cell planning fundamentals_document

CELL PLANNING FUNDAMENTALS Abstract This document is intended to provide a sound understanding of the concept of cell planning and how cellular networks are designed.

Cellular Planning Fundamentals CP03 For intenal use only 2

REVISION LIST Date Revision Description Responsibility Approvals Comments 13th April 2001 1.0 Initial draft Edwin Yapp


TABLE OF CONTENTS

REVISION LIST................................................................................................................................................................................2

1 CELL PLANNING INTRODUCTION............................................................................................................................................5

1.1 CELL PLANNING DEFINITION .......................................................................................................................................................5 1.2 CELL PLANNING GOALS .....................................................................................................................................................................5 1.3 CELL PLANNING FACTORS................................................................................................................................................................6 1.4 TYPE OF CELLS ...................................................................................................................................................................................6

2 CELL PLANNING PROCES S.........................................................................................................................................................9

2.1 STEP 1 : TRAFFIC & COVERAGE ANALYSIS.....................................................................................................................................9 2.2 STEP 2 : NOMINAL CELL PLAN.......................................................................................................................................................10 2.3 STEP 3 : SITE SURVEY .......................................................................................................................................................................10 2.4 STEP 4 : SYSTEM DESIGN ..................................................................................................................................................................10 2.5 STEP 5 : SYSTEM IMPLEMENTATION & TUNING........................................................................................................................10 2.6 SYSTEM GROWTH / CHANGE ...........................................................................................................................................................11

3 CELL PLANNING TOOLS & PROPAGATION MODELS .................................................................................................... 13

3.1 CELL PLANNING TOOLS............................................................................................................................................................... 13 3.2 PROPAGATION MODELS .............................................................................................................................................................13

3.2.1 Flat Conductive Earth.............................................................................................................................................................. 14 3.2.2 Knife Edge Diffraction.............................................................................................................................................................. 15 3.2.3 Fresnel zone clearance............................................................................................................................................................. 17 3.2.4 Field measurements and Semi-Empirical Models ............................................................................................................... 19 3.2.5 Okumura -Hata Model ............................................................................................................................................................... 19 3.2.6 Cost 231 – Hata Model ............................................................................................................................................................ 20 3.2.7 Path Loss (Attenuation) Slope ................................................................................................................................................ 21 3.2.8 Okumura -Hata Corrections..................................................................................................................................................... 21 3.2.9 Walfisch-Ikegami Model ........................................................................................................................................................... 22 3.2.10 Choice of propagation model ............................................................................................................................................ 22

4 FREQUENCY PLANNING CONCEPTS..................................................................................................................................... 24

4.1 FREQUENCY RE-USE......................................................................................................................................................................24 4.2 FREQUENCY PLANNING...................................................................................................................................................................25

5 LINK POWER BUDGET................................................................................................................................................................ 28

5.1 PATH BALANCE ................................................................................................................................................................................28 5.2 MAXIMUM PERMISSIBLE PATH LOSS ............................................................................................................................................29

6 FUTURE GROWTH AND EXPANSION..................................................................................................................................... 31

6.1 INTRODUCTION..............................................................................................................................................................................31 6.2 CELL SPLITS......................................................................................................................................................................................31 6.3 MULTIPLE RE -USE PATTERN.........................................................................................................................................................33 6.4 FRACTIONAL RE -USE PATTERN.....................................................................................................................................................34 6.5 MICROCELLULAR UNDERLAYER...................................................................................................................................................35

6.5.1 Layered Architecture ................................................................................................................................................................ 35 6.5.2 Combine Cell Architecture ...................................................................................................................................................... 36

6.6 DUAL BAND NETWORKS.................................................................................................................................................................37


CHAPTER 1

CELL PLANNING INTRODUCTION

Objectives: This chapter will describe the basic studies on

cell planning concepts, the goals in cell planning and the factors involved. It also describes the types of cells in cellular network.

Upon completion of this chapter, the student will be able to:

• Understand the main reasons for cell planning • Explain the types of cells involved


1 CELL PLANNING INTRODUCTION

1.1 CELL PLANNING DEFINITION Cell planning can be described as all the activities involved in:-

a) Selecting the sites for the radio equipment b) Selecting the radio equipment c) Configuring the radio equipment

Every cellular network requires cell planning in order to provide adequate coverage and call quality.

1.2 CELL PLANNING GOALS

When designing a network, a network planner needs to achieve an overall goal to ensure that there that the plans can be implemented properly. These goals includes:-

a) Providing desired capacity

A plan that meets all design criteria. b) Offering good efficiency A plan that utilizes all the spectrum given in the best possible way. c) Implementing the network at low cost

A plan that uses the right amount of equipment to provide the best possible capacity and coverage.

d) Offers high grade of service A plan that ensures the customers are satisfied.


