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Radio Wave Propagation

Chapter 4

OBJECTIVES:After this chapter the student will know:

The different types of fading

About system balance

About path loss.

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4 Radio Wave Propagation

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Table of Contents

Topic Page

INTRODUCTION..................................................................................35

SIGNAL TO NOISE..............................................................................36

INTERFERENCE..................................................................................37

QUALITY AND CAPACITY..................................................................39

PROPAGATION PREDICTIONS .........................................................40

FADING........................................................................................................................ 41

LOG-NORMAL FADING ......................................................................42

RAYLEIGH FADING ............................................................................43

MULTIPATH.........................................................................................44

KNIFE EDGE OBSTACLE...................................................................45

KNIFE EDGE DIFFRACTION ..............................................................47

PATHLOSS CALCULATIONS.............................................................49

FREE SPACE PATH LOSS:................................................................50

THE ERA PREDICTION MODEL................................................................................. 50

SYSTEM BALANCE ............................................................................51

INTRODUCTION ......................................................................................................... 51

SYSTEM BALANCE ASSUMPTIONS ......................................................................... 51

PARAMETERS ............................................................................................................ 52

FREQUENCY REUSE..........................................................................54

INTERLEAVED CHANNELS ...............................................................55

HANDOVER.........................................................................................57

INTRA-CELL HANDOVER........................................................................................... 58

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HANDOVER TYPES.................................................................................................... 58

DECIBEL LOSS & GAIN.............................................................................................. 58

POWER RATIO............................................................................................................ 60

VOLTAGE RATIO ........................................................................................................ 61


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INTRODUCTION

Radio signals are affected in several ways. We will describe heresome of the mechanisms that affect radio signals.

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SIGNAL TO NOISE

One of the quality criteria in CMS 45/89 is the signal to noiseratio (S/N).

The S/N quality parameter is measured with help of the -signal.

so = ps/pn or Ps =Ps - Pn (dB)

ps = useful signal power (Ps = 10 log ps)

pn = Power from other signals such as noise (Pn = 10 log pn)

A 12 dB S/N is an acceptable signal to noise ratio in a mobiletelephone system. For a fixed telephone line, the S/N is 30 dB.

Figure 4- 1 Signal to noise ratio.


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INTERFERENCE

When two or more signals with the same frequencysimultaneously reach a radio receiver, a mixing of the signals,often termed interference, occurs. If the signals areapproximately equal in power, the information they carry cannotbe separated and the result will be an increased noise level.

If interference occurs on a channel, the power of the utilitysignal must be higher to drown the noise level caused by theinterference.

Figure 4- 2 Interference.

Although the concept interference is general and used forcoinciding frequencies in general, in mobile telepony,interference generally means direct signal frequenciesintentionally generated in transmitters (the operators own aswell as others transmitters). Interference between frequencieswithin the frequency plan for a cellular system is usually termedco-channel interference.

Ultimately the coverage range from the base station will belimited by the co-channel interference rather then by noise.Thus, we say that a small-cell system is interference-limited asopposed to a conventional, noise-limited system.

When designing a cellular system this problem can be partlyalleviated by controlling the channel reuse distance. The larger

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this distance, the less the interference. Obviously, then, we areconcerned with two main factors that affect speech quality. Oneis the received level (C) of the desired carrier, which should behigher than the total co-channel interference level (I). This is theratio named C/I. A suitable value of this ratio is determined bysubjective evaluation (by a large group of listeners) as to what isacceptable speech quality". A typical minimum value used foranalogue mobile telephone systems is C/I =18 dB.


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QUALITY AND CAPACITY

Figure 4- 3 Quality and capacity.

In an analogue mobile telephone system, an acceptable value forC/I is approximately 18 dB. In a digital system, there are othermechanisms that allow us to have lower C/I values. This resultsin higher capacity for a digital system.

