project report of idea

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1 OVERVIEW OF THE PROJECT

1.1 OBJECTIVE

The objective of this document is to give a brief overview of the Global System of Mobile Communication (GSM) and describe in detail the RF design consideration, methodology and RF simulation results for the coverage area of a Model City and. RF design parameters that are considered are described in this report and results of the simulations conducted to ascertain coverage are also included.

1.2 INTRODUCTION

The GSM system design process consists of several levels or phases. These levels range from an initial budgetary design to a final design used to implement the system. The amount of time and effort required to complete a design increases as one moves from a budgetary design to a final design. However, this additional time and effort results in a more accurate system design & predictions.

The design of a wireless system revolves around three main requirements. Those principles are coverage, capacity and quality and all three of these quantities are interrelated.

The coverage of a system relates to the area within the network that has sufficient signal strength to provide for a call of acceptable quality.

The capacity of a system relates to the ability of the system to support a given number of users

The quality of the system reflects the degree of naturalism or reproduction of speech & data and ease of two-way communication

This report starts with an overview of the GSM 900 network. It is worth mentioning that DCS 1800 is also a part of the GSM standard. Hence the general concepts of GSM 900 can also be applied to DCS 1800. In India both these systems are used either separately or together. In this report GSM is explained using the GSM 900 and then a shift is gradually made to the 1800 band.

IDEA CELLULAR LTD. NEW DELHI 1

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2 INTRODUCTION TO GSM

Global System for Mobile Communication (GSM)

2.1 Definition

Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. It is estimated that many countries outside of Europe will join the GSM partnership.

2.2 Introduction

The concept of cellular service is the use of low-power transmitters where frequencies can be reused within a geographic area. The idea of cell-based mobile radio service was formulated in the United States at Bell Labs in the early 1970s. However, the Nordic countries were the first to introduce cellular services for commercial use with the introduction of the Nordic Mobile Telephone (NMT) in 1981.

The Development of Mobile Telephone Systems

Year Mobile System

1981 Nordic Mobile Telephone (NMT) 4501983 American Mobile Phone System (AMPS)1985 Total Access Communication System (TACS)1986 Nordic Mobile Telephony (NMT) 9001991 American Digital Cellular (ADC)1991 Global System for Mobile Communication (GSM)1992 Digital Cellular System (DCS) 18001994 Personal Digital Cellular (PDC)1995 PCS 1900-Canada1996 PCS-United States

2.3 GSM Standards

Since the scope of the project deals with the Radio Frequency planning, the full specifications of GSM are not mentioned here. However the relevant details regarding the air interface and the mobile handset are mentioned.


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The system specifications for GSM networks are:

Frequency Band: Uplink 890MHz-915Mhz

Downlink: 935MHZ-960MHz

The GSM system is originally specified to operate in the 900MHz band, so even before a commercially viable system could be in place governments of various countries were told to reserve the frequency band for GSM. The frequencies are arranged into pairs so that unique sets can be defined. There are 125 channels in GSM 900, however only 124 are used the first pair are not used, as it is employed as a Guard Band. Shown below is the GSM frequency band, although this is not the only band over which GSM operates it also operate in the 1800Mhz and 1900Mhz band also.

At this moment it would be important to mention here the need to use a lower frequency for uplink. The reason is, since this carries the information from the MS to the BTS over the Air Interface, using a higher frequency means higher attenuation. Secondly to compensate for the attenuation we need to send the signal at a higher power, which consumes more battery, power and leading to a smaller talk time.

Duplex Distance: 45MHz

This is the standard distance between the uplink and the downlink frequencies. This is not constant for all versions of GSM however the separation between the uplink & downlink bands is constant to 20kHz.

Carrier Separation: 200kHz

In GSM we have uplink and downlink carriers. These individual carriers are separated 200kHz apart; therefore we get 125 uplink & downlink carriers. These carriers are then so arranged so that we get 124 ARFCN’s (absolute radio frequency carrier numbers), for GSM 900 they start form 1-124. Henceforth any mention of channels will be done using their ARFCN.


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The figure below shows the carriers.

Each carrier frequency is then divided according to time using a TDMA scheme. Each of the carrier frequencies is divided into a 120ms multiframe. A multiframe is made up of 26 frames. Two of these frames are used for control purposes, while the remaining 24 frames are used for traffic as shown below.

Modulation: Gaussian Minimum Phase shift Keying

The modulation method used in GSM had to be very specific, according to the needs of communication and also to cater for the anomalies in the radio interface. However this would be taken in detail in a later section of this report.

Transmission Rate: 270kbps

Access Method: Time Division Multiple Access(TDMA)

TDMA is used in GSM in conjunction with FDMA to allow voice communication. The 200khz channel is divided into 8 slots and each slot represents a call. However the whole channel is available to the caller, were a caller gets particular time duration in a round robin fashion to proceed with his call as described in the figure below


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Speech Coder: Rapid Pulse Excitation linear Predictive Coder coding at 13kbps

In modern landline telephone systems, digital coding is used. The electrical variations induced into the microphone are sampled and each sample is then converted into a digital code. The voice waveform is then sampled at a rate of 8 kHz. Each sample is then converted into an 8 bit binary number representing 256 distinct values. Since we sample 8000 times per second and each sample is 8 binary bits, we have a bit rate of 8kHz X 8 bits = 64kbps. This bit rate is unrealistic to transmit across a radio network since interference will likely ruin the transmitted waveform. In GSM speech encoding works to compress the speech waveform into a sample that results in a lower bit rate using RPE-LPC. The actual process will be discussed later in the section, where the journey from speech to radio waves is considered.

Diversity: Channel Coding

Interleaving

Frequency Hopping

Adaptive Equalisation

Over The Channel Bit Rate: 22.8kbps

Slow Frequency Hopping: 217hops/second

GSM can use slow frequency hopping where the mobile station and the base station transmit each TDMA frame on a different carrier frequency. A form of slow frequency hopping is used by GSM to help combat the multi path burst errors characteristic of cellular environments. Each base station has its own pattern for hopping from one carrier frequency to another from slot to slot, with mobiles using that base station following suit. This frequency hopping also reduces the incidence of co-channel interference between clusters of cells. The frequency-hopping algorithm is broadcast on the Broadcast Control Channel. Since multi-path fading is dependent on carrier frequency, slow frequency hopping help mitigate the problem. Frequency hopping is an option for each individual cell and a base station is not required to support this feature.


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Typical Base Station Transmit Power: 16W

Frame Duration: 4.615ms

Channel Coding: Half rate Convolution.

These were a few of the specification that were described by GSM for the network. However the MS manufactures also had to adhere to certain specific standards which are discussed as below: There are various types of mobile handsets that are used, namely:

1. Vehicle Mounted: Attached to a vehicle and operate on all power ranges and the antenna is physically mounted on the vehicle.

2. Portable: This equipment can be hand carried or vehicle mounted and the antenna is not connected to the Mobile Termination unit, which is MS minus the antenna.

3. Hand-held: This is the normal piece of equipment that we see generally being carried by people.

2.4 Different Forms of GSM

As all things grow and need space, the same happened with GSM too, being the most popular and by far the most used cellular technology, today GSM accounts for 69.5% of the World's digital market and 64.7% of the World's wireless market. This astounding popularity and has put immense burden on the limited number of ARFCN’s, as a result various bands have been covered and now GSM is called P-GSM (primary GSM) and its cousins have emerged. The figure above speaks volumes about the rise in popularity of the mobile communication:

E-GSM: This represents an extension of the lower end of the two sub blocks by 10MHz adding 50 More ARFCN’s to the Primary GSM (P-GSM).

