Chapter 1 Fundamentals of Cellular Communication
In this chapter, all the background knowledge which is required for this project has been discussed.
The area covered by single BTS(base transceiver station) is known as cell.
1.1.1 Shape of cell
The shape of cell depends upon the coverage of the base station. The actual coverage of the base station is called footprint and is found with the help of measurements from the field. We can make our calculations easier by using the shape of circle noting that there would not be spaces between them. As, the purpose is to provide coverage to each and every subscriber. But if there are spaces between the coverage areas then the person in that specific area will not be able to get any coverage.
To cover the problem of interleaving spaces, the shapes that can be used theoretically are:
But in selection criteria one thing must be kept in mind that every person within a cell get same coverage specially the person at the edges of the cell. So hexagon is the shape among these three choices with largest coverage area. Its coverage area and shape is closest to the circle and it helps tessellate. Omnidirectional antenna is used in the center of it, and if we want to use sectored directional antenna then it must be used at any three corners of it.
1.1.2 Area of the Cell
The area of a cell with radius R is shown in figure 1.1(a), is given by:
1.2 Frequency planning
While developing the cellular system, it has limited capacity due to the given bandwidth. So, in order to solve this problem Cellular Systems have to depends on an intelligent and more use of channels through out the area. Every cellular base station is alloted a group of different radio channels to be used in a cell. Base station in the adjacent cells use completely different frequencies. For this purpose antennas are used such that their power may get limited within the cell. In this way the allocated frequencies maybe reused in different cells again. The process of allocating and selecting channel groups for all the base stations in a system is known as frequency reuse or frequency planning.
We use two types of antennas:
- Omnidirectional antenna
- Sectored directional antenna
Omnidirectional antennas are used in the cells which are centrally excited and sectored directional antennas are used in the edge excite cells.
To understand the concept of frequency reuse, let us say that S are the total no. of duplex channels available for use, k number of channels given to each cell i.e. k<s, n=”” are=”” the=”” no.=”” of=”” cells=”” in=”” which=”” s=”” channels=”” divided.=”” total=”” number=”” is=”” denoted=”” by:<=”” p=””></s,>
Where N is no. of cells which uses the complete set of available frequencies known as cluster
frequency reuse factor (1.3)
Each cell is in the cluster is assigned of the available channels.
The radio frequency from 3Hz to 3000GHz are separated into 12 bands, as shown in the table. Frequency spectrum has different propagation characteristics. As far as concerned to the mobile communication, we only pay attention to the UHF spectrum.
1.2.1 Cluster size(N)
If we use N large (a large cluster), the ratio of the cell radius and the distance between co-channel decreases, which causes weaker co-channel interference. But if N is smaller, by keeping the cell size same then we more clusters are needed to cover an area. Hence the capacity is increased. So if we use N larger then the quality of voice is good but the capacity is less and vice versa.
Interference is one of the major factor in the capacity and performance of a cellular network. The interference is due to a call in the neighbouring cell, another base station operating in the same frequency. Interference causes crosstalk and noise. There are two types of interference.
- Adjacent channel interference
- Co-channel interference
1.3.1 Adjacent channel interference
Adjacent channel interference results from the signals which are side by side in frequencies to the desires signal. Adjacent channel interference is caused by wrong filtering, like incomplete filtering of not wanted modulation in frequency modulation (FM) systems, not proper tuning, or poor control of frequency. It causes problem.
Adjacent channel interference can be reduced by careful channel assignment, filtering and power control within a cell.
1.3.2 Co-channel interference
Co-channel cells are the cell which use the same set of frequencies. For example, in the figure 1.2 all the letter ‘A’ are the co-channel cell because they use the same set of frequencies. Interference due to the co-channel cells is called co-channel interference. It can be reduced by using greater value of N(cluster size). If D is the distance between the co-channel cells and R is radius of the cell, then by using greater value of N the ratio between D to R is increased hence reducing co-channel interference.