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PROPAGATION PREDICTIONS

Predicting transmission in a mobile telephony system involvesserious difficulties, mainly because of the movement of themobile station and its low antenna position. These facts result inan ever-changing, transmission path terrain profile. In mostpositions, the mobile station will receive several reflectedsignals.

The formulas below represent a couple of analytical methods tocalculate radio wave propagation loss.

Basic transmission loss in free space is a simple model thatassumes that there are no reflections and the radio path is equalto the line of sight. The formula below is valid for isotropicantennas only.

L bf = 20 log (4 d/)

d

Figure 4- 4 Radio wave in free space.

One approach to calculating the transmission loss is to use theformula for propagation over flat conductive earth, taking onereflection into consideration. As in the previous case, thisformula assumes isotropic antennas.


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h2

h1

Base Mobiled

L =20 log (d/h1 h2)

Figure 4- 5 Radio wave propagation over flat conductive earth.

FADING

In mobile telephony we very rarely have a direct wave thereare often buildings, hills or other obstacles between the mobilestation and the base station.

Fading is a phenomenon that occurs in two forms: log-normalfading or shadowing, and multipath fading (also called Rayleighfading).

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LOG-NORMAL FADING

Figure 4- 6 Log-normal fading.

The problem of shadowing is most severe in heavily built-upurban centres. Early mobile studies found shadows as deep as 20dB over a very short distance, often literally from one street tothe next, depending upon the orientation to the transmitter andlocal building patterns.

If you measure the incoming signal strength in a mobile stationit will vary around some mean value as you move. Thevariations round this mean value are, measured in dB, normallydistributed. This gives us the name ORJQRUPDOIDGLQJ.

7\SHRI)DGLQJ &DXVH 3HDNWR3HDN'LVWDQFH 'LVWULEXWLRQShort term Local scattering /2 Rayleigh (Voltage)Long-term Local obstruction 5 -50 m Log-normal

(Power)Long-term Global obstruction > 50 m Log-normal

(Power)


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RAYLEIGH FADING

Figure 4- 7 Rayleigh fading.

There are both pros and cons with this multipath propagation.On the one hand, reflection and multipath propagation allowradio waves to bend around corners", to reach behind hills andbuildings and into garages and tunnels. On the other hand,multipath propagation creates some of the most difficultproblems associated with the mobile environment, i.e., 5D\OHLJKIDGLQJ.

For a constant signal level we have:

Signal to Noise ( S/N)min = 7-8 dB

Fading margin = 10 dB

At Raylegh fading (S/N)min = 17-18 dB

Because the propagation pathes have different lengths, there willbe a phase difference between the signals, and when the mobilestation moves their phase angle will vary as seen from thereceiving antenna. At one point in time, the signals will coincideand in the next moment they will counteract each other,resulting in the signal being 30-40 dB lower than the meanvalue.

There is no way to predict this signal. However the fading iswithin a statistical distribution know as Rayleigh distribution insuch a way that the distance between two dips is approximatelyone half wave length.

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MULTIPATH

The radio signal will be reflected differently by differentsurfaces. This reflection will cause the radio signal to travel ashorter or longer path. This results in large variations at thereceiving end.

'LVWDQFH

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/RJQRUPDOIDGLQJ

Medianvalue

Figure 4- 8 Multipath Fading.

The radio wave may be reflected by a hill, a building, a truck, anaeroplane, or a discontinuity in the atmosphere. In some cases,the reflected signal is significantly attenuated, while in others,almost all the radio energy is reflected and very little absorbed.For fast moving mobile stations, this results in rapid fadingwhich is heard as a flutter". For stationary or slowly movingmobile stations, the effect of multipath propagation createsholes in the covered area where the quality of the speech isvery poor.


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KNIFE EDGE OBSTACLE

Another phenomenon is the influence of a knife edge obstacle,such as a tall building or a mountain. According to radio wavepropagation theory, an obstacle will attenuate the signaldifferently depending on the extent to which the line of sight isblocked. The formula and the diagrams below could be used tocalculate the additional attenuation caused by a knife edgeobstacle.

