MOBILE DATA NETWORK

INTRODUCTION

The cellular is system level idea which calls for replacing a single transmitter used to cover a large area with many cells, each providing coverage only to a small portion of the service area. This chapter is designed to provide with an overview of basic concepts of wireless communications. This chapter addresses some major problems in the cellular radio environment plus some solution used to resolve these problems.
Upon completion of this chapter we will be able to :
· Briefly describe the frequency reuse, capacity and coverage.
· Briefly describe the time Division Multiple Access Technique (TDMA)
· Transmission problems and the methods used to solve those.

7.1 FREQUENCY CONCEPT

An MS communicate with a BTS by transmitting or receiving radio waves, which consist of electromagnetic energy. The frequency of a radio wave is the number of times that the wave oscillates per second. The frequencies used by the mobile networks vary according to the standard being used.
The following table summarizes the frequency-related specifications of each of the GSM systems




Table 7.1 Frequency related specifications
System
GSM-900
GSM-1800
GSM-1900
Frequencies : Uplink
880-915 MHz
1710-1785 MHz
1850-1910 MHz
Downlink
925-960 MHz
1805-1880 MHz
1930-1990 MHz
Wave length
33 cm
17 cm
16 cm
Band width
35 MHz
75 MHz
60 MHz
Duplex distance
45 MHz
95 MHz
80 MHz
Carrier separation
200 KHz
200 KHz
200 KHz
Radio channels
175
375
300
Transmission
270 kbps
270 kbps
270 kbps

7.1.1 BANDWIDTH

Carrier spacing 200 kHzBandwidth is the term used to describe the amount of frequency range allocated to one application. The bandwidth given to an application depends on the amount of available frequency spectrum. The amount of bandwidth available is an important factor in determining the capacity of mobile system, i.e. the number of calls, which can be handled.

7.1.2 CHANNEL

Another important factor in determining the capacity of a mobile system is the channel. A channel is a frequency or set of frequencies which can be allocated for the transmission and possibly the receipt, of information. Communication channels 0 form can be one of the following types :
Table 7.2 Types of Communication Channels

Type Description Example

Simplex One way only FM radio, television

Half Duplex Two way, only one way at a time Police radio
Full Duplex Two way, both at the same time Mobile system

A simplex channel, such as a FM radio music station, uses a single frequency single direction only. A duplex channel, such as that used during a mobile call uses two frequencies: one to the MS and one from the MS (Uplink, Downlink).

7.2 FREQUENCY RE-USE

Frequency reuse is the design process of selecting and allocating channel group for all of the cellular base station within a system. Modem cellular networks are planned using the technique of frequency re-use. Within a cellular network, the number of calls that the network can support is limited by the amount of radio frequencies allocated to that network. However, a cellular network can overcome this constraint and maximize the number of subscribers that it can service by using frequency re-use.

Frequency re-use means that two radio channel within the same network can use exactly the same pair of frequencies, provided that there is a sufficient geographical distance (the frequency re-use distance) between them so they will not interfere each other. The tighter frequency re-use plan the greater the capacity potential of the network.

These frequency re-use patterns ensure that any frequencies being re-used are located at a sufficient distance apart to ensure that there is little interference between them. The term “frequency re-use distance” is used to describe the distance between two identical frequencies in re-use pattern. The lower frequency re-use distance between two capacities will be available in the network.

7.3 INTERFERENCE

7.3.1 CO-CHANNEL INTERFERENCE (C/I)

Cellular networks are more often limited by problem caused by interference rather by signal strength problems. Co-Channel interference is caused by the use of a frequency close to the exact same frequency. The former will interfere with the latter leading to the terms interfering frequency (I) and carrier frequency (C). The GSM specification recommends that the carrier to interference (C/I) ratio greater than 9 dB.

This C/I ratio is influenced by the following factors :
· The location of the MS
· Local geography and type of local scattersBTS antenna type, site elevation and position

7.3.2 ADJACENT CHANNEL INTERFERENCE (C/A)

Adjacent frequencies (A), that is frequencies shifted 200 kHz from the carrier frequency (C), must be avoided in the same cell and preferably in the neighboring cell also. Although adjacent frequencies are at different frequencies to the carrier frequency they can still cause interference and quality problem.
The GSM specification states that carrier to adjacent ration (C/A) must be larger than 9 dB.

7.3.3 CLUSTER

Groups of frequencies can be placed together into patterns of cells called cluster. A cluster is a group of cell in which all available frequencies have been used once and only once.

Since the same frequencies can be used in neighboring clusters, interference may become a problem. Therefore the frequency re-use distance must be kept as large as possible. However to maximize the frequency re-use distance should be kept as low as possible.

The re-use patterns recommended for GSM are the 4/12 and the 3/9 patterns. 4/12 means that there are four three-sector sites supporting twelve cells using twelve frequency groups.