Uplink Frequency: 880MHz-915MHz

Downlink Frequency: 925MHz-960MHz

DCS 1800: At a late stage in GSM development the existing technology was modified to meet the need for PCN networks. This involves changes to the radio interface, which moves spectrum allocation up to around 1.8 GHz. More spectrum is available in this frequency range, for two sub-blocks of 75Mhz with duplex spacing of 95Mhz, giving a total of 374 carriers.

Uplink Frequency: 1710Mhz-1785Mhz

Downlink Frequency: 1805Mhz-1880Mhz


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PCS 1900: Used in the USA. The FCC has split the designated spectrum into six duplex blocks. The USA has been divided into 51 Major Trading Areas and 493 Basic Trading Areas. An MTA is broadly equivalent in size to a state, whilst a BTA approximates to a large city. Each MTA has access to 3x15MHz block and each BTA has access to 3x5MHz blocks.

2.5 COMPONENTS OF NETWORK:

GSM is divided into two separate entities the Switching System (SS) and the Base Station System. Each of these contains a number of functional units, where all systems functions are realised. These functional units are implemented into various hardware components.

Functional units within the system are separated by interfaces. Such interfaces are the Air interface (MS-BSS), the Abis interface (BTS-BSC) and the A interface (BSC-MSC). However before undertaking to study the main functional components a brief overview of the complete system is deemed necessary.

The SS Includes the Following Subsystems:

Mobile services Switching Centre (MSC)

Visitor Location Register (VLR)

Home Location Register (HLR)

Authentication Centre (AUC)

Equipment Identity Register (EIR)


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The Base Station System (BSS) includes:

Base Station Controller (BSC)

Base Transceiver Station (BTS)

Transcoder Rate Adapter Unit (TRAU)

Base Transceiver Station

Each cell has a Base Transceiver Station (BTS) operating on a set of radio channels. These are different from the channels used in neighbouring cells to avoid interference. The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network.

Base Station Controller

A base station controller (BSC) controls a group of BTS’. BSC controls such information as handover and power control. BSC can be implemented as a stand-alone node or as integrated with the MSC. The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSC's are served by an MSC.

Mobile services Switching Centre

A number of BSC’ are served by a MSC which controls calls to and from other telephony and data communication systems, such as the public switched telephone network (PSTN), integrated services digital network (ISDN), public land mobile networks (PLMN), public data networks (PDN) and possibly, various private networks. The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signalling, and others.

Databases

The above-mentioned units are all involved in carrying out speech connections between an MS and for example a subscriber in a PSTN. If it were not for the possibility of making calls to an MS we would not need any further equipment. The problem arises when we want to make an MS terminated call. The originator hardly ever knows where the called MS is. Due to this we need a number of databases in the network to keep track of the MS.

The most important of these databases is the Home Location Register (HLR). When someone buys a subscription from one of the GSM operators, he will be registered in the


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HLR of that operator; he will be registered in the HLR of that operator. The HLR contains subscriber information, such as supplementary services and authentication parameters. Furthermore, there will be information about the location of the MS, i.e. in which MSC area the MS resides presently. This information changes as the MS moves around. The MS will send location information to its HLR, thus providing means to make a call.

Authentication Centre (AUC) is connected to the HLR. The function of the AUC is provided the HLR with authentication parameters and ciphering keys, both used for security reasons. AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

The Visitor Location Register is a database containing information about all the MS’s currently located in the MSC area. As soon as an MS roams into a new MSC area, the VLR connected to that MSC would request data about the MS from the HLR. At the same time the HLR will be informed in which MSC area the MS resides. If, later on the MS wants to make a call, the VLR will have the information needed for the call set-up without having to interrogate the HLR each time. The VLR can be seen as a distributed HLR. The VLR will also contain more exact information about the location of the MS in the MSC area. The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC would request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call set-up without having to interrogate the HLR each time.

Gateway

A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC. If someone in a fixed network (PSTN) wants to make a call to a GSM subscribe, the exchange in the PSTN will connect the call to a gateway. The gateway is often realised in an MSC. It can be any one of the MSC in the GSM network. The GMSC will have to find the location of the searched MS; interrogating the HLR where the MS is registered can do this. The HLR will reply with the address to the current MSC area. Now the GMSC can re-route the call of the current MSC. When the call reaches that MSC, then the VLR will know in more detail where the MS is. The call can then switched through.

Mobile Station

In GSM there is a difference between the physical equipment and the subscription. The mobile station is piece of equipment, which can be vehicle installed, portable or hand-held. In GSM there is a small unit called the Subscriber Identity Module (SIM), which is a separate physical entity e.g. an IC-card, also called a smart card. SIM and the mobile equipment together make up the mobile station. Without SIM, the MS cannot get access to the GSM network, except for emergency traffic. While the SIM-card is connected to the subscription and not to the MS, the subscriber can use another MS as well as his own. This then raises the problem of stolen MS’, since it is no use barring the subscription if the equipment is stolen. We need a database that contains the unique hardware identity of the equipment, the


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Equipment Identity Register (EIR). The EIR is connected to the MSC over a signalling link. This enables the MSC to check the validity of the equipment. An non-type-approved MS can also be barred in this way. The authentication of the subscription is done by parameters from AUC.

The Location Area (LA)

Each MSC/VLR service area is divided into several Location Areas. A Location Area is a part the MSC/VLR in which an MS may move freely without updating location information to the MSC/VLR exchange that controls the Location Area. A location Area is the area where a paging message is broadcast in order to find the called mobile subscriber. The Location Area can have several cells and depend on one or more BSC’s, but it belongs to only one MSC/VLR. The Location Area can be identified by the system, using the Location Area Identity (LAI). The location Area is used by the system to search for subscriber inactive state.

The Cell

A location area is divided into a number of cells. The cell is an area of radio coverage, which the network identifies with the Cell Global Identity (CGI). The mobile station itself distinguishes between cells using the same carrier frequencies by the use of the Base Station Identity Code (BSCI).


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2.6 The Radio Interface

The channels in GSM are used to carry both user traffic and control data. This control data performs functions such as paging, access and call set-up. The complete range of functions required is defined by a set of Logical Channels.

The radio interface is the general name of the connection between the mobile (MS) and the base transceiver station (BTS). It utilises the TDMA-concept with one TDMA-frame per carrier frequency. Each frame consists of eight time slots (TS). The direction from the BTS to MS is the downlink and the opposite is defined as uplink.

Before we go on to discuss the various channels in GSM, it would be mandatory to list out all the channels that are used in GSM communication.


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Physical Channel

Logical Channel

Control Channel

Broadcast Channels

Common Control Channels

Dedicated Control Channels

Traffic Channel

The Physical Channel

In GSM each carrier or radio channel is divided into the time domain to produce eight time slots. Each time slot on a radio carrier is called a ‘Physical Channel’, and the P-GSM allocation therefore, supports 992(i.e. 8X124 physical Channels). The combination of time division techniques and the access mechanism gives rise to term Time Division Multiple Access. During operations in a GSM system, an individual mobile station will therefore be transmitting and receiving bursts.

Like we said earlier one time slot in a TDMA frame on one carrier is referred to as a physical channel. It could be compared with one physical channel in a FDMA system where every user is connected to the system via one out of a number of frequencies.

Consequently, there are eight physical channels per carrier in GSM, channel 0-7 (timeslot 0-7). The information sent during on single TS is called burst, there are different kinds of bursts that will be discuss when we will discuss the Traffic Channel.

The Logical Channels

A great variety of information must be transmitted between the BTS and the MS e.g. user data and control signalling. Depending on the kind of in information transmitted, we refer to different logical channels i.e. the different types of information are transmitted on the physical channel in a certain order. These logical channels are mapped on to the physical channels. For example speech is sent on the logical channel “Traffic” which during the transmission is allocated a certain physical channel say channel 6 or TS 6. The logical channels are divided into two groups control channel and traffic channels.

