22066202 Chapter3 Radio Propagation Theory

8/8/2019 22066202 Chapter3 Radio Propagation Theory

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Wireless Network Planning Table of Contents

Table of Contents

Chapter 3 Radio Propagation Theory .........................................................................................11.1 Basic knowledge of Radio Propagation .............................................................................11.2 Radio Propagation Environment....................................................................................... 3

1.2.1 Frequency Division Introduction .............................................................................3

1.2.2 Fast Fading and Slow Fading .................................................................................3

1.2.3 Propagation Loss ....................................................................................................6

1.3 Radio Propagation Model................................................................................................ 101.4 Correction for propagation model.................................................................................... 16

1.4.1 CW Basics ............................................................................................................16

1.4.2 CW Test Method ...................................................................................................16

1.4.3 Correction for Propagation Model and Instance ....................................................18

1.5 Doppler Effect and its Impact on Handover .....................................................................191.6 Fresnel Zone ...................................................................................................................221.7 ASSET Software Introduction ..........................................................................................24

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Wireless Network Planning Chapter 3 Radio Propagation Theroy

Chapter 3 Radio Propagation Theory

1.1 Basic knowledge of Radio Propagation

When planning and constructing a mobile communication network, we mustunderstand the features of the electric waves to define the frequency band,frequency allocation, radio coverage, communication probability calculation,electromagnetic interference between systems and final parameters of radio devices.It is the keystone for system design, efficient use of frequency spectrum and EMC(Electronic Magnetic Compatibility).

It is well known that the radio wave can be transmitted from the transmitting antennato the receiving antenna in multiple modes: forward wave or free space wave, earth

wave or surface wave, troposphere reflecting wave, and ionosphere wave.

As shown in Figure 3-1, as far as electronic propagation is concerned, the simplemethod between transmitters and receivers is free space propagation. Free spacerefers to isotropy (identical in axes characters) and uniformity (even texture) in suchzone. Other names for free space are forward wave or stadia wave. As shown inFigure 3-1(a), forward wave transmits along straight lines, so that it can be used for communication between satellite and exterior space. In addition, this definition is alsoused for stadia propagation in land (between two microwave towers), as shown inFigure 3-1(b).

The second method is the earth wave or surface wave. Earth wave is the combinationof three waves: the forward wave, backward wave and surface wave. The Surface

wave transmits along the earth surface. Some energy from the transmitting antennacan directly reach to the receiver; some energy reaches to the receiver after reflectingon the earth surface; some reaches to the receiver through surface wave. Surfacewave transmits on the earth surface. Since the earth surface is not ideal for propagation, some of the energy is absorbed by the ground. When energy isabsorbed by the ground, it can cause ground current. Such three surface waves areshown in Figure 3-1(c).

The third method is that troposphere reflecting wave. It is generated from thetroposphere layer, which is a heterogeneous medium, changing with time because of air conditions. Its reflection factors decrease with an increase of height. Suchreflection factors with gradual change cause electric wave bending, as shown inFigure 3-1(d). Troposphere method is applied to radio communication with thewavelength less than 10 meters (frequency larger than 30MHz). The fourth method ispropagation through ionospheric reflection. When the electric wave is less than 1meter in length (frequency larger than 300 MHz), the troposphere layer is thereflected body. The radio wave reflected from troposphere layer might have one or more leaps, as shown in Figure 3-1(e). Such propagation is used for long-distancecommunication. Besides reflection, troposphere layer can generate electric wavescattering because of uneven refractive rate. In addition, meteors in troposphere layer can also scatter electric waves. Like the troposphere layer, ionosphere layer also hasthe feature of continuous fluctuation, and such fluctuation is rapid fluctuation atrandom. Cellular system radio propagation is adopted the second method of electricwave propagation. It will be discussed in the following parts.

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(b)

(c) (d)

(e)

(a)

Ground wave

Scatterer

Ionization layer

(a) Forward wavetransmits along straightline

(b) Stadiacommunicationapplication

(c) Earth wave propagation

(d) Troposphere lay scatters radio waveirregularly.

(e) Radio wave transmitsthrough ionospherereflection

Figure 3-1 Different Propagation Modes

There are two reasons for propagation study when designing cellular system: first, itprovides necessary tool for calculating signal level covering different cells. In mostcases, coverage area is , therefore earth wave propagation can be adopted in suchcondition. Secondly, it can calculate monkey-chatter interference and cochannelinterference.

There are three methods for predicatingsignal level radio coverage: the first one ispure theory, which is applied to separate objects, such as mountain and other solidobjects. However, it ignores the irregularity of the Earth. The second one is basedupon measurement in various environments, including irregular landform and man-made obstacles, especially the higher frequency and lower mobile antenna commonlyexisting in mobile communication. The third method is the improved model upon theabove two methods, which considers the influence of mountains and other obstacles

upon the measurement and the refraction law.

In the cellular system, there are at least two propagation models: the first one is FCCsuggested model; the second one, established by Okumura, considers the actualexperience data.

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1.2 Radio Propagation Environment

1.2.1 Frequency Division Introduction

The Radio frequency from 3Hz to 3000GHz are separated into 12 bands, as shown inFigure the following table. Frequency in different frequency spectrum has differentpropagation characteristics. As to mobile communication, we only pay attention toUHF spectrum.