The relation can b written as:
1.4 Improving coverage and capacity
The number of channels assigned to a cell became insufficiently as the demand of wireless system increases. To provide more channels per coverage, some techniques are introduced which improve the coverage and capacity. These techniques are:
- Cell splitting
- Microcell zone concept
1.4.1 Cell Splitting
Cell splitting is the process of dividing a cell into smaller cells. In this process we reduce the antenna height and power of the base station. Cell splitting increases the capacity by increasing frequency reuse factor.
In cell splitting
- Channel assignment techniques remain the same.
- SIR remains the same
- Trunking inefficiency do not get suffer.
Trunking efficiency is the measure of the number of users which can be offered a particular Grade of service with the specific configuration of the channels.
The grade of service (GOS) is the measure of the ability to access a trunked system during the busy hours.
The radius of the new cell is reduce to half. So power is also reduced.
Sectoring uses directional antennas for controlling the interferences and frequency reuse of channels. The co-channel interference is reduced and thus increasing system performance by using directional antenna. A cell is normally divided into three 120 sectors or six 60°sectors.
When sectoring is used, the channels used in a particular cell are broken into sectored groups and are used only within a particular sector. The no. of channels get divided into sectored groups, so the trunking efficiency is reduced. In sectoring SIR is improved by reducing interference and trunking efficiency is reduced. Handoff increased in sectoring. The s/I improvement allows to decrease the cluster size N in order to improve the frequency reuse, and thus the system capacity. Further improvements in s/I is achieved by downtilting the sector antennas.
1.4.3 Microcell Zone Concept
Microcell Zone concept distributes the coverage of a cell and extends the cell boundry to hard to reach places. It maintains the S/I and trunking efficiency, and increases the coverage and capacity of an area.
1.5 Radio wave propagation
Radio waves propagate through different channels and by different ways to reach the MS(Mobile Station). It also depends upon the speed of the wave. The propagation of radio waves depends into two types:
- Large scale propagation
- Small scale propagation(Fading)
1.5.1 Large scale propagation
The model predicts that the average signal strength for all transmitter-receiver (TR) distance on a scale known as large scale propogation model.
1.5.2 Small scale propagation
The models that predicts the rapid fluctuation of the received signal strength over a short distance known as small scale propagation model or fading.
1.5.3 Free Space Propagation Model
The free space propagation model is used when the transmitter and receiver have line of sight (LOS) between them to predict the received signal strength.
Pr = received power.
Pt = transmitted power,
Gt and Gr = transmitter and receiver antenna gain,
do= T-R separation,
L = system loss factor
λ = wavelength.
1.6 Propagation Mechanisms
The propagation mechanisms which effect propagation are:
- Reach directly (in case of Line of Sight)
If there is line of sight signal reach the Mobile station directly and signal power is very strong.
Reflection occurs when an electromagnetic wave falls upon an object which is large as compare to the wavelength of the wave. It occurs from buildings, walls, surface of earth etc.
Diffraction happens when the path between the transmitters and receivers is disturbed by a surface with sharp edges. It source is any sharp edge object. Knife edge diffraction Model is used for diffraction.
Scattering occurs when an electromagnetic wave falls upon an object which has small dimension as compared to the wavelength of the wave. Scattering occurs due to small objects, rough surfaces or any irregularities. Objects such as lamp posts, trees scatter the radio waves. Radar Cross Section Model is used for sectoring.
1.7 Small Scale Fading
Fading is the fluctuation in the received signal strength over very short distance. Fading is due to reception of different versions of same signals. Following are the factors which influence Small-Scale Fading are:
Due to absence of LOS signal follows the multipath due to reflection, diffraction, scattering.
Speed of the mobile:
Fading also accurs due to the movement of the mobile as the signal strength changes.
Speed of the surrounding objects:
Fading also occurs due to the movement of mobile, if the speed of the surrounding object is much faster then the speed of the mobile then it also induces Doppler shift.
