INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

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1 INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN ISSN (Print) ISSN (Online) Volume 3, Issue 2, July- September (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJECET I A E M E DOWNLINK SIGNAL EVALUATION OF HAPS M-55 AIRCRAFT ABOVE MALAYSIAN SKIES Wanis A Hasan and Ahmad N Abdulfattah Communication Engineering Department, Higher and Intermediate Institute of Comprehensive Professions, P.O. Box , Bani Walid, +218, Libya, , algazalat@yahoo.com Communication and Computer Engineering Department, CIHAN University, P.O. Box , Erbil, Kurdistan Region, +964, Iraq, , msc.ahmedaldabbagh@gmail.com ABSTRACT HAPS is a promising technology, uses airborne ships for providing narrow and broadband wireless communication and broadcasting services from the sky to users and end terminals on the ground. It is considered as the most viable and cost-effective solution for communication service providers by complementing their existing terrestrial and satellite network investments, to allow their customers have more freedom and convenience to get connected to different communication networks nationwide and worldwide. Malaysia has made significant efforts in the research of HAPS M-55 aircraft deployment and applications in cooperation with some international partners. In this paper, the downlink signal of HAPS M- 55 aircraft will be evaluated within two different reception scenarios in terms of rain attenuation and signal multipath fading, while hovering above Malaysian skies. Racian channel will be introduced as a multipath fading channel. All results will be obtained based on theoretical assumptions and settings and by using the MATHLAB as a simulation tool. 336

2 KEYWORDS: HAPS M-55 aircraft, elevation angle, radio link budget calculation, rain attenuation, signal multipath fading, Racian fading channel. I. INTRODUCTION The necessity of finding a reliable wireless communication infrastructure, which can ensure high quality of service (QoS) and meet the customers communication needs in Malaysia, has shifted the attention to search deeply in HAPS technology and applications [1],[2]. In 2007, the mile stone was created when the federal government of Malaysia signed an agreement with QucomHaps Co. of Ireland and the Russian owner and designer of M-55 GN stratospheric aircraft. That agreement aims to fly the M-55 aircraft above the Malaysian skies to provide a nationwide wireless access to a broadband connectivity at subscription rates lower than anything commercially offered in the local competitive market [1], [2]. Fig.1. Several communication services provided by a HAPS M-55-based system HAPS M-55 is a piloted aircraft manufactured and developed by the Geoscan International Agency (GIA) projects [1]. It is designed to fly at the Stratosphere layer and lasts for about five-hour-time interval for each flight. It hovers in a circular path at an altitude of nearly

3 km high. It has a body weights approximately 24 tons and 37m wingspan long. It can accommodate carriage of payloads of 2 tons and consumes power supply of 40kW. It is a single-seated aircraft and can operate at day and night time, even in critical weather conditions while taking off or landing statuses as stated in. A single M-55 aircraft can form ground coverage of about 400 km radius. This coverage is equivalent to approximately 258 ground terrestrial base stations coverage. It is supposed that only five M-55 aircrafts will be launched and flown concurrently to provide wireless broadband coverage for the entire Malaysian territory [1],[2]. II.SCENARIOS AND ASSUMPTIONS The downlink signal of HAPS M-55 aircraft (service signal) will be evaluated based on two different reception scenarios in terms of rain attenuation and signal multipath fading. It is assumed that the HAPS M-55 aircraft will be flown at an altitude of 21 km above Johor state, which is a part of Malaysian territory. The elevation angle of downlink signal reception will be a vital parameter in both scenarios. Scenario I: two users (ua and ub) receive the downlink signals with 20 o and 90 o elevation angles respectively. That is to evaluate the downlink signal level when one user is at the end of coverage whereas the other is in the center. 20 o was chosen as the lowest elevation angle, at which the user is supposed to receive the downlink signal with the lowest quality. If the lowest elevation angle is assumed, the larger the service coverage can be formed, but the rain propagation path, however, becomes longer and larger fade margin may be needed [3]. Also 90 o was also chosen as the highest elevation angle, at which the user centralizes directly under the Sub-Platform Point (SPP), and can receive the downlink signal with better quality [3]. Communication links of the downlink signals received by both users are considered Lineof-Sight (LOS) paths. While travelling between the HAPS M-55 aircraft and the two users, it is assumed that those signals will experience a rainfall event (rainy sky weather condition). Just for a comparison purpose, they will be first evaluated under clear sky weather condition. Settings of the two users were calculated and inserted in Table (1) as below: 338

