1. John Thornton, Yuriy Zakharov, David Grace
HAP Smart Antenna
George White
John Thornton, Yuriy Zakharov, David Grace
University of York

2. 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

3. 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 elements
Significant capacity enhancement if applied to
existing TDM or FDM downlink
Sufficient directivity (link budget has been developed)
Relatively easy to analyse

4. HAP smart antenna - Issues
Degree of ‘smartness’:
Simple: Multiple fixed beam directions.
Could provide cellular coverage.
Moderate: Beam steering wih fine angular variations
Can compensate for HAP motion
Capacity increase is limited - interference can be high
when co-channel users are in sidelobes
Truly smart: Null steering
Place co-channel users in beampattern nulls
Increased complexity, particularly for high M

5. Minimum variance beamforming
Desired complex weight vector for nth user is given by:
R is the MM spatial correlation matrix
vn 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:

6. Adaptive beamforming from HAP
HAP scenario:
70km diameter
coverage area
HAP at {0,0}
20km altitude
88 array, 60 users
Black=desired user
White=interferers
Beampattern nulls
steered to interferers

7. HAP beamforming: Performance in ideal case
CDF of SIR on the downlink
for a single, reference user
N users, M elements
Ideal case:
Perfect knowledge of
HAP and user positions
N<<M, strong interference
suppression, high SIR
NM, degradation in SIR
Note: This provides spatial re-use gain. Use alongside frequency or time re-use

8. HAP beamforming: Performance with position and attitude errors
GNSS positional information
e.g. GPS, assume average
accuracy to within 15m
Small, uncompensated
variations in HAP attitude
(e.g. pitch, roll or yaw)
may be a limiting factor
on capacity.
E.g. due to turbulence
 = standard deviation of
zero-mean Gaussian-distributed
pitch variation (degrees)

9. HAP beamforming: Performance with element spacings > /2
Interference increases for d >/2
in non-ideal case.
But such spacings may produce
acceptable spatial re-use.
E.g. d=1.85 = 1.85cm at 28GHz d
7d=12.95cm
64 elements

10. circumferential elements
Array configurations
e.g. d=0.5 where =1cm (f=28GHz)
circular array,
circumferential elements
rectangular
circular array, filled d d d
61 elements
64 elements
64 elements
8d=4cm
7d=3.5cm
rings of:
1, 6 ,12, 18, 24
64d/=10.2cm

11. 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

12. Beampatterns: Circular array, circumferential elements
No interferers
59 interferers

13. Beampatterns: Circular array, filled
No interferers
59 interferers

14. 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
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