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 are divided. The total number of channels is denoted by:
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
- FDD/TDD handover (change of cell)
- Handover 2G - 3G (e.g. handover from GSM
3.7 CODES IN CDMA
CDMA uses a technology called the spread-spectrum. In spread-spectrum, the generated signals are spread in the frequency domain. These signals are secure, less affective to noise and resistant to jamming. Spread-spectrum allows multiple users to communicate using same physical channel. This gives the communicated data a much higher data bandwidth than it usually has. Spread-spectrum uses a special coding scheme in which every user is assigned a specific code for communication and in a channel, only users associated with a particular can communicate.
CDMA system has asymmetric links (i.e. the forward and reverse links have different link structures).The differences ranges from the modulation scheme to error control methods. In addition, each link uses different codes to channels individual user. The forward link uses Walsh codes, while reverse link uses Pseudorandom Noise (PN) codes for channelization.
- PN(Pseudorandom Noise) Codes
- Walsh Codes
3.7.1 Walsh Codes
In CDMA all the users are transmitted in same RF band. In order to avoid mutual interference on the forward link, Walsh codes are used to separate individual users while they simultaneously occupy the same RF band. They provide orthogonality in a cell among all the users. Different Walsh codes are assingned by the base station among every user traffic channel. 64 codes are available for IS-95. For pilot code, code 0 is used and for synchronization code 32 is used. Control channel uses the codes 1 though 7, and the remaining codes are available for traffic channels. At the same time Codes 2 through 7 can be used for traffic channels.
They are derived from Haddamard matrices. They also have a outstanding quality that in their family codes are orthogonal to each other and can also create channelization in 1.25 MHz band. They are used to spread over the reverse channel and for modulation and are also used to create anorthogonal modulation on the forward link. They create channels in CDMA and are the backbone of CDMA systems.
3.7.2 PN CODES
The forwad link of IS-95 CDMA has pilot and sync channels to aid synchronization, the reverse link does not have pilot and sync channels.The mobile stations transmits at will, and no attempt is made to synchronize their transmission. Thus Walsh cods cannot be used for the reverse link. The incoherent nature of the reverse link calls for the use of another class of codes, PN codes, for channelization.
Codes used by CDMA are known as Pseudo Noise (PN) code. A PN code is a binary sequence in random, consisting of 1s and 0s, which are produced by an algorithm making it unique for every user. It can be reused in different manner by different users. These PN codes are used to encode and decode the user's signal in CDMA. This helps users to avoid crosstalk, interference and noise as only the signals with special PN codes are received while others appear as noise to the system. These codes have low cross-correlation factor and codes are unique for every user. These codes provide a strong shield against jamming and data theft.
The form of carrier modulation we can use is Amplitude Modulation. However in practice Phase Shift Keying (PSK) is usually used. The process of modulating the carrier with the PN code is called spreading.
PN CODE GENERATION
A usual way of producing a PN code is by using a shift-register. The shift register generators produce a sequence depending on the number of stages and the initial conditions. A shift-register of length ‘n' has a period given by the equation:
3.8 CDMA LOGICAL CHANNELS
The communications between the mobile and the base station takes place using specific channels. A channel is a stream data designated for a specific use or person and is separated by a code. A channel may be a voice data or overhead control data.
There are two types of channels:
- Forward link channels
- Reverse link channels
3.8.1 Forward link channels
The Forward CDMA channel is from cell-to-mobile direction or the
downlink path. It consists of:
- Pilot channel
- Paging channel
- Sync channel
- Traffic channels
184.108.40.206 PILOT CHANNEL
Pilot Channel is a reference channel which the mobile station uses for acquisition, timing and as a phase reference for coherent demodulation. It is transmitted at all times by each base station on each active CDMA frequency. Each mobile station tracks this signal continuously.
220.127.116.11 PAGING CHANNEL
Sync Channel carries a single, repeating message that conveys the timing and system configuration information to the mobile station in the CDMA system.
