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Fundamentals of Radio Network Planning

Objectives

The participant is able to

− explain the basic steps during radio network planning

Contents

1 Mobile Radio Network Planning Tasks1.1 Collection of Basic Planning Data1.2 Terrain Data Acquisition1.3 Coarse Coverage Prediction1.4 Network Configuration1.5 Site Selection1.6 Field Measurements1.7 Tool Tuning1.8 Network Design1.9 Data Base Engineering1.10 Performance Evaluation and Optimization

2 Repetition

3 Radio Wave Propagation3.1 Path Loss3.2 Shadowing - Long Term Fading3.3 Multi Path Propagation - Short Term Fading3.4 Maximum Path Loss and Link Budget

4 Cellular Networks and Frequency Allocation

5 Traffic Models

6 Exercises

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Objectives of Radio Network Planning

To provide service

to many subscribers with high service quality at low costs

Capacity for a traffic model

− service types

− call rate

− mobility

Quality of service

− low blocking

− low wait time

− high speech quality

− low call drop rate

Efficiency

− low number of BS sites

− high frequency re-use

Boundary conditions

Physics: frequency spectrum, radio propagation → coverage & frequency planning

System: receiver characteristics, transmit power

channel configuration

cell design & network structure

link quality improvement

focal point of this course !

algorithms and parameter setting

Fig. 1

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As shown in the figure below, the main topic of this course is adjustment of system parameters for theSiemens Base Station System (SBS) as part of the radio network planning process.

Before going into the details of the system features and control parameters, this introduction chapter summarizes some basics on radio network planning:

In the first and second section of this chapter the steps within the radio network planning process areexplained. In sections 3 - 5 simple models concerning radio propagation, frequency re-use and tele-traffic are presented.

As each model they are only an approximation of reality. Nevertheless

• they reflect the main physical effects,

• they help to understand the meaning of parameters and the way of working the algorithms,

• they allow to estimate parameter values.

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1 Mobile Radio Network Planning Tasks

The mobile radio network is the connecting element between the mobile telephone users and the fixednetwork.

In this network the base transceiver station equipment (BTSE) is the direct interface to the sub-scriber. It has to make radio communication channels available to the users and to care for a satisfac-tory signal quality within a certain area around the base station. This area may be split into differentsectors (cells) which belong to one BTSE.

Planning a mobile radio network is a complex task, because radio propagation along the earth surfaceis submitted to many influences due to the local environment. Furthermore the performance require-ments to a radio network cover a wide field of applications which depend on the operators potentiali-ties and goals. To respond to all these subjects, it is necessary to observe a certain sequence of tasks.

The first step is to get knowledge about the customers/operators objectives and resources (basicplanning data). On this basis it is possible to estimate the size of the project and to establish a coarsenominal cell plan.

Then it is necessary to install a digital terrain data base into a planning tool which contains topo-graphical and morphological information about the planning region. This digital map permits to makemore accurate predictions about the radio signal propagation as compared to the first rough estima-tion, and to create a more realistic cell structure, including the recommendable geographical positionsof the base stations equipment(coarse coverage prediction).

The network elements defined up to this moment have been found on a more or less theoretical basis.Now it has to be checked if the envisaged radio site locations may really be kept. A site survey cam-paign in accordance with the customer, who is responsible for the site acquisition, must clarify allproblems concerning the infrastructure and technical as well as financial issues of the BTSE imple-mentation. Inside a tolerable search area the optimum site meeting all these issues has to be se-lected.

This site selection should also take into account particular properties of the area, e.g. big obstacleswhich are not recognizable in the digital maps.

Field measurements, to be carried out in typical and in complex areas must give detailed informa-tions about the radio characteristics of the planning region. The measurement results will then help toalign the radio prediction tool for the actual type of land usage (tool tuning).

Now, fixed site positions and an area-adapted tool being available, it is possible to start the detailedradio planning. The final network design has to care for both sufficient coverage and proper radiofrequency assignment in respecting the traffic load and the interference requirements.

The last planning step is the generation of a set of control parameters, necessary to maintain acommunication while a subscriber is moving around. These parameters have to comply with the exist-ing cell structure and the needs to handle the traffic load expected in each cell.

After commissioning of the network, the performance must be checked by the network operator byevaluation of statistical data collected in the operation and maintenance center. Situations of conges-tion or frequent call rejections may be treated by modification of the pertinent control parameters andlead to an optimized network.

The individual planning steps are considered more closely in the following sections.

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1.1 Collection of Basic Planning Data

The requirements of the network operator concerning traffic load and service area extension are basic

data for the design of a mobile network . A coarse network structure complying with these require-ments can be created on this basis.

Two fundamental cell types are possible; their properties may be determined

a) by the maximum radio range of the involved transceiver stations and mobile terminals; the range islimited by the available transmit power and the noise figure of the receivers. This type is called anoise limited cell; it is typical for rural regions.

b) or it may be determined by the limited traffic capacity of a cell in the case of high subscriber con-centration. This leads to the implementation of small cells, mainly in urban areas where interfer-ence will become the major problem.