The transmission BW (bandwidth) of the signal:
The received signal is distorted if the transmitted signal bandwidth is greater than the bandwidth of the channel.
The first GSM network was launched in 1991. The GSM network was structured hierarchically. It consists of one administrative region, which is assigned to MSC. Each administrative region is consists of at least one location area (LA). LA is also called the visited area. An LA consists of several cell groups. Each cell group is assigned to a base station controller (BSC). Cells of one BSC may belong to different LAs. GSM distinguishes explicitly between users and identifiers. The user identity associates with a MS by mans of personal chip cards, the subscriber identity module (SIM). The SIM is portable and transferable MSs. The mobile Roaming number is a temporary location-dependent ISDN number. It is assigned by a locally responsible Visited Location Number (VLR).
The GSM network can defined into four major parts.
- Mobile station (MS).
- Base station Sub-system (BSS).
- Network and switching Sub-system (NSS).
- Operation and support Sub-system (OSS).
1.8.1 Mobile station
A mobile station consists of two parts.
- Mobile equipment and terminal.
- Subscriber identity module (SIM).
1.8.2 THE Terminal
There are different types of terminal distinguished principally by their power and application:
- The fixed terminals are installed in cars.
- The GSM portable terminals can be used in the vehicles.
- The hand held terminals have experienced a biggest success depending upon their weight and volume, which are decreasing continuously. These terminals can emit power of 2 w. The evolution of technologies decreases the maxpower to 0.8 watts.
- Sim is a smart card which identifies the terminal.
- Using the sim card in the mobile, the user can access all the services provided by the provider.
- Terminal does not operate without the sim,.
- Personal identification number(PIN) helps protect sim.
1.9 The Base Station Subsystem
- The BSS connects the MS to Network Switching Sub-system. It is incharge of transmission as well as reception.
- The BSS is further divided into two main parts.
- Base transceiver station (BTS) or base station.
- Base Station Controller(BSC).
1.9.1 The Base Transceiver Station
- The BTS deals with the transceivers and antennas which are used in each cell of a network.
- BTS is usually in the center of cell.
- Size of the cell is defined by its transmitting power.
- Each BTS has one to sixteen transceivers which depends upon the density of users.
1.10 The Base Station Controller
- The BSC controllers the group of BTS and manages radio resources.
- The BSC is incharge of handover, frequency hoping and exchange of radio frequency power level of BTSs.
1.11 The Network and Switching Subsystem
- It is to manage the communication between mobile and other users, such as ISDN users, telephony users.
- It store the information in data bases about the subscriber and manage their mobility.
1.12 The Mobile Services Switching Center (MSC)
- It is the central component of the NSS.
- Network Switching Functions are performed by the MSC.
- It provides connection to more other networks.
Chapter 2 Planning
One of the important phase of the project in which all the detail information is gathered about different areas and their population including city boundary, market analysis and roads are the key features in these details are city profiling. This phase is divided into different tasks.
2.1 Lahore City Map
First is to get the detailed map of the Lahore city, which includes all the aspects related to the project. These are following:-
- Dense area
- Sub-urban area
- open area
Boundaries of City
2.2 Boundary Marking
The project “Radio Frequency Planning ” is basically the frequency planning of the city, not to its belongings areas. The exact boundary of the city is marked in order to concentrate on the marked area.
Population of the city plays an important role in the frequency planning. It helps a lot in the estimations and assumptions. The population of the city is around 10 million.
2.4 Estimations and Assumptions
This part is mainly concerned with the frequency planning. When a new telecommunication company comes in the market, it estimates it users. This estimation is done with respect to the total population of the particular area. The estimations are done to estimate the users on urban, suburban and open areas.
2.5 Area Division
The area division depends upon the percentage of population in an area and type of area as it is the important factor in the site as wall as frequency planning. The Lahore city is divided into three major areas.
2.5.1 Urban Area
Urban area is an area which is surrounded by more density of humans and structures in comparison to the areas surrounding it
2.5.2 Sub-Urban Area
Suburban area is districts located either inside a town or city’s outer premises or just outside its limits.