Logical channels are defined functions that can be supported within a physical channel. One physical channel can support a number of logical channels. The frame structure of 8 time slots is relevant only in defining a physical channel. In order to describe allocations of time and frequency used by a particular logical channel it is necessary to utilise a structure of Multiframes, Superframes and Hyperframes, which will be discussed in Frame formation.


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The Control Channels, CCHWhen the MS is witched on and is searching for a radio broadcast station (RBS) to camp on. This is done by scanning the whole frequency band or optionally uses a list containing the allocated BCCH-carriers for this operator. When the MS has found the strongest carrier it has to find out whether this is a BCCH carrier. A BCCH carrier is the frequency used to carry the control channels.

Broadcast Control Channels, BCCH

Frequency Correction Channel: On the FCH a sinusoidal wave signal is transmitted. This serves two purposes: one to make sure this is the BCCH carrier, the other enables the MS to synchronise the frequency. FCCH is transmitted on the downlink point-to-multipoint.

Synchronisation Channel: Next thing for the MS is to synchronise to the structure within this particular cell, and also to make sure that the chosen base station is a GSM base station. Listening to the synchronisation channel, SCH, the MS receives information on the TDMA frame structure in this cell (the TDMA frame number) and also the Base Station identity code (BSIC), of the chosen base station. BSIC can only be decoded if the base station belongs to the GSM network. SCH is transmitted on the downlink point-to-multipoint.

Broadcast Control Channel: The last information the MS must receive in order to start roaming, waiting for calls to arrive or making calls & some general information concerning the cell. This is broadcast on the broadcast control channel BCCH, and doe among others to include the Location Area Identity (LAI), maximum output power allowed in the cell and the BCCH carriers for the neighbouring cells, on which the MS will perform measurements. BCCH is transmitted on the downlink, point-to-multipoint.

Now the MS is tuned to the base station and the synchronised with the frame structure in this cell. The base station are not synchronised to each other, so, every time the MS changes cells FCCH, SCH and BCCH have to be read.

Common Control Channel, CCCH

Paging Channel: Within certain time intervals the MS will listen to the paging channel (PCH) to see if the network wants to get in contact with the MS. The reason could be an incoming call or an incoming short message. The information on PCH is a paging message, including the Ms’s identity number IMSI or a Temporary number TMSI. PCH is transmitted on the downlink, point-to-point


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Random Access Channel: If listening to the PCH, the MS realises it is being paged. The MS answers, requesting a signalling channel, on the random access channel, RACH. RACH can also be used if the MS wants to get in contact with the network, e.g. when setting up a call. RACH is transmitted on the uplink point-to-point.

Access Grant Channel: The network assigns a signalling channel (the stand alone dedicated channel, SDCCH). This assignment is performed on the Access Grant Channel, AGCH. AGCH is transmitted on the downlink, point-to-point.

Dedicated Control Channel, DCCH

Stand Alone Dedicated Control Channel: The MS (as well as the BTS) switches over to the assigned signalling channel-the Stand Alone Dedicated Control Channel, SDDCH. Among others the call set up procedure on the SDDCH as well as the transmission of textual messages (SMS & CellBroadcast).

SDCCH is transmitted on both up &downlink, point-to-point. When the call set up is performed, the MS is told to switch to a traffic channel, TCH, defined by the carrier and the time slot.

Slow Associated Control Channel: Within a certain time interval on the SDCCH, and also on the traffic channel, information on the Slow Associated Control Channel, SAACH is transmitted. On the uplink MS sends averaged measurements on own base station (signal strength and quality) and neighbouring base stations (signal strength). On the downlink the MS receives information on which transmitting power to use and also an instruction on the timing advance. As seen, the SACCH is transmitted on both up & downlink point-to-point.

Fast Associated Control Channel: If, suddenly during the conversation a handover must be performed the Fast Associated Control Channel, FAACH is used. FACCH works in stealing mode, meaning that 20ms segment of speech is exchanged for stealing information necessary for handover. The Subscriber will not recognise this interruption in necessary for speech since the speech coder will repeat the previous speech block.

Traffic Channels, TCH

The traffic channels are if two types, full rate and half rate. Today only full rate TCH is used, in the future, when the half rate speech coders with a tolerable quality have been designed there will be the possibility to use half rate TCH.

The difference between full rate & half rate TCH is that a full rate TCH occupies one complete physical channel (on TS on a carrier), whereas two half rate TCH’s can share one physical channel.


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3 INTRODUCTION TO PROPAGATION

3.1 PROPAGATION is a key process within every radio link. During propagation, many processes act on the radio signal.

3.2 ATTENUATION

The signal amplitude is reduced by various natural mechanisms; if there is too much attenuation, the signal will fall below the reliable detection threshold at the receiver. Attenuation is the most important single factor in propagation.

3.3 MULTIPATH AND GROUP DELAY DISTORTIONS

The signal diffracts and reflects off irregularly shaped objects, producing a host of components which arrive in random timings and random RF phases at the receiver. This blurs pulses and also produces intermittent signal cancellation and reinforcement. These effects are combated through a variety of special techniques.

3.4 TIME VARIABILITY

Signal strength and quality varies with time, often dramatically.

3.5 SPACE VARIABILITY

Signal strength and quality varies with location and distance.

3.6 FREQUENCY VARIABILITY

Signal strength and quality differs on different frequencies.


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3.7 EFFECTIVE MASTERY OF PROPAGATION RELIES ON

Physics: understand the basic propagation processes.

Measurement: obtain data on propagation behavior in area of interest.

Statistics: characterize what is known and extrapolate to predict the unknown.

Model making: formalize all the above into useful models.

INFLUENCE OF WAVELENGTH ON PROPAGATION

Radio signals in the atmosphere propagate at almost speed of light.

λ = wavelength

C = distance propagated in 1 second

F = frequency, Hertz

The wavelength of a radio signal determines many of its propagation characteristics eg:

Antenna elements size are typically in the order of 1/4 to1/2 wavelength.

Objects bigger than a wavelength can reflect or obstruct RF energy.

RF energy can penetrate into a building or vehicle if they have apertures a wavelength in size, or larger.


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Fortunately, Rayleigh fades are very short and last a small percentage of the time.

Two antennas separated by several wavelengths will not generally experience fades at the same time.

“Space Diversity” can be obtained by using two receiving antennas and switching instant by instant to whichever is best.

Required separation D for good de-correlation is

• 12-24 ft. @ 800 MHz.

• 5-10 ft. @ 1900 MHz.


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SPACE DIVERSITY APPLICATION LIMITATIONS

Space Diversity can be applied only on the receiving end of a link.

Transmitting on two antennas would:

• fail to produce diversity, since the two signals combine to produce only one value of signal level at a given point -- no diversity results.

• produce objectionable nulls in the radiation at some angles

Therefore, space diversity is applied only on the “uplink”, i.e. reverse path

• There isn’t room for two sufficiently separated antennas on a mobile or handheld.


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USING POLARIZATION DIVERSITY WHERE SPACE DIVERSITY ISN’T CONVENIENT

Sometimes zoning considerations or aesthetics preclude using separate diversity receive antennas.

Dual-polarized antenna pairs within a single casing are becoming popular

• Environmental clutter scatters RF energy into all possible polarizations

• Differently polarized antennas receive signals which fade independently

• In urban environments, this is almost as good as separate space diversity

Antenna pair within one casing can be V-H polarized, or diagonally polarized

duplexing OK


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THE RECIPROCITY PRINCIPLE DOES IT APPLY TO WIRELESS?

Between two antennas, on the same exact frequency, path loss is the same in both directions

But things aren’t exactly the same in cellular because

• Transmit and receive 45 MHz. apart

• Antenna: gain/frequency slope?

