful [13] [14]. Ranging from tingling sensation in the head, fatigue, sleep disturbance, dizziness, lack of concentration, ringing in the ears, reaction time, loss of memory, headache, disturbance in digestive system and heart palpitation etc are the examples of non-thermal effects. There is another type of effect called genotoxical effect which includes impairment to chromosomes, alterations in the activity of certain genes and a boosted rate of cell division [13].

1.6 Standards of Radiation Exposure and Practices in Different Countries

RF radiation limits in the United States, most Western European countries, and many countries in other parts of the world follow IEEE C95.1-1999. Those in the Russian Federation, China, Switzerland, and a few other countries are as much as a hundred times lower. These limits are two types: science based and precaution based [15].

1.6.1 Science-based Limits

US including most Western European countries have defined radiation limits based on IEEE C95.1-1999 and ICNIRP standard. These limits were centered on a broad evaluation of scientific literature to classify the possibly hazardous effects and their thresholds. The hazards that are classified in the documentation are mostly thermal in nature [15]. The “average times” in the limits are short (6 to 20 minutes) Table 1.2 below compares three different exposure limits for RF energy at 2000 MHz (similar to that used by many cellular telephones throughout the world). Few of the limits are for long-term exposure to the general population [15].

Limit for general public Country exposure to Basis RF fields (2GHz) ICNIRP (adopted in numerous countries worldwide) 10 Science-based U.S. Federal Communications Commission (FCC) Bulletin 65, “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic 10 Science-based Fields”, Washington DC 1997. Generally follows IEEE C95.1-1999 with some modifications India – 2012 1 Science-based China, UDC 614.898.5 GB 9175 –88 0.1 Science-based Russia 0.1 Science-based Sanitary Norms and Regulations 2.2.4/2.1.8.055-96 Switzerland Ordinance on Protection from Non-ionizing Radiation 0.1 Precautionary (NISV) of 23 December 1999 Typical Maximum Exposure from Cellular Base Station 0.01 Mounted on 50m tower (assuming a total effective radiated power of 2500 watts in each sector, summed over all channels

Table 1.2: Country with RF Exposure Limit [14] [15] [16]

The Russian and several Eastern European limits are evidently not designed essentially to protect against thermal hazards. One Russian authority specified that the limits of the Russian Federation for RF exposure at the frequencies used by wireless communications were set on the basis of biological experiments that found that a 3-hour daily exposure at 250 mW/cm2 (950 MHz) could

be considered as a threshold for harmful physiological effects in experimental animals [15]. Hence, Russian limits noticeably reflect the conviction that long-term (hours or more) exposures at levels far below Western limits result in adverse health effects. Indeed, the Russian and Eastern European medical literature contains many reports of health effects from low-level exposure to RF energy. These include, for example, nonspecific problems (such as headaches, fatigability, irritability, sleep disorders, and dizziness) in workers in radio factories, who are exposed to RF energy at undetermined levels. The Chinese literature contains similar reports [15].

1.6.2 Precautionary Limits

Lately, Italy, Switzerland, and a few other countries have set exposure limits that are based on a totally different approach, the precautionary measures [15]. Unlike Russian limits, the Swiss limits were, in the words of an advisory detail associating the limits, “specifically intended to minimize the yet unknown risks” of RF and power-frequency electromagnetic fields. Practically, it aimed to the dropping of ICNIRP limits by a factor of 10 (in field strength) or 100 (in power density) [15].

Various limits of radiation exposures at 900MHz and 1800MHz are given in Table 1.3 and 1.4.

900 MHz 1800 MHz Electric Power Electric Power Organization Document field density field density (V/m) W/m2 (V/m) W/m2 International health based guidelines

International commission of ICNIRP 1998 41.25 4.5 58.3 9.0 Non-ionizing radiation protection

International/ Institute of Electrical and Electronics IEEE,1999 USA 47.6 6.0 67.3 12 Engineer

European/ European Committee for Electro technical CENELEC,1995 41.1 4.5 58.1 9.0 Standardization (Technical committee)

Table 1.3: Radiation exposures limits of various Organizations [13] [14] [16]

900 MHz 1800 MHz Country or Electric Power Electric Power Document Organization field density field density (V/m) W/m2 (V/m) W/m2 National health based guidelines

Australia/ Standard AS/NSZ, 1998 27.5 2.0 27.5 2.0 Association of Australia

East European health based guidelines

Hungary/ Hungarian Hungary, 1986 6.1 0.1 6.1 0.1 Standard Institution

National guidelines based on precautionary approaches

-- Belgium 20.6 1.1 30 2.4

Italy/ Ministry of Environment Italy 1, 1998 20 1.0 20 1.0

Italy/ Ministry of Environment Italy 2, 1998 6 0.1 6 0.1

Switzerland/Schweizer NISV, 1999 4 0.04 6 0.1 Bunndesrat

Local recommendations, based on precautionary approaches

Austria Local S vorGW 1998 0.6 0.001 0.6 0.001

Table 1.4: Radiation exposures limits of various Counties [14].

1.6.3 IEEE Standards

To address RF radiation exposure, Institute of Electrical and Electronics Engineers (IEEE) has defined two main standards. One of these is IEEE C95.1–2005 and is associated with the human exposure standard. It is named as “Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz”. The other one is IEEE C95.3–1999, which defines the measurement practices standard [17].

Related limits of IEEE C95.1–2005 are shown in Table 1.5 and 1.6.

Maximum Permissible Exposure (MPE) Limits for Controlled Environments Frequency (MHz) Power Density (W/m²) 0.1–1.0 9,000 1.0–30 9,000/f² 30–300 10 300–3,000 (cellular communication in this range) f/30 3,000–300,000 100

Table 1.5: MPE Limits for Controlled Environments [17]

Maximum Permissible Exposure (MPE) Limits for Uncontrolled Environments Frequency (MHz) Power Density (W/m²) 0.1–1.34 1,000 1.34–30 1,800/f² 30–400 2.0 400–2,000 (Cellular Communication) f/200 2,000–100,000 (LTE) 10 100,000–300,000 Increases from 10 to 100

Table 1.6: MPE Limits for Uncontrolled Environments [17]

1.7 Radiation in Traditional Cellular System

In traditional cellular system, the radiation to a user can be found based on type of transmission power use, height of antenna, directivity, distance, scheduling, fading, pathloss and related parameters. A typical free space scenario is given in Table 1.7.

Distance (m) P퐨퐰퐞퐫 퐃퐞퐧퐬퐢퐭퐲 (W/퐦ퟐ) 1 79.7 3 8.86 5 3.19 10 0.797 20 0.199 50 0.0319 100 0.0079 500 0.0003189

Table 1.7: Radiation exposure from Traditional Cellular system for Tx power=20W and Antenna Gain=17dBi

This measurement is done based on free space path loss formula. The calculation is done for a single carrier and a single operator. The above values will increase many times for multiple operators and multiple carriers including 2G/3G/4G on the same tower or rooftop.