2.5.3 Open Area
Open area is partially settled places away from the large cities. Such areas are different from more intensively settled urban and suburban areas. There are less population as compared to urban and sub-urban areas.
2.6 Site Planning
2.6.1 Map of Lahore
2.6.2 Urban Area
2.6.3 Sub-Urban Area
2.6.4 Open Area
HATA Model for Urban Area
= Path loss in Urban Areas in decibel (dB)
= Height of base station in meters (m)
= Height of mobile station Antenna in meters (m)
= Frequency of Transmission in megahertz (MHz).
= Distance between the base station and mobile stations in kilometers
To calculate radius of a site of Urban Area
=-75 dBm(this power covers both indoor and outdoor coverage range -70 to -90 dBm )
= 35 m(Average height of antenna in city is 30 to 200 m)
= 1.5 m
= 13 dBm
= 46 dBm (Max Power transmitted by Base Station)
= Cable loss = 2.01 dBm
= 945 Mhz (Downlink frequency 935 to 960 MHz)
= Combine Loss= 5.5 dBm
Putting in HATA equation
= -102 dbm(Min Power received by Base Station)
= 29.1 dBm (Max transmitted power mobile)
= 900 MHz (890 to 915 MHz)
Putting in HATA equation
We will be using d=0.90 Km as it covers both Uplink and Downlink.
For Sub-Urban Area
For downlink of Suburban parameters are same as for Urban.
Uplink parameters are also same as Urban Areas
We will be using d=2.32 Km for Suburban Area.
For Open Areas
For downlink parameters are same as Urban Areas
We will be using d=8 for Open Areas.
We will be using 65 degree directional Antennas.
Angle between 2 consecutive lobes is 120 degree.
r=Radius of lobes
For Full Lobe
For All 3 Lobes
Area of site in Urban
Area of site in Suburban
Area of site in Fields(Open Area)
Calculations for Number of BTS
2.7 Frequency Planning
One of the breakthrough in solving the problem of congestion and user capacity is the cellular concept. Cellular radio systems rely on reuse of channels throughout a coverage region. A group of radio channels are allocated to each cellular base station to be used within a area known as cell. Different channels are assingned in the adjacent cells of the base station. The same group of channels can be used by limiting the coverage area, within the boundaries of a cell to cover different levels, within tolerable limits. Frequency planning is the design process of selecting, allocating or assinging channel group stations within a system.
The theoretical calculations, and fixed size of a cell is assumed, that can differentiate no of channels in a cell and from that can differentiate cluster size and will differ, the capacity of the cellular system. There is a trade between the interference abd capacity in theoretical calculation as if we reduce the cluster size more cells are needed to cover the area and more capacity. But from another perceptive small cluster size causes the ratio between cell radius, and the distance between co-channels cells to increase, leading to stronger co-channels interference.
In practical calculations, a fixed no of channels are allocated to a cell. One channel per lobe 3channels are allocated to a cell. The capacity can be increased by allocating 2 channels per lobe or 6 channels per cell. But after allocating channels once, they will remain fixed for the whole cellular system and frequency planning.
Now as with the fixed no of channels as per cell, the capacity will remain constant of the system and we can achieve weaker co-channel interference, by having a small cluster size(N). A cluster size of 7 is selected in this project, which is also discussed. So in later practical world , there is not a trade-off between capacity and co-channel interference.
The city of Lahore is divided into 120 cells. We take 3 channels per cell that gives us
1 cell = 3 channels
Reuse factor = 1/N = 1/7
Which means that frequency can be reused after a cluster of 7 cells. That gives us the total of
7 x 3 =21+ 2(guard cells)=23 channels
We will be using 23 channels with a reuse factor of 1/7.