• Different Rayleigh fades up/downlink

• Often, different TX & RX antennas

• RX diversity

Notice also the noise/interference environment may be substantially different at the two ends.


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3.8 Antenna Down tilt-What’s the goal?

Down tilt is commonly used for two reasons

1. Reduce Interference

• Reduce radiation toward a distant co-channel cell• Concentrate radiation within the serving cell

2. Prevent “Overshoot”

• Improve coverage of nearby targets far below the antenna otherwise withinnull” of antenna pattern


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Types Of Down tilt

Mechanical down tilt

• Physically tilt the antenna

• The pattern in front goes down, and behind goes up

• Popular for sectorization and special omni applications

Electrical down tilt

• Incremental phase shift is applied in the feed network

• The pattern “droops” all around, like an inverted saucer

• Common technique when down tilting omni cells


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4 RF Planning

4.1RF PLANNINGThe GSM/DCS system design process consists of several levels or phases. These levels range from an initial budgetary design to a final design used to implement the system. The amount of time and effort required to complete a design increases as one moves from a budgetary design to a final design. However, this additional time and effort results in a more accurate system design & predictions.

The design of a wireless system revolves around three main criteria. Those principles are coverage, capacity and quality and all three of these quantities are interrelated.

The coverage of a system relates to the area within the network that has sufficient signal strength to provide for a call of acceptable quality.

The capacity of a system relates to the ability of the system to support a given number of users

The quality of the system reflects the degree of naturalism or reproduction of speech & data and ease of two-way communication.

Thus a well-planned cell should meet the following requirements Required and predicted coverage. Predicted levels of Co channel and adjacent channel interference. Minimum antenna adjustments during the optimization process. Minimum changes in the BSS parameters Facilitate easy expansion of the network with minimal changes in the system.

4.2 Problem Definition:

To plan and optimize the network for DCS-1800 of a model city.

Initially, neither the capacity nor the coverage requirements are known to the planners. The planning is based on projections given by the customer. The customers may specify the maximum number of sites they desire in the city or the total number of subscribers that need to be catered to. Based on either of these inputs the planning process begins.

Planning is done based on the following steps: Procurement of land data from vendors. Selection of location of sites Selection of Antenna Selection of Antenna parameters Optimizing the Network Generation of Polygon statistics Frequency Planning Co-channel Interference Statistics Adjacent channel Interference Statistics

Procurement of land data


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City survey report is procured from the map vendors. It comprises of the following:1. Map2. Land use cover3. Heights4. Vectors

This data is then fed into the NetPlan tool.

4.2.1Link Budget

Link budget spreadsheet is used to make the "first attempt" at system design. The link budget provides an estimate of the cell radius for a given coverage reliability and thus provides an estimate of the number of cells required to cover a certain area.

Ideally elaborate RF propagation tests are necessary to characterize the radio environment and determine the standard deviation, In-building losses etc. However for this design purpose, standard propagation models have been used combined with idea’s expertise and experience in this field and based on the design planning done by idea for its various customers in India. Two different link budgets were used in calculation the path loss and cell radius for Nokia and Lucent BTS’s. The details for both the link budgets (Street Level) are given below.

Link Budget Assumptions (Nokia BTS’s)

Mobile

Transmit Power = 1 watt (30 dBm)

Sensitivity = -102 dBm

Body Loss = 3dB

Tx/Rx Antenna Gain = 0 dBi

Cable Loss = 3 dB

Base Station

Transmit Power = 16 watt (42dBm +/-1 dBm )

Sensitivity = -110dBm (This is the typical sensitivity of nokia BTS)

Tx/Rx Gain = 17.0/18 dBi

Diversity Gain = 3 dB

Cable Loss (Feeder) = 3 dB (This is of course is dependent on the length of the feeder which varies from site to site depending on the height the antenna is mounted, but here an approximation has been made)

Interference Degradation Margin = 3 dB (This is margin for other interference sources)


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Uplink Link Power Budget

Allowable Path Loss = [MS transmit power – Body Loss] – [BTS Sensitivity + BTS Cable Loss – BTS Receive antenna gain – Diversity gain + Duplexer loss + Fading Margin + Interference degradation margin]

4.2.2 Selection of location of sites

Once provided with the map and the population density, the process involves selection on the basis of the following considerations:

Population density in the area Type of Land clutter

Dense Urban Urban Suburban Suburban with dense vegetation Rural Industrial area Utilities (marshalling yards, docks, container depots etc.) Quasi open area Forest Water

Significance of area (markets, business center, airport, VIP areas etc.) Permission to install site

4.2.3 Selection of Antenna

For this design, the k739495 (Kathereine) antenna was chosen, which has the following specifications:

Gain of 15.85 dBd Horizontal Beam width of 65o

Vertical Beam width of 7 o

Less power radiation from the back lobe of the antenna Electrical tilt of 0 o , 2 o

4.2.4 Selection of Antenna parameters:

Antenna parameters of height, orientation and tilt are so chosen to obtain the best possible coverage with the least possible signal suppression.


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Height: Height ranges from 20m to 30m (for rooftop antennas) and 35m to 50m (for ground base towers) depending upon the height of clutter and the area in vicinity.Orientation: Orientation of the antennas is the direction at which the specified area to be covered. The orientation angle for the sectors is taken in the clockwise direction considering north as the reference at 0.

Tilt: This is done to carry out the process of either decreasing or increasing the coverage area of a sector. This is done to avoid interference arising out of the fact that the sector under consideration is radiating out of the desired area of its coverage and hence affecting other neighboring sectors.Tilt is given by the following means:

Electrical tilt : This tilt is given by changing the phase relations in between various elements of an antenna. If a down tilt is being given electrically, then there will be a down tilt in both the front and the back lobe of the antenna.

Mechanical tilt : This tilt is given manually by changing the angle antenna makes with the vertical. If a down tilt is being given mechanically, then there will be a down tilt in the front lobe but an up tilt in the backlobe.

Mechanical tilt is more often used than electrical tilt.

After simulating the above inputs planning tool can be used to generate various images to check the resultant network. Some of the important images that are used to analyze the network are as follows:

Downlink best signal strength : The image provides a comprehensive knowledge of the zones receiving signal sent out by Base Transceiver Station (BTS) and theamount of signal received. This signal is the signal received by our handsets or Mobile Stations (MS).

Downlink best Server/Sector : Different antennas of the site are called sectors. With the help of this image an RF engineer can visualize the amount of signal that is being released by each sector of the site.

The initial stage of planning and designing the network is followed by a second stage of optimization of network.

4.3 Frequency Planning

This allows us to allot frequencies to all the sectors of the network. Frequency reuse is the central part of this frequency planning because most of the times frequency channels are much less than the number of sectors. This has to be done in such a manner so that to minimize co-channel and adjacent channel interference. This is done as follows:

For example, let there be 12 sites with 3 sectors each and channels given be 12 (32 to 43), then we categorize channels as follows:

A1 A2 A3 A4


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32 33 34 35

36 37 38 39

40 41 42 43

Now allocation is done so that the antennas of a particular site are allotted channels from one group out of A1, A2, A3 and A4.

4.3.1 Neighbour Planning

Once the frequency planning is over, the next step is neighbour planning. A mobile station keeps track of its six nearby BTS so that it can undergo Handover when required.Now of the numerous BTS sites that a mobile station may be looking at, we have to define at least six neighbours, i.e. the most probable BTS to which a call handover process may take place.This process of defining the handover candidates is called as Neighbour Planning.

Main considerations-:

1. The neighbouring cell must be neither a Co-channel nor an Adjacent channel to our serving cell.

2. The handover candidate of first layer (generally in city center and urban areas) is not very far (600-1000m) from the mobile station as the signal strength is very important for successful handover.

3. Second and third layers candidates (for less urban and suburban areas) are also marked, though they are a little far (1000-6000m) but that is just to avoid congestion in the network at certain times.

