Operator Diversity in Forest and Rural Applications
ELIANE SEMAAN
Master of Science Thesis
Stockholm, Sweden 2011
Operator Diversity in Forest and Rural Applications
ELIANE SEMAAN
Master of Science Thesis performed at the Radio Communication Systems Group, KTH.
June 2011
Examiner: Professor S. Ben Slimane
KTH School of Information and Communications Technology (ICT) Radio Communication Systems (RCS)
TRITA-ICT-EX-2011:132
Eliane Semaan, June 2011 c
Tryck: Universitetsservice AB
Abstract
Information and communications technology is increasingly important for ru- ral areas, not just for individual needs but also for the highly automated and demanding forest industry. The ability to communicate wireless in rural areas greatly improves the personal safety of forest workers and provides economical gain for the forest industry.
In the absence of a mobile operator that can solely cover rural areas, a so called
”operator diversity model” seems to be a natural fit as it allows access to all available operators and communication networks (2G to 4G) at a specific rural location. To enable the design of the operator diversity model, it is essential to identify and study all possible communication standards and their respective properties that could be included in this model.
This thesis investigates the improvement of coverage probability if the operator diversity concept is applied in rural areas. The simulation results show that a coverage probability of 100 percent can be reached in some scenarios.
In addition, a case study is carried out in the forest areas around Nykvarn with the intention of demonstrating the substantial benefits of adopting the operator diversity concept. Moreover, bit rate measurements are performed in the same area, thereby providing an insight as to what bit rates to expect in Swedish rural areas.
Furthermore, the user business case is considered in order to estimate the addi- tional costs associated with the operator diversity concept.
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Acknowledgements
I would like to express my deep and sincere gratitude to my advisor, Mats Nil- son, Wireless@KTH, for his inspiration, constructive comments and extensive support throughout this thesis. His wide knowledge and his logical way of think- ing have been of great value for me.
I would like to extend my thanks to G¨oran Andersson, COS Communication Systems KTH, for his time, guidance and support. I would also like to thank Prof. Ben Slimane for agreeing to be my examiner.
A special acknowledgement goes to my sponsors at Skogforsk, represented by Bertil Lid´en for their help during the course of the thesis. I also really appreciate the indispensable help during the field measurements from my fellow student Ahmed Aslam.
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Contents
1 Introduction 1
1.1 Mobile Communications in Rural Sweden . . . 1
1.2 Thesis Motivation . . . 2
1.3 Previous Work . . . 4
1.4 Thesis Outline . . . 4
2 Review of System Standards 7 2.1 The Second Generation System . . . 7
2.1.1 Frequency bands . . . 7
2.1.2 System features . . . 8
2.2 The 2.5 and 2.9 Generation Systems . . . 8
2.2.1 GPRS . . . 8
2.2.2 EDGE . . . 8
2.3 The Third Generation Systems . . . 9
2.3.1 WCDMA and TD-CDMA . . . 9
2.3.2 HSPA . . . 10
2.3.3 HSPA+ . . . 11
2.4 The Long-Term Evolution System . . . 12
2.4.1 System features . . . 12
2.4.2 Supported frequency bands . . . 14
2.5 The CDMA2000/450 MHz System . . . 15
2.5.1 The CDMA2000 technologies . . . 16
2.5.2 System features . . . 16
3 Radio Propagation and Coverage 19 3.1 Radio Propagation and Coverage . . . 19
3.2 Path Loss Propagation Model . . . 19
3.2.1 The Cost 231 extension . . . 21
3.2.2 The ITU-R extension . . . 21
3.3 Fading . . . 21
3.3.1 Shadow fading . . . 22
3.3.2 Fast fading . . . 23
3.4 Interference . . . 24
3.5 Link Budget Calculations . . . 26 vii
viii Contents
4 Simulation 29
4.1 Simulation Tool . . . 29
4.2 Performance Measures . . . 29
4.3 Simulation Model . . . 30
4.3.1 Data communication model . . . 30
4.3.2 Voice communication model . . . 31
5 Field Measurements 33 5.1 Received Signal Strength Measurements . . . 33
5.1.1 Equipment . . . 33
5.1.2 Purpose of measurements . . . 33
5.2 Bit Rate Measurements . . . 35
6 Results 37 6.1 Simulation Results . . . 37
6.2 Field Measurement Results . . . 39
7 User Business Case 43 7.1 Proposed Solutions . . . 43
7.1.1 Broadband services . . . 43
7.1.2 Voice services . . . 44
7.2 Cost Estimation . . . 44
8 Conclusion 47
9 Future Work 49
References 51
A Radio Link Budgets 55
B Field Measurement Results 57
C Sample Code 59
List of Tables
2.1 EDGE data rates for the different coding schemes . . . 9
2.2 Channel bandwidths specified in LTE . . . 13
2.3 Paired frequency bands defined by the 3rd Generation Partner- ship Project (3GPP) for LTE . . . 15
2.4 Unpaired frequency bands defined by the 3rd Generation Part- nership Project (3GPP) for LTE . . . 15
3.1 Hata path loss model . . . 20
3.2 The extended path loss model . . . 21
3.3 Interference margin as a function of the load in LTE systems . . 26
3.4 Link budget parameters . . . 27
4.1 Simulation parameters . . . 32
6.1 Field measurement results . . . 41
A.1 Uplink budget calculations (Data communications) . . . 55
A.2 Uplink budget calculations (Voice communications) . . . 56
B.1 Detailed measurement results . . . 57
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List of Figures
1.1 Speech signal coverage . . . 2
2.1 Minimum required received signal-to-noise ratio as a function of bandwidth utilization . . . 11
2.2 Coverage provided by 450, 900, 1800 and 2100 MHz . . . 17
3.1 The log-normal probability density function . . . 22
3.2 Useful fraction of cell area - Hexagonal cell . . . 23
3.3 Probability density function of the Rayleigh distribution . . . 24
3.4 Probability density function of the Rician distribution . . . 25
3.5 Interference margin as a function of the load in CDMA 450 systems 26 4.1 Illustration of the diversity model . . . 30
5.1 Measurement equipment . . . 34
5.2 Measurement area . . . 34
5.3 Measurement setup . . . 35
6.1 Median coverage probability for data communications with four networks . . . 38
6.2 Median coverage probability for data communications with six networks . . . 39
6.3 Median coverage probability for voice communications with four networks . . . 40
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Chapter 1
Introduction
Swedish rural areas, covering more than 60 percent of the country’s total area, suffer from two major problems. These problems have arisen as a result of the extensive focus of mobile operators on covering densely populated areas rather than rural areas. The first major issue is the lack of coverage for mobile com- munications in extensive rural areas, which affects, in addition to data services, regular telephone calls that are necessary in order to meet certain service and security levels in rural areas. The second common problem is that Internet connections are too slow for some services that are of a certain importance for the forest industry. Sending maps, instructions and ongoing work information from and to forest equipment is an example of such services. This situation has resulted in the need for new or at least, improved technologies in order to support communications in such areas.
In order to get further insight into the current situation in rural Sweden, some background information and related facts are presented in the first part of this Chapter. The second part aims at explaining the problem to be investigated as well as providing the outline of the thesis report.
1.1 Mobile Communications in Rural Sweden
Currently, Mobile communications in rural Sweden highly rely on the use of GSM 900 and CDMA2000/450 MHz. The latter is a 3G system, operating in the 450 MHz frequency band and well suited for providing telecommunications coverage, especially over rural regions due to the propagation characteristics of the 450 MHz band [1]. The CDMA2000/450 MHz communication system, also known as CDMA 450, thus allows base stations to achieve greater coverage areas, which in turn, results in fewer base stations needed to cover a given area.