18.104.22.168 SYNC CHANNEL
Paging Channels' primary purpose is to send out pages, that is, notifications of incoming calls, to the mobile stations. The base station uses them to transmit system overhead information and mobile station- specific messages.
22.214.171.124 TRAFFIC CHANNEL
Forward Traffic Channels are code channels used to assign call (usually voice) and signaling traffic to individual users.
3.8.2 Reverse link channels
The Reverse CDMA channel is from mobile-to-cell direction of communication or the uplink path.
- Access channel
- Traffic channel
126.96.36.199 ACCESS CHANNEL
Access Channels are used by mobile stations to initiate communication with the base station or to respond to Paging Channel messages. The Access Channel is used for short signaling message exchanges such as call originations, responses to pages, and registrations.
188.8.131.52 TRAFFIC CHANNEL
Reverse Traffic Channels are used by individual users during their actual calls to transmit traffic from a single mobile station to one or more base stations.
3.9 POWER CONTROL IN CDMA
Power control ensures that each user in the network receives and transmits just enough energy to convey information while causing minimal interference to other users. For the WCDMA standard, power control is applied in both the uplink and downlink. Power control helps to reduce co-channel interference, increasing the cell capacity by decreasing interference and prolonging the battery life by using a minimum transmitter power.
With appropriate power control, the CDMA offers high capacity in comparison to FDMA and TDMA. Power control also known, as Transmit Power Control (TPC) is a significant design problem in CDMA systems.
Power control manages problems mentioned above by constantly controlling the received power of the mobiles and continuously adjust its transmit power in order to achieve some predefined performance level such as SINR (signal to interference noise ratio).
3.9.1 REQUIEREMENT OF POWER CONTROL
The transmit power for each user is reduced to limit interference, so, the power control is needed to maintain the required Eb/No (signal to noise ratio) for a acceptable call quality. On both the links the dynamic power control is also required to limit transmitted power as to maintain the quality of link under all conditions, mobile battery life and life span of BTS power amplifiers can be prolonged. To achieve the maximum capacity every user's Eb/No should be at minimum level, needed for acceptable channel performance.
Chapter 4 PLANNING OF 3G
4.1 CITY PROFILING
City profiling in 3g is the same as done in 2G and the area calculations are the same as done earlier.
4.2 Link Budgeting of 3G
We have used Cost 231 Hata model for planning of 3G network. The values used are estimated values which are tested in the field practically and are most effective. This model is the most reliable model which is being used in the field till now with some amendments which have been used also.
The equation of Cost 231 Hata model is as follows:
Calculations of 3G are as follows
As the height of mobile station is 1.5m a(hm) becomes ‘0'.
Area of site as calculated in 2G was
Now as r = 0.84
Number of BTS required
The area of Urban as calculated earlier was 136 sq km. so the number of BTS required
Number of BTS = 136/1.19
Chapter 5 CHANNEL MODELING OF 4G NETWORKS
5.1 INTRODUCTION OF 4G
4G technology is the very latest and ultra fast technology and it is getting more attention day by day even though this technology is still at its initial stages. 4G technology aims to provide fastest speed to all users, provide data transfer rate up to 100 Mbps while for its stationary users, it aims to the highest speed of 1 Gbps. 4G Technology is basically an extension in 3G technology with more Data rate and services offered in 3G. The expectation in the 4G is the high quality audio and video streaming over Internet Protocol.
If 4G implemented correctly, will truly protects global roaming, super high speed connectivity, with transparent user performance on each mobile communications. It allows video conferencing, streaming picture and much more. Some standards for the 4G system include 802.20, WiMAX (802.16), TDD UMTS, HSDPA and future versions of UMTS. 4G is based on OFDM. Other aspects of 4G are smart antennas and adaptive processing. OFDM is designed so that to send data over parallel streams, increasing the amount of information that we can send at a time over CDMA networks.
5.2 AIM OF THE PROJECT
As the wireless industry is evolving, it has developed an infrastructure which aims to provide services to the market. But the designing, production of this type of technological infrastructure comes along with high cost. This high cost may push manufacturers from building the whole systems to test the designs. Therefore manufacturers go for different alternatives to avoid high costs; one of the best alternatives is to simulate a real wireless system. A simulation of a wireless system could depend on many different components. A major component of a wireless system simulation is the wireless channel model. Many Different approaches could be used to simulate diverse types of channels and their conditions.
