Wireless Communication Using High-Altitude Platform
Communication using high-altitude platform (HAP) is a cost-effective and an easily deployable alternative to existing satellite and terrestrial communications. This article describes the technology behind HAP communication while also covering signal propagation, losses associated with this communication, antennae used and challenges before researchers
Because of a high demand for various types of communication services, wireless solutions are becoming increasingly important. Next-generation wireless communication with high data rate and multimedia services needs broadband wireless access. Wireless broadband multimedia services will provide a convergence of telecommunication, TV, Internet, video-on-demand, etc.
Satellite systems can be used for broadband personal communications through mobile and fixed wireless communication devices. Satellite offers a moderate capacity and is mostly used by corporate users. But, because of the high cost and high signal attenuation in satellite communication, it is very difficult to use satellite services for general public communication purposes.
An alternative to terrestrial and satellite infrastructure, using high-altitude platform (HAP) in stratospheric altitude, was first proposed in 1992. Broadband wireless communication using HAP is a low-cost and an easily deployable satellite service, where HAP is placed at a lower altitude of atmosphere (stratosphere) than a satellite orbit. Geostationary earth orbit (GEO), low-earth orbit (LEO) or medium-earth orbit (MEO) satellites can be used for onboard processing in HAP communication. The satellite can use forward channel towards user terminals (fixed or mobile), control and management stations, and HAP or a set of HAPs.
There are several advantages of broadband communication using HAP, including easy deployment, lower cost of operation than satellite communication, point-to-point and point-to-multipoint communication, easy maintainance, low path loss, high elevation and hence wide coverage area, flexibility, reconfigurability, mobility and lighter payload.
HAP architecture
HAP can take any form such as a balloon, a powered unmanned airship or a powered manned aeroplane that keeps station in the winds (that is, floating or quasi-stationed) at a high altitude of 18-25 km. International Telecommunication Union (ITU) has recommended 28GHz and 31GHz frequency bands for HAP communications.
The basic architecture of HAP is shown in Fig. 1. Satellite signal is downloaded by the HAP, placed at an altitude of 18–25 km. The signal is communicated to the user terminals on the earth. Each HAP covers a wide zone (like zone 1, zone 2 and zone 3, as shown in Fig. 1) and each zone is divided into smaller cells. Channel allocation in different cells within a zone uses frequency reuse technique (where the same frequency is used after a certain spatial separation). One HAP may communicate with another HAP covering another zone. HAP acts as a hub for communication. Altitude of 18-25 km is chosen because the average wind speed at this height is minimum and the coverage of the antennae can be a footprint of up to 80km diameter with HAP altitude of 20 km, resulting in a cellular service for a large number of users over a wide area. The variation of wind speed with altitude is shown in Fig. 2.
Propagation and losses
Line-of-sight (LOS) path is required for communication through HAP. Signals at this frequency band are attenuated by rain. Polarisation diversity (where signals with different polarisations are used) may be acceptable in faded environments for good-quality signal reception.
In HAP communication, signal delay is negligible compared to direct satellite communication. Several studies verify that propagation path loss on the HAP link increases with the square of the distance (d2) instead of d4 as in terrestrial systems. The path loss in dB may be obtained using the relationship:
L = 32.4 + 10 log f2 + 10 log d2
where ‘f’ is the frequency in MHz and ‘d’ is the distance between the HAP antenna and the user in kilometres. Here the curvature of the earth is neglected for a coverage diameter smaller than 100 km. This behaviour of the path loss is shown in Fig. 3.
If HAP is at an altitude of 20 km, the free-space loss at 1800MHz frequency band may be 120-130 dB. To handle the increased losses due to multipath fading in non-line-of-sight (NLOS) environment, automatic repeat request (ARQ) may be introduced. In ARQ scheme, after reception of erratic information from the transmitter, the receiver sends a request to the transmitter through a feedback path to repeat the transmission again.
Presence of raindrops can severely degrade the reliability and performance of communication links at frequencies above 10 GHz. The attenuation due to rain can be expressed as:
A = aRb
where ‘A’ is the attenuation (in dB/km), ‘R’ is the rain rate (in mm/hour), and ‘a’ and ‘b’ are factors depending on the rain drop size and frequency.