4 Elevation angle (Degree) Altitude Table (1) Settings of the two users (ua & ub) (m) Distance to SPP (km) Propagation path length (km) Total signal path No. Antenna gain (dbi) Feeder ua 20` ub Loss (db) Fig.2. Two users receive HAPS M-55 downlink signal with 20 o and 90 o elevation angles Operational parameters of HAPS M-55 aircraft were assumed for both downlink signals while travelling under a clear sky condition and inserted in Table (2) as below: 339

5 Table (2) Operational parameters of HAPS M-55 aircraft Parameter Downlink Elevation angle (degrees) Altitude (Km) Frequency (GHz) Data Rate (Mbit/s) 2 2 Modulation Scheme QPSK QPSK Output power (dbw) Feeder loss (db) Gain (dbi) EIRP (dbw) Propagation path length (Km) Free space loss (db) Atmospheric gas loss (db) Rain attenuation (db) 0 0 In order to evaluate the downlink signal under a rainy sky condition, data of rainfall rate of Johor was collected and used to predict the overall rain attenuation based on [4], [5], [6] as shown in Table (3) below: Table (3) Rain attenuation predicted in Johor State Elevation angle (degree) Service reliability (%) Rain rate (mm/h) Rain attenuation predicted (db) The equation, which is used in the calculations, is the basic simplified received signal power equation as shown below [4]: P =P L +G L L L +G L (1) Where: 340

6 P : Downlink received signal power (dbw) P : Transmitted signal power (dbw) L : Transmitted signal power loss because of feeder inefficiency (db). G : Transmitting antenna gain (dbi) L : Free space loss (db) L : Power loss because of Atmospheric gas (db). L : Rain attenuation loss (db) G : Receiving antenna gain (dbi) L : Received power loss because of feeder inefficiency (db). Scenario II: only one user, ua, receives the downlink signal through direct and diffuse propagation paths due to the nature of vicinity. It is assumed that one Line-of-Sight path (LOS) and five delayed paths (NLOS) will be absorbed by the ua s antenna. In this scenario, the downlink signal which is received by the ub will be ignored, because it is supposed to have only one Line-of-Sight path (direct path) [3], [7]. The vicinity of the user, ua, includes many clutters and obstructions whose locations relative to the user ua will determine the time delay length the diffuse downlink signal may experience [7], [8], [9]. It is known that the most delayed path will travel the longest distance from the HAPS M-55 aircraft to the ua and the vise-versa. For this purpose, the time delay of the five paths will be set based on into two assumptions; assumax specifies the maximum time delay of the last diffuse path (the fifth delayed path) does not exceed the time duration of the transmitted symbol, and the maximum range of the surrounding clutter is less than 300 m. Also assumin specifies that the minimum time delay of the first diffuse path (the first delayed path) is longer than the time duration of the transmitted symbol, and the minimum range of surrounding clutter is more than 300 m. This scenario will focus on the multipath effects on the downlink signal based on this manner, and by using the Racian multipath fading channel [2], [8], [10], [11], [12]. Settings used for the user, ua, in this scenario are shown in Table (4) as below: 341