Frequency Classification Designation

3 to 30Hz

30 to 300HzExtremely Low

FrequencyELF

300 to 3000Hz Voice Frequency VF

3 to 30KHz Very-low Frequency VLF

30 to 300KHz Low Frequency LF

300 to 3000KHz Medium Frequency MF

3 to 30MHz High Frequency HF

30 to 300MHz Very High Frequency VHF

300 to 3000MHz Ultra High Frequency UHF

3 to 30GHz Super High Frequency SHF

30 to 300GHzExtremely High

FrequencyEHF

300 to 3000GHz

1.2.2 Fast Fading and Slow Fading

According to the last section, in a typical cellular mobile communication environment,direct path between the receiver and the transmitter is obstructed by buildings or other objects. Thus, communication between the cellular base station and mobilestation completes not through direct path but many other paths. In UHF frequency,the main propagation mode for electromagnetic wave from the transmitter to thereceiver is scattering, i.e. reflection from the surface of building or refraction fromartificial and natural objects, as shown in Figure 3-2.

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① building reflected wave ② diffracted wave ③ forward wave ④ ground reflected wave

Figure 3-2 Multipath Propagation Model

All the signal components compose a multi standing wave, the signal level of whichincreases or decreases with corresponding changes of the components. Thesynthesis signal level fades 20 to 30dB in a few car bodies away, the differencebetween the maximum and the minimum is about 1/4 wavelength. A great number of propagation paths result in so called multipath phenomenon, whose synthesisamplitude and phase will undergo great fluctuation with the movement of mobilestations. Usually, such phenomenon is called multipath fading or fast fading, asshown in Figure 3-3. Essentially, multipath fading is a fast change. Besides, suchpropagation character causes time dispersion phenomenon. The distribution of deepfading point in space is approximately half wavelength away (900MHz is 17cm, 1800or 1900Mhz is 8cm). If the mobile antenna is at the deep fading point at that time(when mobile user in a car stay at the deep fading point because of redlight, we call it

Redlight Problem), voice quality is very poor. Therefore, related technologies likehopping should be applied to solve this problem.

Studies show that if the mobile cell receives the amplitude, phase and angle of respective component at random, then the azimuth angle of the synthesis signal andthe probability density function of amplitude are as follows:

() =1

2 0 ≤ ≤ 2π (3-1)

(r ) =r 2 e

(−r

22 )

r ≥ 0 (3-2)

Among them, “r” is the standard deviation. (3-1) and (3-2) represent the azimuth

angle

is even distribution between 0 to 2

π

, while the probability density function of electric field abides by Rayleigh Distribution. Therefore, multi path is also called

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Rayleigh fading. As to this fast fading, the base station adopts the methods of timediversity, frequency diversity and space diversity (polarity diversity). Time diversitymainly adopts the methods of symbol interleave, error code checking and correcting.Different code has different anti-fading characteristics. As to the air channel coding of GSM mobile communication, please see related GSM protocol. The basic of

frequency diversity theory is the correlation bandwidth, i.e. after more than an intervalbetween two frequencies, their space fading characteristics are considered irrelevant.A large number of test data shows that such irrelevancy can be obtained if the intervalbetween the two frequencies is larger than 200 KHz; frequency diversity mainlyadopts spread spectrum. In GSM mobile communication, hopping is simply applied toobtain hopping gain, while in CDMA mobile communication, each channel works inwide band (narrow band CDMA is 1. 25 MHz), which actually, is a spread frequencycommunication. Space diversity mainly adopts the master diversity antenna receivingmethod. Signals the base station receiving from the master and diversity channels arerespectively combined after equalization through the Maximum Likelihood SequenceEqualizer (MLSE). Such master diversity receiving effect is guaranteed by theirrelevancy received by the master diversity. Irrelevance refers to the signals receivedrespectively by the master antenna and diversity antenna having no fading at the

same time. It requires that the spacing between the master and diversity antenna is10 times more than the radio signal wavelength (the antenna spacing is more than 4meters in GSM900), or adopting polarity diversity to guarantee the signals receivedby the master and diversity antenna having different fading characteristics. Mobilestation (mobile phone) has no such space diversity function with only one antenna.The equalizing ability to different ranges (time window) of the base station receiver isalso a form of space diversity. In CDMA communication, when soft switching isperformed, the mobile station and multi base stations communicate at the same timeto select the best signal for handover, such is also a form of space diversity.

A great number of studies shows that the average signal levels received by themobile station, except for fast Rayleigh fading in instantaneous value, appear slowchanges as changing position, such change is called slow fading, as shown in Figure

3-3. It is caused by the shadow effect, and also called shadow fading. Buildings,forest and topographical relief in the way of radio propagation will cause shadow inelectromagnetic field. The medium value of receiving signal level will change whenelectromagnetic shadow is produced by different obstacles the mobile stationencounters. The change is depended upon the obstacle condition and workingfrequency; changing rate has relations with obstacles and driving speed.

By studying this fading law, it shows that its medium value variation abides byLogarithmic Normal Distribution.

Additionally, radio refraction coefficient changes as the climate conditions change withtimes, as well as slow changes in vertical gradient of atmosphere dielectric constant,which results in slow changes in signal level medium value in the same place as timechanging.

Statistics show that such medium value variation also abides by Logarithmic NormalDistribution. The distribution standard deviation is r t. Variation of signal medium valuein a larger range of distribution with time and place all abides by Logarithmic NormalDistribution, so that their synthesis distribution still abides by Logarithmic NormalDistribution. When communicating in land, usually, signal medium value variation astime changing is less than that as place changing, so that such slow fading can beignored, r=r L. However, in fixed-point communication, slow fading shall be considered.