2.8 Implementation in GAIA
Figure 2.1 is a snapshot of GAIA planning tool showing us the structure of an urban area. This figure illustrates the urban boundary which we calculate during city profiling. It also shows the antenna system used, in this case 3 sectors with 120 degree azimuth spacing is used. Antennas are installed on the rooftop of buildings or houses due to dense population and to provide a better coverage.
Figure 2.2 shows us the planning of a Sub-Urban area with sites more distance apart as population is less, compared to urban. In Sub-Urban 3 sector cell is used which is similar to the ones used in Urban
Figure 2.3 shows us the coverage planning of a network in an open area. Here the sites are further apart as open area has least population. 3 sector cell is used with the antennas installed above a steel structure for better coverage.
Figure 2.4 shows the sector wise cell area of the sites in the urban area of the city in GAIA, which can be differentiated with the help of different color for each sector, also it shows the coverage area of every site. We have used grid approach in this planning, it is the most widely used and most effective technique used theoretically and practically.
Figure 2.5 shows the cell boundary of sites in Sub-urban area of the city.
Figure 2.6 shows the cell boundary in the open area of the city.
Figure 2.7 illustrates the signal strength in the urban area of the city. Because of the dense population the signal power is strong throughout to ensure high quality calls to the subscribers with minimum interference and call drop.
Figure 2.8 shows the 2G signal strength in the Sub-urban areas where population density is low and so the power required is less as compared to urban areas.
Figure 2.9 shows the serving signal strength in open area. The signal is the weakest as there is the least number of people in open area.
CHAPTER 3 FUNDAMENTALS OF 3G
The Universal Mobile Telephony System (UMTS) or 3G as it is known is the next big thing in the world of mobile telecommunications. It provides convergence between mobile telephony broadband access and Internet Protocol (IP) backbones.
This introduces very variable data rates on the air interface, as well as the independence of the radio access infrastructure and the service platform. For users this makes available a wide spectrum of circuit-switched or packet data services through the newly developed high bit rate radio technology named Wideband Code Division Multiple Access (WCDMA). The variable bit rate and variety of traffic on the air interface have presented completely new possibilities for both operators and users, but also new challenges to network planning and optimization.
The success of the technology lies in optimum utilization of resources by efficient planning of the network for maximum coverage, capacity and quality of service. This part of our project aims to detail method of UMTS Radio Network (UTRAN) Planning.
The new technologies and services have brought vast changes within the network planning; the planning of a 3G network is now a complex balancing act between all the variables in order to achieve the optimal coverage, capacity and Quality of Service simultaneously.
In UMTS access scheme is DS-CDMA (Direct Sequence CDMA) which involves that a code sequence is directly used to modulate the transmitted radio signal with information which is spreaded over approximately 5 MHz bandwidth and data rate up to 2 Mbps.
Every user is assigned a separate code/s depending upon the transaction, thus separation is not based on frequency or time but on the basis of codes. The major advantage of using WCDMA is that there is no plan for frequency re-use.
3.3 NODE B
Node B functions as a RBS (Radio Base Station) and provides radio coverage to a geographical area, by providing physical radio link between the UE (User Equipment) and the network. Node B also refer the codes that are important to identify channels in a WCDMA system.
It contains the RF transceiver, combiner, network interface and system controller, timing card, channel card and backplane.
The Main Functions of Node B are:
- Closed loop power control
- CDMA Physical Channel coding
- Modulation /Demodulation
- Micro Diversity
- Air interface Transmission /Reception
- Error handling
Both FDD and TDD modes are supported by Single node B and it can be co-located with a GSM BTS to reduce implementation costs. The conversion of data from the Radio interface is the main task of Node B. It measures strength and quality of the connection. The Node B participates in power control and is also responsible for the FDD softer handover.
On the basis of coverage, capacity and antenna arrangement Node B can be categorizes as Omni directional and Sectorial:
- OTSR (Omni Transmitter Sector Receiver)
- STSR (Sector Transmitter Sector Receiver)
3.3.1 OTSR (Omni Transmit Sector Receive)
The OTSR configuration uses a single (PA) Power Amplifier, whose output is fed to a transmit splitter. The power of the RF signal is divided by three and fed to the duplexers of the three sectors, which are connected to sectorized antennas.
