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

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

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

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

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:

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

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

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)

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

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

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

Beampatterns: Circular array, circumferential elements No interferers 59 interferers

Beampatterns: Circular array, filled No interferers 59 interferers

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