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( m)10 20 30

- 20

-40

-60

Received power

Fast fading

Slow fading

Distance

Figure 3-3 Fast fading and slow fading

In general, there are two influences in cellular environment: the first one is fast fading;and the second one is slow changes in receiving signal level resulted from directlyvisible path, i.e. long-term signal level change. That is to say, the channel works infast fading in accordance with Rayleigh distribution, and superimposes amplitude withsignal to meet with slow fading in Logarithmic Normal Distribution.

1.2.3 Propagation Loss

In propagation studies, signal level received by specialized receiver is a major feature. Owing to the interference of propagation path and landform, propagation

signal level decreases. Such signal level decrease is called propagation loss.

In radio propagation studies, first study the characteristics of the two antennas in freespace (homogeneous medium with isotropy, no absorption, zero electric conductivity).Take the ideal omnidirectional antenna as an example. The propagation loss of freespace is:

L p = 32.4 + 20 lglg( f MHz ) + 20 lglg(d km ) (3-3)

Among which, f is frequency, d is distance (kilometers). In the above equation,propagation loss is in inverse proportion to d . When d doubles, free space path lossincreases by 6 dB. Meanwhile, when wavelength λ decreases (increase frequency f ),path loss increases. We can compensate these losses by increasing radiation and

receiving antenna gain. If the working frequency is already known, (3-3) can be alsowritten as:

L p = L0 + 10 lglg(d km ) (3-4)

Of the equation, = 2, is called path loss slope. In the actual cellular system,

according to measurement result, value ranges from 3 to 5.

Having the equation of path loss in free space, the actual propagation can beconsidered between the two antennas on plain but imperfect surface. Suppose thewhole propagation path surface is absolutely plain (without refraction). The antenna

height of the mobile station is hc and hm respectively (A represents hc, and B

represents

hm), as shown in Figure 3-4.

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A

A

A

A'

B

B

B

(a)

(b)

(c)

(a) multireflection (b) simple reflection (c) mapping method of finding path difference between stadia andground reflection

Figure 3-4 Propagation on Plain Surface

As compared with the path loss in free space, propagation path loss on plain groundsis:

L p = 10 lglgd − 20 lglghc − 20 lglghm (3-5)

Of which, = 4. This equation shows that if antenna height doubles, 6 dB can becompensated for loss; while the receiving power of the mobile station changes withthe fourth power of distance, i.e. if distance doubles, the power received reduced by12 dB.

Various landforms and ground objects differ greatly, so the impact on radiopropagation loss in mobile communication also varies. It is impossible to haveabsolutely plain landform in actual application. Such complex landforms can be

divided into two types: “quasi smooth landform” and “irregular landform”.

“Quasi smooth landform” refers to the landform with gentle rolling topography, rollingheight less than or equal to 20 meters as well as slight difference in average surfaceheight. Okumura defines the rolling height as the difference between 10% and 90% of rolling topography 10 kilometers ahead of the mobile station. CCIR defines it as thedifference between over 90% and over 10% of rolling topography 10 to 50 kilometersahead of the receiver. Other landforms are generally called “irregular landform”, whichcan be divided into the following types based upon their conditions: hills, separatedmountains, slopping landform and water-and-land mixed landform and so on.

When analyzing propagation loss in urban areas and their nearby areas, we can alsoclassify “irregular landform” by congestion in regions as open area, dense urban area,medium urban area and suburb area.

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In general, we also analyze diffraction loss when analyzing propagation loss inmountainous area or dense urban areas with close skyscrapers. Diffraction loss isused to measure the height of obstacles and antenna. The obstacle height must becompared with propagation wavelength. As to the same obstacle, the diffraction lossto long wavelength is less than that to short one. When predicating path loss, we can

view these obstacles as pointed obstacles, i. e. “knife-shaped”. Loss can becalculated by the method commonly used in physical optics. Two kinds of obstaclesshown in Figure 3-5. Under the first condition, no obstacles appear in stadium path atH. Under the second condition, obstacles appear in radio path. In the first condition,we assume that the height of obstacle is negative number, while positive number inthe second condition. Diffraction loss can be calculated through the diffractionconstant v, which is known from the following equation.

v = − H 2/(1/d 1 + 1/d 2) (3-6)

The approximate value of diffraction loss can be calculated from the followingequations:

F = 0 v 1

= 20 lglg(0.5 + 0.62v) 0 v < 1

= 20 lglg(0.5e0.45v) − 1v0

= 20 lglg(0.4 − 0.12 − (0.1v + 0.38)2 ) − 2.4v < −1

= 20 lglg(−0.225/v) v < −2.4 (3-7)

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(a) Negative height (b) positive height

Figure 3-5 Radio propagation past the cutting edge

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1.3 Radio Propagation Model

Propagation model is very important. It is the basic of mobile communication in cellplanning. Its value is to guarantee accuracy and to save labor, expense and time.Before planning a cellular system in an area, it is an essential task to select thecellular station address with signal coverage so as to avoid interference. If predictivemethod is not adopted, then the only one is cut-and-try method, which is carried outthrough actual measurement. Measure the coverage area of cellular station addressto select the best one from all the suggested solutions. It is money-wasting and labor-wasting by adopting this method. We can easily select the best layout solution for cellular station address with accurate predictive method through computer calculation,by comparing and evaluating the performance of all the solutions output from thecomputer. Therefore, we can say that the accuracy of propagation model not only

influences on whether the cell planning is proper, but also on whether the operatorscan invest rationally to satisfy users’ needs. With a vast territory, radio propagationenvironment is various in provinces and cities. For instance, propagation environmentand propagation models have great differences between cities in plain area and theones in hills area. Therefore, to ignore different factors of landforms, physiognomy,buildings and vegetation and consider experience will only result in network problemsof coverage and quality or in resource wasting because of too close base stations.