1.3 CELL PLANNING FACTORS

Many factors can affect the design of a network. Among these cell planning factors are:

a) Effects of propagation – how the propagation of electromagnetic

waves travels and propagates in the air and through buildings and obstacles.

b) Efficient traffic capacity – how subscriber traffic is spread throughout the network and how these needs are met.

c) Subscriber environment – the behavior of subscribers with respect to its environment and circumstances.

d) Frequency spectrum – how much physical resources is available to the network.

e) Site planning – how sites are planned and designed, and the cost of these sites.

1.4 TYPE OF CELLS

A cell may be defined as an area of radio coverage from one BTS antenna system. There are two main types of cell:

a) Omni Cell

This cell is served by a BTS, with an antenna, which transmit in all directions. This is shown in Figure 1-1.

b) Sectored Cell

This cell is an area of coverage with an antenna, which transmit in a given direction only. This is shown in Figure 1-2.

Figure 1-1

Figure 1-2


The cells in a network can be divided into different categories, defined on the position of the antenna as shown in Figure 1-3.

Among these different types of cells are:-

a) Macro Cell A macro cell is a traditional cell with the antenna above the average obstacle (building) height. This gives a good outdoor coverage, and pretty good indoor coverage.

b) Mini Cell

The mini cell is a low macrocell or a roof micro cell. The antennas are mounted just below the rooftop level. The buildings shadow the line-of-sight waves, making the cell area more contained. The minicells are most effective in areas where the buildings have approximately the same height.

c) Micro Cell

A microcell uses low antenna height (typically 4 to 10 m). The antenna is mounted outdoors. The waves propagate between the buildings and not over the roof tops. The cell size is typically 150 to 500 meters.

d) Indoor Cell

The indoor cell (or pico cell) has the antennas inside a building. This indoor cell covers a building or a part of a building. This is the best solution when high capacity and very good indoor coverage is required.

END OF CHAPTER 1

Figure 1-3


CHAPTER 2 CELL PLANNING PROCESS

Objectives: This chapter describes briefly the cell planning

process and some of the factors involved.


• Describe the processes involved in cell planning


2 CELL PLANNING PROCESS

The major activities involved in cell planning are shown in Figure 2-1.

Figure 2-1

2.1 STEP 1 : TRAFFIC & COVERAGE ANALYSIS

The cell planning process starts with traffic and coverage analysis. The analysis should produce information about the geographical area and the expected need of capacity. The types of data collected are:

§ Cost

§ Traffic distribution

§ Coverage

§ Grade of Service (GoS)

§ Available frequencies

§ Speech Quality Index

§ System growth capability


The traffic demand provides the basis for cellular network engineering. Geographical distribution of traffic demand can be estimated by using demographic data.

2.2 STEP 2 : NOMINAL CELL PLAN

Nominal cell plans are the first cell plans and form the basis for further planning. It is a graphical representation of the network and simply looks like a cell pattern on a map.

It is also based on some measurable and educated forecast of data or market research data. At this stage of nominal cell plan, coverage and interference predictions is often included using some planning tools.

2.3 STEP 3 : SITE SURVEY

Site surveys are performed for all proposed site locations. The following must be checked for all the sites:

a) Exact location b) Space for equipment, including antennas c) Power facilities d) Contract with site owner

This is a critical step because it is necessary to assess the real environment to determine whether it is a suitable site location when planning a cellular network.

2.4 STEP 4 : SYSTEM DESIGN

Once the planning parameters have been adjusted to match the actual measurements, dimensioning of the BSC, TRC and MSC/VLR can be adjusted and the final cell plan is produced. The final cell plan is then completed. In conjunction with this, a cell design parameter document is produced specifying all the cell parameters for the cell to be entered into the site database.

2.5 STEP 5 : SYSTEM IMPLEMENTATION & TUNING

Once the system has been installed, it is continuously monitored to determine how well it meets demand. This is called system tuning. It involves:


§ Checking that the final cell plan was implemented successfully § Evaluating customer complaints § Checking that the network performance is acceptable § Changing parameters and taking other measurements, if

necessary

The system needs constant retuning because the traffic and number of subscribers increases continuously. Eventually, the system reaches a point where it must be expanded so that it can manage the increasing load and new traffic. At this point, a traffic and coverage analysis is performed and the cell planning process cycle begins again.