7.4 ACCESS METHOD : TIME DIVISION MULTIPLE ACCESS (TDMA)

Most of the cellular systems use the technique of TDMA to transmit and receive speech signals. With TDMA, one carrier is used to carry a number of calls, each call using that carrier at designated periods in time. These periods of time are referred to as time slots. Each MS on a call is assigned one time slot on the up link frequency and one on the down link frequency. Information sent during one time slot is called a burst. In GSM, a TDMA frame consists of 8 time slots. This means that a GSM radio carrier can carry 8 calls.

7.5 TRANSMISSION PROBLEMS

Many problems may occur during the transmission of a radio signal. Some of common problems are described below

7.5.1 PATH LOSS

Path loss occurs when the received signal becomes weaker and weaker due to increasing distance between MS and BTS, even if there are no obstacles transmitting (Tx) and receiving (Rx) antenna.

7.5.2 SHADOWING

Shadowing occurs when there are physical obstacles including hills and buildings between the BTS and the MS. The obstacles create a shadowing effect, which can decrease the received signal strength. When the MS moves, thesignal strength fluctuates depending on the obstacles between the MS and BTS.

7.5.3 MULTIPATH FADING

Multipath fading occurs when there is more than one transmission path t the MS or 4 BTS, and therefore more than one signal is arriving at the receiver. This may be due t buildings or mountains, either close to or far from the receiving device.

Time Dispersion
Time dispersion is another problem relating to multiple paths to Rx antenna either an MS or BTS. However in contrast to Rayleigh fading the reflected signal comes from an object faraway from the Rx antenna.

Time dispersion causes Inter-Symbol Interference (ISI) where consecutive symbols (bits) interfere with each other making it difficult for the receiver to determine which symbol is the correct one. An example of this is shown in the figure below where the sequence 1,0 is sent from the BTS.
If the reflected signal arrives one bit time after the direct signal, then the receive detects a 1 from the reflected wave at the same time it detects 0 from the direct wave the symbol 1 interferes with the symbol 0 and the MS does not know which is correct.

7.5.4 TIME ALIGNMENT

Each MS on a call is allocated a time slot on a TDMS frame. This is an amount of time during which the MS transmits information to the BTS. The information must all arrive at the BTS within that time slot. The time alignment problem occurs when a part of information transmitted by an MS does not arrive within the allocated time slot.

7.6.1 CHANNEL CODING

In digital transmission, the quality of the transmitted signal is often expressed in tern is of how many of the received bits are incorrect. This is called Bit Error Rate.

7.6.2 ANTENNA DIVERSITY

Antenna diversity increases the received signal strength by taking advantage of natural properties of radio waves. There are two primary diversity method diversity and polarization diversity.

Space Diversity
An increased received signal strength at the BTS may he achieved by mounting two receiver antenna instead of one. If the two Rx antenna are physically separated & probability that both of them are affected by a deep fading dip at the same time. By choosing the best of each signal, the impact of fading can be reduced.
Polarization Diversity
With polarization diversity the two space Diversity, antennae are replaced by one din polarized antenna. This antenna has normal size but contains two differently polarized antenna arrays.

7.6.3 FREQUENCY HOPPING

As mentioned previously, Raleigh fading is frequency dependent. This means that the fading dips occur at different-places for different frequencies. To benefit from this, it is possible for the BTS and MS to hop from frequency to frequency during a call. The frequency hopping of the BTS and MS is synchronized.

In GSM there are 64 patterns of frequency hopping, one of them is a simple cyclic or sequential pattern. The remaining 63 are known as pseudo-random patterns, which an operator can choose from.

7.6.4 TIMING ADVANCE

Timing advance is a solution specifically designed to counteract the problem of time alignment. It works by instructing the misaligned MS to transmit its burst earlier 0 later than it normally would.

7.7 MODULATION METHOD

In GSM 900, the frequency that is used to transfer the information over the air interface is around 900 MHz. Since this is not the frequency at which the information into the usable frequency band. Frequency translation is implemented by modulating the amplitude, frequency or phase of the so-called carrier wave in accordance with waveform of the input signal. Any modulation scheme increases the carrier bandwidth and hence is a limit on the capacity of the frequency band available.
In GSM, the carrier bandwidth is 200 kHz.
The modulation technique used in GSM is Gaussian Minimum Shift Keying (GMSK). GMSK enables the transmission of 270 kbit/s within 200 kHz channel. This gives bit rate of 1.3 bit/s per Hz.

7.8 ANALOG TO DIGITAL CONVERSION

There are three steps for conversion analog signal to digital signal :
· Sampling
· QuantizationCoding

GLOBAL SYMTEN FOR MOBILE TELECOMMUNICATION OPERATORS IN BANGLADESH

INTRODUCTION
There are four private and one government owned digital cellular mobile telephone companies in Bangladesh namely,
1. Grameen Phone (GP) Limited.
2. Sheba Telecom
3. Aktel
4. Tele talk and
5. City cell.
The former four companies use the GSM system while the later one uses the CDMA system.

