Polarization, Antenna Gain and Diversity considerations

Chapter 6Polarization, Antenna Gain

and DiversityConsiderations

6- i

Table of Contents

6 Polarization, Antenna Gain and Diversity Considerations___________________ 6-1

6.1 Background ________________________________ _________________________ 6-1

6.2 Depolarization Effects ________________________________ _________________ 6-1

6.3 Distributions from Low - and High -Gain Receiving Antennas ________________ 6-2

6.4 Fade Reduction Due to Lane Diversity ________________________________ ___ 6-4

6.5 Antenna Sep aration Diversity Gain ________________________________ ______ 6-76.5.1 Joint Probabilities ________________________________ _______________________ 6-86.5.2 Diversity Improvement Factor, DIF ________________________________ _________ 6-86.5.3 Diversity Gain ________________________________ _________________________ 6-106.5.4 Space Diversity for Expressway and Trunk Road Driving in Japan ________________ 6-11

6.6 Satellite Diversity ________________________________ ____________________ 6-126.6.1 Background ________________________________ ___________________________ 6-126.6.2 Cumulative Distributions ________________________________ _________________ 6-126.6.3 Satellite Diversity Gain ________________________________ __________________ 6-136.6.4 Satellite Diversity Measurements at S -Band Employing TDRSS __________________ 6-14

6.7 Conclusions and Recommendations ________________________________ _____ 6-16

6.8 References ________________________________ _________________________ 6-16

Table of Figures

Figure 6-1: Cross polarization isolation ( CPI ) as a function of co-polarization fade at equi-probability levels. .........................................................................................................................6-2

Figure 6-2: Fades measured by high - and low -gain systems at equi-probability levels. ...........................6-4

Figure 6-3: Mobile satellite scenario showing larger intersecting path length through tree canopy forinner lane driving relative to the outer lane case. ...........................................................................6-5

Figure 6-4: Fade reduction due to switching lanes at 870 MHz versus equi-probability attenuation atthe indicated path elevation angles. ...............................................................................................6-6

Figure 6-5: Fade reduction due to switching lanes at 1.5 GHz versus equi-probability attenuation(inner lane) at the indicated path elevation angles. .........................................................................6-6

Figure 6-6: Single and joint probability fade distributions for mobile communications operating in aspace diversity mode with antennas separated by the distance, d....................................................6-9

Figure 6-7: Diversity Improvement Factor ( DIF ) as a function of fade depth for a family of antennaseparation distances. .....................................................................................................................6-9

Figure 6-8: Diversity gain versus antenna separation distance for a family of single terminal fadelevels. .........................................................................................................................................6-1 0

Figure 6-9: Cumulative fade distributions at L Band ( f 1.6 GHz) for the simulated Globalstarconstellation with combining diversity for Tokyo, Japan [Vogel, 1997, Akturan and Vogel,1997]. ........................................................................................................................................6-13

6- ii

Figure 6-10: Path diversity gains at f 1.6 GHz derived from distributions for the simulatedGlobalstar constellation with combining diversity and handoff for Tokyo, Japan [Vogel, 1997;Akturan and Vogel, 1997]. ..........................................................................................................6-14

Figure 6-11: Single and joint probabilities for clear, shadowed and blocked scenarios derivedemploying S -Band (f = 2 GHz) TDRSS measurements [Vogel, 1997]. .........................................6-15

Table of Tables

Table 6-1: Summary of pertinent antenna characteristics [Vogel et al., 1992]. ........................................6-3

Table 6-2: Coefficients of the fade reduction formulation at f = 870 MHz. .............................................6-7

Table 6-3: Coefficients of the fade reduction formulation at f = 1.5 GHz. ..............................................6-7

Table 6-4: Single terminal fade distribution and diversity gain values for Japan roads, Ryuko andSaruwatari [1991]. ......................................................................................................................6-11

Chapter 6Polarization , Antenna Gain and Diversity

Considerations

6.1 BackgroundThis chapter reviews a number of L -Band and UHF propagation effects associated withland-mobile propagation scenarios. These include fading effects related to:(1) depolarization, (2) antenna gain , (3) lane changing, (4) antenna space diversity , and(5) satellite diversity . A number of the effects examined were derived frommeasurements by Vogel et al. [1992], Akturan and Vogel [1997], Vogel [1997],Goldhirsh and Vogel [1987; 1989], and Ryuko and Saruwatari [1991].