2.6 SYSTEM GROWTH / CHANGE

Cell planning is an ongoing process. If the network needs to be expanded because of an increase in traffic or because of a change in the environment (e.g. a new building), then the operator must perform the cell planning process again, starting with a new traffic and coverage analysis. Some re-engineering work that need to be done are:

a) Antenna downtilts, azimuths and antenna heights

b) Cells splits

c) Frequency re-tuning

d) Implementation of microcell underlayers

END OF CHAPTER 2


CHAPTER 3 CELL PLANNING TOOLS & PROPAGATION MODELS

Objectives: This chapter describes briefly the cell planning

process and some of the factors involved. In this chapter as well, some of the planning tools used will also be introduced.


• Explain the various types of cell planning tools. • Describe the few kinds of propagation models.


3 CELL PLANNING TOOLS & PROPAGATION MODELS

3.1 CELL PLANNING TOOLS

Normally, coverage and interference predictions are needed for a cell plan. Hence, at this stage, computer-aided cell planning tools are used for radio propagation analysis. These cell planning tools are needed for:

a) Predictions for coverage, interference, traffic and etc.

b) Simulations for frequency planning

These tools are needed to simplify the planners’ task through the use of simulations and calculations as well as for the planner to have a starting point to work on.

Among the commercial cell planning tools are:-

a) Asset Planning Tool

b) TEMS Cell Planner

c) Planet from MSI

d) Odyssey from Aethos

e) TOTEM from Nokia

f) Netplan from Motorola

3.2 PROPAGATION MODELS

It is important to be able to estimate cell coverage to determine the size of the cell and also the interference. The definition of coverage is that an area is considered covered if in 95% of that area, the signal received by the MS is larger than some required value. Hence, in order to achieve this, the predicted signal strength at the cell border must be larger than some design value (e.g. SSdesign = -85dBm) giving Pin(MS)(predicted) ≥ SSdesign .

The signal strength required and design values are estimated by adding margins to the MS receiver sensitivity. These are fast and slow fading margins, interference margins, margins for body loss and possibly additional margins for in-car and indoor coverage. The margins depend on the type of environment and operator requirements. It is very important to be able to estimate or predict the pathloss. Improvements can be made by taking into account the following:


a) The fact that radio waves are reflected towards the earth surface (the Conductivity of the earth is then an important parameter)

b) The transmission losses due to obstructions in the line of sight. c) The finite radius of the curvature of the earth.

d) The terrain type in a real case, as well as the different

attenuation properties of different land usages such as forest, urban areas and etc.

Propagation model is essentially a curve fitting exercise. Tests are conducted at various frequencies , locations, periods, distances and antenna heights. The received signal is analysed and fitted into an appropriate curve. Hence, the formulas used to match these curves are generated and used as models. In order that these models be understood fully, the following concepts needs to be explained.

a) Flat Conductive Earth

b) Knife Edge Diffraction

c) Fresnel zone clearence

3.2.1 Flat Conductive Earth

In Figure 3-1, reflections against the surface of the earth are taken into account. If we assume an unobstructed propagation through free space, the signal at the receiving antenna can be seen as the sum of one direct signal and the reflected signal, if we also assume that the earth is a perfect conductor (hardly a good assumption, except possibly for sea water), i.e. loss free reflection, this yields (for the received power at the receiving antenna) the interference term:

Pr = [PtGrGtλ

2sin2(2πh1h2/λd)] / (2πd)2

Which is the squared sum of the field amplitudes from the direct and reflected wave. Assuming that h1h2 << λd (i.e., small angles), the sine function can be replaced with its argument (radians) and so,

Pr = [PtGrGt(h1h2)2] / d4

or L = 10 log(Pt/Pr) = 20 log(d2/h1h2) – 10 log(Gr) – 10 log(Gt) and the term 20 log(d2/h1h2) corresponds to the path loss.


.

Figure 3-1

3.2.2 Knife Edge Diffraction

Additional path loss due to objects obstructing the line of sight can be taken into account by calculating the (Fresnel) diffraction pattern at the receiver. The intensity is a function of the height of the obstruction above (or below) the line of sight as well as the distances transmitter-object and receiver-object as shown in Figure 3-2 below.