4. Also in outskirts of cities and on highways the handover candidates can be as far as 10km or more.

These distances are for a 900Mhz network.

However as the losses in case of 1800 MHz network is more , the cell radius decreases considerably , thus the above stated values for neighbor site distance also decreases and is approximately half of that in a 900MHz network.


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4.4 PLANNING TOOL

Idea cellular Pvt. Ltd. has emerged as the global leader in Radio Frequency (RF) Planning for the Cellular operators. This is carried out using the highly acclaimed idea RF planning tool namely “Network Planning (Netact Plan)” tool which is based on Sun Solaris platform. The planning tool takes input data in the form of land use cover sets, heights and vectors (roads and railways etc) from vendors and follows already fed in path loss algorithms to simulate a virtual network based on which real time networks are implemented.

Radio frequency planning is carried out using the idea proprietary Netact Plan tool.This tool enables RF Engineers to carry out exhaustive analysis prior to actual installation of network components, which are Base Transceiver Stations (BTS). Its fruitions are evident from the overwhelming response Nokia receives from cellular operators worldwide.

Once data has been procured in the form of land use cover sets, heights and vectors from vendors, the ideaRF team can provide efficient and good capacity network based on the customer requirement.

An overview how to use this tool for planning a MODEL CITY is given below:

Open the Net act Plan Interface

Fig: Netact Plan Interface (Courtesy Nokia)


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Open the analysis of a Model City

To start , a new analysis is created using the various data like

Clutter Elevation Vectors (roads etc)

Here an existing analysis is being opened named Model City.

Fig: Opening analysis of Model City (Courtesy Nokia)


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Identify the areas for sites to be placed on the basis of user densityThe next step involves studying the clutter data so that sites could be placed on the map of the city.

The downlink signal strength in the coverage area should be –90 dbm or better in 90% of the coverage area.In order to define predicted signal strength in outdoors, following values may be considered for penetration loss of different clutter profiles:

Clutter Profile LossDense urban area 25dBUrban area 20dBSuburban area 10-15dBRural area 5-10dBIn car loss 6dBBody loss 3dB

The following figure shows the clutter (i.e, type of population in a particular area).


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Fig: Clutter Image with Map (Courtesy Nokia)


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The sites are selected according to the Clutter data

Fig: Sites with Clutter image and Map (Courtesy Nokia)


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Create the image for Best Signal Strength (Downlink)

Once the sites are placed with all the antenna parameters the image of this analysis is created so as to have an idea of the predicted coverage in the city

Here the image is being created using the XLOS Propagation model that is a proprietary of Nokia and gives fairly accurate results . Extensive resources have been employed to test, calibrate and validate the accuracy of the algorithms of Xlos. To provide maximum flexibility when conducting certain propagation studies, Netact Plan also incorporates the COST 231 Hata, Walfish-Ikegami, and JTC Microcell propagation models.

Due to the complexities of simulating real world phenomena, no prediction model will be completely accurate all of the time, thus Netact Plan has some additional features like:

An automated fine tuning feature to correct the propagation model for different environments can improve coverage prediction for a region in which no measured data is available.

A matrix merge between the predicted and measured data that provides the optimal solution for a particular region.

Comparison between measured and predicted data.

Fig: Creating the Downlink Best Signal Strength Image (Courtesy Nokia)


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Fig: The beginning and end of creation of the image for best signal strength.(Courtesy Nokia)

Displaying the created image

Once the image is created it is displayed on the analysis showing the coverage prediction for the specified area and with specified parameters .The signal strength is measured in dbm at any particular point with the foll0wingcriteria :

Signal Strength(SS)(in dbm ) CoverageSS <= -65 Excellent-75 <= SS < -65 Very good-85 <= SS < -75 Good-95 <= SS < -85 Average-110 <= SS < -95 Poor-110 <= SS No Calls


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Fig: The Coverage Plot with Sites and Map (Courtesy Nokia)

Studying the various site parameters

The various site parameters can be viewed and modified if required. Theseinclude -:

Latitude Longitude Site Id Site Name Antenna Model Antenna height ,bore and tilt Parameter set that defines Propagation Model, and other losses etc.


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Fig: The Site Data Window (Courtesy Nokia)

Frequency Plan window

o Though we can go for automatic frequency plan but as it suffers from few limitations, we plan it manually and then feed it in NetPlan.

o The frequencies are loaded as ARFCNs and then they can be marked as BCCH and also they are locked so as to prevent any accidental change in the frequencies assigned.


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Fig: A typical channel assigner Window with the Network Analysis window (courtesy Nokia)

Once we have assigned the frequencies to the various sites we can easily take a look at

Co-ChannelAdjacent Channel Channels along withThe handover Candidates for any site.


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Fig: Sites with Frequencies along with the Co-Channel and Adjacent Channels marked in Red and Yellow while handover Candidates are in Blue.

Creating C/I and C/A Image


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Once we have assigned frequencies to the sites we can also go for generating Co Channel and Adjacent Channel C/I image.

The image thus created gives us a fair idea about the areas at which we have interference so that we could mark those areas and induce the necessary changes in the frequency plan.

Fig: Creating Co Channel and Adjacent Channel C/I image.


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Fig: Co Channel C/I image with legend.


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Fig: Adjacent Channel C/I with legend.

4.5 RF surveys and site acquisition

Once the process of planning of planning is over then teams are sent on the field for performing RF surveys. These surveys are meant for finding the most suitable place near our proposed place where a tower can be installed.

Generally the search ring for a site in city center is about 50m and outside the city it is up to 150m. If we go beyond that, there is a probability that what we predicted would not be available to us on the field

For site surveys and feasibility of a location there is a site survey form that gives us a fair idea about the situation at a particular site.

During site survey the engineer locates the best building or plot where a tower can be put and then a detailed survey of the surrounding area is done and compiled. A sample RF survey form is depicted here to give a better understanding of the whole procedure.

After the survey is complete and compiled then the acquisition team has the responsibility to proceed with the legal formalities regarding the acquisition of the building or plot . Various requirements such as the strength of the building etc. must also be taken into account .

After the survey and acquisition is complete then the construction of the tower and tower shelter starts . The BTS is installed and commissioned. Various sites are linked with each other using microwave links or fiber cables.

Once the sites have been commissioned and interconnected to the BSC the are made to radiate. The site is then said to be on air

4.6 RF Optimization :

Once the sites are acquired and everything installed, the next most important process is that of optimization.

I Optimization is done to check the performance of the network, just after it is made operational and to get best possible quality of service. The objective of optimisation procedure is:

a) To check whether the network meets the customer’s given requirements, on the basis of which network was designed.

b) To check whether the parameters and configurations are defined correctly or not.c) To find out and suggest changes in the defined parameters and configurations to achieve

best possible quality of service.


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Quality of service can be characterized by factors such as contiguity of coverage, accessibility to the network, speech quality and number of dropped and blocked calls. A number of parameters are checked as a measure of quality of service by using a drive test system.

Drive Test system comprises of a test mobile phone, software to control and log data from the phone and a Global positioning system receiver for position information as shown in figure (1). A drive test system can only indicate the type of problem in the network that exists, it doesn’t indicate cause of the problem but with the help of knowledge of possible reasons of a problem, one can trace the cause. Following steps are taken to fulfill the objective of network optimization using a drive test tool.

a) Collection of Data and extraction of relevant information from it.b) Analysis of the extracted data.c) Suggesting changes in the network configurations based on the analysis.

Collection of Data and extraction of relevant information:

Drive test involves setting up a call to best carrier and driving along the roads. While driving the radio parameters and air interference signal data are collected as a log file. In general following parameters are checked during the drive test for different categories of terrains like dense urban, sub-urban, rural, highways and for different clutters like in building, residential areas, commercial areas, industrial areas etc.