1.8 Coordinated Multi-Point (CoMP)

The crux theme of (Coordinated Multipoint) CoMP is to allow geographically separated BSs to cooperate in serving the UEs, which may or may not belong to the same physical cell; thus minimizing the inter-cell interference (ICI) which is typically the primary source of interference. In a downlink scenario, two main transmissions schemes are considered for CoMP namely Coordinated Scheduling/Beamforming (CS/CB) and Joint Processing (JP) [18] [19].

1.8.1 Coordinated Scheduling/Beamforming (CS/CB)

For CS/CB , data for UE is only available at the serving cell where, radio beams signal processing are formed to enhance the signal strength of its serving UEs while focusing on

eliminating the ICI with null steering towards UEs from neighboring . As depicted in Fig. 1.4, BS1 forms the radio beam toward UE1 then, in order to reduce the interference to UE2 served by BS2, BS1 forms the null steering toward UE2.

Null steering Beamforming

BS1 UE1 UE2 BS2

Fig 1.4 - Coordinated Scheduling/Beamforming concept [18] [19].

1.8.2 Joint Processing (JP-CoMP)

Joint processing is categorized into Coordinated Dynamic Cell Selection (DCS-CoMP) and Joint Transmission (JT-CoMP). In JP-CoMP, data for UE is available at more than one cooperating BS.

(a) Dynamic Cell Selection (DCS-CoMP)

For DSC-CoMP, the coordination of the scheduling decisions is made among all cooperating BSs set. A UE can reselect dynamically another serving cell based on the highest received Signal-to- Interference-plus-Noise-Ratio (SINR) and minimum path loss. When a UE reselect a cell, its resource is transmitted from the first serving cell to one cell among the coordinated cells. The first serving cell resource is muted in order to transmit UE Resource Block (RB) to the second serving cell. In this process, data transmit occurs only by one BS at the time [18].

As depicted in Fig. 1.5, UE1 is initially served by BS1 at the resource block RB1 (i.e. the minimum resource in an LTE network that can be assigned to a UE), can balance its resource from BS1 to BS2 (i.e. from RB1 to RB2) according to the conditions of decision of DSC-CoMP mentioned above.

RB1

RB2

Fig 1.5 : Coordinated Scheduling/Beamforming concept [18] [19].

(b) Coordinated Multi-Point with Joint Transmission (CoMP-JT)

Due to the current trend towards smaller cells, an increasing number of users of cellular networks reside at the edge between two cells; these users typically receive poor service as a result of the relatively weak signal and strong interference [18]. Coordinated Multi- Point (CoMP) with Joint Transmission (JT) is a cellular networking technique allowing multiple Base Stations (BSs) to jointly transmit to a single user. This improves the users’ reception quality and facilitates better service to cell-edge users. In fig 1.6, a CoMP-JT is shown where all the neighbouring BSs of the edge users are being served jointly.

Fig 1.6: CoMP-JT Procedure [20]

1.9 Reduction of Radiation in Cellular System

Radiation is a burning issue in these days [21]. With the rise of mobility support of Internet, the increased numbers of BSs are triggering more and more radiation [22]. Around the globe, a good number of scientists and researchers are working to make arrangements so that the radiation from cellular BS can be reduced. With advancement in digital technology and modulation, the newer generations of cellular systems have more power efficient technology and thus transmit less power compare to that of the older system. However the technique for mitigation of radiation exposure is not up to the desired standard. Use of highly sectorial antennas, directional antennas and smart antennas are playing a vital role to reduce exposure, specially to the unwanted user in cellular communication. More so use of distributed antenna system (DAS) and deployment of smaller cells can also mitigate the hazard of radiation. Reduction of RF power in an efficient way from the BTS is yet to be discovered to mitigate the RF exposure. So it's high time to ponder on this issue and make an out for the mitigation of the radiation exposure.

In 4G/LTE system, BS or eNodeB has a very advanced feature for the management of the radio resources. Coordinated multipoint transmission and coordinated multiple joint transmissions specially for the edge users have a very good prospect in reduction of radiation exposure. It is to be kept in mind that the radio signal sent to the distant user cause more hazard to the nearer users though the signal is not intended for the nearer users. Thus reduction of radiation for the distant users will definitely benefit the nearer users.

The farthest radiation reaching to the edge users has a good amount of path loss. To use a modulation technique with higher spectral efficiency a good SNR is needed. To get good action or more or if power is demanded causing more radiation hazards. However the radiation can be reduced if more time is allotted for transmission with smaller signal to noise ratio so that modulation that can tolerate error can be used.

1.10 Motivation

Recent development in Information and Communication Technology (ICT) has brought great changes in our life style. Cell phone communication system with voice, data and video facility has made our life easy and fast. More so, use of mobile Internet is growing exponentially. As such, operators have enhanced their cell phone network capacity installing more number of BS. This has and will result in increase of RF power density, mostly in city areas. For optimum utilization of telecom resources infra-structure sharing ie towers are being shared by many operators. Though it is good for economy but on the contrary, there is a possibility of concentration of more RF power.

It is encouraging that more and more people around the globe are being sensitized about the RF radiation hazards. So it is high time to find out ways and means to minimize the radiation exposure and thus serve the creations of Almighty. There are couples of ideas in reducing radiation exposure that is power control, uses of DAS and CoMP- JT. However there is no study on how much radiation can be reduced by these techniques. In this research, an endeavour is made to provide algorithms for power controlling, scheduling, CoMP-JT and analyze the performance of the algorithms.

1.11 Contributions

In this research, downlink transmission with FDD technique is considered for cellular networks. Various techniques and algorithms are proposed to reduce radiation exposure from mobile phone BSs. The contributions are:

 A model is provided to calculate radiation exposure in traditional cellular system. The model is used to determine power density at the mobile users for a network scenario. The result shows that the radiation exposure is significant in traditional cellular networks.

 A power control and scheduling algorithm is developed to reduce the power density at the mobile users by maintaining the traffic demand of the users. The performance of the algorithm is evaluated for the network scenario and found to be very effective.

 Considering CoMP transmission scheme, an algorithm is developed to determine transmission power level and CoMP transmission scheduling for the different users. Evaluating the performance of the developed algorithm it is found that it can reduce radiation exposure significantly.

1.12 Outline of the Thesis

The remaining part of the thesis is arranged in the following manner. In Chapter 2, a literature review on estimation and reduction of radiation is discussed. Chapter 3 discusses the analytical procedure for measurement of radiation exposure in traditional cellular system. Then radiation to a user for various network and system parameters is calculated by considering the radiation from corresponding and neighbouring BSs. Chapter 4 is about reduction of radiation by power controlling and user scheduling. In this chapter, an algorithm for power allocation and user scheduling is proposed based on allocating the idle time to the users. Then the performance the proposed algorithm is evaluated and compared with the traditional cellular system. In Chapter 5, CoMP transmission technique is considered. First, the criteria for selection of users for CoMP-JT are established. Then the algorithm for power control and CoMP transmission scheduling is described and the performance of the proposed algorithm is evaluated. In Chapter 6, the research work is concluded and the scopes of future works are discussed. 18

CHAPTER 2

RELATED WORK

The related work can be classified as: (i) estimation of radiation exposure and (ii) reduction of radiation exposure.