The result of this first planning step is a rough estimate of the network structure, called a nominal cell

plan, which gives knowledge about the number of radio stations, their required technical equipmentand their approximate geographical positions. Thus allowing to assess the monetary volume of theproject.

1.2 Terrain Data Acquisition

Mobile communication occurs in a natural environment. The radio signal propagation is highly affectedby the existing terrain properties like hills, forests, towns etc. Therefore the real mapping data must betaken into account by the planning tool.

The signal level encountered by a subscriber in the street is influenced by absorbing, screening, re-flecting and diffracting effects of the surrounding objects and along the radio path.

To make realistic signal level predictions, the propagation models implemented in the prediction toolmust be fed with the relevant terrain data.

A very important factor for correct modeling is the morphographic classification of an area :

• building heights and density of built up areas (metropolitan, urban, suburban, village, industrial,residential) or forest, parks, open areas, water etc.

The screening by hills which may affect the coverage of a service area must be made evident by con-sideration of the terrain profile (height contour lines).

The procurement of digital maps with these informations may be rather expensive. The predictionaccuracy is directly related to the size of area elements (resolution) and to the reliability of these in-formation (obsolescence of maps!)

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1.3 Coarse Coverage Prediction

On the basis of the digital terrain data base and by using standard propagation models, which havebeen preselected to fit for special terrain types, it is possible to make field strength predictions withouthaving a very detailed knowledge of the particular local conditions.

By variation and modification of the site positions and antenna orientations, coverage predictions of rather good quality may be attained.

Yet, the definitive site locations are subject to a later scheduled site selection process in accordanceand by cooperation with the customer.

The particular local characteristics must be introduced later by comprehensive survey measurements.These measurements will be used to upgrade the propagation models.

1.4 Network Configuration

The results of the “coarse prediction“ steps will allow to define the radio network configuration and thelayout of individual base stations.

A first frequency allocation plan may also be derived from these predictions. The result might alreadybe a well functioning network. But it is still based on assumptions. The actual impact of the naturalenvironment must be considered in the following steps. Nevertheless, the “coarse planning“ resultswill help to better assess the special details brought in by the real situation.

In designing the radio network one has to keep in mind the requirements emerging from an increasingsubscriber number. A multiple phase implementation plan has to govern the network configurationconcepts.

In the initial phase a relatively low number of users has to be carried. On the other hand completecoverage of the service area has to be provided from the beginning. Existing sites of the first imple-mentation phase must be useable in later phases. Increasing subscriber numbers (synonymous withincreasing interference tendency!) should be responded by completion of the existing TRX-equipmentand by addition of new sites. This means reconfiguration of the existing cell patterns and frequencyreassignment. The planner should anticipate the future subscriber repartitions and concentrationsfrom the beginning, in creating cell structures capable to respond to future needs. Increasing interfer-ence problems arising with higher site density may be overcome by downtilting of directional antennasinitially mounted for maximum signal range, as now the radio cell areas will be smaller.

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1.5 Site Selection

The site positions found in the coarse planning process on a theoretical basis, must now be verified ina joint campaign, called site survey, between the customer and the radio network planner. All sitecandidates within a tolerable search area around the theoretical site positions must be checked.

This check includes the availability of electric power and of data transmission lines.

The most important topic is the possibility to install the antennas in a suitable height above the roofs or above ground.

Environmental influences (screening obstacles, reflectors) have also to be regarded. The best fittingsite should be selected.

Another important task of this campaign is to declare a certain number of the radio sites be suitable toserve as „survey sites“. This means that radio field measurements shall be done with these stations astransmitters. The resulting measurements will be used for the alignment of radio propagation models.

The environment of the survey sites should be typical for a considerable number of other radio sites.

1.6 Field Measurements

Digital terrain data bases (DTDB) as derived from topographical maps or satellite pictures do not con-tain all details and particularities of the existing environment. Especially in fast developing urban areasmaps cannot keep pace with reality and thus reflect an obsolete status. Keeping maps on this qualitylevel would be very expensive.

The characteristics of built up zones and vegetation areas with respect to radio propagation differ in awide range if we regard different countries. Even climatic conditions may influence the signal level.Knowledge about this specific behavior must be acquired by measurements.

The survey measurements have to be carried out in typical areas. Evaluation of these measurementswill result in models that can be applied in comparable areas as well.

Special measurements must be carried out in very complex topographical regions where standardizedpropagation models will fail. The resulting models are valid exclusively for this measurement zone.

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1.7 Tool Tuning

The measurement results have to be compared with the predictions of proven standard models. Thestandard parameters will be slightly modified to achieve minimum discrepancies with the measure-ments, i.e. to keep the mean error and rms-error as low as possible. As the signal level is subject to

statistical variations which cannot be predicted, the rms-error will never be zero.

The reliability of the created models increases with the number of measurement runs that can be ex-ploited.

