High Altitude Platforms (HAPs) has been proposed as an alternative technology within the lTU to provide narrowband and broadband communication services and establish a complementary technology for radio communication networks at terrestrial and satellite levels. The traditional terrestrial system suffers from coverage issue which are mostly eliminated by using HAPs .. HAPs represent an alternative technology for wireless communication system which can provide large coverage, low propagation delay, line of sight and narrowband and broadband services. Therefore it considered as a promising technology in wireless communications for reason which HAPs takes the satellite
advantages, but at lower altitudes[l], For very large coverage area and broadcast, satellite is
required. While HAPs is able to cover sparsely populated areas at low cost as well as to offer broadband services to the users. At densely populated areas, terrestrial system is required for interactive services. Therefore, HAPs must be synergic
integration between terrestrial and satellite which can lead to a truly evolutionary function as shown in Fig,l, HAPs can be either heavier than air (HTA), lighter than air (LTA) and tethered balloon.
A significant path loss will be decreased and minimized the transmit power for providing high quality of service (QoS), that happened due to reduction in distance to the HAPs compared to a satellite, The provision of telecommunications services using a cellular network structure within a well define
coverage area is one of the applications of HAPs L2}
In the cellular systems, by taking into account the hexagonal shape, the cell geometry can be determined as well as the cell location will be easy to find. In the case of using HAPs, the cell defined as an ellipse by using the half power contour on the ground which can be considered.
A single HAP which provides almost service coverage of a wide rural area is expected to be more technologically complex and every macro cell served by a platform can be subdivided into a number of sectors, with each sector connected to an aerial base station on the platform. HAP can be backup base station instead, supplementing the services provided by existing terrestrial base stations, so that pockets of areas which are not covered by the terrestrial network would be included in this integrated system. The planning cell is used
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to optimize coverage, make better usage of the frequency and enhance signal quality.
The rest of this article is organized as follows; section II explains the related work in this area and the coverage of HAP is described in section III. The antenna radiation pattern is spelled out in section IV. Section V deals with the results and concluding remarks are drawn in section VI.
II. RELATED WORK
HAPs considered as one of the most important and potential options in network deployment for narrowband and broadband services. Rec. ITU-R M. 1456, Explained the operational conditions for HAPs stations and minimum performance characteristics to provide frequency band for
different regions. NeliNet Project [�:J suggest that HAP platform should be stationed within a large cylinder with a probability of 99.9% or a small one with a probability of 99%,
that for guarantee high QoS.
References [�, �] studied the cell architectures and
radiation pattern of antenna. In [�] estimated optimum beam widths for each cell of a regular hexagonal layout. The method is then applied to 121 cell architecture. These cells have been studied and extended method to the used of more radiation patterns based on the theoretical aperture antenna patterns. In
[Q] shown that Madrid could be covered by a HAPs with 169
beams. Then the total practical capacity of the HAPs system could be in the order of 10000 voice users or 1250 data
users@.
III. HAP COVERAGE
Coverage is the geographical area in which a wireless network offers cellular service for subscribers. A base station is part of wireless networks and fIxed location. It often referred to as a cell site, a base station allows user of service to work within a local area. Furthermore, cell is defIned as a radio network distributed over the area. These cells can be jointed together over network coverage to provide radio coverage. Basically, both cell and network coverage depends on natural factors like propagation conditions, human factors such landscape (urban, suburban, and rural) and subscriber behaviour etc.
The coverage of the cell is depending on the different of environment. The environments may be Urban Area Coverage (UAC), Sub-urban Area Coverage (SAC) and Rural Area Coverage (RAC). Four main reasons for the reduction in the cell coverage are: reflection, diffraction, scattering and
multipath [1]. • Reflection The electromagnetic wave lockouts against a smooth
surface and smooth surfaces are large compared with the signal wavelength, thus the reflections will be occurred.
Reflection occurs from the surface of the earth's and from buildings and walls.
• Diffraction
Diffraction is called shadowing; because of the diffracted of signal can arrive the receiver at shadowed by an impenetrable obstruction. When the electromagnetic wave strikes a surface whose dimensions are larger than the signal wavelength; Secondary "wavelets" propagate into the shadowed region, the diffraction will be occurred. Diffraction gives rise to bending of waves around the obstacle.
• Scattering Scattering happens, when a radio wave lockout against a
rough surface and this dimensions are equal to or less than the signal wavelength. Small objects may be street lights and leaves also cause scattering.
• Multipath propagation Multi waves create multipath and this causes large
fluctuations in a signal as shown in fIg.2. These fluctuations are not the same as the propagation path loss.