The use of channel modeling may rises questions about their validity. This generates a motivation towards the study of these models and the conditions under which they can be used.
This part of our project aims to use current methods of modeling Rayleigh Fading Channels, which include common basic concepts and MATLAB simulation. Based on this model, we will be able to analize the model's capacity, application areas and correctness for future developments.
5.3 WHAT IS CHANNEL MODELLING?
A channel modelling is a mathematical representations of the transfer characteristics of the medium. This model can be based on some underlying physical criterion or it could by fitting the appropriate mathematical or statistical model on the channel behaviour. Most channels modelled are formulated by observing the traits of the received signals for each specific condition. Usually the one that best explains the behaviour of the received signal is used to model the given physical channel. Such analysis reduces our cost of building a complicated system by the reduction of the amount of hardware that has to be developed for assessment. When it comes to theory, models have another advantage in their ability to regenerate a channel for comparison between many different communication strategies, resulting in an exact measure of relative performance.
A channel can be modeled by calculating the physical processes which can effect the transmitted signal. The channel is normally modeled by calculating the reflection of every object present in the environment.
A channel model can either be digital or analog. Physical and statistical modeling can be combined. In wireless communications the channel is usually modeled by a randomly generated attenuation of the transmitted signal, and later with additive noise. The noise in this model captures the external interference and/or the electronic noise in the receiver. If attenuation is more complex, it describes the time of a signal, to get in through the channel. Measurements or simulations decide the statistics of the randomly generated attenuation. Channel models can be continuous as there is no limit as to how precisely their values can be defined.
The purpose of communication systems is to transmit baseband signals through a channel using electromagnetic waves. In communications, modulation is the process of transmitting a message signal. Modulation is conversion of baseband signals into band pass signals at a frequency that is high as compared to the baseband signals frequency. The band pass signal is known as modulated signal and the baseband signal is known as modulating signal. Device, which performs modulation, known as a modulator and device that inverses the operation, known as a demodulator. Jointly this type of device is called a modem. The parameters that can be changed in modulation are:
5.5 Digital Modulation
The problem is to convert digital signals to analogue signal and can be transmitted using a twisted cable pair, via microwave or satellite. Digital modulation is used to transfer digital data over analogue channel. Digital modulation schemes have greater ability to send large amounts of information than analog modulation schemes. That is why digital modulation provides more data capacity, also compatible with digital data services, data security is very high, better quality communications, and quick system availability. There are many factors influencing the choice of modulation technique for a specific use which include:
- Power efficiency
- Spectral efficiency
- Bit error rate (BER)
- Implementation complexity
5.5.1 TYPES OF DIGITAL MODULATION
- Frequency shift key modulation(FSK)
- Amplitude shift key modulation(ASK)
- Phase shift key modulation(PSK)
- Binary-phase shift key modulation(BPSK)
- Quadrature-phase shift key modulation(QPSK)
- Quadrature amplitude modulation(QAM)
184.108.40.206 FREQUENCY SHIFT KEY MODULATION(FSK)
Frequency-shift keying (FSK) is the modulation in which digital information is send through discrete changes in frequency of a carrier wave. The frequency of the carrier is changed, as a function of the modulating signal, which is being conveyed. Amplitude is remains unchanged. Two constant amplitude carriers are used, one for a binary zero, the second one for a binary one.
The simplest frequency shift keying used is binary FSK (BFSK). Binary FSK implies using discrete frequencies to send binary information. "1" is called the mark frequency whereas "0" is called the space frequency.
220.127.116.11 AMPLITUDE SHIFT KEY MODULATION (ASK)
Amplitude-shift keying (ASK) is a modulation that shows digital data, as change in the Amplitude of the carrier wave. The amplitude of analog carrier signal, changes according to the bit stream containing frequency and phase constant. Level of amplitude is used to represent binary 0(s) and 1(s). We pursue the carrier signal as an ON or OFF switch. In modulated signal, logic 0 is represented by the absence of the carrier, hence giving ON/OFF operation.