The new specific model may also be applied in other base stations located in similar environment.

1.8 Network Design

The area-specific models are the basis for the final planning steps. The detailed network design has tocare for

• a suitable signal level throughout the planning area

• sufficient traffic capacity according to the operators requirements

• assignment of the pertinent number of RF-carriers to all cells

sufficient decoupling of frequency reuse cells to respect the interference requirements for co-channels and adjacent channels.

Moreover, attention has to be payed to an optimized handover scenario in heavy traffic zones.

The detailed planning process commits the final structure of the radio network and the configuration of the base stations.

The capacity of digital data links connecting the radio stations to the fixed network elements may nowbe defined.

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1.9 Data Base Engineering

A cellular network is a living system with moving subscribers. The service must be maintained whilemobiles change radio cells and superior organization units, called location areas. All control parame-ters, necessary to support this task, have to be administered and supervised in central data bases.

There is a permanent signaling information exchange between mobiles, base stations and controlcenters.

This signaling communication occurs on predefined time slots, called control channels which are as-signed to one of the RF-carriers of each radio cell.

Important control informations for each radio cell are :

• cell identification within the network

• control carrier frequency

• potential neighbor cells

• minimum received signal level

• maximum transmit power of a mobile

• power reduction factor to perform power control

• power margin for handover to neighbor cells

1.10 Performance Evaluation and Optimization

Regular performance checks must be carried out after commissioning of the network. These checkscomprise the evaluation of statistical data collected in the “operations and maintenance center“ (OMC)as well as measurements by means of test mobile stations to explore e.g. handover events under realistic conditions; unwanted handover may lead to traffic congestions in certain cells, or may drainoff traffic from other cells.

Detection of multipath propagation problems caused by big reflecting objects is also subject to meas-urements.

Another goal of these checks is to investigate the real traffic load and its distribution, as subscriber behavior in a living system will not necessarily reflect the original assumptions of the operator; as-sumed hot traffic spots may have been changed or shifted after a couple of years.

Careful evaluation of the measurement data will help to optimize the network performance by modifi-

cation of the system parameters. As the number of subscribers will normally increase in course of time, supervision and control of these parameters should become a permanent maintenance proce-dure.

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2 Repetition

Mobile Radio Network Planning Tasks

• Collection of basic planning data

• Terrain data acquisition

• Coarse coverage prediction

• Network configuration

• Site selection and field measurements

• Tool tuning

• Network design

• Data base engineering

• Performance evaluation and optimization

Collection of basic planning data

• Customer must define basic network performance goals :

• Size of service area and area types

• Traffic load and distribution

• Mobile classes and service quality

• Future development (forecast)

• Available RF - bandwidth

The resulting nominal cell plan is a first planning approach

• to determine the required number of radio stations

• to figure out the approximate equipment configuration

• to get an idea of the financial volume of the project

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Terrain data acquisition

Topographical and morphographical properties of the planning region must be compiled in a digitaldata base for further processing

Contents of the digital terrain data base DTDB :

• Height profile ( topography )

• Land coverage and usage ( morphography )

Possible sources :

• Scanning of topographic maps

• Processed satellite pictures or air pictures

Coarse coverage prediction

A coarse coverage prediction based on the nominal cell plan and on the digital terrain data base :

• using standard propagation models

• using standard antenna types

Results :

• Geographical distribution of the radio signal level

• Coarse cell structure

• Nominal position of the radio sites and antenna orientation

• Search areas for final site positions

• Knowledge about the attainable degree of signal quality

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Network configuration

Internal configuration of individual radio station :

• Equipment to be installed

Configuration of the radio network ( network structure ) :

• Number of base station controllers BSC

• Number of location areas

• Definition of data lines between the network elements

Site selection and field measurements

• Selection of definitive radio site locations

• Radio measurements in typical areas

• Radio measurements in complex topographical regions

Tool tuning

• Radio measurements are exploited to adapt standard propagation models to specific environ-mental conditions

• Resulting models may be applied in similar environment

• or are restricted to the special measurement area

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Network design

The final radio planning is performed by means of the area - adapted models

Planning goals:

• Sufficient signal level throughout the planning region

• Sufficient traffic capacity according to subscriber distribution

• Assignment of radio carriers to all cells

• Low interference level for co-channels and adjacent channels

• Definition of neighbor cells

Data base engineering

Control and maintenance of the radio network requires parameters for

• Identification of serving cell and neighbor cells , i.e.:

− cell identity

− location area

− color code

• Cell - allocated control- and traffic carriers

• Maximum transmit power level

• Minimum receive signal level

• Power margin for handover to each neighbor cell

Performance evaluation and optimization

• By analyzing statistical data from maintenance center

• Measurements performed by a test mobile station roaming about the operating radio network

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3 Radio Wave Propagation

There are three main components of radio propagation which are discussed in the next section:

• mean path loss (loss due to the distance between MS-BS),

• shadowing (long term fading),

• multi path propagation → short term (Rayleigh) fading.