6.2 Depolarization Effect sBy making repeated measurements with co- and cross-polarization at 1.5 GHz forselected runs (elevation = 51) during an Australian campaign, equi-probability cross-polar isolation levels, CPI were determined by Vogel et al. [1992]. The crosspolarization isolation is defined as

)()(

)(PCRPSPCOPS

PCPI = , (6 -1 )

where COPS(P) and CRPS(P) represent the co-polarization and cross-polarization signallevels at the equi-probability value P . The CPI (in dB) is plotted in Figure 6-1 and wasfound to follow the linear relation

94.18605.1 +-= ACPI , (6 -2 )

where A is the co-polarized fade (in dB) with a range between 0 and 11.8 dB.

Propagation Effects for Vehicular and Personal Mobile Satellite Systems6-2

The RMS deviation between the best-fit linear relation (6 -2 ) and the data points for thecorresponding runs was 0.4 dB. We note from the plot in Figure 6-1 that the isolationseverely degrades as a function of fade level. For example, an approximate 11 dBisolation is observed at a 5 dB fade. This result suggests that the simultaneousemployment of co- and cross-polarized transmissions in a frequency re-use system isimplausible because of the poor isolation due to multipath scattering into the cross-polarized channel. Although the instantaneous isolation is poor, polarization diversitymay nevertheless be helpful in reducing the statistical interference between two satellitesystems that manage to share the same frequency band by employing code-divisionmultiple access (CDMA). In that case, one systems signals contribute to the othersnoise. Cross-polarizing the alternate system would tend to reduce the noise at the victimsatellite reverse-link receiver while the alternate systems mobile earth terminals are in aclear line-of-sight condition. Estimating the net benefits of such a scheme is notstraightforward, however, because the impact of power control has to be factored in.

0 1 2 3 4 5 6 7 8 9 10 11 12Fade Level (dB)

0

2

4

6

8

10

12

14

16

18

20

Cro

ss P

olar

izat

ion

Isol

atio

n, C

PI (d

B)

Figure 6 -1 : Cross polarization isolation ( CPI ) as a function of co-polarization fade atequi-probability levels.

6.3 Distributions from Low - and High -Gain Receiving AntennasDuring the Australian campaign by Vogel et al. [1992], a number of repeated runs wereexecuted in which high - and low -gain antennas were employed. The characteristics ofthese antennas are given in Table 6-1 . Figure 6-2 shows a plot of the high gain receiver

Polarization, Antenna Gain and Diversity Considerations 6-3

fade versus the low gain fade over the low gain fade interval of 0 to 15 dB. The datapoints were found to follow the linear relation

51.0)(133.1)( += LGAHGA , (6 -3 )

where A(HG) and A(LG) represent the high and low gain fades (in dB), respectively.Agreement between relation (6 -3 ) and the data points for A(HG) is within 0.2 dB RMS.

Table 6 -1 : Summary of pertinent antenna characteristics [Vogel et al., 1992].

Characteristic Low Gain High Gain

Type Crossed Drooping Dipoles Helix

Gain (dB) 4 14

Nominal Pattern (El.) 15-70 45 (Principal Planes)

Nominal Pattern (Az.) omni-directional 45

Polarization LHCP or RHCP RHCP or LHCP

The high -gain antenna system consistently experienced slightly more fading than thelow -gain system. For example, at 3 and 14.5 dB (of low -gain fades), the high gain fadeswere 4 and 17 dB, respectively, which represents 33% and 17% increases. This slightincrease in attenuation for the high -gain case occurs because less average power isreceived via multipath from surrounding obstacles as the associated antenna beam isnarrower. In contrast, the azimuthally omni-directional low gain antenna receives morescattered multipath contributions resulting in an enhanced average received power. Itshould be emphasized that negligible ground specular backscatter was received by eitherantenna because of the gain filtering characteristics at low elevation angle s. The slightincrease of signal for the lower gain azimuthally omni-directional antenna came fromdiffuse scatter from surrounding tree canopies . It is important to note that because thehigh -gain antenna has 10 dB more gain associated with it, the net power received by it isstill significantly higher than that received for the low -gain case. Even at the 15 dB fadelevel (low -gain re ceiver system), the net received power for the high gain mode is largerby 7.5 dB.