G G

K7; 5;

.QLIHHGJHGLIIUDFWLRQ

Figure 4- 9 Knife edge diffraction.

In order to read the attenuation from the diagram, the parameterV must first be calculated.

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G%

G%

G%

G%

9

Figure 4- 10


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KNIFE EDGE DIFFRACTION

V= h 2(D1 + D2)

D1 D2

The formula above can also be used in a situation where theradio wave is shadowed by a number of knife edge obstacles,each of which will contribute to the total attenuation.

The formulas previously described are based on simple modelsof the radio path. Though fairly complicated, none of theformulas takes into account the terrain type.

Okumora reports that there are 5 different terrain classesdefined:

Quasi smooth terrain

Rolling hilly terrain

Isolated mountain ridge

General slope of terrain

Mixed land-sea path.

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Knife-edge diffraction lossesKnife-edge diffraction losses

Rough estimates of knife-edge difraction losses in dB

0 0 6 12 16 20 dB

First Fresnell zone

Transmitter Receiver

Figure 4- 11


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PATHLOSS CALCULATIONS

There are several formulas for calculating Path Loss. Today,computers have taken over the most tedious work. Here are acouple of examples.

The basic formula presented by Harta is the following:

L(urban)= 69.55 + 25.26 log f - 13.82 log h1 + (44.9 - 6.55 logh1)log d

where:

f=carrier frequency band in MHz

h1=the base station antenna height in meters

d=distance in km from the base station

For other areas than urban areas, the formula is corrected asfollows:

L (suburban) = L( urban) - [ 2 (log( f/28))2 + 5.4]

L (open) = L (urban) - [4.78 (logf)2 - 18.33 f + 40.94]

The prediction model used by ERA is a modified Hata formula.

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FREE SPACE PATH LOSS:

/ ORJGNP

ORJIUT0+]

G%

We have to adjust this formula because the terrain and otherelements attenuate the signal.

THE ERA PREDICTION MODEL

Ericsson formula:

/S $

$

ORJI$

ORJKEDKP$ORJG$ORJGORJK//8/'

Lp = Path loss

I= frequency

KE

= Effective base station antenna height

KP

= Vehicular station antenna height

G = distance (km)

/'= Diffraction loss (Loss more than -6 dB)

//8= Land usage attention (set for each project)

a = ?

$

= 30 - 35

$

= 10-12

$

= 30 - 35

$

is set to a lower value if diffraction loss is more than -6 dB

Ericsson uses up to nine different land usage codes.


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SYSTEM BALANCE

INTRODUCTION

Coverage in a two-way radio communication system is decidedby the weakest transmission direction. Expressions such as up-link and down-link are used to denote the two communicationdirections:

8SOLQN From a mobile subscriber to the base station.

'RZQOLQNFrom the base station to a mobile subscriber.

The two transmission directions should be balanced in order toavoid interference, reduced system access or extra costs. Asystem balance calculation must be done before any coveragecalculation can start.

SYSTEM BALANCE ASSUMPTIONS

It is necessary to make some assumptions and simplifications inthe balance calculation in this example:

Sensitivity of the mobiles and base stations, respectively, isassumed to be equal.

Sensitivity degradation due to man-made noise does notaffect the balance.

The base station multicoupler system is assumed to betransparent.

Base station diversity is used.

Same gain for base station TX and RX antennas.

Diversity variations.

Diversity is used to combat the fast signal level variationscaused by multipath radio propagation.

The diversity improvement varies with different radioenvironments. In open flat terrain, for example, the potentialfor diversity improvement is relatively small.

Dense city areas on the other hand, with high buildingsblocking direct waves, and which produce lots of reflectedwave components, are environments where the diversityimprovements are great.

The following values can be used as guidelines for diversitygain:

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$QDORJXHV\VWHPVRSHQDUHDVG%

$QDORJXHV\VWHPVFLW\DUHDVG%

'LJLWDOV\VWHPVJHQHUDOG%

Figure 4- 12 Mobile to base station balance.