3.1 Path Loss

Standard path loss models are of the form:

Lm[dB]= A + B log d [km]

where Lm is the mean propagation path loss between the base station (BS) and the mobile station(MS) at a distance d.

A: unit loss at 1 km,

B: propagation index or loss per decade.

The propagation coefficients A and B depend upon:

• the transmit frequency,

• the MS and BS antenna heights,

• the topography and morphology of the propagation area.

Examples are:

1. Free space loss:

L0 = 32.4 + 20 log f [MHz] + 20 log d [km]

or more important propagation in real environment - the famous Hata model:

2. Hata model

The Hata model describes the mean propagation effects for large cells and distances d > 1 Km. For urban environment one has:

A = 69.55 + 26.16 log f - 13.82 log Hb - a(Hm)

B = 44.9 - 6.55 log Hb

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Frequency: f [Mhz] 150...1000 -Mhz

BS antenna height: Hb [m] 30...200 m

MS antenna height: Hm [m] a(Hm) = 0 for Hm = 1.5 m

Example: Hm = 1.5 m Hb = 50 m f=900 Mhz

→ A = 123.3 B = 33.8

Path Loss for LargeCells - Hata Model (GSM 900)BS height 50 mMS height 1.5 m

90

100

110

120

130

140

150

160

170

180

190

200

210

1 10 100

Cell radius [km]

P a t h

L o s s [ d Urban

Urban Indoor

Suburban

Rural (quasi open)

Rural (open)

Fig. 2

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For other environments (suburban, rural-quasi-open) the path loss per decade remains the same, butthe unit loss is reduced by a certain amount. The free space loss and the Hata model are illustrated inthe figure above.

Models of this type are adequate for estimating the received level for large cells. However for a real

network planning, refinements of the model and adaptations of parameters to morphological and to-pographical data and to measurement values are necessary (refer to section 1).

The smaller the cells, the more important are the details of e.g. the building structure within the cell.

3.2 Shadowing - Long Term Fading

In larger cells where the BS antenna is installed above the roof top level, details of the environmentnear the MS are responsible for a variation of the received level around the mean level calculated bythe models discussed above.

Usually this variation of level - caused by obstacles near the MS (e.g. buildings or trees) - is described

by the statistical model, i.e. the total path loss Ltot is given by the mean „distance“ path loss plus arandom shadowing

Ltot [dB] = Lm + S

S<0: free line of sight,

S>0: strong shadowing by e.g. a high building near the MS.

S has a Gaussian distribution (see figure below) with mean value 0 and a standard deviation s whichtypically lies in the range s = 4...10 dB.

-3 -2 32-1 10

0.1

0.2

0.3

0.5

0.4

Shadowing S/s [dB]

Fig. 3 Gaussian distribution of Shadowing S

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The length scale for variation of the long term fading is in the range 5 ... 100 m, i.e. the typical size of shadowing obstacles.

3.3 Multi Path Propagation - Short Term Fading

The superposition of several reflected waves arriving at the receiver on different paths and thereforewith different amplitudes and phases causes peaks (constructive superposition) and deep fading dips( destructive superposition) of the received level.

The length scale for variation (e.g. peak to peak) is given by the half of the transmission wave length,i.e. about 15 cm for GSM900 or 7.5 cm for DCS1800. An example for the variation of the receivedlevel due to short term fading is shown in the figure below.

A comparison with the length scale for shadowing explains the names for these fading types.

The statistics of the Rayleigh fading is described in the following way:

Consider the received level due the path loss and long term fading which is called local mean:LLOC[dBm]. The received local mean power is then given by

Ploc[mW] = 10LlOC/10

Using this formula the probability density function for the received power P is given by:

f(P) = 1/Ploc* exp(-P/Ploc)

which means that the probability function for the signal amplitude P = A2

is given by a Rayleigh distri-bution.

Using these formulas and some mathematics, one can calculate the probability that the received levelL (affected by Rayleigh fading) is x dB below the local mean level Lloc:

Prob (L - Lloc< x dB) = 1 - exp ( - 10x/10

)

Example:

x = 3 dBx = 0 dBx = -3 dBx = -6 dBx = -10 dBx = -20 dB

Prob = 86,5 %Prob = 63,0 %Prob = 39,5 %Prob = 22,0 %Prob = 9,5 %Prob = 1,0 %

Changing the transmission frequency and therefore the wave length, changes the position of Rayleighpeaks and dips. This means that at a given position, the received level affected by Rayleigh fading ingeneral differs for different transmission frequencies. The higher the frequency difference the lower isthe correlation for the receive signal for the different frequencies. The coherence bandwidth B

cohis

defined as the frequency difference at which this correlation has decreased to 0.5. The coherencebandwidth depends upon the spread of arrival times of the different multi path components of the re-

ceived signal. This spread is called delay spread ∆T:

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Coherence Bandwidth and Delay Spread

BT

coh =1

2π∆

i.e. the higher the delay spread the lower is the coherence bandwidth.