Figure 3-2

Derivations of the expression is somewhat lengthy, so here we must be satisfied with expressing the additional attenuation caused by these so-called “knife edges” in a diagram. The additional attenuation (also known as diffraction attenuation) is read as a function of the parameter ν (also known as the Fresnel diffraction parameter) which is given as (Figure 3-3),

ν = h√[(d1 + d2) / λd1d2]


An illustration of how the diffraction gain/attenuation can be related to its diffraction attenuation parameter is shown below. For a fixed distance of d1(distance of antenna to the diffracting object=5m and d2=495m and a frequency of operation at 900Mhz, we have Table3-1.

h d2 d1 λλ νν

-10 495 5 0.3 -11.6052-9 495 5 0.3 -10.4447-8 495 5 0.3 -9.28414-7 495 5 0.3 -8.12362-6 495 5 0.3 -6.96311-5 495 5 0.3 -5.80259-4 495 5 0.3 -4.64207-3 495 5 0.3 -3.48155-2 495 5 0.3 -2.32104-1 495 5 0.3 -1.160520 495 5 0.3 01 495 5 0.3 1.1605182 495 5 0.3 2.3210353 495 5 0.3 3.4815534 495 5 0.3 4.6420715 495 5 0.3 5.8025896 495 5 0.3 6.9631067 495 5 0.3 8.1236248 495 5 0.3 9.2841429 495 5 0.3 10.44466

10 495 5 0.3 11.60518

Table 3.1

Note that h is negative if the obstruction tip does not protrude into the propagation path, h=0 if the obstruction tip is tangent with the propagation path and h is positive if the obstruction tip does protrude into the propagation path.

Figure 3-3


From the reading of νν in the table and comparing with the graph, we see that if the knife edge does not protrude in to the wave, there exist some diffraction gain. When h becomes positive, and protrudes in to the line of sight of the wave, the diffraction gain quickly turns to diffraction attenuation.

3.2.3 Fresnel zone clearance

The concept of the Fresnel zone is illustrated below:

Tx Rx

d1 d2

1st Fresnel zoneradius

Figure 3-4

When electromagnetic wave encounters objects in its path, it gets diffracted. Imagine that these waves travel as spherical wave fronts. Looking at the cross section, Fresnel Zones are a set of concentric circles, which are loci of all points having the same signal strength. The Fresnel zone are λ/2 apart from one another.

The radius of the first Fresnel zone is dependent on the frequency of the wave and the antenna height. For a given antenna height, the primary energy of the wave is contained inside the first Fresnel zone. In other words, for the wave to have its maximum energy transmitted without any diffraction attenuating the signal, the wave must be within the radius of the first Fresnel zone. To illustrate this further, please refer to figure 3-5.


Transmitter

Receiver

Rn

d1

d2

ObstructionBuilding

Figure 3-5

Figure 3-5 shows a typical setting where an building is obstructing the line of sight path from the transmistter to the receiver. The diagram illustrates that in order for the majority of line of sight energy to get to the receiver, it must be within the first Fresnel radius, Rn.

The radius of the nth Fresnel zone circle is denoted by R n and can be expressed in terms of n, λ , d1 and d2 by

ddddR

nn

21

21

+=

λ

for d1& d2 >>Rn

For the aforementioned parameters of d1(distance of antenna to the diffracting object=5m and d2=495m and a frequency of operation at 900Mhz, we have

Rn = 1.21m


3.2.4 Field measurements and Semi-Empirical Models

The models discussed previously do not take into account the topographical variations in a real environment nor the different attenuation properties of different land usages such as forests, urban areas, etc. Although calculations taking all details into account are possible, they are tremendously time consuming and not practical to use for the cell planner. Indeed, empirical data can be used.

Among these classical empirical models are:

a) Longley-Rice Model – used for irregular terrain model

b) Okumura-Hata Model – used for urban/suburban model at 900MHz

c) Cost 231-Hata Model – used for 1500MHZ to 2000MHz

d) Walfisch-Ikegami Cost 231 – used for dense urban/microcell areas

3.2.5 Okumura-Hata Model

Among the model mentioned above, the Okumura-Hata model is arguably the most important model that has been developed. In the 1960’s, Japanese engineer Okumura carried out a series of detailed propagation test for land-mobile radio services at various different frequencies. The freqeuncies were 200Mhz, 453Mhz, 922Mhz, 1310Mhz, 1430Mhz and 1920Mhz. The results were stastically analysed and described for distance and frequency dependancies of median field strength, location variabilities and antenna height gain factors for the base and mobile stations in urban, suburban and open areas over quasi-smooth terrain.

The results of the median field strength with respect to the distance in kms from the site at the stated frequencies were displayed graphically as shown in Figure 3-6 below. It is interesting to note that the free space model yields consistently higher field strengths. This means that it yields lower path loss than the measurements.

As this is a graphical representation of the results, the Okumura model cannot use computer based analysis to aid cellular engineers to plan a network. However, the results provided by Okumura are the basis on which path loss prediction equations have been formulated. Perhaps the most significant contribution to the extension of this model was carried out by another Japanese engineer named Hata, who took Okumura’s graphical results and derived generic equations to describe the path loss in various environments. It is through his work that computer based analysis present in many of the commercially available cell planning tools have been made possible.