Fig.2. Multipath propagation
As stated in the geometry represented in FigJ , the xy plane represents the earth surface and the z axis symbolizes the platform height h, the distance between the sub-platform point (SPP)., the center of the cell is represented by g but the outer
circle to the hexagonal cell r. e � represents the elevation angle
and (2J � is the azimuth angle. Therefore, eo can be calculated by the following:
Bo = arctan ( % ) (1) (c' -1 ) dsin ( 7r )
¢ o=arcsin g 3 �ns-l);(2) The angle e � and (2J" can be calculated by the
following:
Bi=arctan ( g:r )-arctan ( g�r ) (3)
2014 International Conference on Advances in Computing, Communications and Informatics (ICACCI) 1435
¢ �=2arctan* ( 4) g +h
Hexagonal Cell is used for HAP and we focus in a
hexagonal that represents the coverage area of HAP. The
hexagonal cell of HAP coverage has depicted in FigA. The hexagon side length is 10 Km and HAP coverage area is 260
Km2• The center cell which located in the center of the HAP
coverage area and the cells located at the edge of HAP coverage have always the same position.
Fig.3. Cell geometry[Q]
The idea is that, when there are a large number of tiers, the distance between tier decreases, and the number of the cell
increases. Therefore, decrease the distance between classes affects directly to the overall network performance. The
separation distance between cells is given by:
5 d= 8Km l, )
Number of tiers
Fig.4. Hexagonal HAP cell
One tier and two tiers situation are shown in Fig.5. In case
of one tier, the separation distance between cells is SKm. On
the other hand, when two tier are used, the separation distance
is 4Km. So that, if the mount of tier increases, the separation distance between cells is decreased.
Fig.S. One and two tier
A. HAP movement HAP is an airplane or airship which operating at 17 -21Km
altitude. The radius coverage of the HAP is 30 Km at 20Km
altitude and the HAP coverage area is divided into 127 cells. So that movement of HAP in vertical and horizontal will cause to dropping call and outage call. Therefore, the coverage area and QoS will be influenced.
a. Horizontal movement of HAP
Movement of HAP in horizontal along with X-axis is clarified in Fig.6. It might be analysis the influence of
horizontal movement in X direction. Movements of HAP will shift all cells in its coverage area with same direction.
Meanwhile, shape and size of the cells in the HAP coverage
area will continue the same.
Fig.6. Horizontal shifting of HAP
b. Vertical movement of HAP
When HAP movement is in upward, the antennas will be pushed inward, and the center will move little upward as
shown in fig.7. Rotation modification of HAP can make all of
its antennas pointing to the initial position on the ground, but elevation angle is different from the initial angle. In the other hand, when HAP moves downward, the antennas will be pushed outward, and the antenna at the center will move a
little downward.
1436 20 I4 Internationai Conference on Advances in Computing. Communications and Informatics (ICACCI)
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I I
I I
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R HAP COVE! rage
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Fig.? Vertical HAP movement with steerable antenna
B. HAP Steering Antenna The coverage area will be affected by HAP movement.
Therefore, part of HAP coverage may be left without services because of HAP shifting. Thus, service probability, especially for users who are in that particular area will be influenced
\ spp
/ Fig.S. Coverage of HAP using Steerable antenna
Steerable antennas are considered to solve this problem as shown in Fig.8. When the HAP performs movement, the elevation of the steerable antennas can be dynamically modulated to different directions to recover original cellular on
the ground as much as possible.
IV. ANTENNA RADIATION PATTERN
An antenna is a conductor between a radiated wave and a guided wave or vice versa as well as it can transmit, send and
receive signals such as microwave, radio or satellite signals.
Antenna is the most important element in the wireless
communication and also used between HAP and end user for two reasons. First, antenna pattern defines a footprint,
coverage and also the interference and carrier to interference
ratio values those effects on QoS and network performance.
Second, due to the platform altitude the antenna beam width must be of a few degrees in order to create a layout of cell. The
radiated wave is characterized by the antenna's radiation
pattern. Describing the features of the antenna radiates energy
out in the space and known as the antenna radiation pattern.
Generally, used of antenna arrays are to obtain suitable directive characteristics in order to increase the radiation
towards the serviced area and suppress it towards other ones. Usually the implementations of antenna have narrow main beam and side lobe beam as shown in Fig.9. Additionally,
radiation properties of the antenna patterns are characterized by the horizontal radiation pattern, vertical radiation pattern. These two radiation patterns are used to estimate the gain of
antenna. Azimuth angle and elevation angle are used to determine antenna gain at any point in the footprint. Also the antenna gain can be expressed as antenna loss, which is the difference between the maximum antenna gain and the gain in that direction. The capabilities of antenna radiation should be
known in all the directions, to ascribe each point under the antenna covered area , the correct signal level and the exact antenna gain in that direction.
HAP.r1l���
Fig.9. HAP antenna radiation pattems[� The HAP antenna radiation pattern was modelled on
ITU-RM.1456 [�]. Antenna radiation pattern is used for
different cell radii as a function of the angle from the antenna
boresight [ID as shown in fig. 10. the HAP antenna radiation
modelling based on the antenna power roll off is explained in
detail by ['2J Directional antenna may be in the form of hom
antennas. The most important parameters in the system design
regarding directional antennas are: gain, bandwidth, beam
width, and side lobe level Lilli .
2014 International Conference on Advances in Computing, Communications and 1nformatics (ICACCI) 1437