The delay spread depends upon the propagation environment. Typical values are:

• 10 µs for hilly terrain (corresponding to path length between difference of 3 km).

• 0.1 ... 1 µs for urban area (corresponding to path length between difference of 30 ... 300 m).

Keeping in mind that a Rayleigh fading dip of more than 10 dB occurs with a probability of 10 %,measures should be provided to combat Rayleigh fading:

Means to combat Rayleigh fading:

• Averaging of Rayleigh fading over speech frames (interleaving of 8 bursts)

- Frequency Hoppingspacing between frequencies in hopping sequence >> coherence bandwidth

- Motion (speed v)Example: v=50 km/h, distance between bursts = TDMA frame length T = 4.6 ms

→ distance between MS positions at subsequent bursts D = 6.4 cm

→ distance for 8 bursts_ 8 * D ∼ 50 cm > 3 * wavelength

• Combining of signals received at positions of mutually uncorrelated fading

- Antenna Diversityspacing between RX antennas >> half wavelength

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Fig. 4

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Short Term Fading

Fig. 5

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3.4 Maximum Path Loss and Link Budget

The maximum radius of a cell depends on the maximum possible path loss between transmitter andreceiver, i.e. upon the difference between the maximum output power level EIRP (emitted isotropicradiation power) at the transmitter antenna and the required input power level (RIPL) at the receiver

antenna.

Output BTS:

EIRPBTS = Power Amplifier Output - Combiner Loss - Downlink Cable Loss + Antenna Gain

Power Amplifier Output: 25 Watt = 44 dBm (GSM900)(higher power amplifier output power in further BTS versions)

Combiner Loss

Combiner Type 1:1 2:1 4:1

Duplexer 2.7 dB 2.7 dB 5.9 dB

Hybrid Combiner 2.0 dB 5.2 dB 8.4 dB

Fig. 6

The ratio x:1 denotes the number of carriers which are combined. In the case of hybrid combiners thesignals are fed to 1 transmitter antenna. In the case of duplexers the signals are fed to 2 antennas (onair combining) which are used for transmission as well as for reception.

Using these antennas for reception, a two branch (maximum ratio) antenna diversity combining can berealized. This means that - using Duplexers - two antennas per cell are needed, whereas when usingHybrid Combiners and applying Antenna Diversity two receive plus one transmit antenna is needed.

Downlink Antenna Cable Loss: 3 dB (example)

Antenna Gain (example): 16 dB (typical value for 600

half power beam width antenna)

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Output MS:

For the MS there is no need combining different carriers; and the cable loss and antenna gain reduceto zero. The EIRP depends upon the power class of the MS specified in GSM Rec 05.05:

Power Class (GSM 05.05) Max. Output Power (GSM900) Max. Output Power (DCS1800)

1 -- 1 Watt = 30 dBm

2 8 Watt = 39 dBm 0.25W = 24 dBm

3 5 Watt = 37 dBm 4 Watt = 36 dBm

4 2 Watt = 33 dBm

5 0.8 Watt= 29 dBm

Fig. 7

Input BTS:

The required input power level RIPL at the BTS antenna is given by

RIPLBTS =Receiver Sensitivity Level - Antenna Diversity Gain + Uplink Cable Loss - Antenna Gain

Receiver Sensitivity Level < - 104 dBm

The receiver sensitivity level is defined in GSM Rec. 05.05 for scenarios where short term Rayleighfading is (at least) partly averaged either by motion or by frequency hopping.

The receiver sensitivity level has been measured to be better than required by GSM Rec. 05.05.

Antenna Diversity Gain:4 dB (for a typical scenario).

The gain which can be achieved by antenna diversity strongly depends upon the propagation envi-ronment, the velocity of the mobile and on whether frequency hopping is applied or not.

For a typical urban environment, a mobile speed of 3 km/h and frequency hopping applied the an-tenna diversity gain is about 4 dB.

Uplink Cable Loss 3 dB without tower mounted preamplifier RXAMOD0 dB with tower mounted preamplifier RXAMOD

The (uplink) cable loss from the antenna to the receiver input can be compensated using a tower

mounted amplifier called RXAMOD.It should be noted that this preamplifier cannot be used together with on air combining (Duplexers).

Antenna Gain (example): 16 dB (typical value for 600 half power beam width antenna)

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Input MS:

For the MS there is neither antenna gain nor antenna diversity gain. Cable losses can be neglected.Therefore the required input power level at the MS antenna is given by the MS receiver limit sensitivityas specified by GSM 05.05:

• 104 dBm for class 2 and 3 (GSM900),

• 102 dBm for class 4 and 5 (GSM900),

• 100 dBm for class 1 and 2 (DCS1800)

Maximum allowed path loss (Link Budget)

downlink Ld[dB] = EIRPBTS - RIPLMS uplink Lu[dB] = EIRPMS - RIPLBTS

Example:

Duplexers 2:1: → no RXAMOD, uplink cable loss = 3 dB

MS of Power Class 3: → EIRPMS= 37 dBm Antenna Diversity Gain: 4 dB

→ Lu[dB] = 37 dBm - (- 104 dBm - 16 dBi + 3 dB - 4 dB) = 158 dB

→ Ld[dB] = 44 dBm - 3 dB - 3 dB + 16 dBi - (- 104 dBm) = 158 dB

i.e. there is a symmetric link budget for uplink and downlink.