Both ASK modulation and demodulation processes are comparatively inexpensive. The ASK technique is also used to send data through optical fibre. The low level shows binary 0, while a higher-amplitude light wave represents binary 1.
18.104.22.168 PHASE SHIFT KEY MODULATION (PSK)
Phase-shift keying (PSK) is a digital modulation technique which transmits data by modulation, or, changing phase of the carrier signal. PSK uses many phases which are finite and each phase is assigned a different scheme of binary bits. Every phase encodes an equal numbers of bit. The demodulator is designed specially for the set of symbols used by modulator, finds the phase and maps the received signal back to the symbol it represents, so it recovers the original data.
BPSK (also known as PRK, Phase Reversal Keying), is the simplest form of phase shift keying (PSK). It consists of two phases which are separated by 180° and so they can also be termed 2-PSK. This modulation is the most vigorous of all the PSKs as it takes the highest level of noise and, or distortion to make the demodulator reach incorrect decision. It is only able to modulate at 1 bit/symbol and so is not suitable for high data rate applications if bandwidth is limited.
QPSK (known as quadriphase PSK 4-PSK or 4-QAM), uses four points in the diagram of constellation, equally spaced around a circle. With four phases, QPSK encodes 2 bits/symbol. This can be used to double the information rate as compared to a BPSK system keeping the bandwidths of the signal or can be used to maintain the data-rate of BPSK but with half the bandwidth required. With BPSK, there are many phase ambiguity problems at the receiving end so to avoid it; differentially encoded QPSK is used more often in practice.
22.214.171.124 QUADRATURE AMPLITUDE MODULATION (QAM)
Quadrature amplitude modulation (QAM) is used in both an analog and a digital modulation. It uses two analog message signals, by modulating the amplitude of the two carrier waves, which are using the amplitude-shift keying (ASK) modulation scheme. These two waves are normally sinusoids and are out of phase by 90° and are called quadrature carriers or components. The modulated waves are added, and the resulting wave is a combination of both amplitude-shift keying and phase-shift keying (PSK). PSK modulators are designed using the QAM principle, but these are not considered QAM as the amplitude of the modulated signal is constant.
5.6 CHANNELS MODELS FOR COMMUNICATION
Communication channels refer to the medium through which information is conveyed from a transmitter to a treceiver.
5.6.1 ADDITIVE WHITE GUASSIAN NOISE (AWGN) CHANNEL
An AWGN channel adds white Gaussian noise to the signal that passes through it.
Additive white Gaussian noise is channel in which the only drawback to communication is the addition of white noise having constant spectral density and a Gaussian distribution of amplitude. This model is not able to support frequency selectivity interference, nonlinearity, fading or dispersion. The AWGN channel is a good model for many satellites but is not good model for most terrestrial links because of multipath and interference. For terrestrial links modelling AWGN is commonly used to create background noise. The relative power of noise an AWGN channel offers is usually described by parameters like Signal-to-noise ratio of each sample. This is the input parameter to the AWGN function in the ratio between Bit energy to noise power spectral density (EbNo).
The relationship between EsNo and EbNo, in dB:
k = information bits per symbol.
5.6.2 MULTIPATH CHANNEL
Signal multipath occurs when the conveyed signal reaches at the receiver through multiple preoperational paths. If there is no line of sight between sender and receiver then multipath is produced from reflection in the environment. Where each path can have a separate phase, delay and doppler shift, attenuation associated with it. Due to signal multipath the received signal has some undesirable properties like signal fading, inter-symbol-interference, distortion etc.
The effects of multipath include destructive and constructive interferences and phase shifting of the signal.
Two types of Multipath:
- Discrete: When the signal arrives at the receiver from a limited number of paths.
- Diffuse: The received signal is better modeled as being received from a very large number of scatterers.