The general description of the OH model is given by:


Lp(urban) = 69.55 + 26.16logf - 13.82loghb + (44.9 - 6.55loghb )logd - a(hm )

Where a(hm ) = (1.1 log f - 0.7)hm - (1.56 log f - 0.8)

f = carrier frequency in MHz (150 - 1000 MHz)

hb = the base station antenna height in meters (30 - 200 m)

d = distance in km from the base station (1 - 20 km)

hm = mobile antenna height in meters above ground (1 - 10 m)

However, the figure can only be used as a rough guide since terrain types differ from place to place and local variations in the topography as well as in the land usage cannot be accounted for. Empirical data can be used to improve more elaborate models.

Figure 3-6

3.2.6 Cost 231 – Hata Model

The Cost 231-Hata model is essentially the same as the Okumura-Hata equation except that it is valid for frequencies between 1500Mhz to 2000Mhz. Thus, the general equation is given by:

Lp(urban) = 46.3 + 33.9logf - 13.82loghb + (44.9 - 6.55loghb )logd - a(hm)

Where a(hm ) = (1.1 log f - 0.7)hm - (1.56 log f - 0.8)


f = carrier frequency in MHz (1500 - 2000 MHz) hb= the base station antenna height in meters (30 - 200 m) d = distance in km from the base station (1 - 20 km) hm = mobile antenna height in meters above ground (1 - 10 m)

3.2.7 Path Loss (Attenuation) Slope

The equations derived by Okumura and Hata can be further simplified to represent the path loss in a straight line graph, so that the path loss can be a function of distance traveled. The simplified equation is given by

Lp(urban) = L0 + (44.9 - 6.55loghb) log(d)

Where L0 = 69.55 + 26.16log f - 13.82log hb - a(hm ) for 150-1500Mhz (Okumura-Hata) and L0 = 46.3 + 33.9log f – 13.82loghb – a(hm) for 1500-2000Mhz (Cost 231-Hata)

or this can be further simplified to,

Lp = L0 + 10γ log(d)

Where γ is the slope and is (44.9 – 6.55loghb)/10 and γ varies from 3.5 to 4 for urban environments. The typical value for free space path loss is 2.0 and the typical value for line of sight propagation is approximately 2.6. This is consistent with the fact that for a denser the environment, the greater the path loss (γγ), and hence the signal will attenuate faster given that all else factors are equal.

3.2.8 Okumura-Hata Corrections

The abovementioned equation describes what is generally applicable to suburban to urban terrains. This is because Okumura conducted his empirical experiments using this kind of terrains. To compensate for other types of terrains, there needs to be some corrections made to the urban path loss equation. These corrections are given by:

Lsub = Lp – 2 log2 (f/28) –5.4

Lopen = Lp – 4.78{log (f)}2 +18.33log(f) –40.94

Lsemiopen = Lp – 4.78{log(f)} +18.33log(f) – 35.94

Where Lpsub is area over a suburban terrain

Lpopen is area over a open terrain


Lpsemiopen is area over a semi open terrain

3.2.9 Walfisch-Ikegami Model

This model is applicable to line of sight (LOS) propagation. Usually takes place in street canyons where microcells are implemented.

The pass loss is given by:-

Llos = 42.6 + 20log(f) + 26 log(d) or

Llos = L0 + 10γ log(d)

where L0 = 42.6 + 20log(f) and γ = 2.6

3.2.10 Choice of propagation model

It can be seen that the choice of a propagation model for cell planning becomes critical in determining the correct cell plan. Below is a table that guides a cell planner what are the general models in a given terrain.

Table 3-2

END OF CHAPTER 3

Environment type Model TypeDense Urban Street Canyons (Line of Sight) Walfisch-Ikegami, LOSMacrocells (above roof tops) Okumura-Hata

UrbanUrban areas Walfisch-Ikegami, LOSMix of building of varying heights, open Okumura-Hata areas & vegetation

SuburbanBusiness & residential area Okumura-Hata (with correction factor)

RuralLarge open areas, field, highways Okumura-Hata (with correction factor)


CHAPTER 4

FREQUENCY PLANNING CONCEPTS

Objectives: This chapter describes briefly the frequency

planning process in a network.


• Understand the concept of frequency planning. • Explain the use of frequency re-use pattern.