2.1 Estimation of Radiation Exposure

A significant number of researches have been carrier out to measure/estimate the radiation exposure from the mobile phone BSs. In [23], the authors propose a theoretical prediction model of electromagnetic radiation at multisystem BS. This is based on the distribution characteristic of electromagnetic power for the BS antenna. The antenna characteristics parameters such as the normalized directivity function, antenna gain, gain of the array element, shaped gain so on, the distribution of its power density of the multi radiation source in multi system BS is calculated. In [24], the authors propose for developing a specialist software tool that can gauge the RFR our concentration within a particular area. The tool encompasses among many, the capability of facilitating as well as ensuring that the RFR monitoring activities and planning can be carried out efficiently and effectively.

In [25], the authors summarize two studies in which measurement and calculation methods to determine the exposure of the general public around GSM and UMTS BSs have been developed and applied to different scenarios. The electromagnetic field variations around the stations in space and time are accounted for by appropriate maximization techniques. Measurements show a band width of exposure from 0.01% to more than 10% of field strength exposure limits. The distance to the station is not a main influencing factor, whereas the orientation to the main lobe and the sight conditions greatly influence exposure. In [26], a measurement report on electromagnetic radiation from BSs antenna was carried out by the appropriate authority of Bangladesh Telecom Regulatory Commission. While measuring the radiation exposure level in different area of Dhaka city, they used a RF meter made by “Narda” model SBM-560. The measurement results shows that the RF exposure density for each operator is within the limit of WHO standard but some are about to touch the top limit. In [27] the authors proposed radiation measurement using the Quad Phone mounted on the Quadcopter which provides an excellent monitoring system for auditing the Electromagnetic Radiation (ER) and subsequently determine the Electromagnetic Pollution Index (EPI) from the delineated pockets of pollution regions.

19 2.2 Reduction of Radiation

Globally discussed hazards caused by the RF radiation exposure from the BSs of cellular communication network have become a serious concern to many. To address this, IEEE has set its standard C95.1 and International Commission on Non-Ionizing Radiation Protection (ICNIRP) has published its guidelines which have been endorsed by World Health Organization (WHO) [28]. There has been a limited study on reduction of the radiation exposure from the BSs of cellular networks. In [29], the authors provide an overview of the techniques to reduce the EM radiation, e.g., power control, beamforming, and Coordinated Multi-Point transmission etc. In [30], the authors propose a multi-layer overlaid hierarchical network architecture with macro, micro, pico and femto cells to reduce radiation. They proposed macro for area coverage, micro for pedestrian and a slow moving traffic, pico for indoor coverage and femto for individual high capacity users. They also proposed to replace the sectoral antennas with intelligent adaptive antenna. In [31], the authors show that by controlling the height of BSs, reducing the vertical radiation pattern down-tilt, changing the HRP, increasing the antenna gain, the radiation exposure can be reduced. They also proposed to apply multiple methods. In [32], the authors propose Electromagnetic Bandgap Structures in aperture to reduce radiation. In [33], the authors used a WHIPP path loss prediction tool to minimize radiation in several networks. Power control at the BSs is considered as one of the promising techniques to reduce the radiation exposure [31]. In [34], the authors propose a wide bandwidth and wide beamwidth L-probe-fed patch antenna array with a novel design of grounded structure is proposed and tested. The antenna is made of stacked patches supported by plastic screws. The patches are proximity fed via L-shaped probe. By cutting slots in two vertical side walls of a box-shaped grounded structure, an impedance bandwidth larger than 20% (SWR <1.5), an H-plane beamwidth over 90° and much reduction in back lobe radiation can be obtained. Details of the proposed antenna, simulation, and experimental results are presented and discussed. In [35], the author discusses about the cell zooming technique and BSs sleep mode technique for overall power reduction.

2.3 Summary

As said in the above paragraphs, many authors have proposed different techniques for reduction of RF exposures. However, from the existing research works it is not clear that how to configure the power of the BSs to minimize radiation exposure. CoMP Joint Transmission (JT) is a promising technique to increase the edge user throughput and coverage [36] [37]. It is expected that CoMP-JT technique will reduce the radiation exposure. Unfortunately, there is no clear indication on how to do CoMP-JT to minimize radiation exposure. 20

CHAPTER 3 RADIATION EXPOSURE IN TRADITIONAL CELLULAR SYSTEM

In this chapter, considering a traditional cellular system, the radiation exposure from mobile BSs of the cellular system is determined and various engineering insights are provided.

3.1 Network Architecture

The cellular network is considered to have 푁 users and 퐾 BSs. The set of the users is denoted by 푁 and it is stated that 푁 = {푢1, 푢2, … … … 푢푛} . The set of the BSs is denoted by

ℬ and it is stated that ℬ= {푏1,푏2 … … … 푏퐾,}. The set of users which are associated with the

BS 푏푖 ∈ ℬ is denoted by 푁푖 that is 푁 = 푁1 ∪ 푁2 ∪ … . 푁퐾 and 푁 = |푁1| + |푁2| +

⋯ |푁퐾|. The distribution of users and the BSs over an area are described below:

3.1.1 User Distribution

The users are distributed in quasi random sequence. This type of sequence is very convenient in computational problem. In general the users’ mobile phones are put at a height of 1 m from the ground level.

3.1.2 BS Distribution

It is assumed that BSs are distributed according to the hexagonal pattern. However in practical case the distribution of the BSs may not be hexagonal. For simplicity, the hexagonal structure is assumed which provides maximum coverage in cellular system. Usually the antennas of the BSs are put at a height of 20 m from the ground level.

3.2 Traffic Model

Only the downlink traffic that is the traffic from BSs to the users is taken into consideration. A frequency division duplex (FDD) system is taken such that the uplink traffic that is the traffic from the users to the BSs can be separated from downlink. For simplicity it is assumed that for each user, the downlink traffic rate is S and which is generated at constant bit rate.

3.3 Frequency Distribution

It is assumed that the BSs use different frequency bands. A frequency band is reused according to the traditional system. The cluster size of the system is kept sufficiently large such that co channel interference is very low. The bandwidth of the frequency band allocated per BS for downlink transmission is B Hz.

3.4 Transmission Scheme

As mentioned before, the uplink and downlink transmissions are separated and independent by a FDD system. Since BSs use the different frequency bands, the downlink transmissions of the BSs are also independent. For downlink transmission a BS uses TDMA scheme where the time is divided among the users. In traditional system, the time is divided among the users according to round robin fashion. It is assumed that all the users of a BS obtain required time for data reception and also the transmission powers for all the users are the same.

3.5 Radio Frequency ( RF) Propagation Model

In typical scenario, free spaces are rarely available to the cellular phone users. So various path loss models are used considering the region. It is assumed that the received power 푃푟 at distance d is proportional to 푑−휂 where 훾 is the path loss exponent. The value of depends on the region and application scenario. If the transmit power of a transmitter is 푃 the received power at distance d is given as [5] [6]

푑 −휂 푃푟 = 푃퐾0퐺 ( ) ………………………………………..(3.1) 푑0 where, 퐺 is the antenna gain, 푑0is the far field crossover distance and 퐾0 is a constant path loss factor given as

휆 2 퐾0 = ( ) ……………………………………………..(3.2) 4휋푑0 for operating wavelength 휆. For simplicity of analysis, fading and shadowing is not considered in current propagation model. Fading and shadowing will change the instantaneous power densities to the users. However, long term radiation exposure may not be affected by fading and shadowing.