− Requirement: Area Coverage Probability: 90 % ←→Coverage Probability at Cell Border: 75 %

− Standard Deviation of Shadowing: s= 6 dB → 75 % value of Shadowing: S75%= 4 dB

− allowed loss L - S75% = 154 dB

→ Lm = L - S75% = 154 dB

Path loss model (Hata): Lm [dB] = 123.3 + 33.7 log d [km]

→ Cell Radius: dmax =10(154-123.3)/33.7

= 8.15 km

Example 2:

Designing a radio cell for mainly MS of Power Class 4 (instead of power class 3), the following valuesfor link budget are obtained:

Lu[dB] = 154 dBLd [dB] = 156 dB

To obtain a symmetric link budget, the power amplifier output power of the BTS has to be reduced by2 dB. This is done using the O&M parameter BS_TXPWR_RED:

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Object DB Name Range Meaning

TRX PWRRED 0, 1, ...6 * 2dB Reduction of BTS power amplifier output

Fig. 8

Reducing the BTS output power has the advantage that less downlink interference is caused by thiscell.

If there are also some mobiles of Power Class 2 and 3 within the cell designed for mobiles of Power Class 4, their maximum transmit power has to be limited for a link budget balance. This is the reasonbehind the following parameters:

Specification Name DB Name/ Object Range Meaning

MS_TXPWR_MAX MSTXPMAX / BTS-B 2...15 GSM

0...15 DCS* 2 dB

Maximum TXPWR a MS is al-

lowed to use on a dedicatedchannel (TCH or SDCCH) in theserving cellGSM: 2 = 39 dBm, 15 = 13 dBmDCS: 0 = 30 dBm, 15 = 0 dBmPCS: 0 = 30 dBm, 15 = 0 dBm

30 = 33 dBm, 31 = 32dBm

MS_TXPWR_MAX_CCH MSTXPMAXCH / BTS-C 0...31* 2 dB

Maximum TXPWR a MS is al-lowed to use on the uplink com-mon control channel (Random Access Channel, RACH) in theserving cell:

GSM: 0 = 43 dBm,19 = 5 dBmDCS: 0 = 30 dBm, 15 = 0 dBm

Fig. 9

Another effect illustrated by this example is the following:

Since there is a balanced link budget Lu[dB] = Ld[dB], but a difference of the receiver sensitivity levelfor the MS and BTS of 2 dB, there is difference between the mean downlink and uplink received levelRXLEV of about 2 dB:

RXLEV_DL - RXLEV_UL ∼ 2 dB.

The consequence is that level threshold for e.g. the handover algorithm have to be set 2 dB higher for

the downlink than for the uplink.

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4 Cellular Networks and Frequency Allocation

One important characteristic of cellular networks is the re-use of frequencies in different cells. By re-using frequencies, a high capacity can be achieved. However, the re-use distance has to be highenough, so that the interference caused by subscribers using the same frequency (or an adjacentfrequency) in another cell is sufficiently low.

Cell Radius R

Re-use

Ditance D

MS

Carrier

Co-channel

Re-use

Cells

Interferer

Fig. 10

To guarantee an appropriate speech quality, the carrier-to-interference-power-ratio CIR has to exceeda certain threshold CIRmin which is 9 dB for the GSM System (GSM Rec. 05.05).

taking the situation of the example above and a path loss model L = A + B log d, one has

C/Itot[Watt] = C / (I1 + ... + INI) ∼ C / (NI * I1) NI: number of interferes

or in dB

C/Itot [dB] = C[dB] - Itot[dB] ∼ B log D - B log R - 10 log NI = B log D/R - 10 log NI > CIRmin + LTFM (x%)

By introducing the long term fading margin LTFM (x%) for a required coverage probability of x%, theeffect of shadowing is taken into account.

For homogeneous hexagonal networks frequencies can be allocated to cells in a symmetric way. De-fining the cluster size K as group of cells in which each frequency is used exactly once, the followingrelations between Cluster Size, Cell Radius and Re-use Distance are obtained.

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Frequency Re-use and Cluster Size

m

n

Rr

D

D

Fig. 11

Outer Cell Radius - R

Inner Cell Radius - r = 0.5 x 3 x R

Re-use Distance - D = R x 3 x (n m nm)2 2+ +

D

R = 3 x K

Cluster Size: Group of cells in which each frequency is used exatly once

K = (n + m + nm)2 2

n,m = 0,1, 2, 3, ...

K = 1, 3, 4, 7, 9, 12 16 19, , , ...