Thus, the coverage area of CDMA 450 is proved to be three times larger than in the 900 MHz band, and twelve times larger than in the 1800 MHz and 2100 MHz [2]. The operator Net1 is the only operator deploying CDMA 450 MHz services in Sweden.
The GSM 900 system belongs to the second-generation communication systems (2G) and operates in the 900 MHz band.
On the other hand, a promising technology for rural areas is the Long-Term 1
2 Chapter 1. Introduction
Evolution (LTE) standard. TeliaSonera, Tele2 and Telenor already deploy sys- tems based on this standard and operating in the 2.6 GHz band in Sweden.
A key feature of the LTE (4G) standard is the improved capability in terms of peak data rates (i.e. 5 bits/s per Hz bandwidth in the downlink direction and 2.5 bits/s per Hz bandwidth in the uplink direction) [3]. Moreover, LTE allows for high data rates at cell edges and reduced latency.
However, deploying this system in a lower frequency band has always been at- tractive for rural communications. Such a deployment has been made available in Sweden by the Post-och telestyrelsen (PTS) auctioning of the 800 MHz spec- trum on Feb 28th 2011. The winners of this auction were TeliaSonera, ”3” and Net4Mobility, which is a joint venture between Telenor and Tele2 [4]. Each winner is offered 2x10 MHz, which is half of what is needed for maximum LTE bit rate performance.
According to the coverage maps provided by different operators in Sweden, it is obvious that there are several independent networks outside the main urban areas. An example showing this independency is illustrated in Figure 1.1 [5][6].
The important aspect of this fact is the possibility of providing a higher coverage probability in case access to all possible 2G-4G networks is considered at a specific rural location, in other words, by using operator diversity.
Figure 1.1: Speech signal coverage
1.2 Thesis Motivation
Although the GSM and CDMA 450 networks have been the predominant solu- tions for mobile communications in Swedish rural areas, they have some major disadvantages.
The existing GSM network, first launched at the beginning of the 1990s, still does not have enough coverage in Swedish rural areas, i.e. the operator TeliaSon- era has the best GSM network in Sweden, covering about 90% of the country’s area. Although the forestry industry prioritizes coverage, when it comes to data
1.2. Thesis Motivation 3
transfer, the GSM system, even when upgraded to GSM/EDGE (2.9G), does not have the ability to provide high-speed data transfer, especially at large dis- tances from base stations. Thus, EDGE cannot be considered as the desirable solution, especially for services requiring high data rates.
On the other hand, CDMA 450 provides higher performance than GSM in terms of coverage, basically due to its deployment in low frequency bands.
However, the services offered by this system are limited to Data-Only services, implying the use of relatively large-sized equipment and terminals, which in turn affects flexibility. Moreover, the fact that Net1 does not offer mobile telephony services is a major drawback, affecting the effectiveness of CDMA 450.
In brief, TeliaSonera’s GSM network and Net1’s CDMA 450 network are currently the best, yet insufficient alternatives, providing, speech, respectively data communication services in rural Sweden.
As mentioned in the previous section, the LTE standard is a promising technol- ogy for rural environments. However, the performance of LTE-based systems has not yet been evaluated in rural areas. Besides, it remains unclear at the moment how large investment the operators are willing to make in order to im- prove coverage in rural areas.
All of this yet again makes operator diversity an interesting alternative as it would be interesting to investigate the coverage probability in a specific rural area if access to all possible 2G-4G networks is considered.
The concept of the operator diversity model is to benefit from the availability of these independent mobile operators and networks at a specific location. The user will be able in this case to switch from one operator to another one when experiencing lack of coverage, or in other words, outage. In addition, the user will be able to switch between different networks within the same operator.
Switching from one operator to another one occurs when the user experiences outage with the first operator and hence tends to find another operator providing coverage at that specific location. As a result, the user is considered to be covered if he is covered by at least one network (or operator). On the other hand, the user will experience outage if he is located within a white spot where no operator can provide coverage.
The main objectives of this thesis are listed below:
• Investigate the performance of the operator diversity technique in terms of coverage probability.
• Evaluate the service improvement as a function of the additional costs.
• Provide an insight into the service quality and the achievable bit rates in rural Sweden by means of field measurements.
The study is focused on the uplink transmission, since that typically limits coverage in cellular systems, mainly due to output power considerations for the user equipment. However, field measurements will cover both uplink and downlink directions.
4 Chapter 1. Introduction
1.3 Previous Work
The concept of operator diversity in forest and rural environments has not been studied in any previous work. However, the GSM/EDGE downlink performance in forest environments has been examined in [7] and results showing the coverage and throughput levels of EDGE have been presented. It has been shown that the throughput of EDGE in rural environments was below expectations, mainly due to the high BLER levels caused by the extra attenuation inside forest. The outcome of the coverage study showed that, for a hand-held phone:
• Roughly 20% of the total cell area would be covered by throughputs above 100 Kbps.
• Throughput below 100 Kbps will be experienced in the remaining 80% of the total cell area.
In [8], a path loss model for GSM900 in coniferous forest was created. Further- more, the distribution of the received signal was examined and the Rayleigh distribution was found to be a relatively good approximation for both high and low receive antenna elevations.
Some of the simulations and measurements conducted in [7] and [8] will serve in this thesis work as a basis for defining some propagation characteristics that are specifically related to rural Sweden.
A similar concept to Operator Diversity is National roaming, which enables users to roam between multiple operators within a country. This concept was examined in [9] and it was shown that national roaming more than doubles the data rates for users at the cell border.
A related study of how users should be allocated within one operator’s multi- access network was conducted in [10]. It was concluded that considerable gains could be achieved in a combined GSM/EDGE and WCDMA scenario.
The essential difference between this project and the work conducted in [9] and [10] includes the following:
• In [9], three different operators are assumed to provide three communica- tion networks that are based on the same standard, e.g. WCDMA. The new aspect in this thesis project is that several communication system standards are involved including EDGE, HSPA, LTE and CDMA 450.
• The study conducted in [10] evaluates the gains achieved by combining two communication system standards, GSM/EDGE and WCDMA, provided by the same operator. The key distinction between the study made in [10]
and this thesis project is the fact that in this project many operators and communication systems are involved.
Moreover, the studies made in [9] and [10] are not oriented towards rural areas.
1.4 Thesis Outline
The thesis is organized in the following order:
1.4. Thesis Outline 5
Chapter 2 introduces the system standards involved in this thesis with focus on LTE.
Chapter 3 explains the basics of radio propagation and describes the path loss model used in this project. In addition, the chapter briefly describes the effects of fading and interference on radio propagation. Link budget calculations are also described in the end of the chapter.
Chapter 4 introduces the simulation model as well as the simulation tool.
Chapter 5 describes the measurement purpose, the measurement area and the used equipment.
Chapter 6 presents the obtained results and attempts to discuss them.
Chapter 7 deals with the user business case and cost estimation.
Chapter 8 and 9 conclude the thesis and present some suggestions for future studies.
Chapter 2
Review of System Standards
Mobile network evolution started in the early 1980s and has been categorized into ”generations” along the years. The first generation mobile systems were consisting of analogue systems, enabling speech and some limited related ser- vices. These systems were first introduced in Europe in 1981, operating at 450 and 900 MHz bands. One of the main drawbacks of these systems was their in- compatibility with each other, making inter-system and international roaming impossible. In addition, there was no efficient use of the frequency spectrum and the services that could be offered for the subscribers were very limited.
