Upload
felipehdz
View
226
Download
0
Embed Size (px)
DESCRIPTION
EFY-JSROY Wireless Communication EFY
Citation preview
7/21/2019 EFY-JSROY Wireless Communication EFY
http://slidepdf.com/reader/full/efy-jsroy-wireless-communication-efy 1/3
58 MAY 2013 | ELECTRONICS FOR YOU WWW.EFYMAG.COM
TELECOM-WIRELESS
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
7/21/2019 EFY-JSROY Wireless Communication EFY
http://slidepdf.com/reader/full/efy-jsroy-wireless-communication-efy 2/3
59WWW.EFYMAG.COM ELECTRONICS FOR YOU | MAY 2013
TELECOM-WIRELESS
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)
7/21/2019 EFY-JSROY Wireless Communication EFY
http://slidepdf.com/reader/full/efy-jsroy-wireless-communication-efy 3/3
60 MAY 2013 | ELECTRONICS FOR YOU WWW.EFYMAG.COM
TELECOM-WIRELESS
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
Broadband wireless communication
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.
Broadband 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