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HAP Smart Antenna

George White

John Thornton, Yuriy Zakharov, David Grace

University of York

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HAP smart antenna - Introduction

Research motivations:

• Flexible way of achieving spatial re-use– adapt power and bandwidth distribution on ground according to positions of users

• Compensation for HAP motion • New beamsteering scenario:

– steering in elevation as well as azimuth, c.f. terrestrial large steering angles, c.f. GEO satellite

• ‘Smartness’ – interference suppression

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HAP smart antenna - Issues

Number of elements, M:

Large M:– allows significant spatial re-use of bandwidth– increases directivity for HAP-Earth link– increases complexity (simulation and implementation)

So far, M=64 elementsSignificant capacity enhancement if applied to existing TDM or FDM downlinkSufficient directivity (link budget has been developed)Relatively easy to analyse

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HAP smart antenna - Issues

Degree of ‘smartness’:

• Simple: Multiple fixed beam directions.Could provide cellular coverage.

• Moderate: Beam steering wih fine angular variationsCan compensate for HAP motionCapacity increase is limited - interference can be highwhen co-channel users are in sidelobes

• Truly smart: Null steering Place co-channel users in beampattern nullsIncreased complexity, particularly for high M

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Minimum variance beamforming

nHn

nn

vRv

vRw

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1

R is the MM spatial correlation matrixvn is steering vector in direction of nth user

Complexity (M3). Can use computationally-efficient algorithms, e.g. dichotomous co-ordinate descent (DCD) method [1] to solve:

Desired complex weight vector for nth user is given by:

nn vRh 1

[1] Y. Zakharov and T.C. Tozer, “Multiplication-free Iterative Algorithm for the LS Problem,” IEE Electronics Letters, Vol. 40, No. 9, April 2004, pp. 567-569.

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Adaptive beamforming from HAP

HAP scenario:70km diameter coverage areaHAP at {0,0} 20km altitude

88 array, 60 users

Black=desired userWhite=interferers

Beampattern nullssteered to interferers

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HAP beamforming:Performance in ideal case

CDF of SIR on the downlinkfor a single, reference user

N users, M elementsIdeal case:Perfect knowledge ofHAP and user positions

N<<M, strong interferencesuppression, high SIR

NM, degradation in SIR

Note: This provides spatial re-use gain. Use alongside frequency or time re-use

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HAP beamforming:Performance with position and attitude errors

GNSS positional informatione.g. GPS, assume averageaccuracy to within 15m

Small, uncompensatedvariations in HAP attitude(e.g. pitch, roll or yaw)may be a limiting factoron capacity. E.g. due to turbulence

= standard deviation of zero-mean Gaussian-distributedpitch variation (degrees)

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HAP beamforming:Performance with element spacings > /2

d

7d=12.95cm

64 elements

Interference increases for d >/2 in non-ideal case.But such spacings may produceacceptable spatial re-use.E.g. d=1.85 = 1.85cm at 28GHz

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Array configurations

e.g. d=0.5 where =1cm (f=28GHz)

d

7d=3.5cm

64d/=10.2cm

8d=4cm

64 elements64 elements

rings of:1, 6 ,12, 18, 24

d

d

61 elements

rectangularcircular array,

circumferential elements circular array, filled

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Array configurations – Pros and cons

• Rectangular or circular, filled arrays:– Lack of space for component placement if d =/2 – Robust to position or attitude errors if d =/2

• Circular array, circumferential elements:– More space available for component placement– Could ensure equal length LO feeds– Performance degradation with position or attitude errors

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Beampatterns:Circular array, circumferential elements

No interferers 59 interferers

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Beampatterns:Circular array, filled

No interferers 59 interferers

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Journal submission/Further work

"Adaptive Beamforming for Communications from High-Altitude Platforms", George White, John Thornton, David Grace, Yuriy Zakharov and Tim Tozer submitted to IEEE Trans. on Wireless Comms., Feb. 2005.

• Reduced complexity (DCD) minimum variance beamforming• Channel allocation methods for HAP beamforming• Comparison of cellular and single-user-per-beam coverage strategies• Performance of beamforming over Capanina HAP channel model• Alternative array configurations (contd.). E.g. tilted arrays

RAF Fylingdales, N. Yorks, UK

Sectorised coverage area