Proceedings of Asia-Pacific Microwave Conference 2007

Efficient Highways Coverage using High AltitudePlatforms

Yasser Albagory, Moawad Dessouky and Hamdy ShasharDept. of Electronics and Electrical Communications Eng.

Faculty of Electronic Eng., Egypt.E-mail: y ahom

Abstract- This paper proposes a novel beamforming technique forthe high altitude platforms (HAPs) mobile communications togenerate adaptive radio coverage worm-shaped cells covering themain highways which carry a heavy trafric of mobile users. Thistechnique is based on pattern summation of individual low-sidelobe narrow beams -which constitute the desired cell pattern-weighted by an adaptive amplitude correcting function. The newshaped cell differs from the conventional hexagonal or ellipticalcells as it follows the curvatures of the highway for longdistances, therefore it has an important role in reducing thefrequent handoff and signaling trafric of location updating frommoving users over long highways.

Keywords: High-Altitude platforms, beamforming

I. INTRODUCTION

There is an increasing demand for broadband mobilecommunications which has led to the rapid development oftheconventional terrestrial and satellite wireless communicationssystems. In recent years, another competitive system hasattracted the attention for providing wireless communicationsservices which is based on quasi-stationary platformsoperating in the stratosphere known by high altitude platforms(HAPs), and located 17-22 km above the earth's surface [1-3].The most important advantages of HAP communicationsystem are their low cost, low propagation delay, highelevation angles, easy and incremental deployment, flexibilityin operation, broad coverage, broadcast and broadbandcapability, ability to move around in emergency situations,etc, [3] but there are also some disadvantages, such as themonitoring of the station and the stabilization of the on-boardantenna. HAPs at their altitudes see well the coverage area andany desired cell shape can be obtained utilizing adaptiveantennas with a suitable beamforming technique [4,5], whichis very useful in optimizing the radio coverage. For example,in most world capital cities, there are main heavy traffichighways surrounding these cities like Washington DC,London, Paris, etc. where the moving users introducesfrequent handovers as well as high location updating signalingtraffic which are difficult to solve in terrestrial or satellitesystems. On the other hand, the radio coverage from HAPscan be designed to solve these problems. Therefore in thispaper, the coverage of highways from a HAP station isdiscussed and a new beamforming technique is proposed to

design a suitable coverage to the major highways. Thistechnique utilizes an adaptive uniform concentric circularantenna arrays [6-9] to form a desired cell pattern and acoverage simulation has performed for some main highwaysin London city. The paper is arranged as follows: section 2demonstrates the coverage characteristics from HAPs, section3 proposes the beamforming technique, section 4 introduces acoverage design case study and finally section 5 concludes thepaper.

II. RADIO COVERAGE FROM HIGH ALTITUDE PLATFORM

Figure 1 demonstrates the footprint of a beam directed from aHAP station on the earth' surface forming a single ellipticalcell. Assuming that this beam has a pointing direction of(0o, O) and a cross section halfpower beamwidths of B. and

Bo in the 0 and 0 directions respectively. The footprint ofthis beam is in general an ellipse which can be defined by itsmajor and minor axes (bc and ac respectively) given by [4]:

bc = R sin- K1+ hjsin ( + B)2

- sin-' +h sin0, -Bo) - Bo

(1)

and

ac = 2 R tanK JK) + h i(cos(7i )+ cos(72 ))j

1~~~~~~~~~~12+

I (cos(7i ) + cos(y2 ))2 tan (7, IJ4

(2)where R is the earth's radius, h is the platform height and Yl,

y2 and r0 are given by [4]:

Y, = sin-I I +h sin 00 - 80 0-0 +-B~R 2 2

72 = sin KI + R sin +00 2 ) 2

2o= I(rl + r2 )

(3)

(4)

(5)

1-4244-0749-4/07/$20.00 @2007 IEEE.

This beam can be formed by using either directional antennasor adaptive antenna arrays. Directional antennas has theadvantages of its simplicity and practical implementationwhile adaptive antenna arrays provides more flexibility andreconfigurability in the design of such beams . In this paper,the uniform concentric circular arrays (UCCA) shown in Fig.2 is adopted, which has several applications including radar,sonar, direction finding and mobile communications [7]. Thisarray has M concentric circular subarrays having an elementseparation of half the wavelength and the number of elementsfor the mth ring is Nm where m = 1, 2, ..., M. The elements areweighted in amplitudes and phases by a weighting matrixW(o, 0) which controls the beam footprint on the ground tohave the desired coverage. This matrix can be composed oftwo parts; the first reduces the sidelobe level while the secondpart is responsible for the cell shape, therefore it may berewritten as:

where the operator SUM is the summation of all elements inthe resulted matrix. Assuming free-space propagation scenariobetween the HAP and mobile users, therefore we can write anexpression for the received power Pr given by:

Pr = PtG, (9, O)Gt (6,0 4ZTd(O) (13)

where Pt is the transmitted HAP cell power, Gr (0, 0) is themobile antenna gain, Gt (0, 0) is the HAP array power gaingiven by:

Gt (0, 0) = |AF(, 0)2 (14)

and d(S) is the line-of-sight distance between the HAP andthe mobile determined from the following equation:

W(0, 0) = [WI (0, 0), WV2 (0 0), ...IWm(09 0).... I WM (a, 0)]

Wm(09, ) = amdm(09, ), m = 1, 2,...M,M (7)

is the mth column in W(0,0) which represents the steeringvector of the mth ring, , .is a window function used forsidelobe reduction [7-91 and dm(,) is the mth column in thecell-shaping matrix which can be written as:

D(9, 0) = [d1 (0, 0), d2 (0, 0),..., dm (0, 0),..., dM (0, 0)] (8)

The array factor AF(0, 0) can be defined by determining thearray steering matrix AS(0,p), which is given by [8]:

therefore the received power can be rewritten as:

(16)

The received power in the last equation depends on the arrayfactor and the mobile location with respect to the HAP stationassuming all the other quantities constant. Therefore, we mayrewrite the last equation as:

(17)

where

p(, 0) =AF(6, )2 cos2(0)AS(9, p) = [S1 (0, p) S2 (9, 0). Sm (9, 0). SM (9, A)]

(18)

where each column in the array steering matrix representcorresponding ring steering vector which in general for thring is given by:

S (6, ) = [e jkr. sin 0 cos (¢- 0 ) e jkr. sin 0 cos (04-0m2)

ejkr~sin 0cos(0-0, ejkr sin0cos(O-. Tejm ( ¢m)..e ( m)

where the mth ring has a radius rm and its elements azi]angle is given by:

2,m

9.mn = NNm

n =1,2,3. Nm

and k= 2

Therefore the array factor AF(0, 0) can be written as:

AF(6, 0) = SUM{W(0, ,)H AS(0, ¢5)}

Ls thee mth

(I M

which may be denoted by the power gain profile functionwhere it represents the variation in the received power leveldue to the array factor and the mobile location.

III. THE PROPOSED BEAMFORMING TECHNIQUE

In most capital cities, there is a need to design a highway thatsurrounds the city to bypass the traffic outside which does not

muth intend to enter it. Also in the city itself, there will be almostlong highways that carry large traffic. Terrestrial mobile radiocells are designed to cover the most straight parts of the mainroads by using directed antennas along these roads [10] whilethis can be more easier in the case of using HAPs. The cellscan be adapted to cover very long distances of the highwaysthrough careful radiation pattern synthesis which must havecontinuous radio coverage without holes or droppings in thereceived power. This specialized cell pattern which followsthe highway terrains forms a "wormy" cell structure which

(12) can be formed by contiguous footprints of low-sidelobebeams. These constituting beams are separated by the half-power beamwidth and any desired pattern can be obtained by

where(6) d(e) C=(h) (15)

(9)

x2

0). 2

P, = PtG, (0, o) AF (0, COS2 (0)

4n h

(0, 0)'I

2

P, = PtG, . P(o, o)4;T h

controlling their number, amplitudes, pointing directions andbeamwidths.In this respect, the desired pattern can be formed by setting thecell shaping matrix as:

N,

D(f}, 0) =I ;(i, pi)AS(l,, 0,)i=l

(19)

where NC is the number of spot beams needed to constitutethe desired wormy cell pattern and ;(Oj,,i ) is an amplitudecorrecting function for the ith beam which is needed to reducethe unwanted amplitude variations after summing theindividual beam patterns. This function is proposed to be theinverse of the array factor at the main individual beamsdirections, and for the ith beam it is proposed to be:

0N) )c (20)SUM{ZAS(OI04)H AS )} cOs(Oj)

C(0i,fi)=q ~~1=1

which results in mostly uniform coverage and half-powercontour, noting that the cosine term in the denominatorcompensates the attenuation due to distance variationsbetween the mobile and the HAP station. To improve thesidelobe performance, the array may be tapered in amplitudewith a suitable window such as Dolph-Chebyshev [8] or theGaussian [9] windows.