Mayer [1996] examined the effects of antenna gain s for low elevation anglemeasurements (8) at 20 GHz in Alaska through measurement and analysis oftransmissions from the Advanced Communications Technology Satellite (ACTS ). Hecompared clear line-of-sight multipath effects for aperture antennas with gains ofapproximately 16 dB, 22 dB, and 28 dB. Because of the low elevation angle, groundmultipath is more likely for the lower gain antennas. The 28 dB gain antenna clearlyshowed the smallest clear line-of-sight multipath fading, giving magnitudes between 1 to2 dB. The smaller fading is due to reduced specular scattering from the road. Thiscompares to multipath fading of approximately 10 dB for the lowest gain antenna.However, tracking of the satellite with the higher gain antenna was found to be moredifficult. Tracking errors for the higher gain antenna was found to result in substantialfading of the satellite signals vis--vis the lower gain antenna.


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Low-Gain Antenna Fade (dB)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Hig

h-G

ain

Ant

enna

Fad

e (d

B)

Measured

Equal Values

Figure 6 -2 : Fades measured by high - and low -gain systems at equi-probability levels.

6.4 Fade Reduction Due to Lane DiversityWe examine the extent by which fades are reduced (or increased) when switching lanesfor LMSS configurations. Figure 6-3 shows vehicles on the inner and outer lanes,respectively, where the satellite is to the side and the propagation path passes through thetree canopy on the side of the road. We note that the path length through the canopy isgreater when the vehicle is closest to the tree line (inner lane). Switching lanes from theinner to the outer lanes should therefore produce a fade reduction . The authors measuredthis effect at UHF (870 MHz) [Goldhirsh and Vogel , 1987], and L -Band (1.5 GHz)[Goldhirsh and Vogel , 1989]. Repeated measurements were executed employing ahelicopter as the transmitter platform, and corresponding cumulative fade distributionswere derived for inner and outer lane scenarios at fixed path elevation angles of 30, 45,and 60. To characterize the increase in signal power by switching from the inner to theouter lanes, a quantity known as the fade reduction, FR was defined. This quantity isobtained is obtained by taking the difference between equi-probability fade values fromdistributions pertaining to inner and outer lane driving.


Figure 6 -3 : Mobile satellite scenario showing larger intersecting path length through treecanopy for inner lane driving relative to the outer lane case.

The fade reduction s at UHF and L-Band are plotted in Figure 6-4 and Figure 6-5respectively, for the indicated elevation angles as a function of the maximum fade asderived for inner lane driving. These curves fit the third order polynomial expressed as

FR a a A a A a Ao= + + +1 22

33 , (6 -4 )

where FR (in dB) represents the fade reduction obtained by switching from the inner tothe outer lanes ( Figure 6-3 ) and A represents the maximum fade (in dB) derived for theinner lane driving scenario. The coefficients in (6 -4 ) are tabulated in Table 6-2 and Table6-3 for the UHF and L-Band frequencies, respectively. The best fit polynomials agreewith the FR values derived from the measured distributions to within 0.1 dB RMS.


0 2 4 6 8 10 12 14 16 18 20Maximum Fade (dB)

0

1

2

3

4

5

6

Fade

Red

uctio

n (d

B)

60

45

30

Figure 6 -4 : Fade reduction due to switching lanes at 870 MHz versus equi-probabilityattenuation at the indicated path elevation angles.

2 4 6 8 10 12 14 16 18 20 22 24 26Maximum Fade (dB)

0

1

2

3

4

5

6

7

8

9

10

Fade

Red

uctio

n (d

B)

60

45

30

Figure 6 -5 : Fade reduction due to switching lanes at 1.5 GHz versus equi-probabilityattenuation (inner lane) at the indicated path elevation angles.