3.6 RF Radiation Exposure Model

If a source transmits at power at 푃 , the power density at distance d can be expressed as 푃퐺 푃 = ……………………………………………. (3.3) 푑 4휋푑2

The above model is suitable for far field with direct wave. Let 퐹(휑) and 퐹(휃) be the factors for the directions of horizontal and vertical plane. Considering antenna direction and tilting factors the power density can be written as

푃 퐺 푃 = 퐹2(휑) 퐹2(휃) … … … … … … … … … … … … … … . . (3.4) 푑 4휋푑2

The above formula is applicable when the source continuously transmits with power 푃 . If the source transmitting time per second is τ, the power density can be written as

τ푃 퐺 푃 = 퐹2(휑) 퐹2(휃) … … … … … … … … … … … … … … … . (3.5) 푑 4휋푑2

3.7 Data Rate Model

In this case, Shannon capacity formula to calculate data rate of a transmission is considered. If the distance between a BS and a user is d, the signal to noise ratio (SNR) is given by [38]

푑 −ɳ ( ) 푆푁푅 = 푃퐾 퐺 푑0 …………..……………………………(3.6) 0 푊퐵 where 푊 is the background noise per Hz and 퐵 is the bandwidth in Hz. The data rate of the transmission between the BS and the user is given as

푅 = 퐵. 푙표푔2(1 + 푆푁푅) … … … … … … … … … … … … … … … ….( 3.7)

3.8 Analytical Model to Determine Radiation Exposure

The power density at each the user location is needed to be determined. It is assumed that the people reside in the cellular network have cell phones with them.

Let the distance between a user uj∈N and a BS bi∈ℬ is di,j . The power density to the user uj only for the BS bi is given as

τ푖푃 퐺 2 2 푃푑 = 2 퐹 (휑 푖,푗) 퐹 (휃 푖,푗) ………..…………………….(3.8) 4휋d푖,푗

Where 휏푖 is the fraction of time of transmission by the BS bi and 퐹(휑 푖,푗) and 퐹(휃 푖,푗) and are the corresponding direction and tilting factors of BS bi and user uj. The total radiation power density to the user uj can be written as

τ 푃퐺 ∑ 푖 2 2 푃푑 = 푏푖 ∈ℬ 2 퐹 (휑 푖,푗) 퐹 (휃 푖,푗) …………………………..(3.9) 4휋d푖,푗

Thus, model to calculate the value of the 휏푖 for each of the BS is left open.

3.9 Model to Calculate 훕풊

Let 푅푖,푗 be the data rate between the BS bi and the user uj∈Ni . The value of 푅푖,푗 is given as 푑 −ɳ 푃퐾 퐺 ( 푖,푗) 0 푑 푅 = 퐵. 푙표푔 1 + 0 ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ (3.10) 푖,푗 2 푊퐵 ( ) where 푑푖,푗 is the distance between the BS bi and the user uj. Since the traffic demand for the user uj is S, the fraction of time for the user uj is S/ Ri,j .The aggregated fraction of time for the BS bi is then given as

τ푖 = 푆 ∑푗∈푁푗 1/ R푖,푗 ……………………………………………(3.11)

3.10 Results

3.10.1 Network and System Parameter

A cellular network consisting of 16 BSs in 2000 m X 2000 m area is considered. The 360 users are distributed quasi randomly in the area. The network is shown in Fig 3.1 where the users in different BSs are labeled as Utot. MATLAB tool is used to generate the network scenarios as well as calculate radiation exposure. The other system parameters taken during calculation are given in Table 3.1. Note that for simplicity 퐹(휑) = 1 and 퐹(휃) = 1 are considered.

2500

4 BS#4 Utot=8 12 BS#12 Utot=8 2000

8 BS#8 Utot=22 16 BS#16 Utot=20

1500 3 BS#3 Utot=22 11 BS#11 Utot=29

7 BS#7 Utot=26 15 BS#15 Utot=21

1000 2 BS#2 Utot=21 10 BS#10 Utot=25 Width Width in (m) 6 BS#6 Utot=26 14 BS#14 Utot=23

500 1 BS#1 Utot=28 9 BS#9 Utot=31

5 BS#5 Utot=28 13 BS#13 Utot=22

-500 -500 0 500 1000 1500 2000 2500 Length in (m)

Fig 3.1: Network Scenario

Parameter Notation Value

Transmitting Power P 20W

Traffic Rate S 1Mbps

Gain of Antenna G 17 dBi or 50 Times

Frequency Band F 1800 MHz

Far Field Cross Over Distance d0 12 m

Frequency Bandwidth B 10 MHz Path loss Exponent γ 3 Noise/Hz 푊 -174 dBm Antenna Height h 20 m

Table 3.1: System Parameters 25

3.10.2 Radiation Exposure to the Users

At first, the radiation exposure is calculated considering radiation from the associated BS only. The power density at different users location obtained by calculation is depicted in Fig. 3.2. The results show that power density varies significantly from one user to another. The total radiation density at the different users for all the BSs transmission is shown in Fig. 3.3. It can be seen that the power density is within the limit of ICNIRP or IEEE standards but not to other standards that are followed by many countries like Switzerland or Austria local. Next, the difference between the total power density and the power density to the respective BS is calculated. The difference of the power densities for each user is shown in Fig. 3.4. The results show that the difference is not significant. It means that the power density only from the nearest BS is significant and hence it implies that calculating the radiation exposure of a user from its associated BS may be sufficient.

Bar showing Power Density at users location from respective BSs For Pt=20W 0.06

0.05 )

2 0.04

0.03

0.02 Power densityPower (W/m

0.01

0 0 50 100 150 200 250 300 350 400 User Identity Number

Fig 3.2: Power Density of Users from their Respective BSs.

Bar showing Sum of Power Density at each user location from all BSs with Pt=20W 0.06

0.05 )

2 0.04

0.03

0.02 Power densityPower (W/m

0.01

0 0 50 100 150 200 250 300 350 400 User Identity Number

Fig 3.3: Power Density for Radiation from all the BSs.

Bar showing Differences of Power Density at each user location -5 x 10 from all BSs vs Respective BS with Pt=20W 8

2 5

Power in W/m Power 3

0 0 50 100 150 200 250 300 350 400 User Identity Number

Fig 3.4: Difference of aggregated power density and power density of the respective BS.

3.10.3 Variation of Radiation Exposure due to Changing the BS Antenna Height

The radiation exposures shown in the earlier are calculated for antenna height of 20 m. The distance between the users and BS decrease with decreasing the antenna height. Thus, the radiation exposure will increase for decreasing the antenna height. However, to understand the effect of antenna height, quantitative results are necessary. As a result, calculation is done for the power densities to the users of the BS considering antenna height 10 m. The positions and the transmit power levels and the other parameters are not changed. The comparison of the power densities for different antenna height is shown Fig. 3.5. The results show that the increments of power densities are not the same for all the users. The users who experience lower radiation exposure, the increments of power density for those users are not significant. On the other hand, the users who experience higher radiation exposure, the increments of power density for those users are very significant.