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Inserting the formula for the cluster size into the formula for the minimum CIR one obtains:

0.5 * B log 3 K > CIRmin + LTFM (x%) + 10 log NI

which gives a lower bound for the cluster size which can be used.

For a given cluster size K and total number of frequencies Ntot, the number of frequencies per cell Ncell is given by:

Ncell = Ntot/K

i.e. the capacity of a cell can be increased by reducing the cluster size.

A reduction of cluster size can be achieved by

• reducing the number of interferers → Sectorisation.

• reducing the interference from co-channel cells → Power Control, Discontinued Transmission, ...

Examples for sectorized network structure are shown in the figures below. Methods for interferencereduction are discussed in chapter 6.

Obviously a real network does not have such a regular hexagonal structure and frequency allocationis performed by planning tools using complex algorithms for optimizing the CIR in each cell.

The objective is to achieve a high mean value of frequencies per cell <Ncell>. The ratio

<K> = Ntot/Ncell

can viewed as the mean cluster size in such an inhomogeneous environment.

The capacity of the radio network depends upon the available number N of radio channels per area F(e.g. F = 1 km

2).

N

FN x

N

FCPF x

N

Kx

1

F / NCPF x

N

Kx

1

CAcell

BTS tot

BTS

tot= = =

NBTS: number of BTSCA: cell areaCPF: channel per frequencies

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Omnicells - Cluster 7

5

3

4

7

6

2

15

3

4

7

6

2

1

5

3

4

7

6

2

1

5

3

4

7

6

2

1

5

3

4

7

6

2

1

5

3

4

7

6

2

15

3

4

7

6

2

1

Fig. 12 Example for homogeneous frequency allocation

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3-Sector Cloverleaf - Cluster 3 x 3

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

1a

1b1c

2a

2b2c

3a

3b3c

Fig. 13 Example for homogeneous frequency allocation

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5 Traffic Models

A traffic model reflects the behavior of the subscribers, as their mobility, the mean call rate or call du-ration. It is needed e.g. for calculating the required total number of channels within a cell and how tosplit them between traffic and control channels.

These traffic model information is always a mixture between field observations in similiar networks andarbitrary assumptions.

Traffic data are variable in time, therefore statistical characterization is used.

The goal of planning is to manage traffic even in busy hour.

In mobile networks we have to evaluate two main factors:

• user mobility

• communications

User mobility:

The user moves with a velocity v.

For example the handover and location update rates depend on this velocity.

Communications:

The number of subscriber in a cell, the traffic per subscriber has to be considered.

Furthermore, one needs information the mean call duration, the mean call cell rate (or busy hour callattempt BHCA). separately for mobile originating calls (MOC) and mobile terminating calls (MTC).

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An example for a traffic model is given in the table below:

number of call attempts (MOC+MTC) per subscriber per hour 1,1

percentage of MOC 58 %

percentage of ‘engaged’ in the case of an MOC 19,8 %

duration of TCH occupation in the engaged case 3s

no answer from a person called by MOC 14,4 %

mean TCH occupation for this case 30 s

percentage of successful MOC 65,8 %

mean time for ringing (MOC) 15 s

percentage of MTC 42 %

no paging response 32,5 %

duration of TCH occupation in this case 0 s

no answer from a mobile subscriber 13,5%

means TCH occupation fir this case 30 s

successful MTC 54,0 %

mean time for ringing (MTC) 5 s

mean call duration (MOC/MTC) 115 s

mean TCH occupation call attempt 83 s

TCH load per subscriber 0,025 Erl

time for MOC/MTC setup signaling on SDCCH (authentications, ...) 3 s

time for a location update 5 s

number of location update per subscriber per hour 2,2

resulting SDDCCH load per subscriber (no TCH queuing applied) 0,004 Erl

Fig. 14 Standard traffic model for GSM

The formula for calculating the load on the respective dedicated channel are given on the next page.

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Load on Dedicated Channels

SDCCH load [Erl]: SUBSCR * ((MTC_PR_ph + MOC_ph) * T_SETUP + LU_ph * T_LU+ IMSI_ph * T_IMSI + SMS_ph * T_SMS)

TCH load [Erl]: SUBSCR * (MTC_PR_ph + MOC_ph) * T_CALL

SUBSCR: number of subscribers within the cell

MTC_PR_ph: mobile terminating calls per subscriber per hour with paging response

MOC_ph: mobile terminating calls per subscriber per hour

LU_ph: location updates per subscriber per hour

IMSI_ph: IMSI attach/detach per subscriber per hour

SMS_ph short message service per hour

T_SETUP: mean time [sec] for call setup signaling on SDCCH

T_LU: mean time [sec] for location update signaling

T_IMSI: mean time [sec] for IMSI attach/detach signaling on SDCCH

T_SMS: mean time [sec] for short message service

T_Call: mean TCH occupation time per call

Fig. 15

For the values of the traffic model above one has

TCH load per subscriber: 25 mErlSDCCH load per subscriber: 4 mErl

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n p = 1 % p = 3 % p = 5 % p = 7 % n p = 1 % p = 3 % p = 5 % p = 7 %