Table 6 -2 : Coefficients of the fade reduction formulation at f = 870 MHz.

El. Angle () a 0 a 1 a 2 a 3 dB Range

30 -5.020 x 10 -3 0.3354 -2.439 x 10 -2 7.764 x 10 -4 2-20

45 -0.8193 0.8430 -5.758 x 10 -2 1.222 x 10 -3 2-12

60 -0.2305 0.2288 6.773 x 10 -2 -3.608 x 10 -3 2-11

Table 6 -3 : Coefficients of the fade reduction formulation at f = 1.5 GHz.

El. Angle () a 0 a 1 a 2 a 3 dB Range

30 0.3181 0.26153 -1.573x 10 -2 3.734 x 10 -4 3-25

45 -1.073 0.8816 -4.651 x 10 -2 7.942 x 10 -4 3-17

60 -8.127 x 10 -2 0.2044 5.781 x 10 -2 -2.235 x 10 -3 3-15

It is interesting to note that larger fade reduction s occur at the greater elevation angles.This arises because at the larger angles, a change of lanes may radically alter the earth-satellite path from a shadowed to a non-shadowed state. At the lower elevation angles,this change of state becomes less likely. It is noted from Figure 6-5 that the L -Band fadeis reduced from 10 dB to approximately 8 dB, 6 dB, and 4.5 dB at 30, 45, an d 60,respectively. At UHF (Figure 6-4 ), the 10 dB fade is reduced to 8 dB, 7 dB, and 5 dB at30, 45, and 60, respectively.

6.5 Antenna Separation Diversity GainA space diversity simulation has been carried out employing the database correspondingto 400 km of roadside tree shadowing measurements taken during the Australiancampaign [Vogel et al., 1992]. Space diversity operation for LMSS configurations maybe envisaged by the scenario of two spaced antennas mounted atop a vehicle where eachantenna is fed to a separate receiver system. Because the signal levels at the twoantennas are expected to be different at any instant of time, rapid switching between thetwo receiver outputs followed by subsequent processing should enable the larger signal tobe accessed. Such a system should therefore require smaller fade margins for the samesignal access distance than single terminal systems. The signal access distancerepresents that distance over which the received signal level operates within the designedfade margin.

Questions addressed here are: (1) what is the increase in signal access distance asa function of antenna spacing along the driving direction , and (2) what is theimprovement in terms of reduced fading (enhanced signal) for a given signal accessdistance as a function of antenna spacing? The first question is addressed employing theconcept of diversity improvement factor , DIF and the second diversity gain , DG ,both of which are characterized in the following paragraphs.


6.5.1 Joint Probabilities

In Figure 6-6 are shown a family of cumulative fade distribution functions derived fromthe above mentioned simulation. The curve labeled d = 0 represents the single terminalcumulative fade distribution corresponding to data acquired from over 400 km of drivingin Australia . The curves labeled d = 1 to 10 m represent the individual joint probabilitycumulative fade distribution for the indicated antenna separation s (in the direction ofvehicle motion). Such a distribution represents the joint probability that two antennasspaced a distance d mutually exceed the abscissa value of fade. Finally, the curve labeledindependent fading corresponds to the joint distribution of two links with singleterminal fading, assuming that the two are independent. We note that the jointprobabilities tend to coalesce with increasing antenna separation at about 2/3 the dB-distance to the independent fading case. That is, the fade distributions for 8 m and 10 mseparations have insignificant differences.

6.5.2 Diversity Improvement Factor, DIF

The DIF is defined as

DIF A dP AP A

o

d

( , )( )( )

= , (6 -5 )

where P 0(A) represents the single terminal probability distribution at the fade depth A ,and P d(A) represents the joint probability distribution for an antenna spacing d assumingthe same attenuation A is exceeded. These probabilities may be obtained from Figure 6-6 .