0.16 Antenna Height = 20m Antenna Height = 10m 0.14

0.12 ) 2 0.1

0.08

0.06 Power Density (W/m PowerDensity

0.04

0.02

0 0 50 100 150 200 250 300 350 400 User Index

Fig 3.5: Comparison of power density at each user location considering the BS antenna height 10 m and 20 m.

It can be attributed to the fact that the distance between a user and the BS decreases significantly with decreasing the antenna height when the user is closer to the BS. On the 28 other hand, the distance between a user and the BS does not decrease significantly with decreasing the antenna height when the user is far away from the BS. The results show that the radiation exposure becomes approximately double for the worst user for decreasing the antenna height 20 m to 10 m. Thus, there is a significant negative impact in reducing the BS antenna height.

3.10.4 Safe Distance for Radiation Exposure

It is very important to know the safe distance of a user from mobile phone BSs. To determine the safe distance of a user from the mobile phone BS, the power density is determined by varying the distance of the user from the BS. The height of the antenna is kept fixed to 20 m. The power density with respect to the distance is shown in Figures 3.6 and 3.7 for BS transmit power 320 watts and 20 watts respectively. The power density limits according to the standards of ICNIRP, India and China are also shown in the figures.

10 Power Density 9 ICNIRP Limit IndiaChina/India Limit (2012)Limit ).ChinaAustria Limit Local Limit 8

7 ) 2 6

4 Power Density (W/m PowerDensity 3

10 20 30 40 50 60 70 80 90 100 110 120 Distance (m)

Fig 3.6: Power density varying the distance between the user and BS with BS transmit power 320 watts. 29

10 Power Density ICNIRP Limit 9 IndiaChina/India Limit Limit ChinaAustria LimitLocal Limit 8

7 ) 2 6

4 Power Density (W/m PowerDensity 3

5 10 15 20 25 30 35 40 45 50 55 60 Distance (m)

Fig 3.7: Power density by varying the distance between the user and BS with BS transmits power 20 watts.

The results show that the safe distances is 13m for BS transmit power of 320 watts as per ICNIRP and this is 3m for 20 watts under the ICNIRP standard. However, the safe distances are 40m and 120m for BS transmission power of 20 watts and 320 watts respectively according to the standard followed in China. According to the radiation limit in India, the safe distances are 10m and 37m for BS transmission power 20 watts and 320 watts, respectively. It should be noted that in a highly populated country, e.g., Bangladesh, India and Pakistan the distance of the BS from the users are not kept very large and hence, in those country lots of people may live in the danger zone.

3.10.5 The Effect of Downlink Traffic on Power Density

So far the radiation exposure from the mobile phone BS for constant downlink traffic of 1 Mbps is studied. With the increasing use of Internet, the traffic demand can be increased up to 10 Mbps. As a result, the power density to the users for traffic demand from 1 to 10 Mbps is calculated. The worst power density to the users for different traffic demand is shown in Figures 3.8 and 3.9 for frequency bandwidth of 10 MHz and 5 MHz, respectively. 30

-10

-20

) 2 -30

-40

-50 Power Density (dBm/m PowerDensity

-60

-70

-80 1 2 3 4 5 6 7 8 9 10 Data Bandwidth (Mbps)

Fig 3.8: Power density for different traffic demand by the users for bandwidth of 10MHz.

-10

) 2

-20

-30

-40 PowerDensity(dBm/m -50

-60

-70

-80 1 2 3 4 5 6 7 8 9 10 Data Bandwidth (Mbps) Fig 3.9: Power density for different traffic demand by the users for bandwidth of 5 MHz

The results show that power density increases with increasing the traffic demand to the users. It can be attributed to the fact that the fraction of time that a BS needs to transmit increases with increasing traffic demand of the users and hence, the radiation exposure also increases.

3.11 Summary

In this Chapter, the radiation exposure from mobile phone BSs with a fixed transmit power at the BSs is analyzed. The results show that if antenna height is kept sufficiently high from the user location, the radiation is not significant. It is also found that most of the radiation power is received by a user from the nearest BS of that user. The comparison of safety distance for various exposure standards is shown. Also, the effect of the antenna height and traffic demand on the radiation exposure is studied. It is found that the radiation exposure increases with decreasing antenna height and with increasing traffic demand. 32

CHAPTER 4

REDUCTION OF RADIATION BY POWER CONTROLLING

In this section, the traditional cellular system is considered. However power controlling is well thought-out to reduce radiation exposure from mobile phone BSs. The algorithm for configuring the TDMA scheduling of the user and power allocation to reduce the traditional exposure is described. The effectiveness of the algorithm is verified by numerical results.

4.1 System Model

The network architecture, frequency allocation, traffic, RF propagation, RF radiation exposure and data rate models of the system are taken to be as in Chapter 3 except that the user data rate is varied from traditional system which will be found in the simulation results. It is assumed that the BSs can adjust the transmit power for the different users. The transmission scheduling of the system is also different from the traditional transmission scheme. The data transmission times of the users are not equal and they depend on the data rate of the users.

4.2 Scheduling and Power Allocation Algorithm

4.2.1 Transmission Time of a User

Data rate of a user uj∈Ni from its associate BS bi with transmission power P is given as [38]:

   d     i, j   PK      do   Ri, j  B log2 1 ...... (4.1)  WB     

Thus, under traditional transmission, the transmission time of the user uj per second is given as

S t  ...... (4.2) j R i, j

4.2.2 Total Transmission Time in a BS

The total transmission time per second of the BS bi can be found as

Ti  t j ...... (4.3) ujNi

4.2.3 Transmission Time Allocation to a User

The residual transmission time per second to the BS bi is (1-푇푖). The residual transmission time is distributed among the users of the BS bi according to their data rate. Intuitively, if a user has higher data rate it requires lower transmission time. Thus, the additional time given to the user uj a is inversely proportional to R . Let be the additional time to the user u such that i, j t j j

a t j 1Ti...... (4.4) jNi

a Since t j =C/ Ri, j with a constant C, it is obtained

1 C  1Ti, …………………………………………………………………..(4.5) j'NiRi, j'

Thus 1T C  i, ...... ( 4.6) 1  jNiRi, j'

and 1T t a = i, …………………………………………………………….(4.7) j 1 Ri, j  jNiRi, j 34

Thus, the transmission time to a user uj is then

new a t j  t j  t j …………………………………………...(4.8)

Using (4.2) and (4.7) in (4.8),

new S 1Ti t j   ...... (4.9) R Ri, j i, j  jNi Ri, j

Using Using (4.3) in (4.9) it can be shown that, 1 t S  j t new   jNi ...... (4.10) j R R i, j  i, j jNi Ri, j