2.1 The Second Generation System
In order to overcome these limitations, a new standard called Global System for Mobile communications was developed and first introduced in Europe at the beginning of 1991. By the end of 1993, this standard became adopted by more European countries as well as Australia, Hong Kong, part of Asia, South America and United States [11] and continued to evolve to meet the requirements of data services and to allow for new services.
2.1.1 Frequency bands
The GSM system uses a duplex frequency band around 900 MHz [12]. The first band is dedicated to the uplink and operates at 890 to 915 MHz and the second band is dedicated to the downlink and operates at 935 to 960 MHz which means that the available bandwidth in each direction is equal to 25 MHz. A frequency division multiplexing (FDM) scheme is then used to allocate each GSM channel 200 KHz of bandwidth, resulting in 25 MHz/200 KHz = 125 channels available in each direction. One of these channels is used as a guard band, leaving 124 effective channels available for data transfer and communication [13]. Each of these frequency channels is in turn divided into TDM frames using time division multiplexing (TDM). Each frame is 4.615 ms long and consists of eight time slots or bursts, each assigned to a user, resulting in an effective bandwidth of 200 KHz/8 slots = 25 KHz per user.
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8 Chapter 2. Review of System Standards
The GSM frequency band was later extended by 10 MHz or 50 carriers (10 MHz/200 KHz). This extension is referred to as ”E-GSM” and consists of the uplink band (880 - 915 MHz) and the downlink band (925-960 MHz).
2.1.2 System features
The modulation method used for GSM systems is a constant-amplitude modula- tion called the Gaussian Minimum Phase-shift Keying (GMSK). In this scheme, the modulated signal is able to carry 1 bit per modulated symbol over the radio path originally supporting data rates up to 9.6 Kbps on a single time slot [11].
All basic services such as speech, fax and data services up to 9.6 Kbps were provided by this network.
2.2 The 2.5 and 2.9 Generation Systems
The GSM system continued to evolve; the next version of this system was the GSM and VAS (Value Added Services). At this phase, two platforms were added to the original GSM system, the Voice Mail System (VMS) and the Short Message Service Centre (SMSC) [14].
2.2.1 GPRS
The next step was to introduce new elements such as SGSN (Serving GPRS) and GGSN (Gateway GPRS) to the already existing system. This part of the network is referred to as the packet core network. This enhancement in the system led to the GPRS (General Packet Radio Services) or 2.5G and made it possible to send packet data on the air-interface and to access the Internet wire- less with bit rates between 8 and 20 kbps on a single time slot. The modulation scheme used at this stage was the same as the one used for GSM (GMSK).
The reason behind this enhanced data rate is the introduction of a packet- switched network; the network resources became more dynamic and efficient due to the fact that the subscriber became able to log into the network, use all the eight time slots dynamically and be charged only when using the resources.
2.2.2 EDGE
During this period of time, a more advanced set of specifications and require- ments (Third-generation systems) were defined by the International Telecom- munications Union - Radio communication sector (ITU-R) and aiming at pro- viding multimedia services and high-speed data rates. This fact has led to the emergence of a technology known as EDGE (Enhanced Data rates in GSM En- vironment) or 2.9G, which is able to deliver services and data rates similar to the recently specified requirements, yet with implementation on the existing second generation network (GSM).
For this purpose, the Octagonal Phase-shift Keying (8-PSK) modulation scheme was introduced and added to the GMSK allowing the modulated signal to carry 3 bits compared to 1 bit in the case of GSM and GPRS. As a result, EDGE became four times as efficient as GPRS [15] due to the nine modulation and coding schemes (MCS-1 to MCS-9) used in EDGE systems and providing the
2.3. The Third Generation Systems 9
different throughputs given in Table 2.1 [14].
Table 2.1: EDGE data rates for the different coding schemes
MCS Modulation User rate (kbps per time slot)
1 GMSK 8.8
2 GMSK 11.2
3 GMSK 14.8
4 GMSK 17.6
5 8-PSK 22.4
6 8-PSK 29.6
7 8-PSK 44.8
8 8-PSK 54.4
9 8-PSK 59.2
2.3 The Third Generation Systems
The initial steps towards 3G technologies, also known as IMT-2000, were first taken by the radio communication sector of the International Telecommunica- tion Union (ITU R) in the mid 1980s. A spectrum of 230 MHz was identified for IMT2000 by the World Administrative Radio Congress (WARC-92) such that 2x60 MHz of this spectrum was identified as paired spectrum for Frequency Division Duplex (FDD) and 35 MHz as unpaired spectrum for Time Division Duplex (TDD). The frequency bands defined for the UMTS standard are 1885- 2025 MHz for the uplink and 2110-2200 MHz for the downlink.
2.3.1 WCDMA and TD-CDMA
The very first target data rates for the 3G circuit-switched and packet-switched data services were [16]:
• Up to 64 kbps in a vehicular environment.
• Up to 144 kbps in a pedestrian environment.
• Up to 2 Mbps in an indoor environment.
This new generation of cellular standards has resulted in the UMTS system, first offered in 2001 and standardized by 3GPP. This system was based on wideband CDMA (WCDMA) in the paired spectrum (FDD) and Time Division CDMA (TD-CDMA) in the unpaired spectrum (TDD).
In the case of WCDMA, a multiple access scheme known as Direct-Sequence Code Division Multiple Access (DS-CDMA) is used. This scheme is based on the Direct-Sequence Spread Spectrum (DSSS) modulation technique, operat- ing by spreading the signals from and to different users with different codes.
10 Chapter 2. Review of System Standards
WCDMA is, as mentioned above, used for UMTS in the paired spectrum; the uplink frequency band extends from 1920 to 1980 MHz, while the downlink fre- quency band is in the range 2110 to 2170 MHz [14].
Although several bandwidths were defined for the WCDMA system (e.g. 5, 10 and 20 MHz), the one that is currently being used is 5MHz. However, it should be noted that the effective bandwidth is 3.84 MHz, as the guard band takes up 0.6 MHz from each side.
On the other hand, TD-CDMA is the channel access technique used for UMTS in the unpaired spectrum. This technique uses increments of 5 MHz of spectrum, each portion is split into 10 ms frames containing 15 time slots which in turn are allocated in fixed percentage for the downlink and uplink directions. The main difference between TD-CDMA and WCDMA is that TD- CDMA allows deployment in narrow frequency bands as it does not require separate frequency bands for the two directions. The frequency bands 1900- 1920 and 2010-2025 MHz are the most commonly used bands for UMTS-TDD in Europe [17].
2.3.2 HSPA
The evolution of WCDMA started with the introduction of High-Speed Down- link Packet Access (HSDPA) in Release 5 of the 3GPP/WCDMA specifica- tions. The second evolutionary step was characterized by the complementary Enhanced Uplink (HSUPA) and introduced in Release 6 of the 3GPP/WCDMA specifications. These two steps in the evolution of WCDMA are commonly re- ferred to as High-Speed Packet Access (HSPA).
A crucial aspect of this evolution was the constraint on backwards com- patibility to operate on already deployed networks. However, several new tech- niques have been introduced in order to support this evolution in both downlink and uplink directions, including high-order modulation, rate control, channel- dependent scheduling, and hybrid ARQ with soft combining. As a result, HSPA provides enhanced data rates, up to 14 Mbps in the downlink and 5.7 Mbps in the uplink. Furthermore, a significant improvement in terms of round trip times and capacity is allowed by HSPA. As we will see in the next section, these tech- niques are also used for the 4G systems. The concepts behind these techniques are briefly described in this section and consist of:
• High-order modulation: In [3], it has been shown that in case of band- width utilization smaller than 1 (Bandwidthutilization = γ = BWR < 1), achieving higher data rates implies a similar increase in the minimum re- quired signal power at the receiver. However, in case of data rates in the same order or larger than the available bandwidth (Bandwidthutilization = γ = BWR > 1), increasing the data rate implies a relatively much larger increase in the minimum required received signal power. The relationship between bandwidth utilization and minimum required received power is illustrated in Figure 2.1 [3].