PARAMETERS

In order to achieve balance between two transmitting directions(from a base station to a mobile and the reverse), a number ofradio parameters must be clarified.

P bt = Base transmitter output power (dBm)

(Output from the power amplifier)

P br= Base receiver power (dBm)

(Signal level in the receiver)

P mt = Mobile transmitter power (dBm)

(chassis output power)

P mr = Mobile receiver power (dBm)

(Signal in the receiver)


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G b = Base antenna gain (dBi)

G m = Mobile antenna gain (dBi)

G d = Diversity gain (dB)

L p= Propagation loss (dB)

L c = Combinder loss (dB)

L bf = Base feeder loss (dB)

L mf = Mobile feeder loss (dB)

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The signal level into the mobile station is affected in thefollowing way:

P mr=P bt - L c-L bf + G b-L p+G m-L mf

Analogous for the base station is:

P br=P mt-L mf+G m-L p+G d+G b-L bf

ThenP bt-L c-L bf+G b-L p+G m-L mf=P tm-L mf+G m-L p +G b-L bf+G d

Eliminate L bf,G b,L p,G m AND L mf

Then: P bt-Lc=P mt+G d

The communication balance formula can thus be written:

P bt=P mt+G d+L c

The base station transmit power (Pbt) needs to be higher thanthe mobiles transmitter power (Pmt) with a value correspondingto the sum of the diversity gain (improvement, Gd) and the basestation combiner loss (Lc)!

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FREQUENCY REUSE

Frequency reuse refers to the use of radio channels on the samecarrier frequency. The use of the same frequency is possible,without any co-channel interference, if they are separated bysufficient distance.

Through frequency reuse, a cellular mobile telephone systemcan handle a number of simultaneous calls greatly exceeding thetotal number of allocated channel frequencies.

Figure 4- 13 Frequency reuse pattern.


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INTERLEAVED CHANNELS

The problem with mobile telephone systems is that they usecarriers in a certain limited frequency band. This limits themaximum numbers of users of the system. One way to increasethe capacity is to use interleaved channels.

Interleaved Channels in CMS 45i

The capacity in CMS 45 is limited to 180 channels. To improvethe total capacity, interleaved channels can be used. This meansthat the capacity in the system can be much improved. However,channel assignment becomes more complicated. The frequencydistance between a main channel and its interleaved channel is12.5 kHz. This means that an interleaved channel cannot beassigned to cells that use the adjacent main frequency.Interleaved channels are only available in CMS 45i. The use ofinterleaved channels increases the number of channels available,from 180 to 359 channels.

Figure 4- 14

Interleaved Channels in CMS 89

Interleaved channels in CMS 89 require a frequency plan thatoptimises the reuse of frequencies. It must meet the followingcriteria:

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Each channel group should be repeated only once within acluster of 19. Channel groups with 12.5 kHz distance shouldnot be used on cells bordering each other.

Two channel groups should be available on the same cell.

Figure 4- 15 Use of Interleaved Channels.

Regularity must be maintained so that the same conditions existin areas located on both sides of the border of two clusters.

Figure 4- 16 below shows channel assignment in a cluster usingboth ordinary and interleaved channels. Since each cell uses twochannel groups, the capacity is increased by more than 100percent compared to a system without interleaved channels.


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HANDOVER

Figure 4- 16

Switching a call in progress or handover from one base stationto another is performed to allow the call in progress to continuewhile the mobile subscriber is moving out of the coverage areaof the current base station.

The radio connection quality is measured and evaluatedcontinuously during the call and if the quality becomes too poor,an alarm is sent from the base station to the MTX. The exchangeinvestigates if a better base station can be found, by ordering asignal strength measurement by the surrounding base stations. Ifa better base station with an available radio channel is found , ahandover is initiated.