7 Coverage area Propagation environment Table (4) Settings of the user, ua Total signal path No. User speed (m/s) Max Doppler shift (Hz) K-Factor Suburban Outdoor III. RESULTS AND DISCUSSION Downlink signal level The downlink signal was simulated under two different weather conditions (clear & rainy) and plotted in Fig.3. as shown below: Fig.3. Downlink signal versus elevation angle of reception It is observed that the downlink signal level varies as a function of the elevation angle of reception and the weather condition. In other words, its level drops while a rainfall event is taking place, especially when the elevation angle of reception decreases. ua will only enjoy the same downlink signal level as the ub does when the sky is clear, but will not do during the rainfall events; regardless their downlink signals experience the same rainfall rate. Downlink signal experience The downlink signal was simulated based on the assumax multipath assumption and plotted in Fig.4. as shown below: 342

8 Fig.4. Magnitude of downlink signal versus time delay and frequency It is noticed that the downlink signal received by the user, ua experiences a frequency-flat fading, caused due to the time dispersion phenomena. The scattering and reflection of the downlink signal on the surfaces and edges of the clutters, which are located at not farther than 300 m from the user, ua, produced several replicas of the downlink signal. Those replicas were delayed and received at irresolvable time points within the time duration of the direct received symbol. It seen that the five delayed components combine and get clustered at approximately the time zero second as shown in Fig.4. Also the downlink signal was simulated based on the assumin multipath assumption and plotted in Fig.5. as shown below: 343

9 Fig.5. Magnitude of downlink signal vs. time delay and frequency It is seen in this time that the downlink signal received by the user, ua experiences a frequency-selective fading, resulted from the delayed signals which were received randomly at different time points higher than time duration of the direct received symbol. It is clear that the five delayed components are aligned to the time line relative to their locations (at distances farther than 300 m) to the user, ua as shown in Fig.5. IV. CONCLUSION It can be concluded that the downlink signal of the HAPS M-55 aircraft (service signal) may be affected by several propagation impairments such as the rain attenuation, especially in Malaysia, where the rainfall events are dominant weather conditions along the year. Also the signal multipath fading is a matter of importance that can affect the downlink signal level either with flat or selective fading unless proper fade mitigation techniques (FMTs) are applied. 344

10 V. REFERENCES 1. GEOSCAN (UK) Plc., et al. (2004), project profile, Moscow, Russia 2. Alejandro.A.Z., et. al. (2008), High-Altitude Platforms for Wireless Communications, 1st ed, John Wiley & Sons Ltd, Chichester, UK, 3. International Telecommunication Union (2002), Technical and operational characteristics for the fixed service using high altitude platform stations in the bands GHz and GHz, Rec. ITU-R F.1569, Geneva, Switzerland 4. Wanis.A.H. (2010), Evaluation of potential interference and rain effects on GHz downlink broadcasting satellite signal in Malaysia, Johor, Malaysia 5. Cheblil.J (1997) Rain rate and rain attenuation distribution for Microwave study in Malaysia, Johor, Malaysia 6. International Telecommunication Union (2007), Propagation Data and Prediction Methods Required for the Design of Earth-Space Telecommunications Systems, Rec. ITU-R P.618-9, Geneva, Switzerland 7. José.L. C., et. al. Channel Modeling and Simulation in HAPS Systems, Catalonia, Spain 8. Fabio.D., et. al. (2002), Small-Scale Fading for High-Altitude Platform (HAP) Propagation Channels, IEEE Journal on Selected Areas in Communications, vol. 20, No. 3, April International Telecommunication Union (2012), Propagation data required for the design of Earth-space aeronautical mobile telecommunication systems, Geneva, Switzerland 10. Vogel, W. J. and J. Goldhirsh (1995), Multipath Fading at L-Band for Low Elevation Angle, Land Mobile Satellite Scenarios, IEEE Journal on Selected Areas in Communications, Vol. 13, No. 2, February International Telecommunication Union (2001), Propagation data required for the design of Earth-space land mobile telecommunication systems, ITU-R P.682-3, Geneva, Switzerland 12. Fernando U-V. and Delgado-P, Performance simulation in high altitude platforms (HAPS) communications systems, Catalonia, Spain 345