Again using (4.2) in (4.10) the following can be derived,

S 1  s jNi R  t new   i, j j R R i, j  i, j jNi Ri, j

S 1 S    R R R i, j  i, j i, j jNi Ri, j

1  R  i, j jNiRi, j

1  Ri, j   1   ...... (4.11)  jNi\ j Ri, j 

Interestingly, the new allocation time does not depend on the other parameters except the data rates of the users. Thus, the transmission time per second for a user can be calculated easily by using (4.11)

4.2.4 Power Allocation for Transmission to a User

Now the value of RF power transmission to the different users is needed to be determined. Let the required transmission power for a user uj∈Ni of the BS bj∈B be Pi,j. Since the per second new transmission time for the user uj is ti,j , the required data rate is given as:

new S Ri, j  new ...... (4.12) ti, j To satisfy the new data rate the required transmit power can be obtained by using the following expression.

   d    P K  i, j    i, j     new  do  Ri, j  B log2 1 ...... (4.13)  WB      Thus, from (4.12) and (4.13) the transmit power for The BS bi∈B for transmitting to the user uj∈Ni is given as

푛푒푤  푅푖,푗 푊퐵  di, j  푃푖,푗 = (2 퐵 − 1)   …………………………………………(4.14) 퐾표  do 

4.3 Analysis of Radiation Exposure under the Proposed Scheduling and Power Allocation Algorithm

Since the transmit power of the BS bi is Pi,j for the user uj∈Ni the power density only for the transmission of BS bi is given as 36

τ 푃 퐺 푖 푖,푗 2 2 푃푑 = 2 퐹 (휑 푖,푗) 퐹 (휃 푖,푗) …………………………………(4.15) 4휋d푖,푗

∑ 푛푒푤 Note that τ푖 = 푗∈푁푖 푡푗 = 1 Thus, 푃 퐺 푖,푗 2 2 푃푑 = 2 퐹 (휑 푖,푗) 퐹 (휃 푖,푗) ……..…………………………(4.16) 4휋d푖,푗

The total radiation power density to the uj∈Ni can be written as

푃 퐺 ∑ 푖,푘 2 2 푃푑 = bi∈B,푢푘∈푁푖 2 퐹 (휑 푖,푗) 퐹 (휃 푖,푗) ……………….....(4.17) 4휋d푖,푗

4.4 Results

To generate result with power controlling and scheduling algorithm, the network shown in Fig 3.1 is considered. The other parameters are also considered as in chapter 3 except the transmit power of the BSs for transmission to the different users.

4.4.1 Comparison with the Traditional System

The power density to each user is determined by using the scheduling and power allocation algorithm. A comparison of power density results between the traditional system and the system under the proposed algorithm is shown in Fig. 4.1. The results show that a significant amount of radiation can be reduced by using the proposed algorithm. The reduction of power density to the different users is shown in Fig. 4.2. The average power density reduction is approximately 60 dB, i.e., the reduction of radiation exposure by using the proposed algorithm is 106.

Comparison of Power Density at Eact User Location for 37 New Modified Power vs Tx Power=20W from Respective BS 0 Power Density For Tx Power = 20W Power Density for New Modified Tx Power

-20 ) 2 -40

-60

-80 Power Density Power (dBm/m

-100

-120 0 50 100 150 200 250 300 350 400 User ID

Fig 4.1: Comparison of Power Density to the users for system with and without power control

Bar showing Reduction of Power Density at each user location for New Tx Power (based on New Tx Time) Vs Pt=20W 0.06

0.05

) 2 0.04

0.03

0.02 Power Density Power (dBm/m

0.01

0 0 50 100 150 200 250 300 350 400

UserUser Identity Index Number

Fig 4.2: Reduction of Radiation Exposure by Power Controlling and Scheduling Algorithm 38

4.4.2 Comparison of Radiation of the Proposed Algorithm with another Existing Scheme

Another existing scheme in cellular system is that the transmit power at the BSs is variable for different transmissions. The transmit power is adjusted at the BSs such that the received power at the users becomes -80 dBm. For this scheme, the radiation exposure is calculated with the required power at the BS such that the received power at the users becomes -80 dBm. A comparison of the power density of this existing scheme and the proposed scheme is shown in Fig 4.3. The results show that the proposed scheme reduce the radiation exposure significantly compare to this existing scheme. The reduction level is very good which just above the noise level. The reduction of power density at the maximum radiation exposure point can be seen as (approximately) 18 dB, i.e., the radiation exposure under the proposed algorithm is 1/63 times of the existing scheme.

Comparison of Power Density at Eact User Location for New Modified Power vs Controlled Rx power of -80 dBm for All BSs -40 Power Density For Controlled Power Tx for -80dBm Rx Level Power Density for New Modified Tx Power -50

) -60 2

-70

-80

Power Density Power (dBm/m -90

-100

-110 0 50 100 150 200 250 300 350 400 User Identity Number

Fig 4.3: Comparison of Radiation Exposure between the Proposed Algorithm and one of the Existing Schemes.

4.4.3 Reduction of Radiation Exposure

The reduction of radiation exposure is determined by using the developed algorithm as follows

푡 푃푑 Ratio= ɳ 푃푑 푡 ɳ where, 푃푑 is the power intensity without power control and 푃푑 is the power intensity with power control. This ratio provides us how many times the radiation is reduced. In Fig 4.4, the ratio of the power densities is shown. The results show that the maximum reduction of radiation exposure is 5x106 times, the minimum reduction of radiation exposure is 0.25x106 times and the average reduction of radiation exposure is approximately 1x106 times.

6 x 10 5

4.5

3.5

2.5

Ratio of Power Density ofRatio Power 1.5

0.5

0 0 50 100 150 200 250 300 350 400 User Index

Fig 4.4: Reduction of Radiation Exposure by Power Controlling and Scheduling Algorithm

4.5 Summary

In this Chapter, the scheduling and power control algorithm to reduce radiation exposure is described. The performance of the proposed algorithm is evaluated for different network scenario via numerical calculations. The results demonstrate that the proposed algorithm is very effective in reduction of radiation exposure from mobile phone BSs.

CHAPTER 5

REDUCTION OF RADIATION BY COMP-JT

In this chapter, it is intended to find the impact of CoMP-JT on reduction of radiation by determining the change of RF power exposure of a specific user for joint transmission (JT) compared to that of transmission from respective BSs of the user without changing the data capacity of the user. First a cluster is taken in consideration where the cluster head [39] [40] will select the edge users for CoMP-JT based on distance parameter. The data rate of the edge users as well as power density of the edge users is determined without CoMP-JT. Next the data rates and power density of the edge users is determined for CoMP-JT scheme. This new data rate is supposed to be higher for CoMP-JT compared to that of the respective BS. Then the reduction of transmission power of the BSs under CoMP-JT is calculated to reduce the new data rate so that it can match the old data rate for that user. At this point of transmission, the power density at that user location is determined. If the RF exposure during CoMP-JT is found to be lower, then the decision of continuing the CoMP-JT scheme will be taken; otherwise the user will be served by the respective BS without CoMP-JT.