Due to the fact that bandwidth is a scarce and expensive resource, high data rates should be provided within a limited bandwidth. This is achieved by extending the modulation alphabet to include additional signaling al-
2.3. The Third Generation Systems 11
ternatives, in other words, by using higher-order modulation. As a result, the number of bits per symbol interval increases and thus allowing higher data rates (e.g. QPSK uses 4 different signaling alternatives resulting in up to 2 bits per modulation symbol. Increasing the number of signaling alternatives to 16 (16 QAM) or 64 (64 QAM) results in an increase in the number of bits per modulation symbol to, respectively, 4 and 6 bits). A related drawback is that higher-order modulation schemes require higher SNR and SIR at the receiving end.
Figure 2.1: Minimum required received signal-to-noise ratio as a function of bandwidth utilization
• Rate control: This technique is implemented by dynamically adjusting the channel-coding rate and at the same time selecting between different modulation schemes. The main objective of the rate control is to track rapid channel variations.
• Channel-dependent scheduling: Scheduling controls to which users the shared resources should be directed at a given time instant, as well as the data rates to be used, based on the channel conditions.
• Hybrid ARQ with soft combining: This technique is used to allow the terminal to rapidly request retransmission of unsuccessfully received data. In addition, soft combining implies that the terminal combines soft information from previous transmission attempts with the retransmitted information in order to improve the decoding process. For this purpose, incremental redundancy (IR) is used as the soft combining strategy.
2.3.3 HSPA+
A more advanced version of HSPA is the Evolved High-Speed Packet Access (HSPA+), first defined in Release 7 of the 3GPP/WCDMA specifications and released late in 2008. This standard provides theoretical data rates up to 84 Mbps in the downlink and 22 Mbps in the uplink (per 5 MHz carrier) [18]. This improvement in the system performance is provided by the use of higher order
12 Chapter 2. Review of System Standards
modulation (64 QAM), in addition to multiple-input and multiple-output tech- nique (MIMO). This multiple antenna technique is based on spatial multiplexing and implying multiple antennas at both transmitter and receiver sides.
2.4 The Long-Term Evolution System
In parallel to the HSPA evolution, a new radio access technology, known as Long-Term Evolution (LTE) has been specified by 3GPP with fewer restrictions on backward compatibility and higher packet-data capabilities. In order to support these new capabilities, an evolved core network has been specified by the System Architecture Evolution (SAE). The targets capabilities and system performance that have been set out by 3GPP in the initial phase of the LTE standard development are outlined below [19]:
• Capability: Peak data rates of 5 bits/s per Hz bandwidth for the down- link and 2.5 bits/s per Hz bandwidth for the uplink. It has to be noted that LTE supports both paired and unpaired spectrum allocations by us- ing, respectively, FDD and TDD. As transmission and reception occur si- multaneously in the case of paired spectrum, these peak data rates should also be reached simultaneously. Moreover, LTE has an attractive feature characterized by the low delay and high data rates that can be achieved at the cell edge and the reduced latency, with a Round Trip Time (RTT) of 10 ms.
It is to be noticed that the achievable performance of LTE has, in many cases, exceeded the preliminary requirements that 3GPP set out in the initial phase; downlink data rates up to 300 Mbps and uplink data rates up to 75 Mbps will be achieved in the future in some cases.
• Mobility: The optimal performance should be reached at low terminal speeds (i.e. 0-15 Km/h). For speeds up to 120 Km/h, the system should be able to provide high performance. In case of speeds above 120 Km/h, the connection across the cellular network should be maintained. In addition, the LTE system should be able to manage speeds up to 350 Km/h and sometimes 500 Km/h depending on the used frequency band.
• Coverage: High performance is required for cell ranges up to 5 Km, whereas a slight degradation in the user throughput is allowed. For cell ranges up to 100 Km, no performance requirements are specified.
• Deployment: LTE is required to coexist with other 3GPP systems (i.e.
GSM, WCDMA/HSPA etc).
2.4.1 System features
A key characteristic of LTE systems is spectrum flexibility which allows for de- ployment in already existing IMT-2000 frequency bands and supports a limited set of spectrum allocations ranging from 1.4 MHz to 20 MHz as shown in Table 2.2 [3]. It should be noted that in LTE, a transmission is structured in the time domain in radio frames. Each of these radio frames is 10 ms long and consists of 10 subframes of 1 ms each. In the frequency domain, the subcarrier spacing is 15 KHz and a ”Resource Block” consists of twelve of these subcarriers, i.e.
2.4. The Long-Term Evolution System 13
12*15 KHz = 180 KHz. A terminal can be allocated a minimum of 1 resource block (RB) during 1 subframe in the uplink or the downlink direction.
Another related property of LTE systems is the possibility to be deployed in both paired and unpaired spectrum allocations, using FDD and TDD as already mentioned in a previous stage. The fact that LTE-based systems can operate in different bands with different bandwidths, compared to HSPA that operates in a fixed bandwidth of 5 MHz, makes the deployment of LTE very beneficial and attractive for operators having their allocated spectrum spread over different bands with different bandwidths.
Although using wide bandwidths is efficient in improving data rates, finding Table 2.2: Channel bandwidths specified in LTE
Channel bandwidth (MHz) Number of resource blocks
1.4 6
3 15
5 25
10 50
15 75
20 100
spectrum allocations of large sizes is difficult. Furthermore, using wide trans- mission and reception bandwidths increases the radio equipment complexity at both base and mobile stations. On the other hand, as the transmission occurs in wide band, the transmitted signal propagates to the destination via multiple paths with different delays, leading to an increased corruption of the signal due to time dispersion. In systems operating in a fixed bandwidth such as WCDMA (i.e. 5 MHz), this problem can be dealt with by receiver-side equalization [20].
However, for LTE systems, and especially for bandwidths above 5 MHz, the high performance equalizing is very complex. In order to deal with this issue, multi-carrier transmissions, using Orthogonal Frequency Division Multiplexing (OFDM), are adopted for the downlink of the LTE systems. This modulation scheme allows the transmission of an overall wide-band signal as several narrow band frequency multiplexed signals, using a subcarrier spacing of 15 KHz for 3GPP LTE and with a number of subcarriers that depends on the transmission bandwidth (e.g. approximately 1300 subcarriers for a transmission bandwidth of 20 MHz) [3].
For the uplink transmissions, a single-carrier modulation scheme known as DFT- spread OFDM (DFTS-OFDM) is adopted in LTE. This modulation scheme is equivalent to a normal OFDM scheme with a DFT-based pre-coding and provides small variations in the instantaneous power of the signal and low- complexity equalization. In addition, DFTS-OFDM is based on orthogonal separation of uplink transmissions in the time and frequency domain, unlike the non-orthogonal WCDMA/HSPA, uplink which is also based on single-carrier transmission. The use of these orthogonal separations is beneficial as it pre- vents intra-cell interference.
14 Chapter 2. Review of System Standards
At the core of the LTE transmission scheme, some advanced techniques are used in order to fulfill the requirements set out by 3GPP. Some of these techniques are commonly adopted by HSPA and LTE as mentioned in an early section:
• Channel-dependent scheduling and rate adaptation: As mentioned before, this technique is already exploited in HSPA. The only difference is that LTE is able to take channel variations in both time and frequency domains into account compared to the restriction to the time domain in the case of HSPA.