4 FREQUENCY PLANNING CONCEPTS

4.1 FREQUENCY RE-USE

Cellular systems re-use frequencies, which is needed to support its system capacity. This means that co-channel interference exists in the network. The way we re-use can vary from totally rigid (systematic) to totally flexible (adhoc). A totally systematic approach makes the job much easier but if a systematic approach is not possible, then flexibility can be introduced. The level of this interference is dependent on the cell radius (r) and the distance (d) between them. The minimum re-use distance is d is given by the equation: d = (√3N)r Where N = re-use pattern = i2 + ij + j2 (i & j are integers) Thus, if i = j = 1, N = 3 and d = 3r And,

i = 2, j = 0, N = 4 and d = (√12)r =3.464r i = 2, j = 1, N = 7 and d = (√21)r = 4.583r i = 2, j = 2, N = 12 and d = (√36)r = 6r

The re-use distance d and cell radius r are related to the C/I as given by:

(d/r)γγ = 6 (C/I)

Where γγ = attenuation slope or path loss coefficient with the assumptions that: i) All the cells have the same size and transmit the same power. ii) The path loss is not of free space and is govern by γ.

For example, in an urban environment, what should be the re-use distance if the required C/I is 11dB? (γ is assumed to be 3.525 in an urban environment) (d/r)3.525 = 6(C/I) = 6 * (12.59) d/r = 3.410 And if the cell radius is 2km, the minimum re-use distance is 6.8kms.


4.2 FREQUENCY PLANNING

Once the frequency re-use pattern has been decided, a system can be assigned a frequency plan.

For example, if a system has 10Mhz, and the frequency re-use is N=4, then 10Mhz/200kHz = 50 channels are available. Assuming that 48 channels are used (76-123), how many channels can be allocated per cell? 48 channels/(N=4) = 12 channels per site

And assuming that a site has 3 sectors,

12 channels /3 sectors cells = 4 channels per sector cell Hence, the frequency plan is shown in Table 4-1:

Table 4-1

Figure 4-1 below shows an example of a typical 4/12 frequency re-use.

Figure 4-1

Frequency group A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3BCCH 76 77 78 79 80 81 82 83 84 85 86 87TCH 1 88 89 90 91 92 93 94 95 96 97 98 99TCH 2 100 101 102 103 104 105 106 107 108 109 110 111TCH 3 112 113 114 115 116 117 118 119 120 121 122 123


However, the example given is of an ideal case. In the real world, channels are not so uniformly assigned because of the few reasons below:

a) Traffic density is never homogenous for every cell. b) The sites built are sometimes not on the exact grid. c) Terrain and clutter issues always affect the coverage.

For the given reasons above, we can conclude that a theoretical frequency plan has its limitation. Frequency planning in practice is a balance between the ideal plan based on equations and the practical plan based on the limitation of physical resources. For example, it is quite rare that two or more neighboring cells need the same amount of channels. It must be always kept in mind that the values calculated for traffic distribution are only crude estimates and the real traffic distribution always deviates from these estimates. Consequently, the frequency plan should be flexible enough to allow for rearrangement of the network to meet real traffic needs.

END OF CHAPTER 4


CHAPTER 5 LINK POWER BUDGET

Objectives: This chapter introduces the calculations involved in link power budget.


• Describe the parameters involved in link power

budget calculation.


5 LINK POWER BUDGET

5.1 PATH BALANCE

Path balance implies that the coverage of the downlink is equal to the coverage of the uplink. The power budget shows whether the uplink or the downlink is the weak link.

A typical power budget is shown Table 5-1 below.

Table 5-1

Why is the link power budget an important parameter to look at? Firstly, a balance link budget will ensure that the coverage be equal between the downlink and uplink. This means that the uplink signal can always be received by the BTS and the downlink signal can always be received by the MS.

Secondly, the link power budget tells us the maximum permissible path loss (Lpmax). The Lpmax will define the absolute minimum signal that must be receive by a BTS in the uplink direction (and conversely in the MS in the downlink direction) beyond which the BTS will not be able to receive each the MS’s signal.

RF LINK BUDGET UL DLTRANSMITTING END MS BTS

Tx RF OUTPUT POWER 33 dBm 40 dBmBody Loss -3 dB 0 dBCombiner Loss 0 dB 0 dBFeeder Loss 0 dB 1.5 dBConnector Losses 0 dB 2 dBTx Antenna Gain 0 dB 18 dBEIRP 30 dBm 55 dBm

(A) (C)

RECEIVING END BTS MS

RX sensitivity -107 dBm -102 dBmRx Antenna Gain 17.5 dBm 0 dBDiversity Gain 3 dB 0 dBConnector Loss 2 dB 0 dBFeeder Loss 1.5 dB 0 dBInterference Degradation Margin 3 dB 3 dBBody Loss 0 dB 3 dBDuplexer Loss 0 dB 0 dBRx Power -121 dBm -96 dBmFade Margin 4 dB 4 dBRequired Isotropic Rx. Power -117 dBm -92 dBm

(B) (D)Maximum Permissible Path Loss 147 dB 147 dB

(B-A) (D-C)


Looking at the table above, we can see that the link for the uplink is balance with the downlink. It is worth noting that, in practice, the link budget is almost never exactly balanced. This is due to a number of factors. For example, the receiver sensitivity of the BTS is taken as –107dBm, but it could be as good as –109dBm. Similarly, the MS receiver sensitivity could be as good as –105dBm. Another point is that practically, the antenna peak transmit power is never exactly 33dBm, and lies more in the region of 31 or 32 dBm. Hence the uplink budget is normally 1-2 dB worse than what is shown in the table. Also, the diversity gain is assumed to be 3dB but actually, it could range from 0-4dB, depending on the propagation environment, location of the mobile and the kind of diversity employed.