5.1 Network Architecture

The cellular network system is considered to similar that is described in chapters 3 and 4. It is assumed that a set of BSs formed a cluster for CoMP transmission. Various algorithms for cluster formation for CoMP transmission can be found in the literature [39] [40]. The cluster head is denoted by Hc.

5.2 Selection of Users for CoMP-JT

The cluster head monitors the channel status and location of the users under the cluster. The BSs will be selected arbitrarily under the cluster and the number of BSs will be either 2 or 3. The decision to select the user(s) for CoMP-JT will be determined if the user(s) are within the intersection of selected BSs where the radius of each BS will be considered to be 10% more distance of the farthest user.

5.3 Measurement of Data Rate for Selected User without CoMP-JT

Data rate of a user uj∈Ni from its associate BS bi with transmission power P is given as

   d    PK  i, j      do   R  B log 1   ...... (5.1) u j 2    WB     

The power density of the user will be:

푑 푃퐺 푃푢푗 = 2 ……………………………………… …….(5.2) 4휋푑푖푗

5.4 Measurement of Data Rate for a Selected User with CoMP-JT

As discussed before, the neighbouring cells of the cellular network use different frequency. So, during CoMP-JT the participating BSs will transmit with the all frequency band used by them. Let m be the number of BSs using different spectrum with bandwidth B for each BS, and they are doing CoMP-JT, then the new data rate can be calculated [41] as:

Where, PCoMP is the transmission power with CoMP-JT.

5.5 Calculation of the Transmission Power for CoMP-JT

Since the new data rate RCoMP is higher than the data rate R the transmission power can be u j u j

CoMP R R reduced. Assume, u j = u j . Thus,

  m  d    P K  i, j     CoMP     i1  do  R  mB log2 1 ...... (5.4) u j  mWB     

From (6.4), it can be shown that

 m  di, j    R   u j i1  do  P K  mB 1 CoMP  mWB 2 and then PCoMP can be found as

 Ru   j 1  mB  mWB 2  PCoMP   ...... (5.5) K m   di, j    i1  do 

P Hence, the value of CoMP is shown in equation (5.5) at which the user data rate of CoMP-JT will be equal to that of in Non-CoMP-JT mode.

5.6 Radiation Exposure under CoMP-JT

Radiation exposure in CoMP-JT mode can be calculated as:

푑 푚 푃퐶표푀푃 퐺 푃퐶표푀푃 = ∑푖=1 2 ………………………………..…(5.6) 4휋푑푖푗

Here the radiation exposure from respective BS vs CoMP-JT using the equation (5.2) and (5.6) is 푑 푑 compared. If it is found that 푃퐶표푀푃 < 푃푢푗 , then the communication will be continued in CoMP-JT mode, else the user will be discarded from CoMP-JT. This procedure will be checked time to time and users will be selected for CoMP-JT.

5.7 Results and Discussion

(a) Network and System Parameters

A cellular network is considered consisting of 16 BSs as shown in Fig 5.1. There are 660 users distributed all over the network in quasi random sequence. It is considered that all of the BSs

transmit with 20 watt in traditional mode and the frequency bandwidth used by each of them is 10 MHz. The antenna of BS is considered as omni-directional & is sited at a height of 20 m. The users are at a height of 1 m from the ground level. The antenna gain is taken as 17dBi. Shadowing and fading effects for the network is not considered here. Frequency band 1800 MHz and path loss component as 3. The receiver antenna is taken as unity gain antenna. The BSs are considered to operate in in two different modes i.e. (a) they use the same frequency where the value of m is 1, (b) they use different frequencies and the frequencies of the neighboring BSs are not same (m=2/3).

Let there be a cluster consisting of BSs 5, 6, 9, 10, 13 and 14 which are marked with yellow colour in Fig 5.1. In this cluster, BS 9 is the cluster head. As mentioned before, cluster head takes decision for CoMP-JT. Then various scenario of CoMP-JT will be considered and the reduction of radiation compared to that of the traditional system will be calculated.

3000 4 BS#4 Utot=19 12 BS#12 Utot=24

8 BS#8 Utot=44 16 BS#16 Utot=46 2500

3 BS#3 Utot=36 11 BS#11 Utot=48

2000

7 BS#7 Utot=45 15 BS#15 Utot=46

1500 Width 2 BS#2 Utot=37 10 BS#10 Utot=49 (m) BS 10

1000 6 BS#6 Utot=46 14 BS#14 Utot=47 BS 6 BS 14

1 BS#1 Utot=43 9 BS#9 Utot=54 500 BS 9 Cluster Head BS 5 BS 13

5 BS#5 Utot=38 13 BS#13 Utot=38

0 0 500 1000 1500 2000 2500 3000 Length (m)

Fig 5.1: The network layout for CoMP-JT

(b) Radiation Exposure Result in CoMP-JT Mode and its Comparison with Traditional One

Let us consider BS 9 13 and 14 are doing CoMP-JT transmissions. Each of the BSs is using different frequency i.e. the value of m=3. For these three stations, common users having index 80 and 152 are selected for CoMP-JT. The comparison of transmission power in CoMP-JT mode and that of the traditional modeis shown in Fig 5.3. It is seen that the traditional mode needs more than 12 dBW transmission power whereas the power requirement in CoMP-JT mode is much below and its around -24 dBW.

For the same scenario, the comparison of power density is shown in Fig 5.3. It can be marked that the power density in CoMP-JT mode is less than -80 dBW per square metre whereas it is around - 48 dBW per square metre. Thus the radiation exposure in CoMP-JT mode is far below compared to that of the traditional mode.

In CoMP-JT mode, if the BSs use the same frequency, then the transmission power requirement and the radiation exposure level will change from the shown values as mentioned above. Figure 5.4 and 5.5 shows the transmission power requirement and the radiation exposure level if the value of m is equal to 1 i.e. BS 9 13 and 14 use the same frequency. If the Fig 5.2 is compared with 5.4, then it can be seen that the transmission power requirement is more for the latter case though it is less than that of traditional mode. Again comparing Fig 5.3 with 5.5, it can be observed that the power density level increases if same frequency is used by BSs doing CoMP-JT and it is almost equal to that the traditional mode. As the transmission power requirement is less in CoMP-JT, so the maximum exposure level reduces for the users of respective BS.