A good mobile radio propagation model is flexible to adjust according to differentlandforms, such as plains, hills and mountains, or different man-made environment,such as open areas, suburb and urban areas, etc. These environmental factors,involved in many variables in propagation model, play an important role. Therefore, itis not easy to form a good mobile radio propagation model. In order to improve

models, statistical method is used to measure a large number of data and correctmodels. Correction for propagation model will be introduced in section 3. 4.

Also, a good model should be easy to use. Models should be clear enough not to giveusers any subjective judgment and explanation, for different predictive value can bededuced from that in the same area. A good model shall have good recognition andacceptability. Using different models might have different structures. Good recognitionis very important.

Most of models predict path loss in radio propagation path. Therefore, propagationenvironment plays an important role in radio propagation model. Main factors involvedin propagation environment in a specific area are:

Natural (mountains, hills, plains and water area);

Quantity, height, distribution and material characteristics of man-madebuildings;

Characteristics of vegetation in the area;

Climate conditions;

Conditions of natural and man-made electromagnetic noise.

In addition, radio propagation model is affected by system working frequency andmobile station movement. In the same area, different working frequency results indifferent receiving signal fading; stationary mobile station differs high-speed movingmobile station in propagation environment. Generally, it is divided into two types:

outdoor propagation model and indoor propagation model. Commonly used modelsare shown in Table 3-1.

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Table 3-1 Common Propagation Models

Model name Scope of ApplicationOkumura-Hata

Applied to 150-1000 MHz macrocellular predication

Cost231-HataApplied to 150-2000 MHz macrocellular predication

Cost231 Walfish-IkegamiApplied to 900 and 1800 MHz microcellular predication

Keenan-MotleyApplied to 900 and 1800 MHz indoorpredication

Used in ASSET planningApplied to 900 and 1800 MHz macrocellular predication

Below is the brief introduction of Okumura-Hata model and Cost231-Hata model aswell as the propagation model used in ASSET network planning software. Hata modelis composed of the average data measured in Japan. Path loss value in generalareas can be approximately represented with the following equation:

L p = 69.55 + 26.16 lglg f − 13.82 lglghb

+ (44.9 − 6.55 lglghb ) lglgd − AOkumurahm (3-8-1:Okumura-Hata)

L p = 46.3 + 33.9 lglg f − 13.82 lglghb

+ (44.9 − 6.55 lglghb ) lglgd − ACost 231hm + C m (3-8-2:Cost231-Hata)

L p---Path loss from the base station to the mobile station, unit: dB

---Carrier wave frequency, unit: MHz;

hb---Antenna height of the base station, unit: m;

hm---Mobile station antenna height (1-10 m), having average value 1. 5 m, unit: m;

d ---Distance between mobile stations, unit: km;

C m--The value is 0dB in medium-size cities or in suburb with medium woods density,while 3 dB in big cities.

Okumurahm－－ (1.1lglg f −0.7)hm−(1.56lglg f 0.8)MS height correction, value in mediumsized citiesOkumurahm－－ (1.1lglg f −0.7)hm−(1.56lglg f 0.8)MS height correction, value in mediumsized cities

;The value in big city is 3.2(logloglog(11.75hm))2 − 4.97 (with frequency more than

400MHz);

ACost 231hm＝（1.1 lglg f − 0.7）hm − (1.56 lglg f − 0.8);

In suburb area, propagation model can be revised as

L ps= L p −2[lglg( f /28)]2−5.4

Urban area

L ps= L p −2[lglg( f /28)]2−5.4

Urban area

(3-9)

In open areas, the propagation is revised as

L po= L p −4.78(lglg f )2+18.33 lglg f −40.94Urbanarea L po= L p −4.78(lglg f )

2+18.33 lglg f −40.94Urbanarea

(3-10

)

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In the actual radio propagation environment, various relief shall be considered, whichis considered in ASSET planning software to improve propagation model. Consider various ground objects and relief having influence on radio propagation in actualenvironment so as to guarantee the accuracy of prediction result.

The model expression is as follows:

L p = K 1 + K 2 lglgd + K 3(hm) + K 4 lglghm + K 5 lglg( H eff )

+ K 6 lglg( H eff ) lglgd + K 7 diffn + K clutter

In the above expression (the following expressions are applied to macro cell):

K 1---the constant related to frequency;

The center of medium-size city:

K1=69. 55+(26. 16+1. 56lg(Fc))-0. 8 {Fc=150-1000MHz}

K1=46. 3+(33. 9+1. 56)lg(Fc)-0. 8 {Fc=1500-2000MHz}Center of big city

K1=69. 55+26. 16lg(Fc) {Fc=150-1000MHz}

K1=46. 3+Cm+(33. 9+1. 56)lg(Fc)-0. 8 {Fc=1500-2000MHz}

Suburb area:

K1=69. 55+(26. 16+1. 56lg(Fc))-0. 8 -2(log(Fc/28))2 - 5. 4{Fc=150-1000MHz}

K1=46. 3+(33. 9+1. 56)lg(Fc)-0. 8 -2(log(Fc/28))2 - 5. 4{Fc=1500-2000MHz}

Open area:

K1=69. 55+(26. 16+1. 56lg(Fc))-0. 8-4. 78(log(Fc))2+18. 33log(Fc)-40. 94 {Fc=150-1000MHz}

K1=46. 3+(33. 9+1. 56)lg(Fc)-0. 8-4. 78[log(Fc)]2+18. 33log(Fc)-40. 94 {Fc=1500-2000MHz}

K 2---Distance fading constant;

K 3、 K 4---The revision coefficient of mobile station antenna height;

K 5、 K 6---The revision coefficient of base station height;

K 7---The revision coefficient of diffraction;

K clutter ---The revision coefficient of ground object in the prediction is: the field densityof the prediction point is revised based upon the clutter type of that point, and hasnothing to do with the clutter type in the propagation path. And all the losses in thepropagation path lie in the medium value loss;

d ---Distance between the base station and the mobile station, unit: km;

hm、heff ---The available height of mobile station and base station antenna, unit: m.

As to the radio propagation in different areas and cities, K value will have differentvalue owing to different landform and relief as well as different city environment. Kvalue and some clutter fading values used in radio propagation analysis in medium-

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size cities are shown in Table 3-2.

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Table3-2 K parameter value

K parameter name Parameter value

K 1

150/900MHz Urban, 160/1800MHzUrban

146/900MHz Large city, 163/1800MHzLarge city

K 2 44. 90

K 3

-2. 54/900MHz Urban,-2. 88/1800MHzUrban

0/900MHz Large city,-2. 88/1800MHzLarge city

K 4 0. 00K 5 -13. 82K 6 -6. 55K 7 -0. 8

Clutter fading valueInland Water -3. 00

Wetland -3. 00

Open Areas -2. 00Rangeland -1. 00Forest 13. 00

Industrial & Commercial Areas 5. 00Village -2. 90

Parallel_Low_Buildings -2. 50Suburban -2. 50

Urban 0Dense urban 5High Building 16

Medium value of propagation loss can be calculated according to these K values.However, thanks to the complicated environment, some revision is required. Buildingloss is to be considered when the cellular mobile communication is used indoors.

Building loss refers to the functions of wall structure (steel, glass and bricks, etc),building height, building direction, percentage coverage of the window area. Owing tocomplicated variables, building loss can be only calculated based upon thesurrounding environment. Below are some conclusions we draw:

The average penetration loss in urban buildings is more than those insuburb areas and remote areas.

Loss in the area with window zone is generally less than that withoutwindow zone.

Loss in the open area within buildings is less than that in the wall areawith corridors.

Fading in street wall with aluminum support frame is more than thatwithout aluminum support frame.

Loss in the building with isolation only added to the ceiling is less thanthat in the building with isolation both added to the ceiling and insidewalls.

There are two frequencies in GSM mobile communication system, i.e.e. 900MHz and 1800MHz. Different frequency results in differentpropagation characteristics. The longer the wavelength is, the less thediffraction loss is. While the relation between wavelength andpenetration loss is worth further study, or is uncertain. In addition,indoor radio components are the superimposition of penetration

components and diffraction components, and the diffraction accountsfor the majority. Therefore, generally speaking, 1800MHz level

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difference between indoors and outdoors is larger than 900MHz.However, the problems of complicated propagation environment andthe direction of incident wave make it impossible to quantize indoor-and-outdoor level difference. The best method is to test indoor-and-outdoor level difference in a specific environment, so as to optimize the

plan.

The average floor penetration loss refers to the function of the floor height. Accordingto record data, the slope of loss line is -1. 9dB/story. The average penetration loss inthe first floor is about 18dB in urban area, and 13dB in suburb area. Themeasurement of specific floors shows that loss characteristic inside buildings can betreated as a waveguide with fading. For example, when radio propagates along thecorridor direction, which is vertical to the outdoor window, the loss can reach to 0.4dB/m.

Tunnel propagation loss shall be considered when calculating radio propagation intunnels. At this moment, simply regard the tunnel as a wave-guide with loss. Theexperiment result shows that propagation loss in a specific distance reduces as thefrequency increases. When the working frequency band is below 2GHz, the relationbetween the loss curve and working frequency show exponential fading. As to GSMfrequency, it can be approximately considered that loss and distance appear theinverse exponential change of fourth power, i.e. e. if the distance between the twoantennas doubles, then the loss increases by 12dB.

Besides, the influence of leaves on propagation in UHF frequency shall beconsidered. Studies show that, in general, the signal loss in summer is about 10 dBmore than that in winter, vertically polarized signal loss is more than the horizontally-polarized one, for leaves flourish in summer.

Radio battle-sight distance might be very far in wide coverage, such as desert or sea.The earth curvature shall be considered under such conditions. Assume that the earth

radium is (unit: m, the equator radium is 6378000m), hm、heff is the height of mobile

station antenna and the base station antenna respectively, the unit is m,hb is theheight of base station antenna, the unit is also m, then the battle-sight range of radio

wave is d (unit: m).

d = 2 heff + 2 hm

Per contra, if the expectation coverage range is known (when path loss is not themajor factor), the base station height can be calculated.