5.6.3 FADING CHANNELS
Fading channels in communication are those which experience fading of the signal. Where fading is the change in the signal amplitude over time at the receiver.In wireless systems fading may be due to the multipath propagation called multipath fading or due to shadowing called shadow fading.Due to the loss of signal power overall performance of the system decreases. Fading channel models are used in order to model the effect of transmission of information over a channel.
Fading channels are multiplicative-noise channels and result in bursts of errors.The multiplicative nature of the channel means increasing signal power may not yield a proportional improvement in performance. When signals reach at the receiving antenna with traversed different paths, they may combine destructively. This, multipath, phenomenon can induce signal fading.
- Frequency-Selective: The effects of the channel on the information signal are frequency-dependent
TYPES EXAMPLE OF FADING CHANNELS
- Rayleigh Fading
- Ricean Fading
5.6.5 RICEAN FADING CHANNEL
Ricean fading is the fading in which the cancellation of the signal is by itself. Rayleigh fading with a strong line of sight is said to have a Ricean distribution and is called Ricean fading.
5.7 TECHNOLOGIES USED IN 4G:
- ORTHOGNAL FREQUENCY DIVISION MULTIPLEXING(OFDM)
- ULTRA WIDEBAND(UWB)
- MILLIMETER WIRELESS
- SMART ANTENNAS
- LONG TERM POWER PREDICTION
- SHEDULING AMONG USERS
- ADAPTIVE MODULATION AND POWER CONTROL
5.8 ORTHOGNAL FREQUENCY DIVISION MULTIPLEXING
Orthogonal frequency division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. OFDM is defined as a form of multi-carrier modulation where the carrier spacing is vigilantly selected so that every sub carrier is orthogonal to the remaining sub carriers. Two signals are orthogonal when their dot product is zero. If you multiply two signals together, and if their integral over an interval is zero, then two signals are orthogonal in that interval. Orthogonality can be attained by vigilantly selecting carrier spacing, such as letting the carrier spacing be equal to the reciprocal of the useful symbol period. As the sub carriers are orthogonal, the spectrum of every carrier has a null at the center frequency of each of the other carriers in the system. This results in no interference between the carriers.
The major principle of OFDM is to split a high information Stream into a number of lower data-rate streams which are conveyed over a number of subcarriers simultaneously. Because the symbol duration for lower data-rate parallel subcarriers increases, the amount of dispersion in time caused by multi-path delay spread is decreased. The help of introducing a guard time in each OFDM symbol finishes inter-symbol interference almost completely. The symbol is extended in the guard time to avoid inter-carrier interference.
Orthogonal Frequency Division Multiplexing (OFDM) is a very important technique for communication over frequency selective channel. By dividing the bandwidth available into orthogonal and non-interfering subcarriers and using a parallel transmission strategy, it gives better immunity to the multipath fading effect of the wireless channel than single-carrier transmission system. OFDM is extensively used in commercial systems such as xDSL modems, and wireless LAN. It is also part of WiMax, and a strong candidate for future wireless cellular systems. Even OFDM multiplexes low data rate sub-streams from a user and divide it to multiple subcarriers, a cellular network uses orthogonal frequency division multiple access (OFDMA), in which the data streams from different users are multiplexed onto different sets of the subcarriers.
5.8.1ADAVANTAGES OF OFDM
- Immune to delay spread
- Symbol duration greater than channel delay spread
- Needed Guard interval
- More Resistance to frequency fading
- Every sub-channel is flat fading
- More Efficient bandwidth usage
5.8.2DISADVATAGES OF OFDM
- problem of synchronization
- errors due to Timing
- Carrier noise
- Sampling synchronization
- Frequency synchronization
- FFT units needed at transmitter and receiver
5.9 Simulation of Rayliegh Fading in MATLAB
We computed the BER for BPSK in OFDM modulation in the presence of Rayeligh fading channel. The equation used to find BER is
Rayliegh fading was implemented in MATLAB as a part of our project the simulations results are as follows:
Matlab simulation performs the following:
- Generation of random sequence in binary.
- BPSK modulation
- Assigning to multiple OFDM symbols