Employing the above results, a least square estimate of DIF was derived given by,

[ ]23.0)ln(2.01),( ++= dAdADIF , (6 -6 )where d is the antenna separation expressed in m and A is the fade depth in dB. In Figure6-7 are plotted a family of curves of DIF as a function of fade depth for antennaseparations between 1 and 10 m. We note, for example, that DIF (8,1) 3. This impliesthat when the antennas are separated 1 m, the distance over which the signal is receivedabove noise is three times greater for diversity operation relative to the single terminalcase assuming an 8 dB fade margin. At the larger separations for any given fade depth,the rate at which DIF increases is shown to diminish rapidly.


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14Fade Depth (dB)

2

4

68

2

4

68

2

4

68

0.1

1.0

10.0

100.0

Per

cent

age

of D

ista

nce

Fade

> A

bsci

ssa

d = 0 m

d = 1 mIndependent

Fading

d = 10, 8, 4, 2 m

Figure 6 -6 : Single and joint probability fade distributions for mobile communicationsoperating in a space diversity mode with antennas separated by the distance, d.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Fade Depth (dB)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Div

ersi

ty Im

prov

emen

t Fac

tor

1 m

2 m

4 m

8 m

10 m

Figure 6 -7 : Diversity Improvement Factor ( DIF ) as a function of fade depth for a familyof antenna separation distances.


6.5.3 Diversity Gain

Diversity gain is a concept defined by Hodge [1978] for an earth-satellitecommunications system involving two spaced antennas operating in a diversity mode inthe presence of precipitation. This concept may also be applied to antennas separatedatop a vehicle for LMSS scenarios. The diversity gain is defined as the fade reductionexperienced while operating in the diversity mode at a given fade margin. It is equal tothe difference in fades between the single terminal and joint probability distributions at afixed probability level. For example, from Figure 6-6 we note that the diversity gain at aprobability of 1% for a 1 m antenna separation is 4 dB. Hence, while the single terminaloperation at 1% probability will experience a 12 dB fade, the diversity pair for a 1 mseparation will experience only an 8 dB fade.

In Figure 6-8 are plotted the diversity gain s versus antenna separation s for a familyof single terminal fade levels. Each single terminal fade uniquely defines a probabilitylevel. For example, an 8 dB fade occurs at a probability level of 3% as is noted fromFigure 6-6 (for d = 0). Figure 6-8 shows that for any given fade margin, the effect of theantenna separation is dramatic the first 2 meters, whereas at larg er spacing, relativelylittle additional fade reduction ensues.

1 2 3 4 5 6 7 8 9 10Antenna Separation, d (m)

0

1

2

3

4

5

6

7

8

Div

ersi

ty G

ain

(dB

)

1 dB

2 dB

4 dB

6 dB

8 dB

10 dB

12 dB

14 dB

3 dB

Figure 6 -8 : Diversity gain versus antenna separation distance for a family of singleterminal fade levels.


6.5.4 Space Diversity for Expressway and Trunk Road Driving in Japan

Ryuko and Saruwatari [1991] describe 1.5 GHz cumulative fade distributions derivedfrom road measurements in Japan using the Japanese Engineering Test Satellite V(ETS -V ) as the transmitter platform. Using these measurements, joint probabilitydistributions were calculated as a function of antenna spacing on the roof of a mobilevehicle. Measurements were made on roads labeled expressways and trunk roads.The expressways in Japan run through mountainous areas and have many overpasseswith local roads. The trunk roads are not as wide and run through urban areas. Themajor fading for expressway measurements was observed to depend primarily on thedensity of overpasses. On the other hand, tall buildings primarily cause trunk roadfades. Table 6-4 summarizes the fading and diversity gain results for the Kan-etsuExpressway and the trunk road which correspond to measurements at an elevation angleof 46 to 47. The Kan-etsu Expressway has a total length of 150 km between Tokyo andYuzawa. The trunk road runs alongside the Kan-etsu Expressway, passes through localurban areas, suburbs, farming areas and has many bridge crossings for pedestrians.Pedestrian-bridge crossings, tall buildings, trees, utility poles and road signs causedfading for this road. Since this route runs approximately in the same direction as thesatellite path, fading along other trunk roads not so favored by direction is expected to bemore severe. The expressway case shows that a diversity gain of 4 dB exists at the 1%probability level. Negligible diversity gain differences exist when the antenna spacing isincreased from 5 m to 10 m over the percentage interval shown. The trunk road exhibitssimilar results at the higher percentages. At the 0.5% fade (13 dB), diversity gains of5 dB and 8 dB occur at an antenna spacing of 5 m and 10 m, respectively.