Co-channel interference and adjacent channel interference are the other important factors which may cause signal losses at the receiver antenna in HAP communication. Co-channel interference may increase due to cross-polarisation losses at the receiver antenna.
Requirement of antennae
Various types of antennae are used for broadband wireless communications, many of which are omnidirectional. In broadband wireless communication using HAP, directional antennae are required. Normal horn antennae, multibeam horn antennae or digitally-controlled array antennae may also be used for HAP. Very small aperture terminal (VSAT) is one of the effective antennae for HAP communication.
At the subscriber’s end, for small terminals like mobile handsets, small vehicles and laptops, VSAT cannot be used. In this case, small dipoles and planar antennae can be used as directional antennae. The antennae should have relatively high gain. Gain decreases as the size of the antenna decreases. Planar antenna array can be a good option for small terminals in HAP communication.
Smart antenna technology can play a key role in broadband communication using HAP for high-speed vehicles like trains and helicopters. In smart antenna technology the antenna radiates a directive beam towards the subscriber based on the direction of arrival and the time of arrival of the signal coming from the subscriber. Signal processing part of the antenna system performs this job.
Smart antenna technology increases the efficiency of radio resource management. Due to wind, the displacement of HAP can be both in horizontal and vertical directions. In this situation, angular variation can be used to determine whether fixed or phased array antennae are required to achieve a given link budget in HAP communication.
The beam width of a receiving antenna may vary from two degrees to 20 degrees. Typical HAP antenna gain is 22-25 dBi, whereas typical gain of a train antenna is 17 dBi.
Diversity techniques using two or more antennae may be useful for vehicular applications where space is not a constraint.
Modulation and coding
For good network capacity and highest spectral efficiency, suitable modulation and coding schemes are required in broadband communication using HAP. Application-based adaptive techniques may provide better communication with specified quality of service and bit-error rate. Quadrature amplitude modulation, Quadrature phase-shift keying and Gaussian minimum-shift keying are recommended modulation techniques for HAP communication. Powerful forward-error correction (FEC) code may be useful when channel conditions are poor to maintain the communication link. Also, codes like convolutional code, turbo code and Reed-Solomon code can be used for better performance.
Research challenges in HAP-based broadband communication
Broadband wireless communication using HAP is a relatively new technology in the area of communications. One big task for the researchers is development of a suitably-shaped HAP that is light-weight and provides wide coverage. The HAP structure should use renewable energy efficiently. Efficient channel assignment and resource allocation schemes need to be developed for HAP communication.
Another research problem is modeling of HAP channel, where three cases are to be considered: first is the line-of-sight, second is the shadowing (by trees and/or small obstacles) and third is the full blockage of signal (by large obstacles like mountains and big buildings).
Since HAP communication uses wide frequency bandwidth, frequency band-wise studies of propagation and losses at high altitudes are important research topics. These studies must include both LOS and NLOS environments. Proper management and planning are necessary for integration of HAP technology with terrestrial and satellite infrastructures.
Development of traffic management algorithm is another issue. The feasibility of communication of inter-platform links at high altitude is to be investigated. Research on adaptive-based modulation, coding and networking is necessary for HAP communication. Design and development of directive antennae with a high gain is necessary, especially for HAP communication with small-size and low-profile terminals.
To sum up
HAP can be a very important technology for 4G wireless communication. Broadband wireless communication using HAP is effective for many purposes including disaster management, communications in rugged terrains, monitoring of sports events (motor cycling, car racing, cycle racing, etc), certain military purposes and time-limited additional support to existing communication. Broadband wireless access through HAP may be used to provide broadband Internet access and broadband multimedia services in high-speed trains. In this case, Doppler frequency shift plays an important role in HAP communication. HAPs can also be used for applications like remote sensing, navigation and surveillance.
The difficulties with HAP communication are efficient monitoring of stations, high-end antenna technology and airship manufacturing. However, there are some hurdles to overcome. For instance, it is difficult to maintain the position of the airships (HAPs), above a fixed position on the ground, by producing high power using renewable sources.
