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Communication using high-altitude platform (HAP) is a cost-effective and an easilydeployable alternative to existing satellite and terrestrial communications. This articledescribes the technology behind HAP communication while also covering signalpropagation, losses associated with this communication, antennae used and challengesbefore researchers

Wireless Communication UsingHigh-Altitude Platform

DR JIBENDU SEKHAR ROY

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, exibility, recongurability, mo-

bility and lighter payload.

HAP architecture

HAP can take any form such as a bal-

loon, a powered unmanned airship or a

powered manned aeroplane that keeps

station in the winds (that is, oating orquasi-stationed) at a high altitude of

18-25 km. International Telecommuni-

cation 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 al-

location in different cells within a zone

uses frequency reuse technique (where

the same frequency is used after a

certain spatial separation). One HAPmay communicate with another HAP

B

ecause of a high demand for

various types of communica-

tion services, wireless solutions

are becoming increasingly important.

Next-generation wireless communica-

tion with high data rate and multime-

dia services needs broadband wireless

access. Wireless broadband multime-

dia 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 wirelesscommunication 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

difcult to use satellite services for

general public communication pur-

poses.

An alternative to terrestrial and

satellite infrastructure, using high-al-

titude platform (HAP) in stratospheric

altitude, was rst proposed in 1992.

Broadband wireless communication

using HAP is a low-cost and an eas-

ily deployable satellite service, where

HAP is placed at a lower altitude of at-

mosphere (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 commu-

nication. The satellite can use forward

channel towards user terminals (xedor mobile), control and management

Fig. 1: Basic architecture of HAP

LEO/MEO/GEO

SATELLITE

HAP

AIRSHIP/BALOON

HAP

COVERAGE

SMALL APERTURE

HAP

COVERAGE

HAP

COVERAGE UHF/VHF

TOWER

BIG APERTURE

ZONE-1ZONE-2

ZONE-3



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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 numberof 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 stud-

ies 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 alti-

tude 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 re-

ception of erratic infor-

mation from the trans-

mitter, the receiver sends

a request to the transmitter through a

feedback path to repeat the transmis-

sion again.

Presence of raindrops can se-

verely degrade the reliability and

performance of communication links at

frequencies above 10 GHz. The attenu-

ation 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 adja-

cent 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 communica-

tion using HAP, directional antennae

are required. Normal horn antennae,

multibeam horn antennae or digitally-

controlled array antennae may alsobe used for HAP. Very small aperture

Fig. 2: Wind velocity with respect to the altitude

MINIMUM

WIND SPEED

A L T I T U D E ( k m )

40

35

30

25

20

15

10

5

0

0 10 20 30 40 50 60 70

WIND SPEED (m/sec)



60 MAY 2013 | ELECTRONICS FOR YOU WWW.EFYMAG.COM

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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 appli-

cations where space is not a

constraint.

Modulation andcoding

For good network capacity

and highest spectral efciency,

suitable modulation and cod-

ing schemes are required in

broadband communication

using HAP. Application-based

adaptive techniques may pro-

vide better communication

with specied 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 chan-

nel 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 inHAP-based broadbandcommunication


using HAP is a relatively new technol-

ogy in the area of communications.

One big task for the researchers is de-

velopment of a suitably-shaped HAP

that is light-weight and provides wide

coverage. The HAP structure should

use renewable energy efciently. Ef-

cient channel assignment and resource

allocation schemes need to be devel-

oped for HAP communication.

Another research problem is mod-

eling of HAP channel, where three

cases are to be considered: rst 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 environ-

ments. Proper management and plan-

ning are necessary for integration of

HAP technology with terrestrial andsatellite infrastructures.

Development of traffic manage-

ment 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 develop-

ment of directive antennae with a high

gain is necessary, especially for HAP

communication with small-size and

low-prole terminals.

To sum up

HAP can be a very important technol-

ogy for 4G wireless communication.


using HAP is effective for many pur-

poses 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 com-

munication are efcient monitoring of

stations, high-end antenna technology

and airship manufacturing. However,

there are some hurdles to overcome.

For instance, it is difcult to maintain

the position of the airships (HAPs),

above a xed position on the ground,

by producing high power using renew-

able sources.

The author is a professor at the School of Elec-

tronics Engineering, KIIT University, Bhubaneswar,Odisha

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 com-

munication 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 in-

creases the efciency of radio resource

management. Due to wind, the dis-

placement of HAP can be both in

horizontal and vertical directions. In

this situation, angular variation can

be used to determine whether xed

or phased array antennae are required

to achieve a given link budget in HAP

communication.

The beam width of a receiving an-

tenna may vary from two

degrees to 20

degrees. Typical HAP antenna gain is

Fig. 3: Path losses for terrestrial and HAP network

TERRESTRIAL LINK

HAP LINK

DISTANCE FROM THE CENTRE OF A CELL (km)

P A T H

L O S S

( d B )

170

150

130

90

0 10 20 30 40 5070

110

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