Table 6 -4 : Single terminal fade distribution and diversity gain values for Japan roads,Ryuko and Saruwatari [1991].

Road Type Single TerminalDiversity

Gain for GivenAntenna Separation

(dB)

Prob (%) Fade (dB) d = 5 m d = 10 m

Expressway 2.0 3 1 1

1.0 6 4 4

0.5 14 11 11.5

Trunk Road 2.0 3 1 1

1.0 6 3 4

0.5 13 5 8


6.6 Satellite Diversity

6.6.1 Background

Akturan and Vogel [1997] and Vogel [1997] describe a method by which they derivesingle and joint probability distributions and diversity gain s associated withcommunications employing multiple satellites. The method consists of: (1) videorecording hemispherical images of the surrounding environment through a fisheye lensmounted atop a mobile vehicle or photographing still images of the surroundingenvironment through a fisheye lens held head-high, (2) performing image analysis ofsequences of the hemispherical scenes, (3) simulating a constellation of potentiallyvisible satellite locations for the particular region of the world and different times of theday, (4) extracting path-state information associated with the line-of-sight for eachpotentially visible satellite (e.g., clear, shadowed, or blocked) for different times of theday for each scene, (5) injecting the path-state information into an appropriate densitydistribution model, and (6) computing single and joint cumulative distributions associatedwith different satellite-look scenarios. Details concerning the density function models forthe different path states are described in Chapter 10.

6.6.2 Cumulative Distributions

Figure 6-9 depicts a series of L -Band distributions ( f 1.6 GHz) for different diversityscenarios to the satellite for urban Japan , assuming a simulated Globalstar constellationof 48 satellites [Schindall , 1995]. In deriving the distributions given in Figure 6-9 , 236images were combined with approximately 1000 independent constellation snapshotsencompassing a 24 hour period (for each image). Hence, an equivalence of 236,000 setsof path states went into the database, where approximately 50% of the time threesatellites were potentially visible. The distribution labeled Highest Satellite representsthe distribution associated with the satellite having the greatest elevation angle. Thisdistribution was derived under the condition that the mobile antenna transmits to orreceives radiation from a different satellite position every time a new satellite achievesthe highest elevation angle, independent of azimuth. The highest elevation path may notnecessarily have a clear path state. That is, depending upon the scene at the time, itmay be representative of a blocked path state. The distribution labeled Best Satelliteis also derived from multiple satellites where the antenna is pointed to the satellite givingthe smallest fade.

In calculating this distribution, a decision for best satellite was madeapproximately every 20 seconds befor e hand-over was potentially executed. Thedistribution labeled 2 Best Satellites represents the joint distribution associated with thetwo satellites giving jointly the smallest fades. At any instant of time, different pairs ofsatellites may fall under the 2 Best Satellite category. The distributions labeled 3 BestSatellites and 4 Best Satellites are analogously defined. The above joint distributionswere derived assuming combining diversity where the signals received are added, asopposed to hand-off where the satellite with the highest signal is processed. It isapparent that each of the above distributions is calculated from many different satellitesat variable elevation and azimuth angles. Using the Highest Satellite distribution as the


reference, the fade is considerably reduced by switching to the other diversity modes.For example, at the 20% probability, a 17 dB fade for the highest satellite may becompared to 6 dB for the Best satellite scenario, giving rise to an 11 dB diversity gainat that probability. We note that the higher diversity combinations (e.g., 2, 3, and 4 BestSatellites) do not significantly contribute to an increased diversity gain at percentagesgreater than 20%. Figure 6-9 shows that using the 3 Best Satellite diversity mode, 1%probability gives rise to a 20 dB fade margin for an urban environment. Thissubstantially high fade may preclude voice communications for urban environments atsmall probabilities even with satellite diversity .