The various data observed and analyzed are

1. Rx Level.2. Rx Quality3. Timing Advance4. Handover parameters5. Data of six best neighbor cells.6. Layer 2 and layer 3 messages.

From the data collected various information can be extracted which depict the performance of BTS sites and the network as a whole. Following information can be extracted from this drive test data.


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1) Coverage edge probability2) Coverage area probability3) Speech quality4) Frequency and BSIC reuse5) Neighbor cell definition details.6) Handover details.

Edge Probability:

To get an idea of coverage area, coverage boundary of all the cells based on received signal level (RXLEV), is obtained and is plotted over the geographical map of the area. The coverage boundary of a cell is considered to be made up of equal received level points on the field.

With the help of this coverage plot the edge probability or the probability of getting a signal level better than a specific value over the boundary of all cells is obtained which helps in determining the performance of the network with respect to coverage boundary requirements given by the customer.

Area Probability:

The obtained signal levels from the cells at all the points of the network, are then used to make, a best server plot. This best server plot is drawn by categorizing it on the basis of in building coverage, in car coverage and on street coverage. These categories are defined on the basis of the coverage area where a good quality conversation is required. The details about threshold defined for these categories are discussed in ‘coverage planning report’.

Speech Quality:

Speech quality is a very important aspect for determining the quality of service for whole of the network. Speech quality is inferred by the RXQUAL measurements during the drive test. RXQUAL, is the Bit error rate (BER) derived from the 26 bits midamble on TDMA burst. Its level characterizes speech quality where 0 indicates the highest quality and 7 the worst. Thus during drive test, poor quality areas can be found and marked by looking over the quality on the scale of 0 to 7. RXQUAL can be poor due to poor RXLEV, Co-channel interference, adjacent channel interference or multi-path. RXQUAL is measured and tested for all the categories of clutter and terrain.

Frequency and BSIC reuse:


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From the collected data the frequency reuse pattern with the BSIC (Base Station Identity Codes) planning of all the cells of the network can be obtained. The reuse distance for all the reused frequencies can be determined

Neighbor cells definition details:

With the help of collected data 6 best serving neighbours of all the cells can be determined. The drive test window of the antenna system gives details of 6 best neighbors at an instance.

Handover details:

There are certain other very important parameters which has to be checked during drive test as these parameters directly reflects on the performance of the network, like handover margin, handover threshold, values of handover timers, offset and penalty for the handovers.

With a call established, and measuring on the cell edge, we can display the phone measurements of serving and neighbor cells. The difference between the RXLEV of the server and that of neighbors can be monitored on the amplitude and time scale. At some point on the drive-test route, one of the neighbor’s RXLEV will become stronger than the server’s signal level and when this difference of the two exceeds the handover margin, for at least a timing set in the ‘handover required’ counter in BSS, a handover will occur. Thus by simultaneously monitoring RXQUAL during the handover, the value of the handover margin can be determined and a decision can be made whether that value is appropriate for the quality of service desired.Analysis of extracted data:

The information extracted from the collected data is then analyzed to compare it with the agreed benchmarks related to coverage, quality, handover success rate etc and is used to infer the cause of the deviation from given requirements and set benchmarks. It is also used to infer cause of detected problem in the network if there is any.

There are special coverage requirement which are discussed in ‘coverage planning report’ under ‘special coverage category’ these specific coverage requirements are matched to find out whether the requirement of customer is taken care of or not.

RXQUAL is also matched with the given requirement. If RXQUAL is poor and RXLEV is sufficiently good it can reasonably be deducted that the cause is interference. Generally a test frequency which has no adjacent or Co channel present in that area is used to find out if interference is because of multi-path. If it is not because of multi-path then spectrum analyzer can be used to find out whether it is adjacent channel interference or it can be deducted that it is Co channel interference.

A handover margin on the high side will result in a handover occurring after the user has experienced some deterioration in quality. High handover margins can result in poor reception and dropped calls, while very low values of handover margin can produce “Ping-Pong” effects as mobile switches too often between cells.


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With the help of collected data it can be found out weather uplink and downlink are balanced or not. If even after having good RXLEV and RXQUAL, calls are dropping or even when RXLEV and RXQUAL of serving cell is better than that of neighbor cell, handover is taking place, it indicates that the link needs to be balanced.

BSIC for all the cells are also checked and verified with what is defined in the BSS. If same BSIC is defined for cells having same BCCH frequency and these cells coexist in the neighbor list then understandably lot of handovers will be unsuccessful.

Layer 2 and 3 messages can be used for analyzing cause of a particular handover failure, call drop, very poor speech quality or any other abnormality in the performance of the network.

Suggesting changes in the network configurations based on the analysis:

After detection of the causes of the deviation from the requirement or network related problems, measures are taken to improve the performance of the network and to match customer’s requirement.

Network performance can be influenced by the network parameters. The configuration parameters can be divided into two groups hard configuration and soft configuration, depending on the type of control and action required to modify them.

Hard Configuration:

The hard configuration parameters are aspects of base station configuration and include antenna type, antenna gain, antenna orientation, and effective height of antenna radiation centre, use of space diversity, antenna feeder loss and effective isotropic radiated power (EIRP).

Changes in this configuration are made to meet the requirements and to deal with the analyzed problems. For an example if certain area is affected by interference resulting in poor quality then one of the way to reduce interference level is by shrinking the coverage area. Shrinking of coverage area can be achieved by reducing EIRP that is by replacing the existing antenna with a lower gain or narrower horizontal beam width antenna system and by reducing transmitted power under limitation of not loosing the link balance. Most effective solution used to shrink coverage area is by increasing antenna down tilt and/or reducing antenna height. Similarly to improve coverage in certain areas the transmitted power of BTS can be changed, antennas with different gain or beam width can be used and the height of antenna system can be changed.

For further specific coverage and quality requirements Pico or micro cells can be installed inside the residential places, commercial buildings, stadiums and car parks etc. A pico cell is nothing but a cell with very low EIRP in comparison to a Macro Cell. Note that the neighbor


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list for these pico cells is defined differently than that for normal Macro cells. Micro cell has also got lesser coverage area than that of Macro cells.

Repeaters can also be used for providing coverage to specific areas. There can be Channel selective or Band selective Repeaters where band selective repeaters amplifies the whole GSM band and transmit it towards the area required to be covered while channel selective repeaters receive power from selected channels of one or more than one parent cells, amplify it and direct it towards the area required to be covered. In the similar way if capacity requirement of certain area is more, then the coverage of a cell is to be compressed by any of the means discussed above so that it may cater to lesser number of customers.

If the mentioned measures don’t work for matching coverage and capacity requirement then relocation or addition of site can also be suggested. If interference is observed during drive test then apart from reducing coverage area, frequency plan for the network can be redefined and reuse distances can be increased.

After carefully studying the statistical data about the network performance if it is found that congestion for some particular sites are more and call successful rate is less, then more resources (TRX) can be added to improve availability of the traffic channels or additional BTS sites can also be added but this addition has a limit because of limited available frequency spectrum hence with higher number of sites or frequency used, reuse distance of the sites will reduce which will increase interference and hence the quality will go poorer.

There are lots of other ways by which capacity can be increased without much affecting the speech quality.

a) Addition of Micro and pico cells.b) Using Underlay and overlay cells.c) Deploying frequency hopping

Every time TRXs are added in the network, frequency plan of the network or a portion of the network has to be changed which will further require to analyze the network using drive test system, to monitor the network’s performance. It is possible that after addition of certain TRXs frequency reuse distance will decrease to such a level that it will introduce unacceptable amount of interference and deployed frequency plan will require to be redefined.