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1.4 Correction for propagation model

1.4.1 CW Basics

Correction for propagation model is required to obtain the radio propagation model inaccordance with local actual environment and to improve the accuracy of coveragepredication so as to lay a good foundation for network planning. CW test, say,continuous wave test, is a necessary step for model correction by correcting dataobtained from CW test and digital map. The information of latitude and longitude of these test data and incoming level form the data source of model correction.

Random process theory is used to analyze mobile communication propagation, whichcan be expressed as follows:

r ( x) = m( x)r 0( x) (3-11)

In which, x is distance, r(x) is incoming signal; r 0(x) is Rayleigh fading; m(x) is local value, i. e.

the mixture of long-term fading and space propagation loss, which can be expressedas follows:

m( x) =1

2 L

x+ L

x− L

r ( y)dy (3-12)

In which, 2L is the average sample interval length, also called intrinsic length.

CW test is aimed to obtain the local average value of various locations in an area onwhole way, that is, the difference between r (x) and m(x) is as small as possible.

Therefore, The influence of Raleigh fading must be removed so as to obtain the localaverage value. When a group of signal data r (x) is averaged, if the intrinsic length 2Lis too short, then the influence of Raleigh fading still exists; if 2L is too long, then thenormal fading will be averaged. Therefore, in CW test, to determine 2L has greatinfluence on the degree of approximation between the tested data and the actual localaverage, as well as on the accuracy of the propagation model prediction correctedthrough CW test. Li Jianye, a famous communication expert, has proved that, in GSMsystem, the intrinsic length is 40 wave lengths; the difference between the tested dataand the actual local value is less than 1 dB by sampling 50 sampling points (the testequipment and the error of digital map are ignored).

1.4.2 CW Test Method

I. Select station for CW test

Before testing, test stations and quantity need to be determined. According toexperiences, at least 5 test stations in big cities with dense population; as to medium-size and small-size cities, 1 test station is enough, which is mainly depended on theantenna height of test base station and the effective radiant power (EIRP). Theprinciple of station selection is to cover ground objects as many as possible (theseground objects come from digital map)

In actual test, proper test stations can be selected according to the followingstandards:

(1) The antenna height is above 20 meters;

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(2) The antenna height is above 5 meters over the nearest obstacle.

5 m

Figure 3-6 Diagrammatic representation of station selection standard

The obstacle here refers to the highest building at the top of which the antenna islocated. The building as a station shall be higher than the average height of the

surrounding buildings.

II. CW test preparation

CW test first needs a test base station to transmit RF signal with or without FMmodulation, then make a drive test by using CW test equipment. The base stationincludes transmitting antenna, feeder cable, power amplifier and HF signal source.The test system includes test receiver, GPS receiver, distance measuring instrument,test software as well as portable PC. The sampling rate of the test receiver is as fastas possible.

After the equipment of base station is installed in the selected test station, using thepower meter to measure the forward power and reflection power of the antenna.

Calculate the EIPR of the test base station. The calculation formula is below:

EIRP = 10 lglg[ P _ forward (mW ) − P _ reflect (mW )]

+Tx _ Antenna _ Gain + Rx _ Antenna _ Gain

− Rx _ Feeder _ Loss (3-13)

In which, P_forward is forward transmitting power, P_reflected is reflection power,Tx_Antenna_Gain is the transmitting antenna gain of the test station (dBi),Rx_Antenna_Gain is the antenna gain of the test receiver (dBi), Rx_Feeder_Loss isthe feeder cable loss of the test receiver.

After normal installation and debugging of the base station equipment, record the

EIRP of the base station. Use GPS to measure the latitude and longitude of thestation; use triangulation method to measure the height of the building, and use angleinstrument to test the slope angle of the antenna. The antenna height is the height of the building plus antenna mast height and half the antenna height. Sweep frequencyby using portable test equipment to ensure the normal work of the test base stationequipment, without any interference signal in surroundings.

III. CW test

There are three sampling ways of the professional CW test equipment: sampling bytime, pulse and distance. General test equipment samples by time only. Test bydistance sampling can meet the Theorem of Lee’s requirement of sampling 36~50sampling points with 40 wave lengths. The measure accuracy is very high. Speed isnot strict in distance sampling, but there exists an upper speed limit. The upper speed

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limit (Vmax) has relation with the maximum sampling speed of CW equipment:

V maxmaxmax = 0.8/T sample (3-14)

During the test, test paths with various ground objects are selected as random drive

test. When the mobile station is within the distance of 3km away from the test basestation, the receiving signals are affected greatly by the building structure around thebase station, and the antenna height. The intensity difference between the signal levelparallel to the signal propagation direction and that vertical one is around 10dB.Therefore, when testing on the street within 3km in radium of the base station, it isbetter to sample the same amount of samples in longitudinal and lateral streets toremove their effects. Test paths should not be selected on highways and on the wideand flat streets, but on the narrow streets. Sample as much data as possible in eachtest base station. Generally, it is better to test in each station over 4 hours. Stoprecording when the car stops for redlight.

The landform and ground objects are fixed within a period of time, so that in adeterministic base station, the local average value is determined in a deterministiclocation. The local average value is the data tested through CW test expectation,which is also the closest value to propagation model predication value.

1.4.3 Correction for Propagation Model and Instance

Digital map is needed for model correction. The digital map used in mobilecommunication contains the geographical information such as relief height andground usage, which effect radio propagation in mobile communication. It is theimportant fundamental data for planning software in model correction, coverageprediction, interference analysis and frequency planning.