• Hybrid ARQ with soft combining: This technique is used in both HSPA and LTE for very similar reasons.
• Inter-cell interference coordination (ICIC): Interference in LTE sys- tems is, at least theoretically, restricted to inter-cell interference as intra- cell interference is dealt with by the orthogonal modulation schemes. In order to encounter the effects of the inter-cell interference on the cell edge users, the inter-cell interference coordination scheduling strategy increases the cell-edge data rates by taking into account the inter-cell interference.
• Multiple antenna support: This technique is supported by LTE and can be used in different ways and for different purposes. As dual receive antennas is the baseline for all LTE terminals, suppressing fading and in- terference is made possible. Another way to benefit from this technique is to use multiple transmit antennas at the base station allowing for trans- mit diversity and beam-forming. Beam-forming is mainly used to raise the received SNR and SIR which in turn improves system coverage and capacity. An additional way of using the multiple antenna technique is, the so called, spatial multiplexing (MIMO), based on the use of multi- ple antennas at both transmitter and receiver, resulting in increased data rates.
2.4.2 Supported frequency bands
The spectrum and bandwidth flexibility of LTE made it possible for LTE-based systems to operate in spectrum currently used for other communication systems.
In addition, LTE can be deployed in future bands that may be specified (e.g.
the 800 MHz). The supported frequency bands for both paired and unpaired spectrum are shown in Tables 2.3 and 2.4 [3][21].
In May 2008, the PTS auctioned 190 MHz of the 2.6 GHz band. 50 MHz of TDD was won by Intel Capital Corporation; 2x20 MHz FDD were awarded to Tele2, Telenor and TeliaSonera; Hi3G won 2x10 MHz FDD [22].
In addition, the 800 MHz spectrum was auctioned by the PTS in February 2011 enabling the deployment of LTE in the 800 MHz band. The winners of this auction were TeliaSonera, ”3” and Net4Mobility, which is a joint venture between Telenor and Tele2 [4]. Each winner is offered 2x10 MHz, which is half of what is needed for maximum LTE performance.
2.5. The CDMA2000/450 MHz System 15
Table 2.3: Paired frequency bands defined by the 3rd Generation Partnership Project (3GPP) for LTE
Band Uplink range (MHz) Downlink range (MHz) Main region(s)
1 1920-1980 2110-2170 Europe, Asia
2 1850-1910 1930-1990 Americas (Asia)
3 1710-1785 1805-1880 Europe, Asia (Americas)
4 1710-1755 2110-2155 Americas
5 824-849 869-894 Americas
6 830-840 875-885 Japan
7 2500-2570 2620-2690 Europe, Asia
8 880-915 925-960 Europe, Asia
9 1749.9-1784.9 1844.9-1879.9 Japan
10 1710-1770 2110-2170 Americas
11 1427.9-1452.9 1475.9-1500.9 Japan
12 698-716 728-746 Americas
13 777-787 746-756 Americas
14 788-798 758-768 Americas
15 Reserved Reserved Americas
16 Reserved Reserved Americas
17 704-716 734-746 Americas
18 815-830 860-875 Japan
19 830-845 875-890 Japan
20 832-862 791-821 Europe
21 1447.9-1462.9 1495.5-1510.9 Japan
22 3410-3500 3510-3600 -
Table 2.4: Unpaired frequency bands defined by the 3rd Generation Partnership Project (3GPP) for LTE
Band Frequency range (MHz) Main region(s)
33 1900-1920 Europe, Asia (not Japan)
34 2010-2025 Europe, Asia
35 1850-1910 -
36 1930-1990 -
37 1910-1930 -
38 2570-2620 Europe
39 1880-1920 China
40 2300-2400 Europe, Asia
41 3400-3600 Americas
2.5 The CDMA2000/450 MHz System
The fact that using low frequency ranges increases the geographical coverage for terrestrial IMT-2000 systems has led to the rapid adoption of CDMA2000/450
16 Chapter 2. Review of System Standards
MHz, also known as CDMA 450. This system, using frequency ranges between 450 and 470 MHz, was identified by the ITU-R World Radio communication Conference 2007 (WRC 2007) to be used for International Mobile Telecom- munications (IMT) services on a global basis and as the only 3G (IMT-2000) solution in this frequency band.
Taking advantage of the favorable propagation characteristics of its low fre- quency band, this system is well-suited for providing telecommunications cov- erage, especially over regions with low population densities or difficult terrains.
In other words, the propagation characteristics of the 450-470 MHz band make it possible for base stations to achieve greater coverage areas, leading to fewer base stations needed to cover a certain area.
2.5.1 The CDMA2000 technologies
CDMA2000 is represented by a number of technologies, all of them are based on 3G IMT-2000 standards and use 1.25 MHz carriers [2]:
• CDMA2000 1X: is a circuit-switch voice communications and packet data services, with peak data rates reaching 153.4 kbps in both uplink and downlink directions.
• CDMA2000 1xEV-DO Release 0: is a packet data service, allowing peak data rates up to 2.4 Mbps in the downlink and average throughputs between 300 and 600 kbps. In the uplink direction, peak data rates up to 153.4 kbps are provided by this technology, with average throughputs of 70 to 90 kbps. In addition, this technology is backward compatible with CDMA2000 1X.
• CDMA2000 1xEV-DO Revision A: is an IP-based low latency only supporting packet data services. The peak data rates in the downlink direction are of the order of 3.1 Mbps with average throughputs of 600 to 1400 kbps; while the uplink peak data rates can reach 1.8 Mbps and the average throughputs ranges from 500 to 800 kbps. This technology is also backward compatible with CDMA2000 1X and CDMA2000 1xEV-DO Release 0.
• CDMA2000 Multicarrier EV-DO: is a part of the 1xEV-DO Revi- sion B standard and is realized by a software upgrade of the Revision A networks and intends to increase the peak data rates to 9.3 Mbps and 5.4 Mbps, respectively in the uplink and downlink directions, within 5 MHz.
2.5.2 System features
The system operates initially across the 410-470 MHz band and includes the following sub-bands: 410-430 MHz, 450-470 MHz and 470-490 MHz, but only the 450-470 MHz spectrum allocation was identified by the ITU WRC-07 to deliver IMT-2000 3G services as mentioned earlier.
The frequency band dedicated to this system is divided into an uplink band ranging from 452.5 to 457.5 MHz, and a downlink band ranging from 462.5 to 467.5 MHz [23]. These bands can accommodate cellular broadband services en- abling high-speed data, IP-packet and video.
2.5. The CDMA2000/450 MHz System 17
CDMA (Code Division Multiple Access) is a spread spectrum technology used for CDMA 450 networks; it allows multiple users to occupy the same time and frequency allocations in a certain band [1]. In contrast to some of the 2G, 2.5G and 2.9G standards (e.g. GSM, GPRS, EDGE), CDMA operates by assigning unique codes to each communication in order to differentiate it from other on- going communications that use the same spectrum. This technology was used in some of the 2G networks (e.g. cdmaOne) and represented a basis for 3G services as well (e.g. CDMA2000 and W-CDMA).
Due to the propagation characteristics of the lower frequency band com- bined with the 3G CDMA technology, CDMA2000/450 MHz requires fewer base stations and has better propagation characteristics which, in turn, lead to bigger cells. Furthermore, it allows for more diffraction over undulating terrains, less penetration loss through buildings, less maintenance, lower capital and op- erating costs [1]. A comparison between this system and systems with higher frequency bands shows that the coverage area of CDMA450 is three times larger than in the 900 MHz band, and twelve times larger than in the 1800 MHz and 2100 MHz bands and moreover, with excellent signal-to-noise ratio [2]; These results are presented in Figure 2.2.