IV. SIMULATIONS AND DISCUSSIONS

Assuming a highway coverage in London city is to bedesigned as shown in Fig. 3 utilizing a Dolph-ChebyshevUCCA [8] of N1 = 5 and M = 20 onboard a HAP stationat 20 km high, therefore we may need about 38 individualbeams separated by a half-power beamwidth which equals 5.2degrees. Adopting the amplitude correction function given inEq. 20 and plotted in Fig. 4 for the different 38 beamsprovides the required wormy cell. Fig. 5 depicts thenormalized received power profile in dB from a HAP stationwhile in Fig. 6, the half-power contour is plotted.Wormy cellular structure for HAP mobile communicationssystem will improve the system performance especially incovering long highways carrying mobile users because:

1- The handoff will be reduced because the cell is very

long even if the mobile users are talking for longperiods of time.

2- The location updating rate and its correspondingsignaling traffic will be reduced for users movingalong the wormy cell.

3- The wormy cells can be designed to optimize thecoverage depending on the terrains of the coveredregions.

4- It can be adapted in shape to accommodate anychange in the highways design due to the varyingtraffic or population conditions.

5- Adopting the tapered UCCA beamforming willreduce the radiation in other uncovered regions whichreduces the interference to the cochannel cellsallowing frequency reuse.

On the other hand, this cellular structure needs a carefulfrequency planning to have the desired carrier-to-interferenceratio as the cells are not distributed uniformly.

V. CONCLUSIONS

The coverage of highways is examined and a wormy cellshapes for the radio coverage is proposed. This novel cellstructure can cover very long distances along the highwaysand has several advantages like reduced handoff rate as wellas reduced signaling traffic due to location updating. Thiscellular structure can be designed by a beamforming techniqueadopting tapered uniform concentric circular arrays in whichthe total desired radiation pattern can be obtained by summingthe patterns of individual contiguous beams weighted by anamplitude correction function to equalize the pattern to havemostly uniform coverage without holes over the highway.

REFERENCES

[1] B. El-Jabu and R. Steele," Cellular Communications Using AerialPlatforms", IEEE Transactions on Vehicular Technology, Vol. 50, No. 3, May2001.[2] J. Thornton, D. Grace, C. Spillard, T. Konefal, and T. C. Tozer "BroadbandCommunications from a High-Altitude Platform: the European HeliNetprogramme", Electronics & Communications Engineering Journal, June 2001.[3] S. Karapantazis and F. Pavlidou "Broadband Communications via High-Altitude Platforms: A Survey", IEEE Communications Surveys & Tutorials,First Quarter 2005, pp. 1-3 1, 2005.[4] M. Dessouky, H. Sharshar and Y. Albagory "Geometrical-Analysis-of-HighAltitude Platforms Cellular Footprint Progress in ElectromagneticsResearch, PIER67, pp. 263-274, 2007.[5] M. Dessouky, H. Sharshar and Y. Albagory "Design of High AltitudePlatforms- Ccilular Communications- Progrcss in ElcctromagnticsResearch, PIER67, pp. 251- 261, 2007.[6] M. Dessouky, H. Sharshar and Y. Albagory "Efficient SidelobeReductionTechnique for Small-Sized Concentric Circular Arrays Progress inElectromagneticsResearch, PIER-65, pp. 187-200, 2006.[7] M. Dessouky, H. Sharshar and Y. Albagory "A Novel TaperedBeam_forming Window-for Uniform Concentric CircularArrays Journal-ofElectromagnetic Waves-and-Applications, JEMWA, Vol. 20, No. 14, pp.2077-2089, 2006.[8] M. Dessouky, H. Sharshar and Y. Albagory "An-Approach-for-Dolph-Chebyshev Uniform Concentric Circular Arrays Journal ofElectromagnetic Waves-and-Applications, JEMWA, Vol. 21, No. 6, pp. 781-794, 2007.[9] M. Dessouky, H. Sharshar and Y. Albagory "Optimum NormalizedGaussian Tapering Window-for SidelobeReduction in Uniform ConcentricCircular-Arrays Progress in Electromagnetics-Research, PIER-69, pp. 35-46, 2007.[10] W. Lee, "Mobile Cellular Communications," McGraw Hill, New York,1989.

HAP station

Figure 2. Uniform concentric circular arrays (UCCA)

Figure 1. Footprint of a HAP cell

.

a)3C3

,.u

,,

1.4

1.2

Figure 3. 38 footprints of individual spot beams constituting awormy cell for one of the major London highways. These beamsare formed using Dolph-Chebyshev UCCA ofN1 = 5 and M = 20.

Bam number, i

Figure 4. The amplitude correction function for thedesign in figure 3.

10

o

-10

-15 -10 -5 O 5 10Distance in km

15

Distance in km

Figure 5. Normalized received power pattern in dB for thedesign in figure 3. Figure 6. The half-power contour of the wormy cell

designed in figure 3.

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0-5

--10

.-15

-20

-26

-30

-36

-40

-46

-60

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