Soft Configurations:

Other parts of the system can be controlled with soft parameters. These affect operation of algorithms within the system, and include categories such as common BTS parameters, cell access parameters etc. GSM defines around 150 soft parameters. For an example if it is


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found from the BSS statistics details that excessive handovers hence more utilization of resources is taking place then reduction of overlap of the cell coverage areas can avoid them.

Defined BSIC’s for the cells especially for cells transmitting same frequencies are set to be different otherwise lot of unsuccessful handovers will take place. Even then, if it is found that number of unsuccessful handover is high then redefining the neighbor list in BSS can control it. Several neighbors for a serving cell can be defined in GSM. Usually, we want a handover to be made to the strongest neighbor, but in some cases frequent handovers to this best neighbor can result in congestion in this cell, affecting the users initiating calls from that cell. The situation can also occur in reverse, when a handover required to the best neighbor can result in a rejection due to unavailability of resources, causing the handover to be attempted to the next best neighbor, which can delay the process and deteriorate the quality further. Under certain circumstances, we may need to remove a potential neighbor from the neighbor list and provide alternatives.

In the idle mode, the mobile always prefers to remain with or move to the best serving cell. The best cell is decided on the basis of uplink and downlink path balance in the cells. This balance is calculated by GSM defined C1 calculations. C1 calculations force the mobile to move to the strongest cell. In certain cases, such as macro-micro cell architecture, optimization may require that in certain areas the mobile not remain in the best cell, but instead remain in a cell depending on traffic loading. C2 parameters provide the option of adding fixed positive or negative offsets to the C1 calculation in each cell. So, although C1 might be better for a neighbor cell, the application of C2 parameters could delay reselection. C2 parameters also allow the mobile to apply temporary offsets for a period known as penalty time, which helps reduce Ping-Pong effects. With the help of carefully done drive test these parameters like offset or penalty time for handovers can also be checked and verified.

Optimization philosophy:

Setting the parameters that control mobility have equal importance to the frequency plan. In GSM there are a series of parameters that control mobility. Tuning these parameters for improved GSM operations, in terms of maximizing calls carried, improved handover performance and increased call success rate, is termed ‘Optimization’.

The aim of optimization is to maximize the Quality of Service of the GSM network. In order to do this you need to measure the QOS, compare the measured value with the desired value, and then take steps to correct the causes of any deviations from the desired value.

Basic Optimization Philosophy

It is typical that during optimization the choice of cell frequency, the neighbor list and any margins/timers will be examined and optimized for improved performance.


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Optimization is traditionally undertaken immediately after the commissioning stage, or after a new frequency plan in a deployed network. Several teams of field personnel drive around each site making a number of calls, concentrating on testing the handover between each cell. Each call is investigated and any potential problems resolved by classical fault-reasoning/resolution methods. Most network operators use this methodology, termed “drive-testing” as a tried and tested way to improve their network.

Optimization is divided into the following criteria when tuning a cell:

-Frequency plan

-Topology (neighbors)

-Cell dynamics (handover timers and margins)

-Real-estate (antenna tilts etc.)

Only Basic optimization can be done in the network, if the network does not have a substantial amount of active subscribers. For statistical data to be used as in the advanced optimization process, the network must be carrying a significant amount of traffic

Basic Optimization pre-requisites

Personnel Requirements

The intention here is to show the engineers required in the optimization process and not the amount of engineers. The amount of engineers will depend on the size of the network, the amount of area to be covered and the roll out schedule.

Once the above information is known a more precise proposal can be done detailing specific numbers of people required.

The engineers required in the optimization process is as follows

Change Control Procedure

A procedure to manage changes within the network is required to maintain the integrity and quality of the network.


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The procedure ensures all changes required to improve the quality of the network are valid and that precautions against failure of the change have been considered.

This procedure below is the change control procedure and ensures changes on the network have been fully evaluated before implementation and that each change has a test plan and a back plan in case of failure..

Drive Test Routes

Before drive testing is started drive test route need to be agreed with the operator. These routes should cover the following points before agreement is reached.

All sites and sectors should be tested within the drive test routes at least once.

All major roads and highways should be tested at least twice within the agreed routes.

All cells should be tested for handout and hand in within the routes if possible.

The routes should be approximately 2 - 3 hours in duration. This is required to manage the data collected.

Routes of major importance should be identified prior to starting and should be driven first. i.e Airports to the city center

RF Design and Database Parameters

Before Optimization can begin the RF design and database parameters will be required. This is normally presented in spreadsheets from the RF planning tools.

This information is required to help the drive test engineers to identify possible sources of interference. The information is also used to evaluate possible changes to improve the quality of service.

Switch Test Number

To aid in the optimization of the network a test number is required within the MSC. This is required for the drive test teams to access from the test mobiles in the car, a test number in the MSC is preferred as this removes any contact with the land system, so in the event of any dropped or unavailable calls they will all be in the mobile network .


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Customer Feedback Procedure

A procedure is required to feed back customer information on the performance and coverage of the network. The received information is used to target areas of optimization and to verify coverage against the RF design.

The information feed back is also used in the growth of the network by identifying were subscribers are using there mobiles.

The process of feeding information back is an internal process for each operator, GSM vendor however have closely worked with customer care departments to assist in providing information as to the coverage and quality of the network. The format of providing information is usually graphically based around the Mapinfo this provides coverage maps from the RF planning tool and live data gathered from drive test data. This support can be provided to any future customers.

Timescales

The optimization process never comes to an end within a network, the process usually evolves into the performance engineering department as the network evolves.

The rate of growth in most cellular networks means the network continues to expand with new sites or more capacity with different RF design techniques, this will always mean that optimization will be required to maintain and improve the quality of the network.

The initial optimization of a system is somewhat variable depending on many factors i.e amount of sites, area to be optimized, road traffic density e.t.c. However as an indication until the precise information is known on the network it takes approximately 2-4 weeks for one drive test team to optimize a BSC, a BSC usually consists of about 14 - 18 sites at with present software load.

If a faster optimization process is required more engineers will be required, another team would see a reduction of 10 days in the process.

Basic optimization procedure

The optimization process starts immediately the network is brought into operational service or an enhancement takes place on the network. During times of little change to the network Performance Engineering monitor the quality of the network and will seek assistance from Optimization Engineers if the quality of the network begins to fall.


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The procedure below is the basic method for optimizing a system and can be modified to each operator to maximize results.

Before Optimization starts all of the pre-requisites must have been done or be in place. There must be a change control procedure in place with the operator that GSM vendor are familiar with, the RF design and Database parameters must be presented to the Optimization control manager. All drive test teams must have test mobiles and SIM cards provided by the operator. Note - The Optimization control personnel are usually situated with the OMC personnel for maximum efficiency as OMC and Optimization control are continually passing information between each other.

The drive test routes must be agreed with the operator and a priority set on the routes for testing.

The drive test teams make test calls on the network of 2 minute duration with a 15 second break between calls to the MSC test number with the Test Mobile equipment (TEMS) and all data is logged to the computer, location information is also taken using a GPS receiver to provide location information.

The drive test routes are usually 3 - 4 hours in duration so that the data collected can be managed.

During or after completion of the drive test route analysis of the data collected is done to find areas of dropped or noisy calls. This can either be done on the RF planning tool .

Should the analysis of the route indicate problems of either dropped or noisy calls ,with the aid of the RF design and Database parameters an assessment is made to identify the possible source of interference causing the noisy or dropped call. If a call is dropped and no interference is present a retest is made in the same area, if the scenario of the dropped call can be repeated, information of the problem cell should be obtained, this will then be escalated to Optimization control to seek assistance from the BSS maintenance engineers to investigate the cell dropping calls.

After conformation as to what is causing the problem with the drive test route, the drive test engineer will attempt to find a solution to the problem. This can be one of a number of possibilities i.e. Power Change to BTS, Frequency Plan change, Neighbor addition required e.t.c.