Figure 2.2: Coverage provided by 450, 900, 1800 and 2100 MHz
Chapter 3
Radio Propagation and Coverage
This chapter is intended to give an overview on radio propagation characteristics as well as on radio link budget calculations.
3.1 Radio Propagation and Coverage
The coverage area of a base station can be defined as the area within which the base station should be capable of communicating with mobile stations, while maintaining a certain quality of service (QoS). The term ”cell” is very common in mobile networks and is defined as the area covered by one sector or one antenna system, resulting in one or more cells within the coverage area of a single base station. Although the real shape of a cell is non-geometric, with some areas not having the required quality of service, known as ”holes”, they are usually represented by an artificial hexagonal shape not showing any overlapping areas or holes.
These cells can be classified as indoor and outdoor cells. Outdoor cells can, in turn, be classified as macro-cells, micro-cells and pico-cells. In the case of macro-cells, the base station antennas are located above the average roof-top level and the cell range varies from a couple of kilometers to 35 km. Hence, this type of cells is normally used for suburban and rural environments [14]. A more limited type of cells is the micro-cell, ranging from a few hundred meters to a couple of kilometers. In the case of a micro-cell, the base station antennas are installed below the average roof-top level, primarily serving urban and suburban areas. On the other hand, pico-cells are typically used to cover small areas, to extend coverage to indoor areas or to enhance network capacity in areas with high phone usage density.
3.2 Path Loss Propagation Model
In fact, the interaction between the electromagnetic waves and the environment affects the signal strength, resulting in a certain path loss. Different models are used to calculate this path loss and can be categorized into three types:
19
20 Chapter 3. Radio Propagation and Coverage
• Empirical Models: By definition, an empirical model is based on ob- servations and measurements and is used to predict, not explain a system [24]. This category can be in turn split into two subcategories, time dis- persive and non-time dispersive [25]. The former provides information about the time dispersive characteristics of a channel.
• Deterministic Models: A deterministic model tends to determine the received signal power in a particular location.
• Stochastic Models: A stochastic model is used to model the environ- ment as a set of random variables. As a result, least information is required to create this model but its accuracy is questionable.
Since we are interested in studying the propagation characteristics in rural en- vironments, where macro-cells are usually used, the most suitable propagation model is the Hata model, an empirical and non-time dispersive model. The use of this already available model, with respect to some constraints, is proved to be efficient and can be extended to cover a broader range of input parameters.
This model aims at predicting the distance-dependent path loss between a base station and a mobile station and is briefly described in Table 3.1 [26][27]
Table 3.1: Hata path loss model
Urban areas Lu = 69.55 + 26.16 ∗ log(f) − 13.82 ∗ log(Hb)
−a + [44.9 − 6.55 ∗ log(Hb)] ∗ log(d) Suburban areas Lsu = Lu − 2 ∗ [log(f/28)]2− 5.4
Rural (Quasi-open) Lrqo = Lu − 4.78 ∗ [log(f)]2+ 18.33 ∗ log(f) − 35.94 Rural (Open) Lro = Lu − 4.78 ∗ [log(f)]2+ 18.33 ∗ log(f) − 40.94
For suburban and rural areas:
a = [1.1 ∗ log(f) − 0.7] ∗ Hm − [1.56 ∗ log(f) − 0.8] (3.1) For urban areas:
a = 8.29 ∗ [log(1.54 ∗ Hm)]2− 1.1 f or 150 ≤ f ≤ 200 M Hz (3.2) a = 3.2 ∗ [log(11.75 ∗ Hm)]2− 4.97 f or 200 < f ≤ 1500 M Hz (3.3) Where
d : Distance between transmitter and receiver antenna [km]
f : Frequency [MHz]
Hb: Transmitter antenna height [m]
Hm: Mobile station height [m]
These equations are valid for frequencies ranging between 150 and 1500 MHz, base station heights between 30 and 200 m, mobile heights between 1 and 10 m and distances between base station and mobile station in the order of 1 to
3.3. Fading 21
20 km.
Two limitations of the Hata model are the limited path length and the limited frequency range. However, a number of modified models have been produced to extend the path length and frequency range.
3.2.1 The Cost 231 extension
Cost 231-Hata model is initiated as an extension of Hata model, enabling path loss prediction in the frequency range 1500 to 2000 MHz [28]:
Lu = 46.3+33.9∗log(f)−13.82∗log(Hb)−a+[44.9−6.55∗log(Hb)]∗log(d)+Cm (3.4) Where Cm = 0 dB for median sized city and suburban centres with moderate tree density and Cm = 3 dB for metropolitan centres.
3.2.2 The ITU-R extension
Another modified model uses the ITU-R extension in order to extend the range of the original Hata model to 100 km [29].
Based on a comparison between the Hata path loss model and a number of curves obtained from field measurements performed in Swedish rural areas [8], it can be seen that the Hata model for suburban areas is the most suitable model to represent path loss in rural Sweden. The complete path loss model used in this work, including the needed extensions, is described in Table 3.2.
Table 3.2: The extended path loss model
Urban areas
Frequency range: 150-1500 MHz Lu = 69.55 + 26.16 ∗ log(f) − 13.82 ∗ log(Hb) Distance range: 1-20 km −a + [44.9 − 6.55 ∗ log(Hb)] ∗ log(d)
Urban areas
Frequency range: 150-1500 MHz Lu = 69.82 + 26.16 ∗ log(f) − 13.82 ∗ log(Hb) Distance range: 20-100 km −a + [44.9 − 6.55 ∗ log(Hb)] ∗ (log(d))b
Urban areas
Frequency range: 1500-2000 MHz Lu = 46.3 + 33.9 ∗ log(f) − 13.82 ∗ log(Hb) Distance range: 1-20 km −a + [44.9 − 6.55 ∗ log(Hb)] ∗ log(d) + Cm
Suburban areas Lsu = Lu − 2 ∗ [log(f/28)]2− 5.4
b = 1 + [0.14 + 0.000187 ∗ f + 0.00107 ∗ Hb
(1 + Hb2∗ 7 ∗ 10−6)0.5] ∗ (log(d
20))0.8 (3.5)
3.3 Fading
The wave propagation between a Base Transceiver Station and a Mobile Station may be affected by many factors, resulting in a variation in the signal level which
22 Chapter 3. Radio Propagation and Coverage
leads in turn to a varying coverage and quality of service. The following two sections will explain the concept of two such factors, the shadow fading and the fast fading effects.
3.3.1 Shadow fading
Depending on the environment and the surrounding objects, the received signal strength at a given distance from the transmitter will be different. In effect, distance-dependent path loss models, such as the Hata-model described in the previous section, provide the mean value of the path loss that can be expected if the distance between the transmitter and receiver is r. However, the actual path loss will vary around this mean value depending on the location of the receiver. This variation is mainly caused due to the signal being blocked from the receiver by buildings or other objects, and is referred to as Shadow fading, even known as Slow fading. Several measurements and simulations have shown that, at a given distance r from the transmitter, the path loss Lp(r) is a random variable having a log-normal distribution about the mean distance-dependent value predicted by the Hata path loss model. Thus, the path loss Lp(r) can be expressed in terms of Lp(r) Hata plus a random variable X:
Lp(r) = Lp(r) Hata(dB) + X(dB) (3.6) Here X is normally distributed in the logarithm domain with zero mean. The standard deviation of this log-normal distribution depends on the environment, with typical values about 8 dB in urban areas, 10 dB in dense urban areas and 6 dB in suburban and rural areas [30].