Communication using high-altitude platform (HAP) is a cost-effective and an easily deployable alternative to existing satellite and terrestrial communications. This article describes the technology behind HAP communication while also covering signal propagation, losses associated with this communication, antennae used and challenges before researchers
Because of a high demand for various types of communication services, wireless solutions are becoming increasingly important. Next-generation wireless communication with high data rate and multimedia services needs broadband wireless access. Wireless broadband multimedia services will provide a convergence of telecommunication, TV, Internet, video-on-demand, etc.
Satellite systems can be used for broadband personal communications through mobile and fixed wireless communication devices. Satellite offers a moderate capacity and is mostly used by corporate users. But, because of the high cost and high signal attenuation in satellite communication, it is very difficult to use satellite services for general public communication purposes.
 Fig. 1: Basic architecture of HAP |
An alternative to terrestrial and satellite infrastructure, using high-altitude platform (HAP) in stratospheric altitude, was first proposed in 1992. Broadband wireless communication using HAP is a low-cost and an easily deployable satellite service, where HAP is placed at a lower altitude of atmosphere (stratosphere) than a satellite orbit. Geostationary earth orbit (GEO), low-earth orbit (LEO) or medium-earth orbit (MEO) satellites can be used for onboard processing in HAP communication. The satellite can use forward channel towards user terminals (fixed or mobile), control and management stations, and HAP or a set of HAPs.
There are several advantages of broadband communication using HAP, including easy deployment, lower cost of operation than satellite communication, point-to-point and point-to-multipoint communication, easy maintainance, low path loss, high elevation and hence wide coverage area, flexibility, reconfigurability, mobility and lighter payload.
HAP architecture
HAP can take any form such as a balloon, a powered unmanned airship or a powered manned aeroplane that keeps station in the winds (that is, floating or quasi-stationed) at a high altitude of 18-25 km. International Telecommunication Union (ITU) has recommended 28GHz and 31GHz frequency bands for HAP communications.
The basic architecture of HAP is shown in Fig. 1. Satellite signal is downloaded by the HAP, placed at an altitude of 18–25 km. The signal is communicated to the user terminals on the earth. Each HAP covers a wide zone (like zone 1, zone 2 and zone 3, as shown in Fig. 1) and each zone is divided into smaller cells. Channel allocation in different cells within a zone uses frequency reuse technique (where the same frequency is used after a certain spatial separation). One HAP may communicate with another HAP covering another zone. HAP acts as a hub for communication. Altitude of 18-25 km is chosen because the average wind speed at this height is minimum and the coverage of the antennae can be a footprint of up to 80km diameter with HAP altitude of 20 km, resulting in a cellular service for a large number of users over a wide area. The variation of wind speed with altitude is shown in Fig. 2.
 Fig. 2: Wind velocity with respect to the altitude |
Propagation and losses
Line-of-sight (LOS) path is required for communication through HAP. Signals at this frequency band are attenuated by rain. Polarisation diversity (where signals with different polarisations are used) may be acceptable in faded environments for good-quality signal reception.
In HAP communication, signal delay is negligible compared to direct satellite communication. Several studies verify that propagation path loss on the HAP link increases with the square of the distance (d2) instead of d4 as in terrestrial systems. The path loss in dB may be obtained using the relationship:
L = 32.4 + 10 log f2 + 10 log d2
where ‘f’ is the frequency in MHz and ‘d’ is the distance between the HAP antenna and the user in kilometres. Here the curvature of the earth is neglected for a coverage diameter smaller than 100 km. This behaviour of the path loss is shown in Fig. 3.
If HAP is at an altitude of 20 km, the free-space loss at 1800MHz frequency band may be 120-130 dB. To handle the increased losses due to multipath fading in non-line-of-sight (NLOS) environment, automatic repeat request (ARQ) may be introduced. In ARQ scheme, after reception of erratic information from the transmitter, the receiver sends a request to the transmitter through a feedback path to repeat the transmission again.
Presence of raindrops can severely degrade the reliability and performance of communication links at frequencies above 10 GHz. The attenuation due to rain can be expressed as:
A = aRb
where ‘A’ is the attenuation (in dB/km), ‘R’ is the rain rate (in mm/hour), and ‘a’ and ‘b’ are factors depending on the rain drop size and frequency.