Propagation models developed for computer aided analysis are different, but basedon Okumura basic models, and provide modified parameters. Below is the specific

method of model correction based upon the above-mentioned ASSET planningsoftware. It needs to be pointed out that if the model parameters of the city similar tothe existing landform and ground objects, they can be directly applied to planningprediction. It is unnecessary to redo CW test and model correction, thus saving labor.

Parameters from K1 to K7 in ASSET model are determined by specific propagationenvironment, K(clutter) is the correction factor depended on different ground objects.Different ground objects determine different K (clutter), these K parameters are graduallyfitted from CW test data. When CW test data obtained, K parameters can be acquiredin two ways: K parameter testing method and the minimum variance method.

Among a great many of K parameters in the standard model, the degree of influenceof each K parameter is different. By analyzing the models, we know that K1 and

K(clutter) are constant, which has nothing to do with the propagation distance andantenna height; K3 and K4 are the height modifying factor of the mobile station. Themobile station has slight changes in height (about 1. 5m), so that K3 and K4 can beeventually classified as micro-adjustment in the final stage, while the adjustment of K2, K5 and K6 are determined by specific test data and test path.

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1.5 Doppler Effect and its Impact on Handover

In GSM system, the relation of frequency change caused by Doppler effect is giventhrough the following formula:

(1) the base station is the frequency source f , the frequency f ́ received by the mobile

phone is

f ́ =f(1±V/c) (3-15)

In the formula, v is the travel speed of MS, c is the radio signal propagation speed(3E8 m/sec)

Select “+” when MS moves towards the base station and select “-” when it is away

from the base station.

(2) MS is the frequency source f, and the frequency f ́ received by the base station is

f ́ =f/(1±U/c) (3-16)

In the formula, u is the travel speed of MS, c is the radio signal propagation speed(3E8 m/sec)

Select “-” when MS moves towards the base station and select “+” when it is awayfrom the base station.

Below are several special conditions discussed:

(1) MS moves towards BTS at the speed of v, as shown in Figure 3-7.

f 1

f 2

f 3

V ( k m / h )

Figure 3-7 MS moves towards BTS

The signal frequency of BTS is f1. Through FCH channel of BCCH channel, BTS cancontrol MS to synchronize the frequency with BTS. MS receives the signal frequencyf2 because of the Doppler Effect, and transmits f2 to the base station. f3 is thefrequency received by BTS because of the Doppler Effect. Below is the formulasbased on the above-mentioned:

f2=f1(1+v/c)

f3=f2/(1-v/c)

f3=f1(1+v/c)/(1-v/c)=f1(c+v)/(c-v)

The relative frequency change is (f3-f1)/f1=2v/(c-v) (3-17)

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(1) MS moves away from BTS at the speed of v, as shown in Figure 3-8.

f 1

f 2

f 3

V ( k m / h )

Figure 3-8 MS moves away from BTS

The signal frequency of BTS is f1. Through FCH channel of BCH channel, BTS can

control MS to synchronize the frequency with BTS. MS receives the signal frequencyf2 because of the Doppler Effect, and transmits f2 to the base station. Frequency f3received by BTS because of the Doppler Effect, below are the formulas based uponthe above-mentioned formula:

f2=f1(1-v/c)

f3=f2/(1+v/c)

f3=f1(1-v/c)/(1+v/c)=f1(c-v)/(c+v)

The relative frequency change is (f3-f1)/f1=-2v/(c+v) (3-18)

The travel speed of MS is slow as compared with signal propagation speed; thereforerelative frequency change is almost the same in these two conditions except for the

opposite direction. Frequency increases in the first condition, while decreases in thesecond one.

The relation between the relative frequency and MS speed can be illustrated in Figure3-9.

Figure 3-9 Graph of relation between the relative frequency and MS speed

The graph shows that when MS speed is 100km/h, the relative frequency change is 0.19ppm. As to 900M frequency, the deviation is 171Hz, while 342Hz as to 1800M.

(3) MS moves between the two base stations at the speed of v, as shown in Figure 3-

10. When handover is performed, the deviation is the superimposition of the above

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two conditions. MS obtains the monitoring information of the BCCH channel of theneighboring cells through BA table, controls MS to adjust its frequency and a certainnumber of kHz to monitor the neighboring cell level. Thus, it might appear Doppler frequency changes, which make MS unable to receive the signals of the neighboringcells correctly. Take the Figure 3-10 as an example, MS monitors BTS1 level, the

signal f2ˊ received by MS might appear between the two MS adjustment frequencies.So that MS cannot correctly monitor BTS1 signal level. On the other hand, RXlevinformation reported from SACCH shall be transmitted at least once every 30s. Suchlong time information report will also result in abnormally monitoring the neighboringcells level, which causes unsuccessful handover. The frequency change caused bythe Doppler Effect will effect the signal frequency f1(c+v)/(c-v) received by the basestation, which will receive data by f1 sampling clock. Receiving data error might beanother reason for effecting handover.

f 1

f 2

f 3

V ( k m / h )

f 1 '

f 2 '

f 3 '

B T S 1 B T SM S

Figure 3-10 MS moves between the two base stations

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1.6 Fresnel Zone

There are direct wave and reflected wave in the propagation path from the transmitter to the receiver, and the electric direction of the reflected wave is just opposite to theoriginal with the phase difference of 180 degree; if the antenna height is relatively lowand the distance is relatively far, the difference between the direct way path and thereflected wave path is small, then the reflected wave will cause destruction. In

addition, the path difference between the direct wave and the reflected wave is2h t hr d ,

the phase difference is =4h t hr d , h t ,hr refer to the height of the transmitter and

the receiver above the ground respectively, d is the horizontal distance from thetransmitter to the receiver, as shown in Figure 3-11.

