I. J. Computer Network and Information Security, 2013, 7, 51-57 Published Online June 2013 in MECS (http://www.mecs-press.org/) DOI: 10.5815/ijcnis.2013.07.06 Copyright © 2013 MECS I.J. Computer Network and Information Security, 2013, 7, 51-57 Flat-Top Ring-Shaped Cell Design for High- Altitude Platform Communications Yasser Albagory Taif University, College of Computers & Information Technology, Computer Science Dept. Saudi Arabia Menoufia University, Faculty of Electronic Engineering, Egypt dalbagory@gmail.com Abstract — In this paper, a new design for ring-shaped cells is introduced where to improve the power distribution and carrier-to-interference ratio (CIR) over the cell area. The designed cell has flat-top radiation pattern with minimal ripples in the service area while the out-of-cell area has lower radiation levels. The new design utilizes two weighting functions applied to a vertical linear array; the first is responsible for the flat-top design and the second smoothes the pattern and reduces the sidelobe levels. The resulted power pattern has a uniform distribution over the cell stripe with as small as 0.25 dB ripples and a uniform CIR values greater than 43 dB within the cells which reduces the burden of power control and increases the immunity to propagation problems. Index Terms — High altitude platforms, directional antennas, phased antenna arrays, mobile radio communications I. I NTRODUCTION Wireless mobile communications services with higher data rates have gained an increased demand which need to develop more innovative communications infrastructures. Terrestrial systems and satellite systems are the well-established existing technologies providing mobile communication both have their own advantages and disadvantages. The low-cost, low-power user terminals, short propagation delays and higher capacity are the main advantages of terrestrial systems, while it suffer from the multipath-fading and high scattering of radio signals. In addition, to provide higher system capacity, it should have a large number of base stations creating many other problems such as site acquisition and complex infrastructure. On the other hand, satellite systems provide reduced infrastructure and better radio coverage especially at higher elevation angles; however it suffer from the long delay especially for the geo- stationary satellites, higher launch costs and complexity in constellation for the low-earth orbit satellites. Recently, an innovative communication technology based on high- altitude platforms (HAPs) has gained attention as it preserves many advantages of both terrestrial and satellite systems but also provide special advantages of their own [1-3]. These platforms are positioned at altitudes 17–22 km high and have the potential to deliver broadband services cost effectively. A single HAP with communications payloads onboard can replace a large number of terrestrial base stations and their backhaul infrastructure. The high-altitude property of HAPs indicates good radio coverage and line-of-sight communications as in satellite systems but with lower propagation delays comparable to that of terrestrial systems. HAPs are considered nowadays as a substantial part of the future integrated terrestrial/satellite networks for providing wireless communication services [1]. In addition, HAPs may be used in other applications such as disaster monitoring and mitigation [4] and global positioning [5]. An important parameter that affects the HAP system performance is the type of antennas used to provide the radio coverage especially for the cellular systems. The multi-beam horn (MBH) antenna and antenna arrays were developed for high-speed transmission at 48/47-GHz in [6], while in [7], a low sidelobe level and asymmetric beam antenna was developed using lens antenna to provide almost circular footprint especially at lower elevation angles. In [8], the concentric ring array (CRA) is examined to provide an improved HAP cellular coverage performance where it has many advantages such as independent azimuth beamforming and lower sidelobe levels. The capability of shaping the cells to optimize the system performance is not an easy task in both conventional terrestrial and satellite systems where varying the cell area faces some difficulties in rearranging the new cells and reallocation of channels for each cell as well as outlining the desired cell boundary due to the variations in the propagation conditions. In terrestrial systems the main shaping of the cells was in sectorizing the beams to improve the CIR or to extend the coverage of the cell in one direction as in highway coverage, while in satellite systems, the shaping of cells is possible; however, the resulted cells suffer from its very large area and consequently reducing the system capacity. The HAPs will close the gap between the capability of smartly shaping the cells and the required smaller cell area to provide both the improvement in system performance and the desired higher capacity. On the other hand, HAP stations suffer from some of the positional instabilities especially the rotational motion of the platform [4], where the platform can rotate around its vertical access and cause severe problems in the