If not, the call is continued on the current channel. New signalstrength measurements and handover attempts are madeperiodically until the handover attempt is successful or the callmust be disconnected due to the fact that the subscriber hasmoved too far away from the serving base station. Normally 20-second periods are used between the attempts.

The handover includes seizure of the most suitable channel inthe new base station, starting of the transmission qualitysupervision of the new channel and switching of the path to thenew channel. The MTX then transmits an order to the mobilestation to change the frequency to the selected new traffic

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channel in the new base station. The switch is made in the MTXat the same time as the mobile station changes its frequency.After a successful handover, the old channel is released.

INTRA-CELL HANDOVER

This function is used in order to hand over mobiles from adisturbed traffic channel to another traffic channel in the samebase station, thus improving speech quality.

HANDOVER TYPES

1etwork &ontrolled +andRver NCHO Used in 107 TACSAMPS

0obile $ssisted +andRver MAHO Used in GSM ADC JDC

0obile &ontrolled +andRver MCHO Used in DECT CT3

DECIBEL LOSS & GAIN

On its way from one subscriber to another, speech passes manydifferent devices such as the subscribers telephone set, thelines, the switch. Passing some of these devices results in loss"or attenuation, others in gain or amplification. To expressthe value of loss or gain in a device, the relationship between theinput and the output signals could be used. However, it is moreconvenient to use the logarithm of the input/output signal ratio.

Definition:

LossA = 10 log(P in/ P out) (dB) P in > P out (1)

GainG =10 log(P out/ P in) (dB) P in < P out (2)

The decibel (dB) is a sub-unit of the bell (B) unit.

Note that in the definition above, it is the ratio between inputand output power that is used. To find the correspondingrelationship between voltages, the following is used:

P =U2/R (3)

This will give:

P out = ( Uout)2/R


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P in = ( U in)2 /R

Inserted in (1) the following is obtained:

A = 10 log (U in/U out)2 (dB)

or

A = 20 log(U in/U out)2 (dB)

Note: For the above to be true, the resistances R must be equal.

([DPSOH

P1 = 15 W - 2.3 dB P2 ?

P2 =P1*10 -2.3/10

= 15*10 -0.23

= 8.8w

([DPSOH

P1 = 2w a= ? dB P2 = 17 mW

dB =10 log P2/P1 = 10 log 17 * 10 10 -3

/ 2 00 log 8,5* 10-3

= - 20,7 i.e dB

Definition => 0 dBm is equivalent to 1mW RMS.

P (W)

(P) dBm = 10 log

1mW

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POWER RATIO

3RZHU5DWLR

1/1000 -30 dB

1/100 -20 dB

1/10 -10 dB

EQUAL 0 dB

* 10 +10 dB

* 100 +20 dB

* 1000 +30 dB

1/8 -9 dB

1/4 -6 dB

1/2 -3 dB

* 2 +3 dB

* 4 +6 dB

* 8 +9 dB


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VOLTAGE RATIO

9ROWDJH5DWLR

1/100 -40 dB

1/10 -20 dB

Equal 0 dB

* 10 +20 dB

* 100 +40 dB

1/4 12 dB

1/2 -6 dB

* 2 +6 dB

* 4 +12 dB

Examples

Receiver: -113 dBm =0,5 V

Transmitter: +37 dBm =5 W

+40 dBm =10W

+43 dBm =20 W

Different reference levels can be used:

3RZHUOHYHO /S ORJ33UHI

3RZHU: P ref = : Lp = Log 10 log (P/1)G%:

P ref = P: Lp = 10 Log (P/10-3)G%P

9ROWDJHOHYHO /X ORJ88UHI

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9ROWDJH U ref = P9 Lu = 20 log (U/0.775)G%8

(For dBU: 775 mV in 600 gives 1 mW)

U ref = 9 Lu = 20 log (U/10-6) G%9

([DPSOH

P out = 40 dBm

P out = 10(40/10) x 10 -3 = 10 W

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