-5 0 5 10 15 20 25 30 35Fade (dB) Exceeded at Probability P

1

2

5

10

20

30

40

50

60

70

80

90

95

98

Prob

abili

ty P

(%)

Satellite Scenario

Highest Satellite

Best Satellite

2 Best Satellites

3 Best Satellites

4 Best Satellites

Figure 6 -9 : Cumulative fade distributions at L Band ( f 1.6 GHz) for the simulatedGlobalstar constellation with combining diversity for Tokyo, Japan [Vogel , 1997,Akturan and Vogel , 1997].

6.6.3 Satellite Diversity Gain

Figure 6-10 depicts the diversity gain s for the distributions given in Figure 6-9 associatedwith the combining diversity mode. Also shown are the diversity gain resultsassociated with the hand-off mode as defined above. For any fixed probability, thesetwo processing modes differ by approximately 1 dB (at most), but also note that any


implementation losses have been neglected. All diversity gains are shown to have a peakat approximately 20% probability.

1251020304050Probability (%)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Div

ersi

ty G

ain

(dB

)

Diversity Type

Combining

Handoff

Best Satellite

2 Best Satellites

3 Best Satellites

4 Best Satellites

Figure 6 -10 : Path diversity gain s at f 1.6 GHz derived from distributions for thesimulated Globalstar constellation with combining diversity and handoff for Tokyo,Japan [Vogel , 1997; Akturan and Vogel , 1997].

6.6.4 Satellite Diversity Measurements at S -Band Employing TDRSS

Direct orbital satellite mobile measurements were conducted by Vogel [1997] employinga portable satellite receiver with a prototype personal antenna system using NASA sTracking and Data Relay Satellite System (TDRSS ). During three occasions, NASAmade available two TDRSS satellites for diversity measurements at ~2 GHz. Thereceiver recorded simultaneously the signals from the two satellites located at differentazimuth and elevation angles. The mobile earth station (MES) was hand-carried toseveral typical environments in Austin, Texas during the spring-summer period when thedeciduous trees were in full bloom. Figure 6-11 shows single and joint distributions forthree path states which are characterized as follows for the specific environmentsconsidered: (1) clear denotes an open field with unobstructed line-of-sight paths to bothsatellites, (2) shadowed denotes walking in the vicinity of a grove of tall and thindeciduous trees, and (3) blocked denotes walking on the grounds of an apartmentcomplex with multiple three-story buildings set at various angles. The azimuths andelevation pairs to the satellites denoted as T7 and T1 were respectively, 248, 24 and


146, 49. Hence, these satellites were separated in azimuth and elevation by 102 and25, respectively.

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30Fade Exceeded at Probability (dB)

1

2

5

10

20

30

40

50

60

70

80

90

95

98

99

Pro

babi

lity

(%)

TDRS Satellite

T7 (24 o El, 248 o Az)

T1 (49 o El, 146 o Az)

Max. of T1 and T7

Blocked

Shadowed

Clear

Figure 6 -11 : Single and joint probabilities for clear, shadowed and blockedscenarios derived employing S -Band (f = 2 GHz) TDRSS measurements [Vogel , 1997].

The joint distributions in Figure 6-11 correspond to the hand-off mode (maximum oftwo satellite signals). Fading for the clear case at the smaller percentages is a result ofground specular reflection s and shadowing by the head. At the 1% level, the jointdistribution is shown to reduce the fade margin from a maximum of approximately 6 dBto 2 dB. The blocked distributions show smaller fading than the shadowed ca se forthe following reasons. The blocked environment distribution is in part representativeof an on or off switch where the line of sight is either not blocked or blocked,respectively. In addition, this distribution also includes diffraction and multipath fromthe buildings, and some shadowing from a few trees. The distribution is thereforestrongly dependent upon the density and height of the structures. On the other hand, theshadowed distribution represents an almost continuum of tree fading measured inside agrove; especially relative to the T7 satellite. The diversity gain s at 1% from theshadowed and blocked environments are noted to range from 8 to 13 dB and 8 to10 dB, respectively. These diversity gains have similar magnitudes to those calculatedemploying the optical measurements described in the previous section.