The log-normal probability density function is given by [31]:
f X(x; µ, σ) = 1 xσ√
2π∗ exp[−(lnx − µ)2
2σ2 ] , x > 0 (3.7) The corresponding probability density function is depicted in Figure 3.1 for different standard variation values σ and zero mean µ.
Figure 3.1: The log-normal probability density function
3.3. Fading 23
As shadow fading may result in shadowed areas, where the received signal strength is insufficient to correctly detect the information, a certain margin, known as fade margin, needs to be added to the radio link budget in order to overcome the shadow fading effects and ensure a desired level of signal coverage.
The distribution of X in 3.6 is used to determine the appropriate fade margin.
At the fringe locations, the mean value of the shadow fading is zero dB. Fifty percent of the locations have a positive fading component, and the remaining fifty percent have a negative fading component, which means that the locations having a positive fading component will experience a larger path loss resulting in insufficient signal strength. In fact, the fade margin technique is employed to move most of these locations to within a sufficient received signal strength value.
This fading margin can be applied by either increasing the transmit power while keeping the cell size unchanged, or by reducing the cell size as it will be the case in this project.
As mentioned above, the required fade margin depends on the desired area cov- erage probability, defined as the probability that the signal level in a given area is above a certain threshold value. In addition, this margin depends on the stan- dard deviation of the log-normal fade distribution and the path loss exponent.
As the path loss exponent depends on the environment, a suitable value to be used in this project would be 3.71 [32]. Considering 6 dB as the standard deviation of the log-normal fade distribution, the location probability can be extracted from the curves in Figure 3.2 [33].
For area coverage of 95%, the location probability at the cell edge is 75% (For
Figure 3.2: Useful fraction of cell area - Hexagonal cell
hexagonal cells). As a result, the corresponding fade margin is found to be 4.2 dB [34].
3.3.2 Fast fading
Fast fading, also known as small-scale fading, accounts for the rapid variation of signal levels when the mobile terminal moves within a small area. In most
24 Chapter 3. Radio Propagation and Coverage
cases, this variation is due to the transmitted signal being reflected, scattered, diffracted and absorbed numerous times along its propagation path before reach- ing the receiver. As a result, the receiver will be subject to a number of incoming waves arriving at different angles and with different amplitudes.
Fast fading can be modeled by means of distributions, e.g. Rayleigh distribution and Rician distribution, based on the characteristics of the propagation path.
• The Rayleigh distribution: The Rayleigh distribution is commonly employed to describe the statistical time varying nature of the received envelope where no Line Of Sight (LOS) propagation path exists between the transmitter and the receiver. The probability density function of the Rayleigh distribution is illustrated in Figure 3.3 [35].
Figure 3.3: Probability density function of the Rayleigh distribution
• The Rician distribution: On the other hand, if the signal is transmitted over an environment where, in addition to the presence of many reflecting objects around the receiver, a LOS path between the transmitter and the receiver exists, a suitable distribution would be the Rician distribution represented by the probability density function of Figure 3.4 [36]. How- ever, this distribution represents a special case and will not be used in this report.
3.4 Interference
Interference is a crucial factor affecting mobile communication systems in dif- ferent ways depending on the characteristics of the communication.
• The GSM system: Due to the fact that, in a GSM system, the same fre- quency channels can be reused in many different cells results in Co-channel Interference. As the name indicates, Co-channel Interference refers to the interference caused by the use of the same frequency channel by users in different cells. Assuming that the GSM system has a reuse factor k = 4
3.4. Interference 25
Figure 3.4: Probability density function of the Rician distribution
and that the base stations consist of 3-sectored antennas, the reuse dis- tance can be calculated as: D = 3√2kR. Thus, if we consider a symmetric hexagonal cell plan, each cell will have exactly 6 co-channel neighbors at distance D. In addition there are 6 additional co-channel cells at distance , 6 at distance and so on. As a result, the signal-to-interference ratio at a certain terminal can be expressed as [37]:
SIR =
cP R4
6cP
D4 +6cP9D4 +16D6cP4 + . . . (3.8)
• The 3G systems: This generation of cellular standards operates by spreading the signals from and to different users using different codes.
In addition, the use of non-orthogonal separation of uplink transmissions causes the communication system to be subject to Intra-cell Interference, i.e. within the same cell. The received power from one user end represents interference for other terminals. Hence, the amount of tolerable interfer- ence level in the cell is the limiting factor.
Similarly, Inter-cell Interference is caused by user ends belonging to neigh- boring cells and transmitting with relatively high powers.
In the case of UMTS systems, the interference in the uplink depends on the load of the node B and can be extracted from the expression below:
IU L = −10log10(1 − n) (3.9)
Where n is the load of node B, e.g. 50%
For the CDMA 450 system, the amount of interference in the system is a function of the load as it can be seen in Figure 3.5 [38].
• The LTE system: The use of DFTS-OFDM modulation scheme, which is equivalent to a normal OFDM scheme with a DFT-based pre-coding, provides an orthogonal separation of uplink transmissions in the time and
26 Chapter 3. Radio Propagation and Coverage
Figure 3.5: Interference margin as a function of the load in CDMA 450 systems
frequency domain. As a result, intra-cell interference can be prevented in LTE-based systems and the interference can be limited to inter-cell interference. However, some LTE systems use Inter-cell Interference co- ordination to limit the effects of inter-cell interference, especially on cell edge users. This scheduling strategy aims at assigning different parts of the spectrum to users located near the cell edges while keeping the whole spectrum available for users located at a certain distance from cell edges.
In order to estimate the interference in LTE systems, some simulations and measurements are needed. Table 3.3 shows the results of a simulation done in [39].
Table 3.3: Interference margin as a function of the load in LTE systems
Load [%] Interference margin [dB]
35 1
40 1.3
50 1.8
60 2.4
70 2.9
80 3.3
90 3.7
100 4.2
3.5 Link Budget Calculations
The planning of any Radio Access Network begins with a Radio Link Budget.
As the name indicates, link budget calculations account for all the gains and losses from the transmitter, through the medium to the receiver. Once the link budget calculation is completed, the maximum allowed signal attenuation be- tween the mobile and the base station is obtained. At this level, the maximum
3.5. Link Budget Calculations 27
path loss is mapped to the corresponding cell size using the propagation model introduced in Table 3.2.
As mentioned before, the propagation model converts the maximum allowed path loss into a maximum cell range depending on environment characteristics, frequency and if applicable also atmospheric conditions.
Table 3.4 lists the parameters needed in an uplink budget calculation whereas Table A.1 and Table A.2 in Appendix A show the uplink budget calculations corresponding to the involved Radio Access Network technologies for respec- tively data and voice communications.
Table 3.4: Link budget parameters
Notation Parameter Unit
a UE Maximum TX power:different power classes have different power levels dBm
b TX antenna gain dBi
c Body loss dB
d EIRP:calculated as a+b-c dBm
e Base station RF noise figure:depends on the implementation design dB
f Thermal noise dBm
g Receiver noise floor:calculated as e+f dBm
h SINR:estimated from link simulations or measurements dB
i Receiver sensitivity dBm
j Interference margin dB
k Cable loss dB
l RX antenna gain:depends on the type of the antenna dBi
m Fast fading margin dB
n Soft handover gain dB
Chapter 4
Simulation
The main objective of this thesis project is to estimate the improvement in coverage in case the operator diversity technique is used. For this purpose, a model representing the overlapping communication systems, i.e. the operator diversity technique, has been created using a suitable modeling tool. The model as well as the input parameters can easily be modified allowing the model to be applied in different situations and under completely different assumptions. The following sections give a detailed description of the simulation tool, performance measures and simulation model.