1

23456789

10111213141516171819

202122232425262728293031323334353637

38394041424344454647484950

0.01

0.150.460.871.361.912.503.133.784.465.165.886.617.358.118.889.65

10.4411.23

12.0312.8413.6514.4715.2916.1316.9617.8018.6419.4920.3421.1922.0522.9123.7724.6425.5126.38

27.2528.1329.0129.8930.7731.6632.5433.4334.3235.2236.1137.0037.90

0.03

0.280.721.261.882.543.253.994.755.536.337.147.978.809.65

10.5111.3712.2413.11

14.0014.8915.7816.6817.5818.4819.3920.3121.2222.1423.0623.9924.9125.8426.7827.7128.6529.59

30.5331.4732.4133.3634.3035.2536.2037.1738.1139.0640.0240.9841.93

0.05

0.380.901.532.222.963.744.545.376.227.087.958.849.37

10.6311.5412.4613.3914.31

15.2516.1917.1318.0819.0319.9920.9421.9022.8723.8324.8025.7726.7527.7228.7029.6830.6631.64

32.6233.6134.6035.5836.5737.5738.5639.5540.5441.5442.5443.5344.53

0.08

0.471.061.752.503.304.145.005.886.787.698.619.54

10.4811.4312.3913.3514.3215.29

16.2717.2518.2419.2320.2221.2122.2123.2124.2225.2226.2327.2428.2529.2630.2831.2932.3133.33

34.3535.3736.4037.4238.4539.4740.5041.5342.5643.5944.6245.6546.69

51

525354555657585960616263646566676869

707172737475767778798081828384858687

888990919293949596979899

100

38.80

39.7040.6041.5042.4143.3144.2245.1346.0446.9547.8648.7749.6950.6051.5252.4453.3554.2755.19

56.1157.0357.9658.8859.8060.7361.6562.5863.5164.4365.3666.2967.2268.1569.0870.0270.9571.88

72.8173.7574.6875.6276.5677.4978.4379.3780.3181.2482.1883.1284.06

42.89

43.8544.8145.7846.7447.7048.6749.6350.6051.5752.5453.5154.4855.4556.4257.3958.3759.3460.32

61.2962.2763.2464.2265.2066.1867.1668.1469.1270.1071.0872.0673.0474.0275.0175.9976.9777.96

78.9479.9380.9181.9082.8983.8784.8685.8586.8487.8388.8289.8090.79

45.53

46.5347.5348.5446.5450.5451.5552.5553.5654.5755.5756.5857.5958.6059.6160.6261.6362.6463.65

64.6765.6866.6967.7168.7269.7470.7571.7772.7973.8074.8275.8476.8677.8778.8979.9180.9381.95

82.9783.9985.0186.0487.0688.0889.1090.1291.1592.1793.1994.2295.24

47.72

48.7649.7950.8351.8652.9053.9454.9856.0257.0658.1059.1460.1861.2262.2763.3164.3565.4066.44

67.4968.5369.5870.6271.6772.7273.7774.8175.8676.9177.9679.0180.0681.1182.1683.2184.2685.31

86.3687.4188.4689.5290.5791.6292.6793.7394.7895.839689

97.9498.99

Fig. 16 Erlang B formula

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6 Exercises

Exercise 1: Calculation Loss / Gain

L P

=

10 log

P

10dBm

-3 reference P = 1 mW

LU

=

20 log

U

10dB V

-6µ reference U = 1 µV

Loss B A 10 logP

Pdin

out

=

Gain G B10 logP

Pdout

in

=

P = U

R

2

L BU U

=

20 log

U

0,775d reference = 775 mV, 600 Ω

A 20 logU

Uin

out

=

G =

20 log

U

Uout

in

1. Amplifier: 100 mVin, 1 Vout. Calculate the gain

2. Amplifier: 2 mWin, 5 Wout. Calculate the gain

3. Amplifier: 20 dBmin, two steps amplification with 7 dB, 3 dB gain. Calculate the gain.

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Appendix Exercise 1

Power classes for MS/BTS

Class Watt dBm

12345

20852

0,8

4339373329

MS

1234567

8

32016080402010

5

2,5

55524946434037

34

BTS

Conversion dBm ↔ Watt

Watt dBm

4 • 10-14

10-5

10-4

10-3

10-2

10-1

12

2550

100

-104-20-10

01020

3033444750

Maximum Range

Example: SBS, GSM

Power Amplifier 25 WattCombiner 2:1Cable Antenna gain

Sending power

^ 44 dBm- 8 dB- 3 dB

+ 18 dB

51 dBm ^ 125 Watt

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Solutions

Exercise 1

1. G = 10 log1

0,1= 20 dB

2. G = 10 log5

0,002= 34 dB

3. Power out = 20 + 7 + 3 = 30 dBm

30 dBm ^ 1 Watt

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