From the link budget example, it can be seen that except for the fade margin, all other parameters are more or less fixed, as these are part of equipment specification. The fade margin added into the calculation is to compensate for the slow and fast fading (multipath fading) of the signals will invariably experience in a wireless system. This fade margin effectively gives an extra margin of error for the receiver sensitivity in that it ensures that the receiver is still able to receive severely faded signals.

The main factors that affects the fade margin is the attenuation coefficient (γγ), the mean and standard deviation of the receive signal strength(RSS) and the probability factor (usually 90%) of the RSS receiving equal or better than the design criteria in a given coverage area.

5.2 MAXIMUM PERMISSIBLE PATH LOSS

Once the link budget and Lpmax has been calculated, the corresponding cell radius can be determined by using a suitable wave propagation (that fits the environment), for example the Okumura-Hata model. For a given cell radius, the coverage of a cell can be estimated by using formula:

2

332

r×

Where r= radius of the cell, in km

For example, if we had a maximum permissible path loss of 147dB and a wanted RSS of at least 92dBm at the minimum signal strength, and the frequency = 900Mhz, the hbts = 30m and hm = 3m, the cell radius will be approximated (using the Okumura-Hata equation) to

Lp = 69.55+77.28 –20.41 – 3.81+35.525 log(r) 147 = 122.61 + 35.525 log(r) d = 4.85 kms

Thus, a cell with a radius of 4.85kms, will provide a RSS of –92dBm in 90% of the area, including a fade margin of 4dB.

END OF CHAPTER 5


CHAPTER 6

FUTURE GROWTH AND EXPANSION

Objectives: This chapter describes briefly the different alternatives in handling a network expansion.


• Explain the different options to increase the capacity

of a network.


6 FUTURE GROWTH AND EXPANSION

6.1 INTRODUCTION

Normal practice in network planning is to choose one point of a well known re-use model as a starting point. Even at this early stage the model must be improved because any true traffic density doesn’t not follow the homogeneous pattern assumed in any theoretical model.

Small sized heavy traffic concentrations are characteristic of real traffic distributions. Another well known traffic characteristic feature is the fast descent in the density of the traffic when leaving urban areas. It therefore uneconomical to build the whole network using a fixed cell size covering every area, but rather using cells of varying size. Because of the aforementioned factors, the capacity requirements for a given area will differ from networks to networks and that different networks operators and vendors will have their own solutions to these problems.

General speaking, the following methods are used to cope with the increasing capacity requirements usually found in dense urban areas.

a) Using cell splits by employing a tighter frequency re-use pattern incorporating frequency hopping.

b) Multiple re-use pattern c) Fractional re-use pattern d) Deploy microcellular underlayers. e) Increase the frequency band by deploying dual band network ( if

licensing is possible).

6.2 CELL SPLITS

Cell splitting as the names implies, is a technique use to “break down” the size of an original cell so that more cells can be built to serve the same given geographical area. This can potentially give rise to capacity increase because more cells are being used in the same given area. The compromise is that, the frequency re-use pattern is closer or “tighter” together which then leads to higher interference in the network. This concept is illustrated below.

Before splitting the cells, they look as shown in Figure 6-1.


Figure 6-1

After phase 1 of cell split, the cells look like as shown in Figure 6-2. The grey cells are the original cells and the cells are now split into 3 cells per one original cell.

Figure 6-2

After phase 2 splitting, the cells look as shown in Figure 6-3. Final step in cell splitting yields 4 times more cells than the original cell. This theoretically yields 4 times more capacity.

Figure 6-3


It is clear that a smaller cell size increases the traffic capacity. However, a smaller cell size means more sites and higher cost for the infrastructure. Hence, it is preferable not to work with an unnecessary small cell size. The system is started using a large cell size and when the system capacity needs to be expanded, the cell size is decreased in order to meet the new requirements. This normally also calls for using different cells sizes in different areas.