8.2 GRAMEEN PHONE (GP)

Grameen Phone (GP) is one of the four mobile telephone operators in Bangladesh. It is a joint venture company in which Telenor, the Norwegian State: 6 wend telecommunications company, owns 51 percent of the shares of GP; Grameen Telecom, a subsidiary of Grameen bank owns 35 percent; Marubeni corporation, one of Japan’s leading trading companies, owns 9.5 percent; while Gonofone Development corporation, a New York-based investment firm, owns the remaining 4.5 percent of the shares. Grameen Phone lunched its service on Independent Day on March 26, 1997.

8.3 Some Statistical Data of Grameen Phone (GP)

Standard – GSM
1. Number of mobile subscriber – about 20,00,000
2. Number of base station (BTS) – 1600
3. Number of MSC/VLR – 5 (In Dhaka and Chittagong)
4. Number of HLR – 1 (In Dhaka)
5. Frequency Band used :
Uplink : 900 to 905 MHa, Downlink : 945 to 950 MHz

8.4 Roaming Feature of Grameen Phone

Grameen Phone (GP), early the October, 1999 lunched the first both-way international roaming facility for its subscribers. This should now enable GP subscribers to use their mobile phones in foreign countries where Grameen has a partner network. Presently, Grameen Phone has roaming agreements with 36, mobile telephone operators in foreign countries. Of them, 12 are on the active roaming list : seven with both way roaming and five with one-way roaming facility.

Under the both way international roaming facility, GP subscribers can use their mobile telephones in a country where Grameen Phones partner network exists while subscriber of the partner network can also use their mobile Phones when they are in Bangladesh. Currently GP has both-way international roaming arrangements with sing Tel mobile of Singapore, Mobile One of Singapore, Tele Denmark, Telecom Mobile of Norway, Hutchinson Telecom of Hongkong, Vodafone Ltd. of the United Kingdom and Switzerland.

Under international roaming system, Grameen Phone visiting foreign countries where they have a partner network will be able to make and receive calls using their same GP numbers. This is a totally automatic system; there is noting that the subscribers need to do except carry their handsets along with them when they travel and turn on the mobile phone select that particular operator with whom has a roaming agreement.
To use the international roaming facility, subscribers of Grameen Phones partner networks visiting Bangladesh should bring with them either GSM 900 handsets or dual or tri-bands. Local Grameen Phone subscribers who want to use the international roaming facility should consult the companies Info centers or the customer Relations Division before going abroad.

8.5 EVALUATION OF TRAFFIC LOAD HANDLING CAPACITY OF GEAMEENPHONE

The traffic density of a coverage area of a cellular system is a critical element must be determined before a system designed. This traffic pattern in busy hours is different in different zones within the service area. Choice of the initial cell sites should be based on the signaled covered in zones of heavy traffic. This means that the cell, site would be likely being located at the center of those zones.

GSM ANTENNA CONFIGURATION AND SPECIFICATION

GSM ANTENNA CONFIGURATION AND SPECIFICATION

9.1 ANTENNAS CONFIGURATION

The BTS’s antennas will be physically separated to provide an additional isolation, through the transmitted band received band are widely apart. Several types of antennas are used in BTS of GSM system, such as
Ø Directional antenna
Ø Tilting antenna and
Ø Diversity antenna.

9.1.1 DIRECTIONAL ANTENNA CONFIGURATION

Besides depending on frequency band isolation and physical space separation to reduce signal interference, the directional antenna could be positioned to eliminate unnecessary contamination of radiation in certain areas. In the cellular system using three directional antennas at each cell site can form each sect ionized cell.

In most cases omni directional antennas are used. The coverage of the signal strength from an omnidirectional antenna should take the shape of a smooth circle if the ground is perfectly flat. In hilly terrain the coverage of an omnidirectional antenna may be so intensified also cause severe interference. In these circumstances the directional antenna can be used to eliminate the transmitted power in the other direction.

9.1.2 TILTING ANTENNA CONFIGURATION

An effective way to confine the signal in its own coverage cell and reduce its interference in the other co-channel cells is to tilt a directional antenna beam pattern downward at certain angle. A typical antenna vertical beam pattern is shown in figure. When the beam pattern is tiled downward, the field strength received by a distant mobile unit diminishes. The average signal to interference ration in an interfered cell region interfered by a serving cell antenna site is improved by tilting the antenna gain pattern downward some predetermined amount. It must be understood that the signal-to-interference ratio in any way serving cell will increase if antenna beam pattern are tiled downward from other cell sites. This means that the degree of tilting of the antenna beam pattern at each cell site has to be especially chosen to reduce further co-channel interference in the system other than using directional antenna alone.