Figure 3-11 Graphs of Direct incidence and reflection

Ignore part of the signals from the transmitting point to the receiver through groundwave propagation (signals in ultra-high frequency and very-high frequency band canbe ignored), then the square of the ratio of the total receiving field density an the freespace density (unit: V/m) is:

E rec E s

2 4 sinsinsin( 2

) = 4 sinsinsin22ht hr d (3-19)

The formula shows that n is a natural number, when is（2n－1）, it can

generate 6dB signal power gain; while when

is 2n

, the two signals can be offset.The change from this point is caused or caused together by the change of antennaheight and propagation distance.

The simulation result shows that when d is less than4h t hr ,

2 is more than

2 , then

the gain obtained swings as the mobile station moves towards the base station; when

d is more than4h t hr

,

2 is less than

2 , the gain won’t swing as the mobile station isaway from the base station.

In the actual propagation environment, the first Fresnel zone definition contains someellipsoids of reflection points, on these reflection points, the path difference between

the reflected wave and direct way is half a wavelength, say,2 less than

2 , as shown

in Figure 3-12. The first Fresnel zone is the main propagation zone, when obstacles

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don’t block the first Fresnel zone, the diffraction loss is least. As to a point in the path

with d in length, its radium of the first Fresnel zone (the distance to the transmitter is

d t , and d r is the distance to the receiver) is:

h0(m) = d t d r d = 548 d tkmd rkm

d km f MHz (3-20)

Figure 3-12 The radium of the first Fresnel zone

Take an example to illustrate that: in typical cities, a point in the path with thecoverage range is 2km; suppose that the distance from this point to the transmittingantenna is 100m, as to the frequency of 900MHz, this point’s first Fresnel zone is

h0 5m.

On the definition basis of the first Fresnel zone, define the n th Fresnel zone as thereflection-point set, in which its propagation is half wavelength more than the n-1 th;

the phrase difference between the two reflection paths is 180 degree. The radium of the nth Fresnel zone is:

hn(m) =

nd t d r d = 548

nd tkmd rkmd km f MHz (3-21)

If the direct path jumps over the wavy terrains and ground buildings, then thereflected wave will have positive effect on direct wave; otherwise it might become theobstructive multi-path interference. The obstructive effect grows as the frequencyincreases. Therefore, the height of antenna shall be built as high as possible abovethe ground. This conclusion will be applied to the below-mentioned antenna projectdesigning. As a matter of fact, according to experience, if 55% of the first Fresnelzone, used for stadia microwave link designing remain unobstructed, then the

conditions of other Fresnel zones won’t affect the diffraction loss.

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1.7 ASSET Software Introduction

Below is the brief introduction about the above-mentioned ASSET planning software.

ASSET software is the network planning software designed by Aircom company. Byusing ASSET software, we can configure the system hardware parameters, networkcapacity, frequency allocation and complete the network design (such as coverageprediction, traffic analysis, neighboring cell allocation, frequency plan, interferenceanalysis and microwave propagation and so on) and simulate the network operationeffect to guide the project construction.

Before using the ASSET software, we need to prepare and know the followinginformation:

Digitalized map with proper ratio of accuracy. The accuracy of the

digitalized map includes 20m, 50m, 100, 5m. 20m accuracy is appliedto urban area and suburb area, 50m or 100m accuracy can be appliedto rural area, while 5m accuracy is generally used for micro cellular planning;

Network and base station information mainly includes theconfiguration of MSC and BSC, latitude and longitude of the basestation, antenna type, feeder system parameters;

Design consideration includes the purpose of this planning, networkhierarchy, frequency range and frequency reuse mode, cell frequencyhierarchy, configuration of network functional parameters.

ASSET network planning is carried out on the basis of digital map. Digital map is a

map for record and storage in digital form; digital map is convenient to store, transmitand update, which can be transformed into paper map by processing in computer, or displayed on the computer screen by visual processing. Owing to different storagestructure, digital map can be divided into vector digital map and grid digital map (suchas scanning map). In order to cover prediction, we usually use vector digital map.Map data usually is composed of three data types: digital elevation model (DEM),digital object model (DOM) and linear vector model (LDM). Construction vector data isalso applied to micro cellular prediction. Digital ground elevation model and groundobject disaggregated model has related to prediction. Digital ground elevation modelis used to describe the basic relief of this area and directly participate in thecalculation of radio propagation model; ground object disaggregated data is used todescribe planar ground coverage, such as forest, lakes, open area, industrial area,downtown, high building area and so on, and used to calculate radio propagation path

loss; LDM is used to describe the relation between the plane distribution and thespace of linear ground objects, including highway, streets, rivers and so on. DEM dataand DOM data adopts the grid data format, each grid represents a sampling point;while LDM adopts vector data.

Before officially beginning planning, the following work needs to be done:

(1) Define the parameters related to propagation model and feeder system, and inputantenna database

(2) Define layer, determine the frequency reuse mode

(3) Add network element with multi methods (MSC, BSC, BTS and cell layer)

(4) Improve various parameters in the database

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Thus, we can make use of ASSET software to complete the entire planning process.

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22066202 Chapter3 Radio Propagation Theory

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Transcript of 22066202 Chapter3 Radio Propagation Theory