4.1 Simulation Tool
Mathematica (v.8.0) is a computational software program, developed by Wol- fram Research and used in many areas of technical computing. The main advan- tage of using Mathematica in this project is the availability of various functions, tool boxes and statistical models that are relevant to the wireless communication field. These functions and tools are mainly provided by the package ”Wipack (v.1.9.0 for Mathematica v.8.0)” developed by G¨oran Andersson, KTH-Radio Communication Systems and supporting computation and graphics in the wire- less communication field. A sample code is available in Appendix C and tends to give an insight of how mobile networks as well as operator diversity can be modeled in Mathematica.
4.2 Performance Measures
One essential measure of the benefit with operator diversity is the achievable coverage probability. In this paper, the improvement in coverage is tracked starting from the real case scenario where only one communication system is used and investigating the improvement in coverage at each time an additional communication system is considered.
29
30 Chapter 4. Simulation
4.3 Simulation Model
The main objective of the simulation part is to estimate the improvement in terms of coverage probability in case the operator diversity technique is adopted.
The following two sections will provide a detailed description of the simulation model as well as the different phases of the simulation process corresponding to both data and voice communications.
4.3.1 Data communication model
Throughout the simulations we assume that a number of communication systems coexist. Each communication system represents a different technology and is modeled based on the radio link budget calculations performed in an earlier stage of the project. The concept of the model consists of a number of overlapping communication networks as illustrated in the Figure 4.1.
Figure 4.1: Illustration of the diversity model
The initial state of the simulation model consists of the presence of one communication system, CDMA 450 as it is the one that is currently in use by the forest industry. A corresponding network is modeled in Mathematica based on the simulation parameters summarized in Table 4.1. The mobile stations are considered to be stationary and uniformly distributed, covering the whole cell.
The next step is to calculate the coverage probability obtained in the CDMA 450 network by evaluating the availability of coverage for each mobile station.
It can be predicted that the coverage probability will strongly depend on the load in the system and on the log-normal shadow fading.
The second step is to introduce another communication system, (e.g. EDGE 900) to the model which means that mobile stations will have the possibility to connect to this new system in case they experience a lack of coverage with the CDMA 450 communication system. At this stage, the probability for service is recalculated as well as the improvement in coverage that can be expected when using the two systems.
4.3. Simulation Model 31
The same procedure is followed when introducing the LTE 800 and the HSPA 2100 communication systems. The resulting graphs will show the median coverage probability as well as the improvement in coverage starting with the case in which only CDMA 450 is considered and ending with a model consisting of multiple overlapping communication systems.
4.3.2 Voice communication model
The voice communication model slightly differs from the previously described model. The major difference in this case is the fact that neither CDMA 450 nor LTE are currently available for voice services. It should be noted that the term
”voice communications” is used in this context to refer to the regular mobile phone call services and does not include services such as Voice over Internet Pro- tocol (VoIP). Consequently, these two communication systems will be excluded from the diversity model. However, the fact that there are several independent GSM and 3G networks outside the main urban areas makes it possible to benefit from the presence of different operators. For instance, there are two indepen- dent GSM networks; the first one is deployed by TeliaSonera while the second one is shared by Telenor and Tele2. Similarly, there are two independent 3G networks, TeliaSonera and Tele2 share one of them while Telenor and ”3” share the other one. Thus, the diversity model in this case consists of two independent GSM networks and two independent 3G networks.
A similar concept is adopted for the simulation of the voice communication model. As mentioned before, the main distinction resides in the use of two GSM 900 and two 3G 2100 networks instead of four different networks. Since the forest industry is mainly relying on the use of GSM for voice communications, the coverage probability achieved by utilizing a single GSM network is first estimated and then compared to the cases where more than one network is utilized. The parameters applied in the simulation can be found in Table 4.1.
For a detailed description of the link budget calculations and network design, for both data and voice communications, the reader is referred to Table A.1 and Table A.2 in Appendix A.
32 Chapter 4. Simulation
Table 4.1: Simulation parameters
Parameter Value
Cell load [%] 35, 50, 90
Number of networks 4, 6
Number of sectors per base station 3 Shadow fading correlation between networks 0.5 Standard deviation (Shadow fading) [dB] 6
Shadow fading margin [dB] 4.2
Base station height [m]
EDGE 900 60
HSPA 2100 60
LTE 800 60
CDMA 450 100
Mobile station height [m] 1.5
Cell radius [km]
Data communication model
EDGE 900 6.1
HSPA 2100 10.1
LTE 800 21.6
CDMA 450 15.3
Voice communication model
GSM 900 21.1
3G 2100 10.1
Chapter 5
Field Measurements
At this stage, the achieved service characterization will be estimated by means of field measurements and tests. The field measurements will focus on gather- ing two types of information, the Received Signal Strength and the Bit Rates achieved in each communication system. The main intention is to develop and test a method for performing these field measurements so that it can be made available for future work aiming at obtaining statistical results. In addition, these field measurements will serve as a proof of the importance of the operator diversity technique.
5.1 Received Signal Strength Measurements
5.1.1 Equipment
The Test Mobile System (TEMS v.13.0) is a tool provided by Ascom, mainly used to plan, implement and optimize networks. An important feature of TEMS v.13.0 is that it supports LTE communication systems.
The mobile phone or modem will be communicating with the base station through TEMS which allows the visualization of different types of information including the Received Signal Strength. The operators TeliaSonera (GSM/EDGE and UMTS), Telenor (GSM/GPRS and UMTS) and Net1 (CDMA2000/450) will be involved in the measurements. The equipment used in connection with the field measurements consist of: A TEMS mobile phone (Sony Ericsson Z750i supporting GSM/EDGE/UMTS/HSPA) with a corresponding antenna and usb cable to connect to a laptop, a 4G broadband modem (Samsung GT-B3730), a CDMA EV-DO Rev A broadband modem (D-50) with a corresponding antenna.
The equipment are shown in Figure 5.1.
5.1.2 Purpose of measurements
In order for the field measurements to be relevant to Swedish rural applications, it was desired to carry out the measurements in a rural area of the same type as the one where the forest industry is working, i.e. coniferous forest. In addition, and in order to be able to test the benefits of operator diversity, the area should suffer from white spots, i.e. lack of coverage in some locations, from the involved
33
34 Chapter 5. Field Measurements
Figure 5.1: Measurement equipment
operators TeliaSonera, Telenor and Net1. An area fulfilling the criteria above was found in Nykvarn. Figure 5.2 shows a map over the measurement area and the positions of the 7 different measurement locations.
Figure 5.3 provides a description of the measurement setup including software and hardware equipment used in connection with field measurements.
This first part of the measurements aimed at collecting two types of informa-
Figure 5.2: Measurement area tion:
5.2. Bit Rate Measurements 35
Figure 5.3: Measurement setup
1. The received signal level, which provides real case information about the signal level and quality and would be of interest in comparing the different systems.
2. Coverage information, allowing us to evaluate the coverage at some specific locations (i.e. whether there is coverage or not).
The same measurements were conducted for the three operators and for the different technologies that are provided by each operator (i.e. 2G, 3G and 4G provided by both TeliaSonera and Telenor, while CDMA 450 is provided by Net1).
5.2 Bit Rate Measurements
Bit rate measurements are performed using the speed testing service provided by bredbandskollen.se. The location and the involved communication systems are the same as in the Received signal Strength measurements. The purpose of the bit rate measurements is to get an insight into the bit rates that can be achieved by each communication system in rural Sweden.