6.3 MULTIPLE RE-USE PATTERN

This is a method used to increase the capacity whereby the BCCH is fixed but separately planned. The available frequencies are split into a number of segments each representing a re-use situation on each of the carriers of the cell. This can be illustrated in Figure 6-4.

Figure 6-4

TCH re-use is grouped into 9 frequencies for TCH1, 6 frequencies for TCH2 and 4 frequencies for TCH3. Each cell is only allocated with the necessary number of carriers (starting from the most relaxed re-use) given by the traffic requirements per cell up to a maximum of four TRXs per cell. The average re-use for 12/9/6/4 is (12+9+6+4)/4=7.75 for the cell and (9+6+4)/3=6.3 for the TCH frequencies. Multiple Re-use Pattern (MRP) is a scheme to gradually tighten the frequency re-use in a cellular network. It is also very well suited to handling networks with uneven traffic distribution, i.e. different number of transceivers (TRXs) in each cell. A tighter frequency re-use means an increased interference level.


The idea is that instead of organizing the TCH carriers according to a single re-use scheme, the available frequencies could be split into a number of segments each representing a re-use cluster. The BCCH carriers are already planned separately. These re-use clusters are of different sizes providing different re-use situations on each of the carriers in the cell. By then applying frequency hopping over all the carriers averaging the interference situation in each overlaid re-use cluster, a very efficient system is built. This method allows a gradual tightening of the re-use as more transceivers are installed in the cell. The re-use group on the last transceiver can be tight since it probably will not be used in every cell. This can be traded into tighter re-use.

6.4 FRACTIONAL RE-USE PATTERN

There is another alternative for advance re-use especially when only a limited bandwidth (≤ 6MHz) is allowed. This method is called fractional re-use. With this type of frequency planning the interference is averaged in the time domain and the cumbersome task of frequency planning gets less complicated. This method can be shown in Figure 6-5.

Figure 6-5

Both systems utilizes the strength of frequency hopping, namely interference averaging and frequency diversity.

The diagram shows 2 different re-use pattern namely:


a) 1*3*N fractional re-use With the 1*3*N re-use patterns, the frequency groups are repeated in every 3 cells.

b) 1*1*N fractional re-use

With the 1*1*N re-use patterns, the frequency groups are repeated in every cell.

6.5 MICROCELLULAR UNDERLAYER

6.5.1 Layered Architecture

The basic term “underlayer” is used in the microcellular context to explain how macro cells overlay microcells. It is worth noting that when talking of the traffic capacity of a micro cell, it is additional capacity to that of the macro cell in the areas of microcellular coverage.

MACRO CELL

Micro Cell A Micro Cell B

Top view

MACRO CELL

Micro Cell A Micro Cell B

Side view


The traditional cell architecture design ensures that, as far as possible, the cell gives almost total coverage for mobile subscribers within its area.

6.5.2 Combine Cell Architecture

A combine cell architecture is a multi-layer system of macro and micro cells. The simplest implementation contains two layers. The bulk of the capacity in a combined cell architecture is provided by the micro cells. Combined cell systems can be implemented in a multi vendor environment.

Overlayed macro cells

Contiguous coverage over areas of high slow moving

Underlayed micro cell

(could be a different vendor)


6.5.2.1 Macro cells

Implemented specifically to cater for fast moving mobile subscribers and to provide a fall-back service in the case of coverage holes and pockets of interference in the micro cell layer. Macro cells form an umbrella over the smaller micro cells.

6.5.2.2 Mirco cells

Mirco cells handle the traffic from slow moving mobile subscribers. The micro cells as shown in the diagram can give contiguous coverage over the required areas of heavy subscriber traffic.

Some points to note

• Macro and micro cells networks may be operated as individual systems.

• The micro cell network is more dominant as it handles the greater amount of traffic.

• Microcells can be underlayered into existing systems.

As the GSM network evolves and matures its traffic loading will increase as the number of subscribers grow. Eventually a network will reach a point of traffic saturation. The use of micro cells can provide high traffic capacity in localised areas.

6.6 DUAL BAND NETWORKS

Dual band networks as the name suggest is a network that consist of two physical GSM infrastructure (GSM 900Mhz & 1800Mhz) but only seen as one logical entity. It enables a network operator with licenses in two or more frequency bands to support the use of multiband mobiles in all bands of the licenses.

GSM 1800 for capacity

GSM 900 for coverage


The concept of a dual band network is not unlike the microcellular concept except that a dual band normally employs the GSM 1800Mhz infrastructure as the micro cell layer. Because of the increase number of channels inherent in GSM 1800, the capacity gains far outweighs the capacity increase of other capacity gaining systems. The design of these networks can be quite complicated, but if design and optimized properly, the can be tremendous gains available for the network operator.

END OF CHAPTER 6

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