3.3.2 STSR (Sectorial Transmit Sector Receive)
The STSR configuration uses three (PA) Power Amplifier, whose output is fed directly to the duplexers of the three sectors, which are connected to sectorized antennas.
Node B serve the cells which depend on sectoring.
3.4 ACCESS MODES
3.4.1 FDD (Frequency Division Duplex)
A duplex method whereby uplink and downlink transmissions use two separated radio frequencies. In the FDD, each downlink and uplink uses the different frequency band.
3.4.2 TDD (Time Division Duplex)
It is a method in which same frequency is used for the transmission of downlink and uplink by using synchronized time intervals. Time slots are divided into transmission and reception part in the physical channel.
3.4.3 Frequency Bands
3.4 CELLULAR CONCEPT
The UMTS network is third generation of cellular radio network which operate on the principle of dividing the coverage area into zones or cells (node B in this case), each of which has its own set of resources or transceivers (transmitters /receivers) to provide communication channels, which can be accessed by the users of the network.
A cell is created by transmitting numerous number of low power transmitters. Cell size is determined by the different power levels according to the subscriber demand and density within a specific region. Cells can be added to accommodate growth.
Communication in a cellular network is full duplex, which is attained by sending and receiving messages on two different frequencies.
In order to increase the frequency reuse capability to promote spectrum efficiency of a system, it is desirable to reuse the same channel set in two cells which are close to each other as possible, however this increases the probability of co-channel interference .
The performance of cellular mobile radio is affected by co channel interference. Co-channel interference, when not minimized, decreases the ratio of carrier to interference powers (C/I) at the periphery of cells, causing diminished system capacity, more frequent handoffs, and dropped calls.
Usually cells are represented by a hexagonal cell structure, to demonstrate the concept, however, in practice the shape of cell is determined by the local topography.
3.4.1 Types of Cell
The 3G network is divided on the basis of size of area covered.
- Micro cell – the area of intermediate coverage, e.g., middle of a city.
- Pico cell – the area of smallest coverage, e.g., a “hot spot” in airport or hotel.
- Macro cell – the area of largest coverage, e.g., an complete city.
Fading is another major constraint in wireless communication. All signals regardless of the medium used, lose strength this is known as attenuation/fading. There are three types of fading:
- Rayleigh Fading
Pathloss occurs as the power of the signal steadily decreases over distance from the transmitter.
Shadowing or Log normal Fading is causes by the presence of building, hills or even tree foilage.
3.5.3 Rayleigh Fading
Rayleigh Fading or multipath fading is a sudden decrease in signal strength as a result of interference between direct and reflected signal reaching the mobile station.
3.6 HANDOVER IN CDMA
The term handover or handoff refers to the process of transferring data session or an ongoing call from channel to channel connected to the core network to another. The handover is performed due to the mobility of a user that can be served in another cell more efficiently. Handover is necessary to support mobility of users.
Handover are of following types (also known as handoff):
- Hard Handover
- Soft Handover
- Softer Handover
In Hard handover the old radio links in the UE are dispose of before the new radio links takes place. It can be either seamless or non-seamless. In seamless hard handover, the handover is not detected by the user. A handover that needs a change of the carrier frequency is a hard handover.
Soft handover takes place when cells on the same frequency are changed. Atleast one radio link is always kept to the UTRAN in the removal and addition of the radio links. It is opperated by means of macro diversity in which many radio links are active.
It is one of the important case of soft handover which describe the removal and addition of the radio links which is being belonged by the same Node B. Macro diversity can be performed in the NODE B with maximum ratio combining in softer handover.
There are inter-cell and intra-cell handover.
- Handover 3G – 2G (e.g. handover to GSM)
- FDD inter-frequency hard handover
- TDD/FDD handover (change of cell)
- TDD/TDD handover