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Once a possible solution to the problem has been found it may be possible in some circumstances to immediately attempt the solution via the OMC, this usually relates to minor database changes and adding neighbors. The solution is implemented and proven immediately. If the problem is rectified the change remains in place and a change request is raised for the solution for the purpose of keeping records of all changes in the network. If the solution requires a major database change or antennae work a change request must be raised via the Optimization Control Engineers. After the solution is implemented a retest of the problem area is carried out to confirm the problem has been solved.

In the event of the problem not being solved alternative solutions may be attempted, this process continues until it becomes impossible to find a solution. At this point the problem is discussed with the operator as to the reasons that the problem cannot be solved for example the solution may require a new cell to be built, clearly this is beyond the scope of optimization. If the operator is in agreement this particular problem will be removed from the drive tests until such time a solution is implemented.

Basic optimization tools and software

OMC

The OMC is an integral part of a GSM system, its relationship to the Optimization process is to provide statistics for the quality metrics and information on the status of the network.

Tems

Tems is the drive test mobile and software from Erisoft. The kit consists of a laptop P.C, an Ericsson GH688 test mobile and a GPS receiver for positioning information.

The first snapshot represents a simple drive test of a model city. It contains various information about the Rx-Level , Rx-Qual, Serving Cell , neighbor list , speech quality index SQI , and various other parameters.


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RF Planning Tool – Netact plan

An RF planning tool is required in the Optimization Process for displaying drive test routes for analysis, modifications to the frequency plan and antennae azimuths and down tilt changes.

Mapinfo

Mapinfo is a GIS software tool, it is used to display drive test data for analysis and to produce Optimization reports in a clear and easy manner.

Test Mobiles


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Test mobiles are an invaluable source of information; all field engineers should be equipped with a test mobile to identify problem areas.

The test mobile should be capable of giving the received signal level, RXQual value, Cell I.D and six neighbors with rxlevels.

Advanced optimization pre-requisites

The advanced optimization toolset developed by GSM vendor will be required to undertake the advanced optimizations procedure in the network.

Subscribers

To gather meaningful data for the advanced optimisation tools the network should have a substantial amount of traffic being generated by subscribers on the network.

Personnel Requirements

The type of engineers required will remain the same as the requirements in the basic optimization pre-requisites. There will however be a requirement for extra performance engineers to analyze data gathered from the intelligent optimization tool, the Call Trace Product Tool and the Cell Analysis Tool.

The requirement for drive test engineers will still be valid for verification purposes, but a there should be a significant reduction in the amount of drive test engineers required.

The reduction will be based on the amount of advanced tools available in the network and the timescales involved.

Drive Test -:Once the network is up and running , we have to optimize the network so that what we simulated could be matched with the actual field results .For this process the most common and reliable method is Drive test.

The process involves the use of Tools like TEMS and Agilent Drive test tools.

Once the drive test is over we have to study the Statistics collected during the drive.The log files recorded are processed and converted into text files that could be loaded on NetPlan.

The two most important Parameters that are checked upon are

1. Rx level

2. Rx Quality.

Rx Level -: It is an indication of the signal strength in dbm that is available to a cell phone at any point.


RF PLANNING

The interval used is same as that we have for generating the image of Rx Level .

This gives a fair idea about the efficiency and accuracy of our plan as now we can compare the signal that we predicted at a point and what we are getting at that point.

The aim is to match field results with the predicted values and for that the process of optimization is carried out.

Fig: Drive test image for Rx Level

Rx Quality -: This is another important parameter that gives us an idea about the signal quality or the bit error rate called as BER. This indicates that out of out of the total number of bits that were transmitted what was the percentage of the bits that got corrupted .The


RF PLANNING

calculation is done on the basis of the 26 bit training sequence that is transmitted along with the voice data that is transmitted on the air interface .As the contents of this training sequence is known to the mobile station the error in it is taken as the error in the data packet transmitted and thus the percentage of the total data that got corrupted can be calculated.

The ideal value of Rx Quality should lie between 0 to 4.

Following are the BER that define a particular Rx Quality.

0 :( BER<0.2%)1 :( 0.2 %< BER<0.4%)2 :( 0.4 %< BER<0.8%)3 :( 0.8 %< BER<1.6%)4 :( 1.6 %< BER<3.2%)5 :( 3.2 %< BER<6.4%)6 :( 6.4 %< BER<12.8%)

7 :( 12.8 %< BER)


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Fig: Drive test plot of Rx Qual with Legend.

5 ConclusionRF PLANNING

RF Planning is the main process of network planning. We have to do this in a proper manner for Optimization. Customer satisfaction is also depends on this planning. This planning depends on various actors

FREQUENCY Re-use pattern

The total number of radio frequencies allocated is split into a number of channel groups


RF PLANNING

or sets. These channel groups are assigned on a per cell basis in a regular pattern whichrepeats across all of the cells. Thus, each channel set may be re-used many timesthroughout the coverage area, giving rise to a particular re-use pattern .

Subscriber environmentSystem quality (voice quality, for example), system access and grade of service, asperceived by the customer, are the most significant factors in the success of a cellularnetwork. The everyday subscriber neither knows or really cares about the high level oftechnology incorporated into a cellular network. However, they do care about the qualityof their calls.

Frequency planningThe ultimate goal of frequency planning in a GSM network is attaining and maintainingthe highest possible C/I ratio everywhere within the network coverage area. A generalrequirement is at least 12 dB C/I, allowing tolerance in signal fading above the 9dBspecification of GSM.The actual plan of a real network is a function of its operating environment (geography,RF, etc.) and there is no universal textbook plan that suits every network. Nevertheless,some practical guidelines gathered from experience can help to reduce the planningcycle time.

Features that affect planning


RF PLANNING

This section provides a description of the software features that might affect the requiredequipment, and that should be taken into consideration before planning actual equipment. Check with the appropriate Motorola sales office regarding software availability with respect to these features.Diversity.Frequency hopping.Short message, cell broadcast.Code storage facility processor.Packet Control Unit (PCU) for General Packet Radio Service (GPRS) upgrade.

Initial information requiredThe information required before planning can begin can be categorized into three main areas:

Traffic model and capacity calculations.Category of service.Site planning.

Traffic model and capacity calculations

The following information is required to calculate the capacity required:

Traffic information (Erlangs/BTS) over desired service area.Average traffic per site.Call duration.Number of handovers per call.Ratio of location updates to calls.

Ratio of total pages sent to time in seconds (pages per second).Ratio of intra-BSC handovers to all handovers.Number of TCHs.Ratio of SDCCHs to TCHs.Link utilization (for C7 MSC to BSS links).SMS utilization (both cell broadcast and point to point).Expected (applied and effective) GPRS load.

Site planningThe following information is required to plan each site.

Where the BSC and BTSs will be located.


RF PLANNING

Local restrictions affecting antenna heights, equipment shelters, and so on.Number of sites required (RF planning issues).Re-use plan (frequency planning) omni or sector:- Spectrum availability. - Number of RF carrier frequencies available. - Antenna type(s) and gain specification.Diversity requirement. Diversity doubles the number of Rx antennas and associated equipment.Redundancy level requirements, determined for each item.Supply voltage.

Planning toolsIn order to predict the signal strength in a cell area it would be necessary to make manycalculations, at regular intervals, from the BTS.The result, is the necessity to perform hundreds of calculations for each cell. This wouldbe time consuming in practice, but for the intervention of the software planning tool.This can be fed with all the details of the cell, such as: Type of terrain. Environment. Heights of antennas.Several planning tools are available on the market, such as Netplan or Planet, and it isup to the users to choose the tool(s) which suit them best.

We have done drive test and survey for optimization.


project report of idea

Documents