Tx Power = 20W CoMP-JT Tx Power 10

5 <-----Common User Index 80

0 Common User Index 512 ------>

-5

Tx Power(dBW) Tx -10

-15

-20

-25 0 100 200 300 400 500 600 User Index Fig 5.2: Transmission Power in CoMP-JT Mode for Common Users of BS 9,13,14 with m=3

-10 Power Density For Tx Power = 20W

Power Density for CoMP-JT Scheme -20

) -30 2

-40

-50 Common User Index 80

Power Density (dBW/m Density Power -60 Common User Index 512

-70

-80

-90 0 100 200 300 400 500 600 User Index

Fig 5.3: Comparison of Power Density with and without CoMP-JT for BS 9, 13 & 14 with Frequency Reuse Factor m=3

Tx Power = 20W CoMP-JT Tx Power 12

6 Tx Power(dBW) Tx

Common User Index 80 Common User Index 512

0 0 100 200 300 400 500 600 User Index Fig 5.4: Transmission Power in CoMP-JT mode for Common Users of BS 9,13,14 with Frequency Reuse Factor m=1

0 Power Density For Tx Power = 20W Power Density for New Modified Tx Power

-10

) -20 Common User Index 512 2 Common User Index 80

-30

Power Density (dBW/m PowerDensity -40

-50

-60 0 100 200 300 400 500 600 User Index Fig 5.5: Comparison of Power Density for with and without CoMP-JTfrom BS 9, 13 and 14 for freq reuse factor m=1

In figure 5.6 anf 5.7, the state of transmission power as well as radiation exposure level for CoMP-JT by BS 9 and 5 can be observed. Here, each of the BSs is using different frequency i.e. the value of m=2. For these two stations, 11 common users are selected for CoMP-JT. The comparison of transmission power in CoMP-JT mode and that of the traditional mode shown in Fig 5.6 and 5.7 respectively. It is seen that the power requirement in CoMP-JT mode is much below than that of the traditional mode.

0.05

0.045

0.04

0.035

0.03

0.025

Tx Power Tx(W) Power 0.02

0.015

0.01

0.005

0 0 100 200 300 400 500 600 UserUser Identity Index Number

Fig 5.6: Transmission Power in CoMP-JT mode for Common Users from BS 9 and 5 compared to 20W in Non-CoMP-JT mode for freq reuse factor m=2

-10

-20 )

2 -30

-40

-50

Power Density (dBW/m PowerDensity -60

-70

-80 CoMP-JT Conventional Transmission -90 0 100 200 300 400 500 600 User Index

Fig 5.7: Comparison of Power Density for with and without CoMP-JT from BS 9 and 5for freq reuse factor m=2

Let us consider BSs 9 and 10 have decided to do CoMP-JT. There are 6 common uses within these two BSs. The index of the common users of BS 9 and 10 are 78, 186, 294, 438, 510 and 618 which are shown in the groups under each BS in Fig 5. 8 along with their distance in metre from BS 9 and 10 as plotted in Y axis. It can be seen that the distance of user 78 from BS 9 is more than that of BS 10. Again user 186 is almost at equal distance from BS 9 and 10. If looked at figure 5.9, the comparison of users' received power level for traditional transmission of 20 watt vs CoMP-JT transmission of 10 watt can be seen. The received power level for user 78 is different for traditional and CoMP-JT transmission whereas for user 186, both the power levels are almost equal. The same thing applies for the other users also. It indicates that the edge users those are at almost equal distance from the BSs have better profile for doing CoMP-JT.

600

500

400

300 Distance (m)

200

100

78 186 294 438 510 618 78 186 294 438 510 618 0 9 10 User Index of BS 9 & 10

Fig 5.8: Distance of Common Users from BS 9 and 10

-8 x 10 7 Power Received from Respective BS taking Tx Power = 20W Power Received in CoMP-JT Scheme for Transmission Power=10W 6

3 PowerReceived (W)

0 78 186 294 438 510 618 0 100 200 300 400 500 600 700 User Index

Fig 5.9: Comparison of Power for Commom Users of BS 9&10 with and without CoMP-JT

It can be seen from above Fig 5.1 and 5.2 that the users who are at almost equal distance from CoMP-JT BSs have better profile for CoMP-JT.

5.8 Summary

From the simulation it can be seen that CoMP-JT provides a good opportunity to mitigate the radiation exposure especially for the edge users who are almost at equidistant from the serving CoMP-JT BSs. This method will not be suitable if the difference of relative distance of a user from serving BSs is more. CoMP-JT reduces the radiation exposure significantly especially if the neighbouring BSs use the different frequency.

CHAPTER 6

CONCLUSION AND FUTURE WORKS

6.1 Conclusion

Although radiation hazard due to the mobile communications is a burning issue of these days, there has been conducted a very limited study on the reduction of the radiation exposure from the BSs of cellular networks. In this research, firstly, the radiation exposure of traditional cellular system is studied through numerical calculations and analytical modeling. Several insights on the radiation exposure are given. It is found that (i) if antenna height is kept sufficiently high from the user location, the radiation is not significant, (ii) most of the radiation power is received by a user from the nearest BS of that user, and (iii) the radiation exposure increases with decreasing antenna height and with increasing traffic demand.

Next, the radiation exposure reduction technique by configuring the transmit power at the BSs and user scheduling is proposed. An algorithm is proposed for configuring these parameters through analytical modeling of the data rate and transmission time of the users. The performance of the proposed algorithm is evaluated for different network scenario and it is demonstrated that the proposed technique is very effective in reduction of radiation exposure from mobile phone BSs.

Finally, the radiation exposure reduction technique by configuring the transmit power at the BSs and scheduling CoMP transmission is proposed. It is demonstrated that CoMP-JT provides a good opportunity to mitigate the radiation exposure especially for the edge users who are almost at equidistant from the serving CoMP-JT BSs. CoMP-JT reduces the radiation exposure significantly especially if the neighboring BSs use the different frequency.

6.2 Future Works

In this thesis, new methodology and algorithms have been proposed for reduction of power density from the BSs. Based on the developed algorithms and methodology, future works may be taken as mentioned in the subsequent paragraphs.

6.2.1 Optimization of the Proposed Algorithms

Here nothing is mentioned about the optimization of the proposed scheme, i.e., how to configure the transmit power and user scheduling to minimize the radiation exposure optimally. It is observed that the users located near the BSs are mostly affected by radiation exposure and the need for optimizing the power output is most important so that every user receives almost similar amount of radiated power. This can be done by allocating the transmission time in a way so that the users receive the almost equal amount of power. Optimization of proposed CoMP-JT scheme may also be done by determining the required amount of transmission power for each common user from each BS so that overall radiation exposure reduces. So, future works may be taken on optimizing the developed algorithm proposed in this thesis.

6.2.2 Impact of Fading and Shadowing on the Proposed Algorithms

The impact of fading and shadowing were not considered for the reduction of radiation in present context. The different probability density functions such the Rayleigh, Nakagami, gamma, generalized gamma, Weibull, lognormal, Nakagami-lognormal, K distribution, generalized K distribution, and Nakagami inverse Gaussian distribution etc. may be considered for this. These factors have short and long term effect of the received signal, as such there is a need to take these impacts into the proposed algorithms and modify those as needed.

6.2.3 Impact of Sectoral Antenna and Directivity Gain on the Proposed Algorithm

Sectoral antennas and smart antennas are being used widely now a day. The directivity function has various parameters for different types and orientations of the antenna system. So the radiation exposure will vary widely as the antenna type and user location changes. To verify these impacts on the proposed algorithms may be another scope of future work.

6.2.4 Reduction of Radiation Exposure Based Proposed Algorithms by Applying Available Modulations and Channel Capacity

In this paper the reduction of radiation exposure based on Shannon’s channel capacity has been discussed. So future research work may be taken considering the real world scenario where different data rates, coding and modulation techniques exist as used in latest communication standards. These will have definite impact on the outcome of proposed algorithms. Hence, this is a good area to go for further research.

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