9.1.3 DIVERSITY ANTENNA CONFIGURATION

Diversity antenna configuration is very important in system design. To achieve the better degree of diversity advantage, the separation requirement for a = 0o is much less than that for a = 90o . However, the mobile units travel around base station with no restriction in any direction a (i.e. 0o £ a £ 90o ). At a = 90o the separation is the same as the antenna height for a correlation coefficient of 0.7. The triangular configuration for the omnidirectional antennas arrangement overcome the difficulty of a = 90o.

9.2 ANTENNA LOCATIONS

It is difficult to select an optimum antenna location for a base station. First the signal strength coverage at a distance such as 13 km from a base station antenna does not exhibit a uniform pattern. This irregular pattern is due to the irregular terrain configuration. Another important aspect is avoiding interface. Therefore a plan for a base-station antenna location should consider both its coverage range and its interference with other antennas. In a large system all the potential base-station location should be considered at the same time. If one base station is moved to a different position, then all the other station locations are affected.

There are several steps in choosing a base-station location :
1. First, decide on a reception level at the cell boundary. This is bale on the features of the mobile trans-receiver an the system performance required.
2. Choose a location where land is usually available for a first choice base station.
3. Follow the new path-loss prediction midel to make a point-to –pint predicting. An equal-strength contour can be drawn on the map.
4. Choose other locations and draw -100-dBm equal-strength contour of them. The equal-strength contours of all locations should have roughly the same portion overlapping as shown in fig. A.
5. Avoid the equal-strength contour condition shown in Fig. B.

CONCLUSION

The communication development and the increase of living standard of people are directly related to the more use of cellular mobile. Cellular mobile radio-the high end sophisticated technology that enables every one to communicate anywhere with anybody. The mobile telephony industry rapidly growing and that has become backbone for business success and efficiency and a part of modern lifestyles all over the world.

In this thesis work we have tried to give and over view of the GSM system. We hope that we gave the general flavor of GSM and the philosophy behind its design. The GSM is standard that insures interoperability without stifling competition and innovation among the suppliers to the benefit of the public both in terms of cost and service quality.

The features and benefits expected in the GSM systems are superior speech quality, low terminal, operational and service costs, a high level security, providing international roaming support of low power hand portable terminals and variety of new services and network facilities. In near forth coming days, the third generation mobile telephony becomes available whole over the world, which will give the facility of videoconference in mobile telephone.

10.1 TECHNOLOGY PLATFORMS

Global System for Mobile Communication (GSM) is a standard that embraces all areas of technology, resulting in global, seamless wireless services for its customers. It’s all part of the Wireless Evolution and is achieved through the GSM family of wireless technology platforms. The unrivalled success of GSM can be attributed to many factors, including the unparalleled co-operation and support between all those supplying, running and exploiting the platform. It is based upon a true end-to-end solution, from infrastructure and services to handsets and billing systems.

10.2 THE GLOBAL PAKAT RADIO SERVICE

Today’s second-generation GSM networks deliver high quality and secure mobile voice and data services (such as Short Massaging Services, SMS/Text Massaging) with full Roaming capabilities across the world Networks enabled with General Packet Radio Service (GPRS) offer “ always –on ”, higher capacity, Internet based content and packet-based data services. This enables services such as color Internet browsing, e-mail on the move, powerful visual communications, multimedia messages and location based services.

10.3 THE EDGE TECHNOLOGY

Further enhancements in data capability over the core GSM network will be provided with the introduction of Enhanced Data rates for GSM Evolution known as EDGE. This achievement delivers advanced mobile services such as the downloading of video and music clips, full multimedia messaging, high speed color internet access and e-mail on the move.

10.4 THE 3 Globai system for mobile Telecommunication

3GSM is the generic term used for the next generation of mobile communications services. These new systems will provide enhanced services to those available today, with regard to, voice, text and data. The concepts for 3GSM services are currently being developed across the industry and by global groups such as the Third Generation Partnership Project (3GPP). The vision of 3GSM is based on today’s GSM standard, but evolved to include an additional radio air interface better suited to high speed and multimedia data services. 3GSM has the potential to offer services such as high-speed data and video communications, voice calls, messages, photos, file downloads, music, news and video downloading/conferencing all on a single, compact wireless device and with a single contact address/number. 3GSM services will also include concurrent usage of multiple services and bridge the gap between wireless and internet/computing.

10.5 Suggestions For Further Scope For Future Study

The suggestion for the forth studies on GSM system are as follows :

· Radio Channel Modeling
· Testing the GSM networking equipments
· Billing System of the GSM system
· Software management of the GSM system
· Trouble shooting and maintenance in the GSM system
· Detail advancement technology provided by GSM system such as GPRS,EDGE