6.7 Conclusions and RecommendationsThe following salient conclusions and recommendations are based on the results of thischapter.

1. Simultaneous employment of co- and cross-polarized transmissions in a frequencyre-use system should not be used except to potentially help isolate different CDMAsystems because of the potential for poor isolation caused by multipath scattering intothe cross-polarized channel ( Figure 6-1 ).

2. Where ground multipath effects do not arise, small equi-probability fade differencesof 1 to 2 dB may occur (at L -Band) for mobile antennas having different antennagain s (Figure 6-2 ). At low elevation angle s where ground multipath effects mayarise, higher gain antenna has been found to substantially reduce fading by as muchas 10 dB at 20 GHz.

3. Changing lanes may substantially reduce the equi-probability fading because of theshortening of the path length through roadside tree canopies (Figure 6-3 ). At a 10 dBfade at UHF and L -Band, lane changing has been found to reduce the equi-probabilityfade for 60 elevation by approximately 5 dB ( Figure 6-4 and Figure 6-5 ). Equation(6 -4) gives a model describing the reduced fading at equi-probability levels, wherethe coefficients are summarized in Table 6-2 and Table 6-3 .

4. For a 5 m separation between antennas and single antenna fades of 3 dB and 6 dB,diversity gain s at L -Band (based on simulations using measurements made inAustralia ) are approximately 1.5 dB and 3.5 dB ( Figure 6-8 ). These results are within0.5 dB of those derived from measurements made in Japan (Table 6-4 ).

5. Satellite diversity measurements at L -Band for urban areas gave rise to diversity gain sas high as 14 dB for perce ntages as low as 1% ( Figure 6-10 ). Nevertheless, evenapplying diversity gain methods, fade margin levels have been found to exceed 17 to20 dB at the 1% probability ( Figure 6-9 and Figure 6-11 ). Such high fade margins forurban areas may preclude direct earth-satellite voice communications that requirenear instantaneous two-way responses.

6.8 ReferencesAkturan, R. and W. J. Vogel [1997], Path Diversity for LEO Satellite-PCS in the Urban

Environment, IEEE Transactions on Antennas and Propagation , Vol. 45, No. 7,July, pp. 1107-1116.

Goldhirsh, J. and W. J. Vogel [1987], Roadside Tree Attenuation Measurements at UHFfor Land-Mobile Satellite Systems, IEEE Transactions on Antennas andPropagation , Vol. 35, No. 5, May, pp. 589-596.

Goldhirsh, J. and W. J. Vogel [1989], Mobile Satellite System Fade Statistics forShadowing and Multipath from Roadside Trees at UHF and L -Band, IEEE Trans.Antennas Propagation , Vol. AP-37, No. 4, April, pp. 489-498.

Hodge , D. B. [1978], Path Diversity for Earth-Space Communication Links, RadioScience , Vol. 13, No. 3, pp. 481-487.


Mayer , C. E. [1996], Multipathing Effects on a Low Elevation Angle MobileExperiment at K A -Band, USNC/URSI Radio Science Meeting, July 21-26, p. 232.

Ryuko , H. and T. Saruwatari [1991], Propagation Characteristics for Land MobileSatellite Systems in 1.5 GHz Band, Journal of the Communications ResearchLaboratory , Vol. 38, No. 2, July, pp. 295-302.

Schindall , J. [1995], Concept and Implementation of the Globalstar Mobile SatelliteSystem, Proceedings of the Fourth International Mobile Satellite Conference ,Ottawa, Canada , June 6-8, pp. A-11 to A-22.

Vogel , W. J. [1997], Satellite Diversity for Personal Satellite Communications -Modeling and Measurements, Proceedings of the Tenth International Conferenceon Antennas and Propagation , Publication #436, Edinburgh, UK, 14-17 April, pp.269-272.

Vogel , W.J., J. Goldhirsh, and Y. Hase [1992], Land-Mobile-Satellite FadeMeasurements in Australia , AIAA Journal of Spacecraft and Rockets , Vol. 29,No. 1, Jan-Feb, pp. 123-128.

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Polarization, Antenna Gain and Diversity considerations

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