Co-channel interference and adjacent channel interference are the other important factors which may cause signal losses at the receiver antenna in HAP communication. Co-channel interference may increase due to cross-polarisation losses at the receiver antenna.
Requirement of antennae
Various types of antennae are used for broadband wireless communications, many of which are omnidirectional. In broadband wireless communication using HAP, directional antennae are required. Normal horn antennae, multibeam horn antennae or digitally-controlled array antennae may also be used for HAP. Very small aperture terminal (VSAT) is one of the effective antennae for HAP communication.
At the subscriber’s end, for small terminals like mobile handsets, small vehicles and laptops, VSAT cannot be used. In this case, small dipoles and planar antennae can be used as directional antennae. The antennae should have relatively high gain. Gain decreases as the size of the antenna decreases. Planar antenna array can be a good option for small terminals in HAP communication.
Smart antenna technology can play a key role in broadband communication using HAP for high-speed vehicles like trains and helicopters. In smart antenna technology the antenna radiates a directive beam towards the subscriber based on the direction of arrival and the time of arrival of the signal coming from the subscriber. Signal processing part of the antenna system performs this job.
Smart antenna technology increases the efficiency of radio resource management. Due to wind, the displacement of HAP can be both in horizontal and vertical directions. In this situation, angular variation can be used to determine whether fixed or phased array antennae are required to achieve a given link budget in HAP communication.
The beam width of a receiving antenna may vary from two degrees to 20 degrees. Typical HAP antenna gain is 22-25 dBi, whereas typical gain of a train antenna is 17 dBi.
Diversity techniques using two or more antennae may be useful for vehicular applications where space is not a constraint.
Modulation and coding
For good network capacity and highest spectral efficiency, suitable modulation and coding schemes are required in broadband communication using HAP. Application-based adaptive techniques may provide better communication with specified quality of service and bit-error rate. Quadrature amplitude modulation, Quadrature phase-shift keying and Gaussian minimum-shift keying are recommended modulation techniques for HAP communication. Powerful forward-error correction (FEC) code may be useful when channel conditions are poor to maintain the communication link. Also, codes like convolutional code, turbo code and Reed-Solomon code can be used for better performance.
Research challenges in HAP-based broadband communication
Broadband wireless communication using HAP is a relatively new technology in the area of communications. One big task for the researchers is development of a suitably-shaped HAP that is light-weight and provides wide coverage. The HAP structure should use renewable energy efficiently. Efficient channel assignment and resource allocation schemes need to be developed for HAP communication.
 Fig. 3: Path losses for terrestrial and HAP network |
Another research problem is modeling of HAP channel, where three cases are to be considered: first is the line-of-sight, second is the shadowing (by trees and/or small obstacles) and third is the full blockage of signal (by large obstacles like mountains and big buildings).
Since HAP communication uses wide frequency bandwidth, frequency band-wise studies of propagation and losses at high altitudes are important research topics. These studies must include both LOS and NLOS environments. Proper management and planning are necessary for integration of HAP technology with terrestrial and satellite infrastructures.
Development of traffic management algorithm is another issue. The feasibility of communication of inter-platform links at high altitude is to be investigated. Research on adaptive-based modulation, coding and networking is necessary for HAP communication. Design and development of directive antennae with a high gain is necessary, especially for HAP communication with small-size and low-profile terminals.
To sum up
HAP can be a very important technology for 4G wireless communication. Broadband wireless communication using HAP is effective for many purposes including disaster management, communications in rugged terrains, monitoring of sports events (motor cycling, car racing, cycle racing, etc), certain military purposes and time-limited additional support to existing communication. Broadband wireless access through HAP may be used to provide broadband Internet access and broadband multimedia services in high-speed trains. In this case, Doppler frequency shift plays an important role in HAP communication. HAPs can also be used for applications like remote sensing, navigation and surveillance.
The difficulties with HAP communication are efficient monitoring of stations, high-end antenna technology and airship manufacturing. However, there are some hurdles to overcome. For instance, it is difficult to maintain the position of the airships (HAPs), above a fixed position on the ground, by producing high power using renewable sources.
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